Co-reporter:Joseph A. Burg and Reinhold H. Dauskardt
The Journal of Physical Chemistry B October 19, 2017 Volume 121(Issue 41) pp:9753-9753
Publication Date(Web):October 4, 2017
DOI:10.1021/acs.jpcb.7b09615
An asymmetric elastic modulus is a recently discovered and unexpected property of hybrid molecular materials that has significant implications for their underlying thermomechanical reliability. Elastic asymmetries are inherently related to terminal groups in the molecular structure, which limit network connectivity. Terminal groups sterically interact to stiffen the network in compression, while they disconnect the network and interact significantly less in tension. Here we study the importance of terminal group molecular weight and size (OH, methyl, vinyl, and phenyl) on the resulting elastic asymmetries and find that increasing the terminal group size actually leads to even larger degrees of asymmetry. As a result, we develop a molecular design criterion to predict how molecular structure affects the mechanical properties, a vital step toward integrating hybrid molecular materials into emerging nanotechnologies.
Co-reporter:Brian L. Watson;Nicholas Rolston;Kevin A. Bush;Leila Taleghani
Journal of Materials Chemistry A 2017 vol. 5(Issue 36) pp:19267-19279
Publication Date(Web):2017/09/19
DOI:10.1039/C7TA05004F
Solution-processed organic semiconducting materials feature prominently in modern optoelectronic devices, especially where low-cost and flexibility are specific goals, such as perovskite solar cells. Their intrinsic solubility, poor cohesion and lack of adhesion to underlying substrates, however, curtail their scope of application and durability. To overcome this, a mechanically stiff, light-activated, tetra-azide cross-linking agent, 1,3,5,7-tetrakis-(p-benzylazide)-adamantane (TPBA), has been developed to transform solution processed organic polymers into solvent-resistant and mechanically tough films. The use of 3-azidopropyltrimethoxysilane (AzPTMS) has been developed as a light-activated adhesion promotor, enabling mechanical testing of toughened, cross-linked polymers. Lithium bis(trifluoromethane)sulfonimide (LiTFSI) doped poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine, poly(triaryl amine) (PTAA), a hole-transporting material used in perovskite solar cells, has been selected as a candidate system for demonstrating the utility of TPBA to transform a fragile and highly-soluble hole-transporting organic semiconductor into a mechanically tough and solvent-resistant semiconducting composite. TPBA enables the solvent resistance and mechanical toughness of PTAA to be tuned without compromising the electronic functionality of the semiconducting material. While increasing the fracture toughness of PTAA by over 300%, TPBA cross-linking also enables fabrication of perovskite solar cells with increased photovoltaic efficiencies in n–i–p and p–i–n geometries, and promotes adhesion of the doped polymer to the perovskite layer, mitigating interfacial device failure.
Co-reporter:Brian L. Watson;Nicholas Rolston;Adam D. Printz
Energy & Environmental Science (2008-Present) 2017 vol. 10(Issue 12) pp:2500-2508
Publication Date(Web):2017/12/06
DOI:10.1039/C7EE02185B
The relative insensitivity of the optoelectronic properties of organometal trihalide perovskites to crystallographic defects and impurities has enabled fabrication of highly-efficient perovskite solar cells by scalable solution-state deposition techniques well suited to low-cost manufacturing. Fracture analyses of state-of-the-art devices, however, have revealed that both the perovskite active layer and adjacent carrier selective contacts are mechanically fragile—a major obstacle to technological maturity that stands to significantly compromise their thermomechanical reliability and operational lifetimes. We report a new concept in solar cell design, the compound solar cell (CSC), which addresses the intrinsic fragility of these materials with mechanically reinforcing internal scaffolds. The internal scaffold effectively partitions a conventional monolithic planar solar cell into an array of dimensionally scalable and mechanically shielded individual perovskite cells that are laterally encapsulated by the surrounding scaffold and connected in parallel via the front and back electrodes. The CSCs exhibited a significantly increased fracture energy of ∼13 J m−2—a 30-fold increase over previously reported planar perovskite (∼0.4 J m−2)—while maintaining efficiencies comparable to planar devices. Notably, the efficiency of the microcells formed within the scaffold is comparable to planar devices on an area-adjusted basis. This development is a significant step in demonstrating robust perovskite solar cells to achieve increased reliability and service lifetimes comparable to c-Si, CIGS, and CdTe solar cells.
Co-reporter:Nicholas Rolston;Adam D. Printz;Florian Hilt;Michael Q. Hovish;Karsten Brüning;Christopher J. Tassone
Journal of Materials Chemistry A 2017 vol. 5(Issue 44) pp:22975-22983
Publication Date(Web):2017/11/14
DOI:10.1039/C7TA09178H
We report on submicron organosilicate barrier films produced rapidly in air by a scalable spray plasma process that improves both the stability and efficiency of perovskite solar cells. The plasma is at sufficiently low temperature to prevent damage to the underlying layers. Oxidizing species and heat from the plasma improve device performance by enhancing both interfacial contact and the conductivity of the hole transporting layer. The thickness of the barrier films is tunable and transparent over the entire visible spectrum. The morphology and density of the barrier are shown to improve with the addition of a fluorine-based precursor. Devices with submicron coatings exhibited significant improvements in stability, maintaining 92% of their initial power conversion efficiencies after more than 3000 h in dry heat (85 °C, 25% RH) while also being resistant to degradation under simulated operational conditions of continuous exposure to light, heat, and moisture. X-ray diffraction measurements performed while heating showed the barrier film dramatically slows the formation of PbI2. The barrier films also are compatible with flexible devices, exhibiting no signs of cracking or delamination after 10 000 bending cycles on a 127 μm substrate with a bending radius of 1 cm.
