•The thermal explosion synthesis reaction of Ni-Al system provides auxiliary heat and the joint can be brazed at 670 °C.•The reaction process of the Ni-Al thermal explosion synthesis reaction system and brazing mechanism were analyzed.•The sizes of Ni particles in the Ni-Al system affect the microstructure and property of the joints significantly.•The brazed joint using 10 μm Al and 7 μm Ni obtains the maximum shear strength of 13 MPa.In vacuum, the Gr/2024Al composite and TC4 alloy were brazed using AgCuTi filler alloy with the assist of Ni-Al interlayer as auxiliary heat source. Because of the thermal explosion synthesis reaction of Ni-Al interlayer, lots of heat can be produced and released, which promises that AgCuTi filler alloy melts and the Gr/2024Al composite and TC4 alloy brazes at low temperature. This method avoids the volatilization of Al in the Gr/2024Al composite at high brazing temperature, keeping the property of the Gr/2024Al composite. According to SEM analysis result, the Gr/2024Al composite reacts with the AgCuTi filler alloy to form a Ti3AlC2 layer adjacent to the composite, which is the key factor to obtain a perfect joint. A series of Ni-Al compounds form after the thermal explosion synthesis reaction of the Ni-Al interlayer. The particle sizes of Ni and Al have important effects on the thermal explosion synthesis reaction, and then influence the interfacial reaction and the formation of the Ni-Al compounds. The maximum shear strength reaches 13 MPa when the diameter of the Ni and Al particles are 10 μm and 7 μm respectively. Cracks primarily happen in the Gr/2024Al composite and propagate in the composite away from the brazing seam.Download high-res image (370KB)Download full-size image

A novel composite interlayer with a reinforced network was designed using a SiC ceramic with a network structure and Ti-Ni-Nb composite filler foils, to which the Nb and BN-SiO2 ceramic were successfully brazed under vacuum. For a brazing temperature of 1160 °C and holding time of 10 min, the interfacial microstructure of the Nb/BN-SiO2 ceramic joint was Nb/(βTi,Nb)-TiNi eutectic structure+(βTi,Nb)2Ni+SiC+TiC/TiN+Ti2N+TiB+Ti5Si3+TiO/BN-SiO2 ceramic. In addition, the shear strength and nano-hardness were analyzed to evaluate the effect of the composite interlayer with a network reinforcement architecture on the mechanical properties of the joint. During brazing, the Ti-Ni-Nb filler metal infiltrated and reacted with the SiC to form the network reinforcement architecture, resulting in the residual stress being relieved and the mechanical performance of the joint being significantly improved. A maximum shear strength of 102 MPa was achieved, which was 60 MPa (142%) higher than that of the joint brazed without the network reinforcement architecture. A reduction in the residual stress on the BN-SiO2 ceramic side from 328 MPa to 210 MPa was observed with the network reinforcement architecture, and the fracture path of the joint changed from the surface of the BN-SiO2 ceramic to the interfacial reaction zone.

•Ti6Al4V alloy and Si3N4 ceramic was brazed by adopting AgCuTi composite filler.•Interfacial microstructural morphology strongly depended on brazing holding time.•Element Ti played a significant role in interfacial microstructure evolution.•Microstructure evolution of Si3N4/AgCuTiC/Ti6Al4V joint was analyzed.A novel nano-Si3N4 particle reinforced AgCuTi composite filler (AgCuTiC) was used to braze Ti6Al4V and Si3N4 ceramic in the study and brazing cycles that peak at 880 °C for 0–20 min. Effects of holding time on interfacial microstructure and mechanical property were studied by scanning electron microscopy, energy dispersive spectrometer, transmission electron microscopy, and universal testing machine. TiN and Ti5Si3 were identified as the main product of the reaction between Si3N4 and Ti. The interfacial microstructure evolved considerably with joining time, eventually leading to a high degree of inhomogeneity across the length of the joint, and the maximal shear strength of 78.3 MPa was obtained when the joint was brazed at 880 °C for 10 min. A limited number of Si3N4/Si3N4 and Ti6Al4V/Ti6Al4V joints using AgCuTiC with different content of Ti particles were also studied to clarify the influences of diffusion and dissolution behaviors of element Ti on interfacial microstructure. In terms of characterizing the interfacial phases, efforts were made to understand the microstructural evolution mechanism of Si3N4/Ti6Al4V brazed joints.

