•Gd2Zr2O7 nanocrystalline powders with an average crystallite size of 5.9 nm can be obtained by a non-aqueous gel-combustion method.•Gd2Zr2O7 nanograin ceramics with high relative densities were successfully fabricated at relatively low temperatures by FAST sintering.•The densification and grain growth in the sintering process were divided into three stages.•Nanograin ceramics have comparatively high hardness compared with sample sintered at 1500 °C for 72 h in furnace.For Gd2Zr2O7, nanograin ceramics with high relative densities (95%–97.4%) and small average grain sizes (61 nm - 82 nm) were successfully fabricated for the first time using field assisted sintering technique (FAST), also known as spark plasma sintering (SPS) method, in which nanopowders with an average crystallite size of 5.9 nm were synthesized via a non-aqueous gel-combustion method. Temperature was varied over a broad range to investigate its influence on the densification mechanism, microstructure evolution and hardness. The results revealed that both relative density and grain size increase with the increase of sintering temperature. The densification processes were divided to three stages. 1250 °C–1300 °C is the most suitable temperature range to fabricate dense Gd2Zr2O7 nanograin ceramics with comparatively high hardness by FAST.Download high-res image (459KB)Download full-size image

Transparent polycrystalline Yb/Er:YAG and Yb, Er:YAG ceramics with different Er3+ concentration were successfully fabricated by vacuum sintering co-precipitated raw powders. The structural, optical, and spectroscopic properties of the Er:YAG and Yb, Er:YAG ceramics were investigated in detail. The absorption coefficient of the Er:YAG ceramics increased with the increase of Er3+ dopant level. The absorption coefficient of Yb, Er:YAG ceramic at ~970 nm (0.967 cm−1) is nearly 4.5 times the value for Er:YAG ceramic of 0.214 cm−1. The upconversion luminescence spectra, infrared luminescence spectra, and fluorescence lifetimes of the ceramics were also studied. Results show that the intense green and red emissions located at around 523, 560 and 677 nm are attributed to the radiant transitions of 2H11/2/4S3/2 - 4I15/2 and 4F9/2 - 4I15/2 of Er3+, respectively. The infrared spectra presented many narrow emission peaks in the 1450–1700 nm regions, attributable to the 4I13/2-4I15/2 emission transition. The fluorescence lifetime of the Yb, Er:YAG ceramic (7.53 ms) is also greater than that of Er:YAG (6.67 ms). Furthermore, the underlying mechanism of the IR and visible emission bands were discussed. Experimental results provided evidence that energy transfer from Yb3+ to Er3+ can be achieved, indicating Yb, Er:YAG ceramic could be a potential lasing medium material.

Defect-fluorite structured Gd2Zr2O7 nanoparticles were successfully synthesized via a homogeneous precipitation-solvothermal method using urea as a precipitant. The obtained nanoparticles were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) analysis and transmission electron microscopy (TEM). Compared to the traditional solvothermal method, this homogeneous precipitation-solvothermal method has the advantage of producing nanoparticles with small grain sizes, a narrow size-distribution, high surface areas and little agglomeration. Particularly, the mean crystallite size of Gd2Zr2O7 obtained by this method is 20–30 nm, providing a great opportunity of using these nanoparticles as starting nano-sized building blocks for low temperature preparation of homogeneous and dense ceramics.

We report on the sintering promoting and fluorescent activator roles of Er3+ in AlON:Er3+ transparent ceramics prepared by pressureless sintering with Er2O3 and AlON powder. There shows that the transparency of samples varied with the content of Er2O3 additive. The AlON:Er3+ ceramics showed upconversion luminescence peaking at 546 nm, 662 nm and 840 nm under the 980 nm excitation due to transition of 4S3/2/2H11/2→4I15/2, 4F9/2→4I15/2 and 4S3/2/2H11/2→4I13/2 of Er3+ ions, respectively. The infrared spectra exhibited strong emission at 1534 nm corresponds to 4I13/2→4I15/2 transition. The mechanism of the IR and visible emission bands in AlON:Er3+ ceramics are discussed, which suggest it should be attractive for lighting and display devices applications.