Co-reporter:Nicholas Rolston, Adam D. Printz, Stephanie R. Dupont, Eszter Voroshazi, Reinhold H. Dauskardt
Solar Energy Materials and Solar Cells 2017 Volume 170(Volume 170) pp:
Publication Date(Web):1 October 2017
DOI:10.1016/j.solmat.2017.06.002
•Subcritical debonding test measures in situ the effects of environmental stressors.•Heat in inert conditions weakens ZnO/P3HT:PCBM interface.•Heat, moisture, and UV in ambient strengthens ZnO/P3HT:PCBM interface.•Effect of environmental stressors on stability informs reliable device design.Organic solar cells subjected to environmental stressors such as heat, moisture, and UV radiation can undergo significant mechanical degradation, leading to delamination of layers and device failure. This paper reports the effect these stressors have on the mechanical integrity of active layers and interfaces as measured by subcritical debonding tests, and the in situ evolution of defects and fracture processes is characterized. At elevated temperatures below 50 °C in inert conditions, significant device weakening was observed, an effect we attributed to a temperature-induced P3HT:PCBM delamination mechanism from the underlying ZnO. At 50 °C in ambient conditions with UV exposure—selected to better simulate real-world environments—devices were more resistant to fracture because of an interfacial strengthening effect from increased hydrogen bonding where UV-induced Zn(OH)2 formation reinforced the interface with P3HT:PCBM. This photoinduced hydroxylation mechanism was determined from a decrease in the Zn/O ratio with increased UVA or UVB exposure, and hydroxylation was shown to directly correlate with the resistance to fracture in devices.Download high-res image (198KB)Download full-size image
Carbon-bridges were successfully incorporated into the molecular structure of inorganic silicate films deposited onto polymer substrates using an oxidative atmospheric plasma deposition process. Key process parameters that include the precursor chemistry and delivery rate are discussed in the context of a deposition model. The resulting coating exhibited significantly improved adhesion and a 4-fold increase in moisture resistance as determined from the threshold for debonding in humid air compared to dense silica or commercial sol–gel polysiloxane coatings. Other important parameters for obtaining highly adhesive coating deposition on oxidation-sensitive polymer substrates using atmospheric plasma were also investigated to fully activate but not overoxidize the substrate. The resulting carbon molecular bridged adhesive coating showed enhanced moisture resistance, important for functional membrane applications.Keywords: adhesion; atmospheric plasma; functional membrane; hybrid coating; moisture resistance
The active layers of perovskite solar cells are also structural layers and are central to ensuring that the structural integrity of the device is maintained over its operational lifetime. Our work evaluating the fracture energies of conventional and inverted solution-processed MAPbI3 perovskite solar cells has revealed that the MAPbI3 perovskite exhibits a fracture resistance of only ∼0.5 J/m2, while solar cells containing fullerene electron transport layers fracture at even lower values, below ∼0.25 J/m2. To address this weakness, a novel styrene-functionalized fullerene derivative, MPMIC60, has been developed as a replacement for the fragile PC61BM and C60 transport layers. MPMIC60 can be transformed into a solvent-resistant material through curing at 250 °C. As-deposited films of MPMIC60 exhibit a marked 10-fold enhancement in fracture resistance over PC61BM and a 14-fold enhancement over C60. Conventional-geometry perovskite solar cells utilizing cured films of MPMIC60 showed a significant, 205% improvement in fracture resistance while exhibiting only a 7% drop in PCE (13.8% vs 14.8% PCE) in comparison to the C60 control, enabling larger VOC and JSC values. Inverted cells fabricated with MPMIC60 exhibited a 438% improvement in fracture resistance with only a 6% reduction in PCE (12.3% vs 13.1%) in comparison to those utilizing PC61BM, again producing a higher JSC.Keywords: electron transport material; fracture; perovskite solar cells; reliability; solvent resistance
Co-reporter:Ryan Brock, Raunaq Rewari, Fernando D. Novoa, Peter Hebert, James Ermer, David C. Miller, Reinhold H. Dauskardt
Solar Energy Materials and Solar Cells 2016 Volume 153() pp:78-83
Publication Date(Web):August 2016
DOI:10.1016/j.solmat.2016.04.027
•Developed new composite dual cantilever beam thin-film adhesion testing method.•Applied cDCB to measure adhesion of antireflective layers on germanium MJ cell.•Quantified degradation of adhesion during long-term 85 °C–85% RH exposure.We discuss the development of a new composite dual cantilever beam (cDCB) thin-film adhesion testing method, which enables the quantitative measurement of adhesion on the thin and fragile substrates used in multijunction photovoltaics. In particular, we address the adhesion of several 2- and 3-layer antireflective coating systems on multijunction cells. By varying interface chemistry and morphology through processing, we demonstrate the marked effects on adhesion and help to develop an understanding of how high adhesion can be achieved, as adhesion values ranging from 0.5 J/m2 to 10 J/m2 were measured. Damp heat (85 °C/85% RH) was used to invoke degradation of interfacial adhesion. We demonstrate that even with germanium substrates that fracture relatively easily, quantitative measurements of adhesion can be made at high test yield. The cDCB test is discussed as an important new methodology, which can be broadly applied to any system that makes use of thin, brittle, or otherwise fragile substrates.
The presence of defective native copper-oxide (CuxO) remains a challenge for device technologies owing to its detrimental effects on the adhesion, moisture sensitivity and stress-migration. Here we demonstrate a rapid, single-step, and organic-solvent-free sol-gel deposition process that is capable of simultaneously reducing the weak native Cu-oxide while forming a densely connected Cu/hybrid interface. A marked 9-fold improvement in adhesion is reported, along with a substantial decrease in the Cu stress-migration rate during in-situ isothermal stress-relaxation experiments. The enhanced Cu/hybrid interface adhesion and the improved Cu stress-migration performance were attributed to the partial reduction of the ~2 nm native Cu2O layer as demonstrated via atomic-resolution transmission electron microscopy. The hybrid-layer strategy we developed is expected to be effective in not only being a strong candidate for adhesion improvement to Cu, but in promoting Cu stress- and the related electro-migration performance.
Journal of Sol-Gel Science and Technology 2016 Volume 77( Issue 3) pp:620-626
Publication Date(Web):2016 March
DOI:10.1007/s10971-015-3891-1
The strength of bonding at epoxy/SiO2 interface and its susceptibility to environmental degradation have profound impact on the lifetime and reliability of microelectronic devices. The incorporation of hybrid film at epoxy/SiO2 interfaces has been shown to alleviate this challenge, but the working time to produce these highly-adherent hybrid films on silicon has been limited to ~10 min. In this work we demonstrate that, by lowering sol–gel aging temperature to 5 °C, the processing window for producing highly-adherent hybrid films on silicon can be extended to more than 6 h. In addition to the extended processing time, an Arrhenius type relationship between sol–gel aging temperature and the optimal sol–gel aging time was observed, with an overall sol–gel reaction activation energy of approximately 2.03 eV/atom that includes both hydrolysis and polycondensation. The enhanced interfacial adhesion was explained in terms of the graded hybrid film structure as determined by X-ray photoelectron spectroscopy depth profiling. The work has significant implications for the successful integration of hybrid film strategy in device packaging technologies.
Alternating layers of organic and oxide thin films used as diffusion barriers in emerging flexible device technologies are vulnerable to degradation under the influence of mechanical stresses, temperature cycling, photodegradation, and chemically active environmental species. Delamination of the internal organic to oxide interfaces often limits the operational lifetime of the barrier system. We demonstrate a method for increasing the adhesion of organic and oxide thin films by generating nanostructures at the interface. We show that the adhesion of an acrylate to silicon oxide model system can be increased by up to an order of magnitude (from ∼2 J/m2 to 24 J/m2). By altering the diameter and depth of the patterns in the model systems, the adhesion energy can be changed, and the delamination pathway can be controlled. In addition, we show that a patterned interface maintains a higher adhesion than its planar counterpart for all durations of UV–A and UV–B exposure.
Quantifying cohesion and understanding fracture phenomena in thin-film electronic devices are necessary for improved materials design and processing criteria. For organic photovoltaics (OPVs), the cohesion of the photoactive layer portends its mechanical flexibility, reliability, and lifetime. Here, the molecular mechanism for the initiation of cohesive failure in bulk heterojunction (BHJ) OPV active layers derived from the semiconducting polymer poly(3-hexylthiophene) [P3HT] and two monosubstituted fullerenes is examined experimentally and through molecular-dynamics simulations. The results detail how, under identical conditions, cohesion significantly changes due to minor variations in the fullerene adduct functionality, an important materials consideration that needs to be taken into account across fields where soluble fullerene derivatives are used.Keywords: cohesion and fracture; molecular dynamics; P3HT; solar cells; substituted fullerenes; thin films;
Effective bonding of organic/inorganic interfaces especially in high humidity environments is paramount to the structural reliability of modern multilayer device technologies, such as flexible electronics, photovoltaics, microelectronic devices, and fiber-metal laminates used in aerospace applications. We demonstrate the ability to design compositionally graded hybrid organic/inorganic films with an inorganic zirconium network capable of forming a moisture-insensitive bond at the interface between an oxide and organic material. By controlling the chemistry of the deposited films and utilizing time-dependent debonding studies, we were able to correlate the behavior of the hybrid films at high humidity to their underlying molecular structure. As a result, an outstanding threefold improvement in adhesion of silicon/epoxy interfaces can be obtained with the introduction of these films even in high humidity environments.Keywords: hybrid materials; moisture insensitivity; organic/inorganic interfaces; sol−gel; thin films
We demonstrate a dual organic and inorganic precursor method to deposit transparent organosilicate protective bilayer coatings on poly methyl methacrylate (PMMA) substrates with atmospheric plasma deposition in ambient air. The bottom layer was a hybrid organosilicate adhesive layer deposited with dual organic 1,5-cyclooctadiene (CYC) and widely used inorganic tetraethoxysiline (TEOS) precursors. The selection of the organic CYC precursor allowed incorporation of a carbon chain in the organosilicate adhesive layer, which resulted in improved adhesion. The top layer was a dense silica coating with high Young’s modulus and hardness deposited with TEOS. The deposited bilayer structure showed ∼100% transparency in the visible light wavelength region, twice the adhesion energy, and five times the Young’s modulus of commercial polysiloxane sol–gel coatings.Keywords: adhesion; atmospheric plasma; dual precursor method; organosilicate coatings; transparent multifunctional coatings
Co-reporter:Stephanie R. Dupont, Eszter Voroshazi, Dennis Nordlund, Reinhold H. Dauskardt
Solar Energy Materials and Solar Cells 2015 Volume 132() pp:443-449
Publication Date(Web):January 2015
DOI:10.1016/j.solmat.2014.09.013
•Pre electrode annealing is used to control morphology and fracture properties.•Adhesive failure at P3HT:PCBM/PEDOT:PSS for annealing below Tc of PCBM.•Increased interdiffusion and adhesion at P3HT:PCBM/PEDOT:PSS with annealing temperature.•Cohesive failure in the P3HT:PCBM layer for annealing above Tc of PCBM.•µm-sized PCBM cluster formation weakens P3HT:PCBM and decreases device efficiency.The role of pre-electrode deposition annealing on the morphology and the fracture properties of polymer solar cells is discussed. We found an increase in adhesion at the weak P3HT:PCBM/PEDOT:PSS interface with annealing temperature, caused by increased interdiffusion between the organic layers. The formation of micrometer sized PCBM crystallites, which occurs with annealing above the crystallization temperature of PCBM, initially weakened the P3HT:PCBM layer itself. Further annealing improved the cohesion, due to a pull-out toughening mechanism of the growing PCBM clusters. Understanding how the morphology, tuned by annealing, affects the adhesive and cohesive properties in these organic films is essential for the mechanical integrity of OPV devices.