•SiO2 ceramic and copper were successfully brazed with Ag-Cu-Ti filler alloy.•Brazing temperature and holding time had influence on the interfacial microstructure and joint properties.•Interfacial reaction mechanism of brazed joints was expounded.•The optimal shear strength of joints reached 22 MPa when brazed at 850 °C for 5 min.Vacuum brazing of copper to SiO2 electroceramics was carried out using commercially obtained Ag-Cu-Ti filler alloy. The effects of brazing temperature and bonding time on the interfacial microstructure and mechanical properties were investigated. The microstructure observation revealed that the typical interfacial structure could be expressed as SiO2 ceramic/Ti5Si3 + Ti4O7/Ti2Cu + Ti3Cu3O/Ag (s,s) + Cu (s,s)/Copper. The migration of Ti in brazing seam, which promoted the reaction of Ti with the SiO2 ceramic, was a positive factor on the interfacial microstructure and mechanical properties. With increased the brazing temperature or holding time, the thickness of Ti5Si3 + Ti4O7 reaction layer thickened, meanwhile, the amount of AgCu eutectic structure decreased. The shear strength firstly increased and then decreased with brazing temperature increasing or bonding time prolonging. The maximum shear strength of 22 MPa was obtained at 850 °C for 5 min. Its fracture occurred both along the ceramic substrate adjacent to the bonding interface and along the reaction layer, showing a mixture fracture mode.

•The strong evaporation of Zn in vacuum provides a driving force for the Ag-Cu-Zn liquid spreading.•The Zn evaporation results in the formation of the (Cu, Ni) solid solution layer.•The (Cu, Ni) layer leads to the transition from non-wetting to wetting of the liquid on the solid.In order to investigate the effect of the evaporation on wetting, reactive wetting of TiC-Ni cermet by molten Ag-Cu-Zn alloys with different Zn contents was studied at 810 °C using a sessile drop method in vacuum. The vapor recoil force induced by the strong evaporation of Zn in the Ag-Cu-25Zn alloy drove the spreading of the molten Ag-Cu-25Zn alloy. Furthermore, the sequential evaporation of Zn in the Ag-Cu-25Zn alloy caused the formation of a (Cu, Ni) solid-solution layer at the interface between the TiC-Ni cermet and the Ag-Cu-Zn alloy. Thus, the (Cu, Ni) solid-solution layer could serve as a metallization layer on the surface of the TiC-Ni cermet, which promoted the non-wetting to wetting transition of the TiC-Ni cermet by the Ag-Cu-25Zn alloy. The equilibrium contact angle of the Ag-Cu-25Zn alloy on the TiC-Ni cermet was 19.9°, approximately equal to that of a Ag-28Cu eutectic alloy on a Cu-Ni alloy surface. The final contact angle of the Ag-Cu-Zn liquid alloy on the TiC-Ni cermet first decreased and then increased as the Zn content in the liquid was increased from 0% to 30%. It reached a minimum value in the Ag-Cu-25Zn alloy.

•Few-layer graphene-enhanced Cu composite interlayer was prepared by CVD for brazing.•Our obtained graphene could effectively impede the intensive Al-Cu eutectic reaction.•The graphene contributed to strengthening the joint.Contact Reaction Brazing of aluminum alloy 6061 using pure Cu nanoparticle interlayer usually encounters cracks and holes due to the intensive Al-Cu eutectic reaction. In order to solve these problems, here we report a new type of graphene-enhanced Cu composite (G-Cu) interlayer prepared by CVD to contact reaction braze aluminum alloy 6061. Results show that the G-Cu interlayer is conducive to retarding the intensive Al-Cu eutectic reaction. The average shear strength of the joint brazed with G-Cu interlayer reached 74 MPa.

ZrC-SiC ceramic and TC4 alloy were brazed using AgCuTi alloy. The microstructure and mechanical property of the joints obtained at different brazing parameters were investigated and the reaction mechanism was analyzed. The results indicated that the Ti from the AgCuTi and TC4 reacted with the ZrC in the ceramic to form different shaped TiC crystals adjacent to the ZrC-SiC ceramic. With the increase of brazing temperature or extending of holding time, the dissolution of TC4 became vigorous and much Ti dissolved into the braze alloy. As a result, Ti reacted with the Cu from AgCuTi alloy to form a series of Cu-Ti compounds in the brazing seam due to the strong affinity between Cu and Ti. The Cu-Ti compounds made the hardness and brittleness of brazing seam increase, which deteriorated the property of the brazed joint. The maximum shear strength was 39 MPa obtained at 810 °C for 5 min.