•Prepared the nano-grained YAG transparent ceramic by high pressure technique.•Obtained the compressive yield with different temperature.•Obtained the compressive yield of nano-grained YAG transparent ceramic.Nano-grained ceramics have their unique mechanical characteristics that are not commonly found in their coarse-grained counterparts. In this study, nano-grained YAG transparent ceramics (NG-YAG) were prepared by low-temperature high-pressure technique (LTHP). The peak profile analysis of the X-ray diffraction was employed to investigate the compressive yield strength of NG-YAG. During the temperature at 450 °C, the residual micro-strain (RMS) increased with increasing loading pressure. However when the loading pressure was exceeded to 4.0 GPa the RMS exhibited a severe negative slop. The temperature effects on the compressive yield strength were also studied. It shows that the compressive yield strength of NG-YAG is 4.0 GPa and 5.0 GPa respectively at 450 °C and 350 °C. More importantly according to this investigation, a feasible technique to study the nano-grained ceramics is provided.Fig. 2 shows the significant slope changes of calculated residual micro-strain (RMS) associated with five selected pressure-temperature conditions. Another the grain size estimated from Scherrer's formula, especially when it changes with the pressure-temperature condition is also plotted in Fig. 2.

Aluminum oxynitride (AlON) transparent ceramics were prepared through aqueous gel-casting forming technique. To prevent hydrolysis, the homemade AlON powder was treated in 1% phosphoric acid at 70 °C for 30 min. Then the surface-treated AlON powder was dispersed in an aqueous-organic solution to prepare low viscosity slurry with 40 vol% solids loading, which was shaped by gel-casting forming technique. The hydrolysis of AlON powder was prevented effectively and the transmittance of sintered ceramics had been significantly improved by introducing hydrolysis protection of the powder. The sample sintered with the treated powder can reach an ultimate infrared transmittance of 78% with 2 mm thickness. The surface modification of AlON powder with phosphoric acid was confirmed by various techniques such as scanning electron microscopy (SEM), pH, Viscosity, X-ray photoelectron spectroscopy (XPS) and infra-red spectroscopy (IR). The related hydrolysis protection mechanism via this available method was discussed.

Using the co-precipitated Nd:YAG raw powder, combined with 0.14 wt.% SiO2 as sintering aid, the green compacts were vacuum sintered from 1250 to 1750 °C to track the microstructural evolution during ceramic sintering process. The SEM results showed that a silicon-related liquid-phase sintering (1400–1550 °C) could greatly promote the densification process of ceramics with the ceramics at 1550 °C becoming translucent. Based on the evolution results, the parameters of sintering schedule, especially intermediate temperature holds and heating rates, were changed and optimized. The corresponding SEM morphologies and the influence mechanism were investigated. The results revealed that intermediate temperature holds (1250, 1450 and 1600 °C) and slower heating rates (1–5 °C/min) had a significant role in the pore elimination, the fine microstructure of ceramic and the promotion of ceramic transmittance. Using the optimized sintering schedule, a ceramic with fine microstructure could be obtained with a transmittance of 83.8% at 1064 nm.

•Ball milling for 12 h can break the sintering neck of particles and increase the transmittance of ceramic to 81% at 1100 nm.•Decreasing slurry concentration from 38.5 to 20.3 wt% improved the densification and the transmittance of Yb:YAG ceramics.•With the optimal conditions, Yb:YAG powders showed the homogeneous particle distribution and highest densification rate.•The highest transmittance of Yb:YAG ceramic reaches up to 82% in the visible light region and 83.5% at 1100 nm.Yb:YAG transparent ceramics were fabricated by vacuum sintering method using the co-precipitation synthesis of raw powders. These raw powders were mixed in ethanol doped with TEOS and ball milled for different time periods (0–20 h) and various solid contents of slurry (Sc = 7.8–38.5 wt%). The samples were vacuum sintered from 1350 °C to 1800 °C. Effects of ball-milling time and slurry concentrations on the morphology of powders, as well as densification, microstructure and transmittance of Yb:YAG ceramics were systematically evaluated. Results show that ball milling for 12 h can break the sintering neck of the particles and increase the optical transmittance to 81% at 1100 nm. For the Sc range of 38.5–20.3 wt%, decreasing slurry concentration improved the densification and in-line transmittance of Yb:YAG ceramics in the visible light region. When ball-milled for 12 h with 20.3 wt% slurry concentration, the Yb:YAG powders showed the most homogeneous particle distribution and highest densification rate, from which the ceramics prepared have the pore-free microstructure and the best transmittance. The highest optical transmittance of Yb:YAG ceramic sintered at 1750 °C for 15 h reaches up to 82% in the visible light region and 83.5% at 1100 nm, which is almost close to the theoretical transmittance.