Co-reporter:Veerle Balcaen, Nicholas Rolston, Stephanie R. Dupont, Eszter Voroshazi, Reinhold H. Dauskardt
Solar Energy Materials and Solar Cells 2015 Volume 143() pp:418-423
Publication Date(Web):December 2015
DOI:10.1016/j.solmat.2015.07.019
•Thermal cycling has no significant effect on polymer solar cell reliability.•No evidence of cycle-by-cycle damage accumulation mechanism with cycling.•Slight increase in mechanical integrity with cycling similar to effect of annealing.•Kinetic analysis model shows mechanical stability unaffected after 5 cycles.The role of thermal cycling of inverted P3HT:PCBM-based polymer solar cells is reported. We found that thermal cycling between −40 °C and 85 °C up to 200 cycles had no significant effect on solar cell efficiency and mechanical integrity. On the contrary, the solar cells exhibited a slight increase in fracture resistance, similar to that reported for a post-electrode deposition thermal annealing at 85 °C. Gc increased from 2.6 J/m2 for our control solar cells to a sustained maximum value of 4.0 J/m2 after 25 thermal cycles. Surface analysis on the fractured samples revealed the formation of an intermixed layer between P3HT:PCBM and PEDOT:PSS, causing the debond path to change from adhesive between P3HT:PCBM and PEDOT:PSS to meandering through the intermixed layer. A kinetic analysis was used to model the effect of thermal cycling on the Gc values of polymer cells. The model revealed for cycling between −40 °C and 85 °C that 25 cycles are needed to reach the maximum Gc, which is consistent with our experimental results. After 5 thermal cycles, the effects of heating and cooling have little impact on the mechanical stability of polymer solar cells.
Hybrid organic–inorganic materials that are compositionally graded are excellent candidates for addressing the challenges related to the bonding of polymeric layers and inorganic substrates. Often, the synthesis of these hybrid materials involves the use of kinetically driven and dynamic solution systems where obtaining the desired hybrid molecular structures in the deposited film is not trivial. A coating process is used to selectively deposit a small fraction of the total species in solution with an optimized molecular weight, resulting in compositionally graded hybrid films that are organic-rich toward the top and inorganic-rich toward the bottom. This selective deposition technique provides a unique knob to fine tune the molecular structure of films deposited from dynamic solution systems, resulting in hybrid organic–inorganic films that exhibit an eightfold increase in adhesion of an epoxy/silicon interface.
The highly conductive polymer PEDOT:PSS is a widely used hole transport layer and transparent electrode in organic electronic devices. To date, the mechanical and fracture properties of this conductive polymer layer are not well understood. Notably, the decohesion rate of the PEDOT:PSS layer and its sensitivity to moist environments has not been reported, which is central in determining the lifetimes of organic electronic devices. Here, it is demonstrated that the decohesion rate is highly sensitive to the ambient moisture content, temperature, and mechanical stress. The kinetic mechanisms are elucidated using atomistic bond rupture models and the decohesion process is shown to be facilitated by a chemical reaction between water molecules from the environment and strained hydrogen bonds. Hydrogen bonds are the predominant bonding mechanism between individual PEDOT:PSS grains within the layer and cause a significant loss in cohesion when they are broken. Understanding the decohesion kinetics and mechanisms in these films is essential for the mechanical integrity of devices containing PEDOT:PSS layers and yields general guidelines for the design of more reliable organic electronic devices.
The successful deposition of conductive transparent TiNx/TiO2 hybrid films on both polycarbonate and silicon substrates from a titanium ethoxide precursor is demonstrated in air using atmospheric plasma processing equipped with a high-temperature precursor delivery system. The hybrid film chemical composition, deposition rates, optical and electrical properties along with the adhesion energy to the polycarbonate substrate are investigated as a function of plasma power and plasma gas composition. The film is a hybrid of amorphous and crystalline rutile titanium oxide phases and amorphous titanium nitride that depend on the processing conditions. The visible transmittance increases from 71% to 83% with decreasing plasma power and increasing nitrogen content of the plasma gas. The film resistivity is in the range of ∼8.5 × 101 to 2.4 × 105 ohm cm. The adhesion energy to the polycarbonate substrate varies from ∼1.2 to 8.5 J/m2 with increasing plasma power and decreasing plasma gas nitrogen content. Finally, annealing the film or introducing hydrogen to the primary plasma gas significantly affects the composition and decreases thin-film resistivity.
We investigate the role of molecular weight (MW) of the photoactive polymer poly(3-hexylthiophene) (P3HT) on the temperature-dependent decohesion kinetics of bulk heterojunction (BHJ) organic solar cells (OSCs). The MW of P3HT has been directly correlated to its carrier field effect mobilities and the ambient temperature also affects OSC in-service performance and P3HT arrangement within the BHJ layer. Under inert conditions, time-dependent decohesion readily occurs within the BHJ layer at loads well below its fracture resistance. We observe that by increasing the MW of P3HT, greater resistance to decohesion is achieved. However, failure consistently occurs within the BHJ layer representing the weakest layer within the device stack. Additionally, it was found that at temperatures below the glass transition temperature (∼41–45 °C), decohesion was characterized by brittle failure via molecular bond rupture. Above the glass transition temperature, decohesion growth occurred by a viscoelastic process in the BHJ layer, leading to a significant degree of viscoelastic deformation. We develop a viscoelastic model based on molecular relaxation to describe the resulting behavior. The study has implications for OSC long-term reliability and device performance, which are important for OSC production and implementation.Keywords: fracture; fullerenes; polymer; solar cells; thin films
•Transparent ZnO films were deposited on polymers in ambient air at room temperature.•We study structural, optical, electrical and adhesion properties of the ZnO films.•The visible transmittance increased with decreasing plasma power.•Resistivity as low as 1.8 × 102 ohm cm was obtained with >95% transmittance.•The adhesion energy to acrylic increased with higher plasma power.Transparent zinc oxide (ZnO) thin films have been successfully synthesized on poly (methyl methacrylate) (PMMA), polycarbonate (PC), and polyethylene terephthalate (PET) substrates by atmospheric plasma deposition in ambient air at room temperature. The structural, optical and electrical properties of the ZnO films as well as their adhesion to the polymer substrates were investigated for various deposition conditions. The film surface exhibited a dome-shaped topography comprised of nanometer-sized grains. The size of both the domes and the grains became larger as the plasma power increased. The visible transmittance increased above 95% with decreasing plasma power. The resistivity exhibited a wide variation in the range of 102–108 ohm cm. The adhesion energies to PMMA varied from 0.2 to 1.5 J/m2 with increasing plasma power. While a finer grain structure achieved with lower plasma power was preferable for higher transmittance, it resulted in lower adhesion to the plastic substrates. The study demonstrated the feasibility of depositing semiconducting transparent ZnO films on polymer substrates at low temperature in ambient air using atmospheric plasma deposition.Graphical abstract
For semiconducting polymers, such as regioregular poly(3-hexylthiophene-2,5-diyl) (rr-P3HT), the molecular weight has been correlated to charge carrier field-effect mobilities, surface morphology, and gelation rates in solution and therefore has important implications for long-term reliability, manufacturing, and future applications of electronic organic thin films. In this work, we show that the molecular weight rr-P3HT in organic solar cells can also significantly change the internal cohesion of the photoactive layer using micromechanical testing techniques. Cohesive values ranged from ∼0.5 to ∼17 J m–2, following the general trend of greater cohesion with increasing molecular weight. Using nanodynamic mechanical analysis, we attribute the increase in cohesion to increased plasticity which helps dissipate the applied energy. Finally, we correlate photovoltaic efficiency with cohesion to assess the device physics pertinent to optimizing device reliability. This research elucidates the fundamental parameters which affect both the mechanical stability and efficiency of polymer solar cells.