•The oxide film at the Al surface is removed by plasma etching.•Al4C3 and carbon layer is formed at the interface of VFG/Al.•The VFG/3D-Al electrode shows a higher specific capacitance and low contact resistance.It is a challenge to find low-cost graphene-based Al electrodes with a low resistance for a high-performance supercapacitors. For this purpose, we have developed new 3D hybrid structure electrodes composed of vertically oriented few-layer graphene (VFG) grown in situ on Al foil with a microporous structure (VFG/3D-Al). The 3D VFG-Microporous hybrid structured electrode shows a higher specific capacitance than that of a conventional VFG/plane-Al electrode in both aqueous and organic electrolytes. Furthermore, the VFG shows excellent wettability in organic electrolytes, whereby the effective reaction area between the VFG and electrolyte is higher than that in aqueous electrolytes. Thus, the VFG/3D-Al electrode exhibits a high specific capacitance of 1398 μF/cm2 (the volumetric specific capacitance up to 139 F/cm3) in organic electrolytes, and retains over 90% capacitance after 12,000 cycles. Moreover, the interface of the VFG/Al is modified and shows low contact resistance because the oxide film at the Al surface is removed by plasma etching and Al4C3 is formed at the interface of VFG/Al. This work paves a feasible pathway for the preparation of low-resistance VFG/3D-Al hybrid electrodes for supercapacitors.

A high-performance supercapacitor was successfully developed using graphene-based 3D hybrid nanostructured electrodes. The 3D hybrid nanostructure, consisting of vertically oriented few-layer graphene (VFG) grown on an alveolate Pt film with high-density nanocups, was synthesized by a simple and efficient “one-step” method. The 3D VFG-nanocup hybrid structured electrodes showed a high specific surface area for ion transmission and storage, which contributes to the enhancement of areal capacitance by accommodating more charges in a given footprint area than that of conventional plane-structured electrodes. Electrochemistry results indicate that the 3D VFG-nanocup hybrid structured electrodes exhibit a high specific capacitance up to 1052 μF cm−2 (three times that of the VFG-plane, at approximately 337 μF cm−2), and a good cycling stability with about 93% capacitance retention after 3000 cycles. These easily fabricated, high-performance 3D hybrid nanostructured electrodes offer great promise in energy storage device applications.

Pristine graphene with a 3D structure is desired for use in graphene-based supercapacitors, yet the very poor wettability of such graphene in water has limited its practical application. Here we report a way to simultaneously realize the 3D structure and good wettability in vertically-oriented few-layered graphene (VFG) grown by plasma-enhanced chemical vapor deposition. Based on scanning and transmission electron microscopic, Raman spectroscopic, contact angle (CA) and electrochemical analyses, a mechanism to explain the improved performance of VFG-based supercapacitors by defect-stimulated increases in wettability is proposed. The CA of our VFG samples notably reduces from 131° to 73° as the content of surface defects increases, which confirms that the wettability of VFG is markedly improved with an increased density of surface defects. Electrochemical results indicate that the VFG samples with a high density of defects exhibit high specific capacities of up to 704 μF cm−2 and a good cycling stability with about 91.2% capacitance retention after 3000 cycles. The excellent supercapacitor performance of the VFG samples with a high density of defects makes them attractive candidates for ultrathin high-performance supercapacitor electrodes.