•High quality Yb:YAG transparent ceramics were fabricated with MgO as sintering aids.•The Yb:YAG samples were annealed at 1250–1450 °C for 20 h in air.•The complex color center [Mg2+F+] formed during the annealing.•The complex color center can be eliminated after post-HIP.High quality Yb:Y3Al5O12 (YAG) transparent ceramics were fabricated by vacuum sintering with MgO as sintering aids. The Yb:YAG samples were annealed at 1250–1450 °C for 20 h in air. The experimental results showed that the transparency of Yb:YAG samples declined markedly with the annealing temperatures of 1250–1450 °C. The samples became increasingly orange-yellow in color with the increase of annealing temperature. The potential reasons of discoloration were discussed for the first time. It was attributed to the complex color center [Mg2+F+] formed during the annealing, which was evidenced by optical absorption in the range of 300–500 nm wavelength and the presence of an electron spin resonance (ESR) line at g = 1.9806. The formation mechanism of the complex color center was explained in detail. The complex color center can be eliminated after post-HIP (hot isostatic pressing). And by air annealing and post-HIP, the transmittance of the samples increased from 80.3% to 83.4%.

•The optical transmittance of Yb,Cr:YAG and Cr:YAG reaches 83% at 1200 nm and 81% at 1400 nm, respectively.•The Yb,Cr:YAG ceramics exhibit a pore free structure with an average grain size of about 5 μm.•A sharp emission peak of 694 nm appeared and the emission intensity enhanced with the increase of the Cr3+ concentration.•The fluorescence lifetime and emission intensity of Yb3+ in Yb,Cr:YAG ceramic is lower than that of Yb3+ in Yb:YAG ceramic.•Yb,Cr:YAG transparent ceramics could be a potential material for passive self-Q-switched solid-state laser.Cr:YAG and Yb/Cr:YAG transparent ceramics containing Ca as charge counter element were fabricated by vacuum sintering technique using the co-precipitation synthesis of raw powders. Their spectral and luminescence properties as well as the influence of Cr3+ concentration on the optical properties of Yb,Cr:YAG ceramic were investigated. Results show the transmittance of 10 at.% Yb, 0.25 at.% Cr:YAG and 0.25 at.% Cr:YAG reaches 83% at 1200 nm and 81% at 1400 nm, respectively. And the Yb,Cr:YAG ceramics exhibit a pore free structure with an average grain size of about 5 μm. After annealing, most of Cr3+ ions transform into Cr4+. In the case of excitation wavelength of 440 nm, a sharp emission peak of 694 nm appeared in the Yb,Cr:YAG ceramic before annealing and the band enhanced with the increase of the Cr3+ concentration, which is attributed to the 4T2g−4A2g fluorescence transition. The emission spectrums and fluorescence decays manifest that both the luminescent intensity and the lifetimes of Yb,Cr:YAG are lower than Yb:YAG ceramic and the lifetimes of Yb,Cr:YAG and Yb:YAG are 0.93 and 2.38 ms, respectively. This results demonstrate the existence of the ground state absorption of Cr4+ in the Yb,Cr:YAG ceramic. Experimental evidence proved that Yb,Cr:YAG transparent ceramics could be a potential material for passive self-Q-switched solid-state laser.

Nanocrystalline magnesium aluminate spinel (MgAl2O4) powders were synthesized in this study by employing a modified nitrate–citrate combustion route, in which the heat treatment was continuously performed by two separate processes. One process was in nitrogen during the carbonization of the precursors, and the other was subsequently in air. This can greatly decrease the heat release during the combustion process. X-ray diffraction and high-resolution transmission electron microscopy, differential scanning calorimetry, thermogravimetry, infrared spectroscopy, and inductively coupled plasma atomic emission spectrometer were utilized to reveal the reaction process and characterize the physical properties of precursors and powders. To achieve well-crystallized spinel powders with smallest possible and controllable crystallite sizes, the effect of carbonization in nitrogen on the calcined spinel powders was investigated in detail. The introduction of the inert atmosphere at the pre-calcination stage, with appropriate temperature and holding time, allowed ultra-fine, well-crystallized and size-controllable MgAl2O4 powders with average crystallite sizes of 5–25 nm to be obtained. Despite the benefit from the pre-calcination in nitrogen, it was found that the stoichiometry of precursors can be compromised due to the loss of aluminum during the heat treatment in N2 as the temperature was too high (>900 °C) with sufficient holding time. The mechanism of the carbothermal reduction was investigated in this study.