Co-reporter:Linying Cui, Krystelle Lionti, Alpana Ranade, Kjersta Larson-Smith, Geraud Jean-michel Dubois, and Reinhold H. Dauskardt
ACS Nano 2014 Volume 8(Issue 7) pp:7186
Publication Date(Web):July 2, 2014
DOI:10.1021/nn502161p
We report on the synthesis of hard, adhesive, and highly transparent bilayer organosilicate thin films on large poly(methyl methacrylate) substrates by atmospheric plasma, in ambient air, at room temperature, in a one-step process, using a single precursor. The method overcomes the challenge of fabricating coatings with high mechanical and interfacial properties in a one-step process. The bottom layer is a carbon-bridged hybrid silica with excellent adhesion with the poly(methyl methacrylate) substrate, and the top layer is a dense silica with high Young’s modulus, hardness, and scratch resistance. The bilayer structure exhibited ∼100% transmittance in the visible wavelength range, twice the adhesion energy and three times the Young’s modulus of commercial polysiloxane sol–gel coatings.Keywords: atmospheric plasma deposition; bilayer; low temperature; multifunctional coatings; one-step process; transparent
The phase separated bulk heterojunction (BHJ) layer in BHJ polymer:fullerene organic photovoltaic devices (OPV) are mechanically weak with low values of cohesion. Improved cohesion is important for OPV device thermomechanical reliability. BHJ devices are investigated and how fullerene intercalation within the active layer affects cohesive properties in the BHJ is shown. The intercalation of fullerenes between the side chains of the polymers poly(3,3″′-didocecyl quaterthiophene) (PQT-12) and poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene (pBTTT) is shown to enhance BHJ layer cohesion. Cohesion values range from ≈1 to 5 J m−2, depending on the polymer:fullerene blend, processing conditions, and composition. Devices with non-intercalated BHJ layers are found to have significantly reduced values of cohesion. The resulting device power conversion efficiencies (PCE) are also investigated and correlated with the device cohesion.
Plasticity plays a crucial role in the mechanical behavior of engineering materials. For instance, energy dissipation during plastic deformation is vital to the sufficient fracture resistance of engineering materials. Thus, the lack of plasticity in brittle hybrid organic–inorganic glasses (hybrid glasses) often results in a low fracture resistance and has been a significant challenge for their integration and applications. Here, we demonstrate that hydrogenated amorphous silicon carbide films, a class of hybrid glasses, can exhibit a plasticity that is even tunable by controlling their molecular structure and thereby leads to an increased and adjustable fracture resistance in the films. We decouple the plasticity contribution from the fracture resistance of the films by estimating the “work-of-fracture” using a mean-field approach, which provides some insight into a potential connection between the onset of plasticity in the films and the well-known rigidity percolation threshold.Keywords: chemical vapor deposition; hybrid materials; mechanical properties; rigidity percolation; structure−property relationship; thin films;
Oxygen atmospheric plasma was used to pretreat polycarbonate (PC) and stretched poly(methyl methacrylate) (PMMA) surfaces in order to enhance the adhesion of the dense silica coatings deposited by atmospheric plasma on the polymer substrates. The treatment time and chemical structure of the polymers were found to be important factors. For PC, a short treatment increased the adhesion energy, while longer treatment times decreased the adhesion. In contrast, plasma pretreatment monotonically decreased the adhesion of PMMA, and pristine PMMA exhibited much higher adhesion than the PC counterpart. We found that adhesion enhancement was achieved through improved chemical bonding, chain interdiffusion, and mechanical interlocking at the coating/substrate interface, after a short atmospheric plasma treatment. Decreased adhesion resulted from overoxidation and low-molecular-weight weak layer formation on the polymer surface by prolonged atmospheric plasma treatment. The dramatic differences in the behavior of PC and PMMA in relation to the plasma treatment time were due to their dissimilar resistance to atmospheric plasma exposure.Keywords: adhesion; atmospheric plasma; polymer; silica coating; surface chemical state; surface morphology;
Interfaces between organic and inorganic materials are of critical importance to the lifetime of devices found in microelectronic chips, organic electronics, photovoltaics, and high-performance laminates. Hybrid organic/inorganic materials synthesized through sol–gel processing are best suited to address these challenges because of the intimate mixing of both components. We demonstrate that deposition from heterogeneous sol–gel solutions leads to the unique nanolength-scale control of the through-thickness film composition and therefore the independent optimization of both the bulk and interfacial film properties. Consequently, an outstanding 3-fold improvement in the adhesive/cohesive properties of these hybrid films can be obtained from otherwise identical precursors.Keywords: fracture; homogeneity; hybrid materials; sol−gel; thin film;
Transparent polymers are widely used in many applications ranging from automotive windows to microelectronics packaging. However, their intrinsic characteristics, in particular their mechanical properties, are significantly degraded with exposure to different weather conditions. For instance, under humid environment or UV-irradiation, polycarbonate (PC) undergoes depolymerization, leading to the release of Bisphenol A, a molecule presumed to be a hormonal disruptor, potentially causing health problems. This is a serious concern and the new REACH (Registration, Evaluation, Authorization and Restriction of Chemical substances ) program dictates that materials releasing Bisphenol A should be removed from the market by January 1st, 2015 (2012-1442 law). Manufacturers have tried to satisfy this new regulation by depositing atop the PC a dense oxide-like protective coating that would act as a barrier layer. While high hardness, modulus, and density can be achieved by this approach, these coatings suffer from poor adhesion to the PC as evidenced by the numerous delamination events occurring under low scratch constraints. Here, we show that the combination of a N2/H2-plasma treatment of PC before depositing a hybrid organic-inorganic solution leads to a coating displaying elevated hardness, modulus, and density, along with a very high adherence to PC (> 20 J/m2 as measured by double cantilever beam test). In this study, the sol-gel coatings were composed of hybrid O/I silica (based on organoalkoxysilanes and colloidal silica) and designed to favor covalent bonding between the hybrid network and the surface treated PC, hence increasing the contribution of the plastic deformation from the substrate. Interestingly, double-cantilever beam (DCB) tests showed that the coating’s adhesion to PC was the same irrespective of the organoalkoxysilanes/colloidal silica ratio. The versatility of the sol-gel deposition techniques (dip-coating, spray-coating, etc.), together with the excellent mechanical properties and exceptional adherence of this hybrid material to PC should lead to interesting new applications in diverse fields: optical eye-glasses, medical materials, packaging, and so forth.Keywords: adhesion; double cantilever beam; elastic properties; hybrid silica; plasma treatment; polycarbonate; sol-gel;
We report on the adhesion of weak interfaces in inverted P3HT:PCBM-based polymer solar cells (OPV) with either a conductive polymer, PEDOT:PSS, or a metal oxide, molybdenum trioxide (MoO3), as the hole transport layer. The PEDOT:PSS OPVs were prepared by spin or spray coating on glass substrates, or slot-die coating on flexible PET substrates. In all cases, we observed adhesive failure at the interface between the P3HT:PCBM with PEDOT:PSS layer. The adhesion energy measured for the solar cells made on glass substrates was about 1.8 J/m2, but only 0.5 J/m2 for the roll-to-roll processed flexible solar cells. The adhesion energy was insensitive to the PEDOT:PSS layer thickness in the range of 10–40 nm. A marginal increase in adhesion energy was measured with increased O2 plasma power. Compared to solution processed PEDOT:PSS, we found that thermally evaporated MoO3 adheres less to the P3HT:PCBM layer, which we attributed to the reduced mixing at the MoO3/P3HT:PCBM interface during the thermal evaporation process. Insights into the mechanisms of delamination and the effect of different material properties and processing parameters yield general guidelines for the design of more reliable organic photovoltaic devices.Graphical abstractHighlights► The interfacial adhesion in spin, spray and slot-die coated OPV’s is quantified and studied. ► The adhesion between the BHJ and the PEDOT:PSS HTL was insensitive to the HTL layer thickness. ► The power of the O2 plasma treatment has an enhancing effect on the adhesion. ► Thermally evaporated MoO3 HTLs poorly adhere to the P3HT:PCBM layer. ► Guidelines to improve the fracture properties of OPV’s is provided.