Brazing SiO2–BN ceramics with Nb are often associated with the problems of high residual stress caused by the difference in the thermal expansion coefficients and poor mechanical properties under high temperature. To overcome these problems, here we report a new type of carbon nanotube (CNT)-reinforced TiNi brazing alloy via an “in situ growth” method by PECVD. The CNT/TiNi brazing alloy has very homogeneously dispersed CNTs within the TiNi brazing alloy, produced by in situ growth of CNTs on TiH2 and Ni powders mixed evenly into the CNT/TiH2 powders. It can be applied in the brazing of SiO2–BN and Nb. Results show that the addition of CNTs into the TiNi brazing alloy is very beneficial for the dissolution and diffusion of Nb in the brazed joint, because it can promote more TiNi–(Nb,Ti) eutectics which emerge in most of the brazing seam. Furthermore, the average shear strength of the brazed joint at room temperature is raised from 49 to 85 MPa with 1.5 vol% CNTs added into the TiNi brazing alloy. In particular, at 800 °C the brazed joint still has a very high shear strength of 51 MPa, about 1.7 times stronger than that of the TiNi brazing alloy. These results suggest that the CNTs played a key role in reducing the residual stress and reinforcing the mechanical properties (at both room and high temperature) of the brazed joint. It provides a way for the development of CNT-reinforced brazing alloys.

Cracking in an electron beam weld of titanium to stainless steel occurred during the cooling process because of internal thermal stress. Using a copper filler metal, a crack free joint was obtained, which had a tensile strength of 310 MPa. To determine the reasons for cracking in the Ti/Fe joint and the function of the copper filler metal on the improvement of the cracking resistance of the Ti/Cu/Fe joint, the microstructures of the joints were studied by optical microscopy, scanning electron microscopy and X-ray diffraction. The cracking susceptibilities of the joints were evaluated with microhardness tests on the cross-sections. In addition, microindentation tests were used to compare the brittleness of the intermetallics in the welds. The results showed that the Ti/Fe joint was characterized by continuously distributed brittle intermetallics such as TiFe and TiFe(Cr)2 with high hardness (~ 1200 HV). For the Ti/Cu/Fe joint, most of the weld consisted of a soft solid solution of copper with dispersed TiFe intermetallics. The transition region between the weld and the titanium alloy was made up of a relatively soft Ti–Cu intermetallic layer with a lower hardness (~ 500 HV). The formation of soft phases reduced the cracking susceptibility of the joint.Highlights► Electron beam welded Ti/Fe joint cracked for the brittleness and residual stress. ► Electron beam welded Ti/Cu/Fe joint with tensile strength of 310 MPa was obtained. ► Cu diluted Ti and Fe contents in weld and separated the TiFe2 into individual blocks. ► Interfacial hard Ti–Fe compounds were replaced by soft Ti–Cu compounds in the weld. ► A large amount of solid solution of copper formed in the weld.

To solve the problem of ultrasonic pulse-echo method in the evaluation of kissing bond and unbond in TiAl and 40Cr diffusion bonding, a characteristics extraction algorithm was proposed. The algorithm was based on continuous wavelet transform to convert ultrasonic TiAl and 40Cr diffusion bonding interface signals into time-scale domain. The ultrasonic tests were performed by an ultrasonic C-scan imaging system using a 10 MHz focused transducer. The time-scale amplitude and phase of the interface signals were calculated and analyzed to distinguish the kissing bond and the unbond from the perfectly bonded interface. The kissing bond can be detected by the scale-dependent amplitude combined with phase variation and the unbond can be measured by the opposite phase. The amplitude and phase characteristics were extracted to reconstruct the amplitude and phase characteristics images for TiAl and 40Cr diffusion bonding specimens evaluation. The amplitude and phase characteristics images are effective in the evaluation of bonding quality.Highlights► The algorithm utilizes the scale-dependent amplitude instead of the amplitude. ► Another difference of the algorithm is the application of the phase information. ► Kissing bond can be detected by the scale-dependent amplitude combined with phase variation. ► Unbond can be measured by the opposite phase.

A high-performance supercapacitor was successfully developed using graphene-based 3D hybrid nanostructured electrodes. The 3D hybrid nanostructure, consisting of vertically oriented few-layer graphene (VFG) grown on an alveolate Pt film with high-density nanocups, was synthesized by a simple and efficient “one-step” method. The 3D VFG-nanocup hybrid structured electrodes showed a high specific surface area for ion transmission and storage, which contributes to the enhancement of areal capacitance by accommodating more charges in a given footprint area than that of conventional plane-structured electrodes. Electrochemistry results indicate that the 3D VFG-nanocup hybrid structured electrodes exhibit a high specific capacitance up to 1052 μF cm−2 (three times that of the VFG-plane, at approximately 337 μF cm−2), and a good cycling stability with about 93% capacitance retention after 3000 cycles. These easily fabricated, high-performance 3D hybrid nanostructured electrodes offer great promise in energy storage device applications.