Nanocrystalline 74Ge embedded SiO2 films were prepared by employing ion implantation and neutron transmutation doping methods. Transmission electron microscopy, energy dispersive x-ray spectroscopy, and photoluminescence of the obtained samples were measured. The existence of As dopants transmuted from 74Ge is significant to guarantee the uniformity and higher volume density of Ge nanocrystals by tuning the system׳s crystallinity and activating mass transfer process. It was observed that the photoluminescence intensity of Ge nanocrystals increased first then decreased with the increase of arsenic concentration. The optimized fluence of neutron transmutation doping was found to be 5.5×1017 cm−2 to achieve maximum photoluminescence emission in Ge embedded SiO2 film. This work opens a route in the three-dimensional nanofabrication of uniform Ge nanocrystals.

We reported the growth of VO2 film deposited by an inorganic sol–gel method, followed by post-annealing. An in situ evolution of the grain size in the films with different annealing temperatures (300, 500, and 700 °C for 90 min), annealing times (500 °C for 20, 40, 60, and 90 min), and film thicknesses (30, 150 and 320 nm) was observed. The results indicated that the grain size distribution in the sol–gel-derived VO2 films was mediated by the density of nucleation center, which was varied in the films with different extents of thermal deformation during the annealing. By increasing the film thickness from 30 to 320 nm, a compact nanostructure with uniform distribution of grain size could be formed. It suggested that the in situ-evolved nanostructure in the thicker VO2 film will lead to lower threshold temperature and enhanced transition intensity in the phase transition. The effect of nanoscale grain size on the lower phase transition temperature in the VO2 film was discussed.

Spinel aluminium oxynitride (γ-AlON), as a kind of transparent ceramic material expectable, is studied using the ab initio density functional method, in terms of electronic, elastic, thermodynamic properties and structure disorder. The results show that γ-AlON exhibits strong ionicity, as quantitatively expressed by (AlO2.43+)15(AlT2.41+)8(O1.64-)27(N2.27-)5 from our reassessment of the ionic character. We summarize and speculate that the considered oxynitride single-crystals exhibit highly elastic anisotropy. The interpretation of the thermodynamic properties of γ-AlON according to quasi-harmonic Debye model confirm the available experiments and are extended to a wider temperature/pressure range. This material holds high elastic strength under extreme environments, where dB/dT absolute value is less than 0.03 GPa/K, independent of the pressure. Finally, we study the O/N structure disorder character of γ-AlON solid solution by investigating nine possible crystal structures. It is found that γ-AlON should be partially disordered, and in fact, the O/N ordering has a significant effect on the properties.Highlights► We reassess the chemical bonding character of γ-AlON which shows strong ionicity. ► γ-AlON single-crystals exhibit highly elastic anisotropy. ► The thermodynamic properties are investigated in a wider temperature/pressure range. ► γ-AlON is an O/N partially disordered structure.

The electronic structures and absorption spectra of the perfect MgAl2O4 spinel and the MgAl2O4 containing various vacancies have been studied by using DFT code in VASP package. The optical transition models induced by oxygen vacancy are predicted. Our results show that the absorption peak at 5.3 eV is due to the neutral oxygen vacancy VO0. Both 3.2 and 4.75 eV peaks are attributed to the 1+ charged oxygen vacancy VO1+. And more, our absorption spectra also indicate that the experiment observed red shift in UV transparent spectra of nano-ceramic MgAl2O4 is induced by the various color centers of oxygen vacancies.Research highlights► Theoretical calculation of various point vacancies in MgAl2O4 spinel. ► The electronic structures and absorption spectra have been studied. ► The optical transition models induced by oxygen vacancies are predicted. ► Gives an explanation to the experimental optical properties of the MgAl2O4 nano-ceramics.

Nanosintering behaviour of MgAl2O4 ultra-fine powder was investigated under high pressure. We found that the nanosintering mechanism under high pressure was quite different from that under ambient pressure. To shed light on the mechanism, the residual stress, grain size, and boundary situation were investigated using HRTEM and XRD, respectively. Our results show high pressure can restrain grain growth and initiate plastic deformation to eliminate pores and/or additional phases existing in triple junctions of the grains. However, the conventional sintering process is controlled by grain growth to avoid imperfections between grain boundaries when preparing transparent ceramics.

We report here experimental results of yield strength and stress relaxation measurements of transparent MgAl2O4 nano-ceramics at high pressure and temperature. During compression at ambient temperature, the differential strain deduced from peak broadening increased significantly with pressure up to 2 GPa, with no clear indication of strain saturation. However, by then, warming the sample above 400°C under 4 GPa, stress relaxation was obviously observed, and all subsequent plastic deformation cycles are characterized again by peak broadening. Our results reveal a remarkable reduction in yield strength as the sintering temperature increases from 400 to 900°C. The low temperature for the onset of stress relaxation has attracted attention regarding the performance of transparent MgAl2O4 nano-ceramics as an engineering material.