Co-reporter:Yusuke Matsuda, Sean W. King, Reinhold H. Dauskardt
Thin Solid Films 2013 Volume 531() pp:552-558
Publication Date(Web):15 March 2013
DOI:10.1016/j.tsf.2012.11.141
The effects of N or O doping into hydrogenated amorphous silicon carbide (a-SiC:H) films on molecular structure and resulting material properties with particular attention to elastic constant, cohesive fracture energy, and moisture-assisted cracking were investigated. Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy characterizations demonstrated that doped N primarily formed SiN and NH bonds, and doped O formed SiO suboxide bonds. The elastic constant of both N-doped a-SiC:H (a-SiCN:H) and O-doped a-SiC:H (a-SiCO:H) films increased with increasing N and O atomic concentrations (at.%). The cohesive fracture energy, Gc, of the a-SiCN:H and a-SiCO:H films also increased with increasing N and O at.%. These increases in the mechanical properties of the films were attributed to film densification with increasing N and O at.%. The a-SiCN:H films exhibited a greater increase in Gc than the a-SiCO:H films, which was due to the moisture-insensitivity of the a-SiCN:H films as opposed to the a-SiCO:H films. The a-SiCN:H films exhibited no moisture-assisted fracture behavior, which was attributed to moisture-insensitivity of SiN bonds due to their less polar nature.Highlights► N and O doping were used to enhance the mechanical properties of a-SiC:H films. ► N and O doping significantly improved the elastic and fracture properties. ► N doping made the films insensitive to moisture-assisted cracking. ► Changes in the mechanical properties were related to the molecular structure.
We report cross-linked polycarbosilane (CLPCS) films with superior mechanical properties and insensitivity to moisture. CLPCS are prepared by spin-coating and thermal curing of hexylene-bridged disilacyclobutane (DSCB) rings. The resulting films are siloxane-free and hydrophobic, and present good thermal stability and a low dielectric constant of k = 2.5 without the presence of supermicropores and mesopores. The elastic stiffness and fracture resistance of the films substantially exceed those of traditional porous organosilicate glasses because of their unique molecular structure. Moreover, the films show a remarkable insensitivity to moisture attack, which cannot be achieved by traditional organosilicate glasses containing siloxane bonds. These advantages make the films promising candidates for replacing traditional organosilicate glasses currently used in numerous applications, and for use in emerging nanoscience and energy applications that need protection from moisture and harsh environments.Keywords: and thin films; dielectrics; hybrid material; mechanical properties; moisture-sensitivity; structure−property relationship;
We explore the application of a high-temperature precursor delivery system for depositing high boiling point organosilicate precursors on plastics using atmospheric plasma. Dense silica coatings were deposited on stretched poly(methyl methacrylate), polycarbonate and silicon substrates from the high boiling temperature precursor, 1, 2-bis(triethoxysilyl)ethane, and from two widely used low boiling temperature precursors, tetraethoxysilane and tetramethylcyclotetrasiloxane. The coating deposition rate, molecular network structure, density, Young’s modulus and adhesion to plastics exhibited a strong dependence on the precursor delivery temperature and rate, and the functionality and number of silicon atoms in the precursor molecules. The Young’s modulus of the coatings ranged from 6 to 34 GPa, depending strongly on the coating density. The adhesion of the coatings to plastics was affected by both the chemical structure of the precursor and the extent of exposure of the plastic substrate to the plasma during the initial stage of deposition. The optimum combinations of Young’s modulus and adhesion were achieved with the high boiling point precursor which produced coatings with high Young’s modulus and good adhesion compared to commercial polysiloxane hard coatings on plastics.Keywords: adhesion; atmospheric plasma deposition; elastic property; precursor chemical structure; silica molecular structure;
Environmentally assisted debonding at Cu/barrier interfaces is reported for oxidizing and reducing environments. Both moist and dry oxidizing environments are considered, and the effects of different oxidizing species and their chemical activity on the rate of debonding of a Cu/SiN interface is quantified. The type of oxidizing species is shown to play a critical role in the kinetics of environmentally assisted debonding. Additionally, the effect of varying the activity and temperature of reducing hydrogen environments is investigated. The mechanisms responsible for environmentally assisted debonding of Cu/SiN and Cu/SiCN interfaces are elucidated using an atomistic bond rupture model. An activation energy for debonding of Cu/SiCN interfaces in a hydrogen environment is calculated. Finally, a connection between environmentally assisted debonding and the time-dependent dielectric breakdown properties of Cu interconnects is proposed.
The cohesive fracture properties of hydrogenated amorphous silicon carbide (a-SiC:H) thin films in moist environments are reported. Films with stoichiometric compositions (C/Si ≈ 1) exhibited a decreasing cohesive fracture energy with decreasing film density similar to other silica-based hybrid organic–inorganic films. However, lower density a-SiC:H films with non-stoichiometric compositions (C/Si ≈ 5) exhibited much higher cohesive fracture energy than the films with higher density stoichiometric compositions. One of the non-stoichiometric films exhibited fracture energy (∼9.5 J m−2) greater than that of dense silica glasses. The increased fracture energy was due to crack-tip plasticity, as demonstrated by significant pileup formation during nanoindentation and a fracture energy dependence on film thickness. The a-SiC:H films also exhibited a very low sensitivity to moisture-assisted cracking compared with other silica-based hybrid films. A new atomistic fracture model is presented to describe the observed moisture-assisted cracking in terms of the limited SiOSi suboxide bond formation that occurs in the films.
Co-reporter:Ruiliang Jia, Siming Dong, Takuya Hasegawa, Jiping Ye, Reinhold H. Dauskardt
International Journal of Hydrogen Energy 2012 Volume 37(Issue 8) pp:6790-6797
Publication Date(Web):April 2012
DOI:10.1016/j.ijhydene.2012.01.063
Mechanical degradation of the catalyst coated membrane (CCM), which contains a perfluorosulfonic acid (PFSA) proton transport layer, can significantly deteriorate the performance of proton exchange membrane (PEM) fuel cells. We initially report on the adhesive and cohesive fracture properties of CCMs and show that failure occurs cohesively in the catalyst layer (CL). We then investigate the effects of foreign cations and chloride contamination and moisture absorption on the mechanical properties of CCMs. The fracture resistance of contaminated CCMs is significantly reduced and the time dependent growth of cracks in the CLs in moist air environments occurs at lower crack driving force thresholds. The deterioration in fracture resistance of the CCMs after foreign cation contamination is related to cation interaction with the molecular structure of PFSA polymer. The harmful effect of chloride contamination is attributed to chloride blocking on the surface of catalyst Pt particles, which tends to weaken the catalyst-polymer interface and induces crack initiation and subsequent propagation with lower energy. The accelerated time dependent crack growth at higher humidity is explained by the role of water molecules on weakening ionic interactions and the intermolecular strength of the PFSA polymer.Highlights► Adhesive and cohesive fracture properties of CCMs are initially studied. ► Fracture resistance of foreign cation contaminated CCMs is significantly reduced. ► Impurity chloride contamination reduces fracture resistance of CCMs in fuel cells. ► Morphology of catalyst materials in CCMs is characterized by electron microscopy. ► Time dependent crack growth in catalyst layers is accelerated at higher humidity.
The ubiquitous presence of solar UV radiation in human life is essential for vitamin D production but also leads to skin photoaging,
damage, and malignancies. Photoaging and skin cancer have been extensively studied, but the effects of UV on the critical
mechanical barrier function of the outermost layer of the epidermis, the stratum corneum (SC), are not understood. The SC
is the first line of defense against environmental exposures like solar UV radiation, and its effects on UV targets within
the SC and subsequent alterations in the mechanical properties and related barrier function are unclear. Alteration of the
SC’s mechanical properties can lead to severe macroscopic skin damage such as chapping and cracking and associated inflammation,
infection, scarring, and abnormal desquamation. Here, we show that UV exposure has dramatic effects on cell cohesion and mechanical
integrity that are related to its effects on the SC’s intercellular components, including intercellular lipids and corneodesmosomes.
We found that, although the keratin-controlled stiffness remained surprisingly constant with UV exposure, the intercellular
strength, strain, and cohesion decreased markedly. We further show that solar UV radiation poses a double threat to skin by
both increasing the biomechanical driving force for damage while simultaneously decreasing the skin’s natural ability to resist,
compromising the critical barrier function of the skin.
Co-reporter:Ruiliang Jia, Binghong Han, Kemal Levi, Takuya Hasegawa, Jiping Ye, Reinhold H. Dauskardt
Journal of Power Sources 2011 Volume 196(Issue 20) pp:8234-8240
Publication Date(Web):15 October 2011
DOI:10.1016/j.jpowsour.2011.05.069
The durability of proton exchange membrane (PEM) fuel cells remains a challenging issue for their long term operational use. Degradation of the PEM related to dissolution of the adjacent catalyst and re-deposition into the PEM significantly reduces cell efficiency. We investigate the effects of platinum (Pt) dispersions intended to simulate the re-deposited catalyst on the mechanical durability of the PEM. The bulge technique was applied to characterize the mechanical properties of PEMs simulating pressure loading on fully hydrated membranes in fuel cells. The results showed that with increasing Pt dispersion concentration the stiffness of the PEMs increased, and the membranes became less ductile and inclined to fracture at lower stresses under pressure loading. We also used the out-of-plane tearing test to characterize membrane fracture behavior which revealed the harmful effects of Pt dispersion on the fracture resistance under different environmental conditions. Deterioration in fracture resistance was explained in terms of the Pt distribution and aggregation as defects inside the membranes as characterized by electron microscopy. Fracture was shown to initiate preferentially at the interface of Pt particles and the polymer matrix, and propagate through the defect regions in polymer with lower energy, thus reducing the overall fracture resistance of the PEM.Highlights► PEMs with Pt dispersion are made to simulate dissolution and redeposition of catalyst. ► Pt distribution and aggregation inside PEMs are characterized by electron microscopy. ► Bulge test is used to simulate hydrated pressurized loading on PEMs in fuel cells. ► Catalyst Pt dispersions stiffen PEMs and reduce membrane fracture resistance.
Co-reporter:Ani Kamer, Kjersta Larson-Smith, Liam S.C. Pingree, Reinhold H. Dauskardt
Thin Solid Films 2011 Volume 519(Issue 6) pp:1907-1913
Publication Date(Web):3 January 2011
DOI:10.1016/j.tsf.2010.08.116
The adhesion and time dependent crack growth behavior of polysiloxane based hard coatings on poly (methyl methacrylate) substrates were investigated. The adhesive fracture energies for different coatings were quantitatively characterized and varied between 1.4 J/m2 and 22 J/m2. Significant time dependent crack growth in various moist environments was observed and was consistent with a viscoelastic crack growth model. The effect of selected weathering treatments was also examined and resulted in a significant drop in coating adhesion. The coatings were analyzed using surface sensitive techniques; structural changes in the coatings resulting from various exposure doses were studied and mechanisms responsible for the observed degradation in adhesion were discussed.
Co-reporter:Taek-Soo Kim and Reinhold H. Dauskardt
Nano Letters 2010 Volume 10(Issue 5) pp:1955-1959
Publication Date(Web):April 14, 2010
DOI:10.1021/nl101169s
The mobility of organic molecules under nanoscale confinement differs greatly from that in the bulk. In this study we show that the conventional free volume dependent mobility relationship explained by the free volume theory of diffusion breaks down for diffusion of linear alkane molecules in organosilicate films with connected nanoporosity. Alkane mobility under such nanoscale confinement was observed to decrease with chain length and was lower than that reported in the bulk. While the activation energy for diffusion was similar to that in the bulk, it was found to decrease with chain length exactly opposite to the trend observed in the bulk. This suggests an increasing molecular free volume with chain length. The effects of molecular polarity and pore size on diffusion were also demonstrated. Molecular mobility was found to be suppressed with increasing molecular polarity and decreasing pore size.
Hybrid organic-inorganic glasses exhibit unique electro-optical properties along with excellent thermal stability. Their inherently mechanically fragile nature, however, which derives from the oxide component of the hybrid glass network together with the presence of terminal groups that reduce network connectivity, remains a fundamental challenge for their integration in nanoscience and energy technologies. We report on a combined synthesis and computational strategy to elucidate the effect of molecular structure on mechanical properties of hybrid glass films. We first demonstrate the importance of rigidity percolation to elastic behavior. Secondly, using a novel application of graph theory, we reveal the complex 3-D fracture path at the molecular scale and show that fracture energy in brittle hybrid glasses is fundamentally governed by the bond percolation properties of the network. The computational tools and scaling laws presented provide a robust predictive capability for guiding precursor selection and molecular network design of advanced hybrid organic-inorganic materials.
Journal of Sol-Gel Science and Technology 2010 Volume 55( Issue 3) pp:360-368
Publication Date(Web):2010 September
DOI:10.1007/s10971-010-2262-1
The mechanical reliability of hybrid films depends critically on their fracture properties which are controlled largely by the film composition and molecular structure. We have investigated the adhesive and cohesive fracture properties of hybrid films processed from 3-glycidoxypropyltrimethoxysilane (GPTMS) and tetra n-propoxyzirconium (TPOZ), for which the roles of molecular structure and composition have not been well established. The influences of film Zr/GPTMS ratio, silane crosslinking, and substrate composition on fracture resistance were quantified in terms of the critical strain energy release rate, GC Film fracture energy was found to increase, then decrease with increasing Zr/GPTMS ratio. Removal of the epoxy rings of GPTMS from the film was found to drastically decrease the cohesive fracture energy of the film as well as the adhesive fracture energy of the film/epoxy interface. Finally, films deposited on silicon had much higher fracture energies compared to those deposited onto aluminum and titanium from identical sols. FTIR, XPS, and AFM were used to characterize the film structure and fracture surfaces. The molecular-scale mechanisms responsible for the observed trends are discussed. These results provide new insights into the interaction between the substrate chemistry, molecular structure, and mechanical reliability of hybrid sol-gel films.
Co-reporter:Taek-Soo Kim, Katherine Mackie, Qiping Zhong, Maria Peterson, Tomohisa Konno and Reinhold H. Dauskardt
Nano Letters 2009 Volume 9(Issue 6) pp:2427-2432
Publication Date(Web):May 15, 2009
DOI:10.1021/nl901138p
Polymer molecules when physically confined at nanometer length scales diffuse nonclassically and very differently depending on their molecular weight and the nature of the confinement. Long polymers that exhibit “snakelike” reptation based mobility in melts may diffuse faster in confined nanometer sized cylinders with pore diameter d ∼ 15 nm, and short polymers subject to Rouse dynamics have shown signatures of reptation and slower diffusion when confined in nanoporous glass with d ∼ 4 nm. However, the mobility of short polymers with radii of gyration similar to a smaller pore diameter (d ≤ 2.1 nm) but with extended lengths well larger than the pore diameter has not as yet been studied. In this work, we demonstrate that those short molecules including nonionic surfactants can readily diffuse in strongly hydrophobic nanoporous glasses film with d ≤ 2.1 nm. The diffusivity was found sensitive to molecular weight, hydrophilic−lipophilic balance, and molecular structure of surfactants. Remarkably, analysis of the measured diffusion coefficients reveals that short-chain surfactants exhibit signature of reptation based diffusion in the nanoscopic pore confinements. Such reptation mobility in agreement with theoretical predictions is not even observed in reptating polymer melts due to fluctuations of the entanglement pathway. The fixed pathways in the interconnected nanoporous films provide ideal nanoscale environments to explore mobility of confined molecules, and the results have implications for a number of technologies where nanoporous materials are in contact with surfactant molecules.
The integration of nanoporous organosilicate thin films involving chemical mechanical planarization (CMP) is a significant challenge due the evolution of defects in the films during CMP in the form of cracking and delamination. This study shows that small changes in CMP electrolyte chemistry and surfactant additions can have dramatic effects on crack growth rates in the films. Crack growth rates were sensitive to the type of electrolyte and decreased in the presence of electrolytes that caused crack tip blunting. Growth rates were also sensitive to nonionic surfactant additions where molecular structure and weight were demonstrated to be important variables. An optimized blend of surfactants and electrolytes can significantly retard defect evolution due to molecular bridging. Surfactant self-assembly and resulting molecular bridging were characterized by in situ atomic force microscopy and used to quantify the molecular bridging observed.
Poly(arylene) ether (PAE) polymer films containing controlled nanometer-sized pores are shown to exhibit increasing fracture resistance with porosity. Such surprising behavior is in stark contrast to widely reported behavior for the fracture toughness of porous solids, which decreases markedly with porosity. A ductile nano-void growth and coalescence fracture mechanics-based model is presented to rationalize the increase in fracture resistance of the voided polymer film. The model is shown to explain the behavior in terms of a specific scaling of the size of the pores with pore volume fraction. It is demonstrated that the pore size must increase with close to a linear dependence on the volume fraction in order to increase rather than decrease the fracture energy. Independent characterization of the pore size as a function of volume fraction is shown to confirm predictions made by the model. Implications for the optimum void size and volume fraction are considered for superior fracture resistance of the nanoporous films.
Adhesion and subcritical debonding at the interface between a thin diglycidyl ether of bisphenol F polymer layer and either SiNx or SiO2 passivated silicon substrates are described. The interface is characterized by weak hydrogen bonding. Prolonged exposure to a moist environment resulted in a time-dependent decrease in adhesion. Subcritical debond-growth rates as a function of applied loads were sensitive to temperature and relative humidity, and greatly accelerated in the presence of cyclic loading. Of particular interest was the occurrence of an anomalous region of persistent debonding that developed below ∼10−8 m s−1 under both monotonic and cyclic loading. In this region, debond-growth rates were characterized by a weak dependence on the applied loads, a strong dependence on moisture activity, and the absence of a measurable threshold below which debonding could not be measured. We propose a new stress-dependent transport model that describes the moisture diffusion mechanism responsible for this anomalous behavior.
Scripta Materialia 2007 Volume 57(Issue 1) pp:69-71
Publication Date(Web):July 2007
DOI:10.1016/j.scriptamat.2007.03.019
We respond to comments made regarding the large discrepancies in the high cycle fatigue life of a Zr-based bulk metallic glass when measured using four-point bending and notched tensile rod configurations. We reiterate our initial conclusions that the present of strong stress gradients and reduced stress at the fatigue initiation site together with the reduced sampling volume of the notched tensile rod experiments are the reasons for the large reported differences. Further stress life results measured using uniaxial tensile specimens are reported that are consistent with data from the four-point bending specimens.
Co-reporter:Brian C. Menzel, Reinhold H. Dauskardt
Scripta Materialia 2006 Volume 55(Issue 7) pp:601-604
Publication Date(Web):October 2006
DOI:10.1016/j.scriptamat.2006.06.015
A stress–life study of a bulk metallic glass (Zr41.25Ti13.75Ni10Cu12.5Be22.5) using notched cylindrical bars reported a fatigue endurance limit of ∼1/2 of the ultimate tensile strength. This result is significantly higher than the value of ∼1/10 of the endurance limit previously reported using four-point bend specimens. A careful study of the stress state and final fracture surfaces for the notched specimens together with an error in the stress concentration factor employed in the original paper were found to explain the discrepancies.
Co-reporter:Kimberly K. Cameron, Reinhold H. Dauskardt
Scripta Materialia 2006 Volume 54(Issue 3) pp:349-353
Publication Date(Web):February 2006
DOI:10.1016/j.scriptamat.2005.10.006
The atomic processes underlying cyclic fatigue was investigated in a metallic glass using molecular-dynamics simulations. A simple four component Lennard–Jones model was used to characterize the behavior of the free volume during deformation. The deformation behavior observed was typical of that seen experimentally, involving both elastic and plastic strains, load history dependence and strain rate sensitivity. Changes in free volume and excess strain were monitored during deformation to demonstrate how the stress state affects the distribution of free volume and how regions with excess free volume preferentially deform. Simulation results indicated that free volume levels were increased and localized during deformation, a process that was greatly accelerated under cyclic loading conditions. The results are used to rationalize the poor fatigue properties commonly reported for metallic glasses.
Co-reporter:Peter A. Hess, Brian C. Menzel, Reinhold H. Dauskardt
Scripta Materialia 2006 Volume 54(Issue 3) pp:355-361
Publication Date(Web):February 2006
DOI:10.1016/j.scriptamat.2005.10.007
Cyclic fatigue was investigated in metallic glasses using fracture mechanics-based crack-growth and stress-life experiments. Steady-state crack growth in a Zr-based bulk metallic glass occurred with a Paris law exponent of ∼1.5. Growing planar fatigue cracks were often noted to destabilize into a wavy crack front geometry, and the conditions determining the stability of these cracks are analyzed. Transient crack-growth rates were measured during variable-amplitude fatigue. Stress-life experiments showed low endurance limits. Fatigue damage initiated at an angle to the applied load, but changed direction at a critical length and grew as mode I cracks. However, the initiation stage of fatigue damage was very short, and almost the entire fatigue life was spent in the crack-growth stage.
An in vitro adhesion test method has been adapted to quantify the through-thickness intercellular delamination energy of isolated human stratum corneum (SC). Both untreated and delipidized tissues were tested. Measured delamination energies were found to increase from ∼3 J/m2 near the surface to ∼15 J/m2 for the inner layers of the tissue. For delipidized SC, the location of the initial debond was located closer to the center of the tissue. Delamination energy values were elevated compared to untreated specimens, increasing from ∼7 J/m2 near the surface to ∼18 J/m2 for the inner layers of the SC. Further tests were run to measure delamination energies of SC as a function of hydration (15–100% relative humidity (RH)) at ∼25 °C and as a function of temperature (10–90 °C) at several hydrations (15, 45, 100% RH). Delamination energies were observed to decrease with increasing hydration and increasing temperature with the most significant changes occurring for 100% RH conditioned SC. Additional SC was treated with pH-buffered solutions (pH 4.2, 6.7, 9.9) and selected surfactant solutions (1%, 10% wt/wt sodium dodecyl sulfate (SDS)) for comparison to untreated controls. While statistically significant differences were observed, the SC was found to be resistant to large changes in delamination energy with pH and 1% wt/wt SDS treatments with values in the range 4.2–5.1 J/m2 compared to control values of 4.4 J/m2. More substantially elevated values were observed for SC treated with a 10% wt/wt SDS solution (6.6 J/m2) and a chloroform–methanol extraction (11.2 J/m2).
Co-reporter:Kenneth S. Wu, William W. van Osdol, Reinhold H. Dauskardt
Biomaterials 2006 Volume 27(Issue 5) pp:785-795
Publication Date(Web):February 2006
DOI:10.1016/j.biomaterials.2005.06.019
An in vitro mechanics approach to quantify the intercellular delamination energy and mechanical behavior of isolated human stratum corneum (SC) in a direction perpendicular to the skin surface is presented. The effects of temperature, hydration, and a chloroform–methanol treatment to remove intercellular lipids were explored. The delamination energy for debonding of cells within the SC layer was found to be sensitive to the moisture content of the tissue and to the test temperature. Delamination energies for untreated stratum corneum were measured in the range of 1–8 J/m2 depending on test temperature. Fully hydrated specimen energies decreased with increasing temperature, while room-humidity-hydrated specimens exhibited more constant values of 2–4 J/m2. Lipid-extracted specimens exhibited higher delamination energies of ∼12 J/m2, with values decreasing to ∼4 J/m2 with increasing test temperature. The peak separation stress decreased with increasing temperature and hydration, but lipid-extracted specimens exhibited higher peak stresses than untreated controls. The delaminated surfaces revealed an intercellular failure path with no evidence of tearing or fracture of cells. The highly anisotropic mechanical behavior of the SC is discussed in relation to the underlying SC structure.
Co-reporter:Katharine M. Flores, William L. Johnson, Reinhold H. Dauskardt
Scripta Materialia 2003 Volume 49(Issue 12) pp:1181-1187
Publication Date(Web):December 2003
DOI:10.1016/j.scriptamat.2003.08.020
The fracture and fatigue behavior of a Zr-based metallic glass composite are presented. The fracture resistance and fatigue endurance limit was significantly higher than that of the matrix material. Results are rationalized in terms of the effects of the second phase on shear band formation and distribution.
Journal of Non-Crystalline Solids 2003 Volume 317(1–2) pp:181-186
Publication Date(Web):March 2003
DOI:10.1016/S0022-3093(02)01997-X
The plane strain fracture toughness of a Zr–Ti–Ni–Cu–Be bulk metallic glass was found to be significantly degraded after annealing at 300 °C. The fracture morphology was changed from characteristic vein patterns to cleavage-like features with little evidence of plasticity. Positron annihilation spectroscopy results revealed that atomic scale open-volume regions were reduced by annealing. A dynamic mechanical analysis showed that the atomic arrangement process for viscous flow was retarded as a result of annealing. The observed annealing embrittlement is ascribed to the loss of stress relief by viscous flow at the crack tip that results from the anneal-out of open-volume regions
The effects of hydrogen on the viscoelastic relaxation behavior of a Zr–Ti–Ni–Cu–Be bulk metallic glass have been investigated in an attempt to elucidate hydrogen-affected flow and fracture behavior. Dynamic mechanical testing was performed to study relaxation behavior near the glass transition temperature. Relaxation time constants were increased in the presence of hydrogen with a concomitant increase of thermal activation energy. In addition, the glass transition temperature was increased and crystallization kinetics retarded in the presence of hydrogen leading to enhanced thermal stability. Positron annihilation spectroscopy was employed to study the interaction of hydrogen and open-volume regions. While hydrogen charging was found to decrease the open-volume regions in the amorphous phase, an increase in free volume was observed in the crystalline counterpart. The amorphous phase was found to have a greater hydrogen absorption capacity compared to its crystalline counterpart. Relaxation behavior, crystallization kinetics and the interaction of hydrogen with the amorphous microstructure are discussed. Finally, the effects of retarded relaxation processes on fracture resistance are considered.
Co-reporter:Krysta Biniek, Joseph Kaczvinsky, Paul Matts, Reinhold H. Dauskardt
Journal of Dermatological Science (November 2015) Volume 80(Issue 2) pp:94-101
Publication Date(Web):1 November 2015
DOI:10.1016/j.jdermsci.2015.07.016
•We examined how the elastic stiffness and strength, cellular cohesion, and kinetics of water diffusion of the stratum corneum vary with age.•Aged stratum corneum is stiffer with higher fracture stress.•Corneocyte cohesion increases with age.•Water diffuses more slowly through aged stratum corneum.•The biomechanical barrier function is significantly altered with ageing, which may contribute to the prevalence of skin disorders in the elderly.BackgroundThe appearance and function of human skin are dramatically altered with aging, resulting in higher rates of severe xerosis and other skin complaints. The outermost layer of the epidermis, the stratum corneum (SC), is responsible for the biomechanical barrier function of skin and is also adversely transformed with age. With age the keratin filaments within the corneocytes are prone to crosslinking, the amount of intercellular lipids decreases resulting in fewer lipid bilayers, and the rate of corneocyte turnover decreases.ObjectivesThe effect of these structural changes on the mechanical properties of the SC has not been determined. Here we determine how several aspects of the SC’s mechanical properties are dramatically degraded with age.MethodsWe performed a range of biomechanical experiments, including micro-tension, bulge, double cantilever beam, and substrate curvature testing on abdominal stratum corneum from cadaveric female donors ranging in age from 29 to 93 years old.ResultsWe found that the SC stiffens with age, indicating that the keratin fibers stiffen, similarly to collagen fibers in the dermis. The cellular cohesion also increases with age, a result of the altered intercellular lipid structure. The kinetics of water movement through the SC is also decreased.ConclusionsOur results indicate that the combination of structural and mechanical property changes that occur with age are quite significant and may contribute to the prevalence of skin disorders among the elderly.
Co-reporter:Nicholas Rolston, Brian L. Watson, Colin D. Bailie, Michael D. McGehee, João P. Bastos, Robert Gehlhaar, Jueng-Eun Kim, Doojin Vak, Arun Tej Mallajosyula, Gautam Gupta, Aditya D. Mohite, Reinhold H. Dauskardt
Low-cost solar technologies such as perovskite solar cells are not only required to be efficient, but durable too, exhibiting chemical, thermal and mechanical stability. To determine the mechanical stability of perovskite solar cells, the fracture resistance of a multitude of solution-processed organometal trihalide perovskite films and cells utilizing these films were studied. The influence of stoichiometry, precursor chemistry, deposition techniques, and processing conditions on the fracture resistance of perovskite layers was investigated. In all cases, the perovskites offered negligible resistance to fracture, failing cohesively below 1.5 J/m2. The solar cells studied featured these perovskites and a variety of organic and inorganic charge transporting layers and carrier-selective contacts. These ancillary layers were found to significantly influence the overall mechanical stability of the perovskite solar cells and were repeatedly the primary source of mechanical failure, failing at values below those measured for the isolated fragile perovskite films. A detailed insight into the nature of perovskite and perovskite solar cell fracture is presented and the influence of grain size, device architecture, deposition techniques, environmental variables, and molecular additives on these fracture processes is reported. Understanding the influence of materials selection, deposition techniques and processing variables on the mechanical stability of perovskite solar cells is a crucial step in their development.
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