YAG (Y3Al5O12) powder was prepared by co-precipitation method. To avoid pH values changing intensely during titration process, the NH4HCO3 solution with NH4NO3 was used as precipitant solution. The pH change of precipitant solution during titration process, compositions of precursor, phase formation process and morphology of the prepared powder were investigated by means of pH meter, FT-IR, XRD and TEM. The results show that the presence of NH4NO3 in NH4HCO3 solution is crucial to alleviate the pH change, lower the calcination temperature and meliorate the morphology of YAG powder. The mechanisms were studied in detail. The YAG phase can be obtained at a lower temperature of 900 °C. The obtained powder, composed of elliptical particles, showed a meliorated morphology.

Samples of nanocrystalline 74Ge embedded in amorphous SiO2 film were prepared by 74Ge ion implantation and subsequent primary thermal annealing. These samples were irradiated by neutron flux in a nuclear reactor then the second annealing followed. Irradiation with thermal neutrons leads to doping of nanocrystalline 74Ge with As impurities due to nuclear transmutation of isotope 74Ge into 75As. Transmission electron microscope, X-ray fluorescence, X-ray photoelectron spectroscopy, laser Raman scattering and photoluminescence of the obtained samples were measured. It was observed that with the increase in As-donors concentration, photoluminescence intensity first increased but then significantly decreased. An explanatory model of this non-monotonic behavior was discussed.

Isotope 74Ge ion implantation and neutron transmutation doping methods were carried out to prepare donor impurities (arsenic) doped Ge nanocrystals embedded in amorphous SiO2 film. The isotope 74Ge atoms can react with thermal neutron and transmute to 75As atoms, acting as donor impurities in Ge crystal. The photoluminescence intensity of donor doped Ge nanocrystals is observed increased first then decreased with increase of arsenic concentration. The non-monotonic dependence of photoluminescence on the level of doping is explained by following model: at low donor concentration, donor electrons can passivate non-radiative centers, as increasing the radiative efficiency; at high arsenic concentration, donor electrons remain free and reduce the radiative efficiency due to appearance of auger-like recombination channel.

Gd2Zr2O7 nanoceramics were fabricated using pressureless sintering method, in which the nanopowders were synthesized via solvothermal approach. The effects of starting powders on grain growth and densification during sintering of ceramics were revealed. Two distinct pressureless sintering methods were investigated, including conventional and two-step sintering. The sample grain size increases abruptly as sintering temperature increases during conventional sintering. In contrast, in two-step sintering, abnormal or discontinuous grain growth was suppressed in the second step, leading to Gd2Zr2O7 nanoceramics formation (average grain size 83 nm, relative density ∼93%). Such distinct behaviors may originate from the interplay between kinetic factors such as grain boundary migration and diffusion. Moreover, suppression of grain growth and promotion of densification in the two-step sintering are mainly due to dominant role of grain boundary diffusion during the second-step sintering process.

•YAG powder crystallinity and its influence on ceramics were investigated.•Poor powder crystallinity involved more the surface defects on particle surface, finally inducing more inhomogeneous ceramic sintering and ceramic defects.•Powder crystallinity should be promoted and considered as a further way to improve ceramic properties.The Nd:YAG raw powders of various crystallinities were obtained from the co-precipitated precursor by calcining at 1100–1250 °C for 2–6 h. XRD, HRTEM and EDX were employed to analyze the powder crystallinity, e.g., its phases, changes of lattice constants, surface defects and morphologies. The subsequent ceramics were fabricated by simply vacuum sintering at 1780 °C for 10 h to well compare the initial intrinsic effect of various powder crystallinities. The induced defect states and corresponding properties of ceramics were explored and discussed. The results show that poorly crystallized powder involves a thick layer on particle surface, which is not well crystallized but can escape XRD detection. This poorly crystallized powder contributes more the inhomogeneous ceramic sintering and introduces more the defects in final ceramics, e.g., impure phase inclusions and dislocations. The results reveal that powder crystallinity should be promoted and considered as a further way to improve ceramic properties.

•Our calculation indicates that the α-LiAlO2 is an indirect band gap insulator of 6.319 eV.•The mechanical properties of α-LiAlO2 are predicted.•The complete phonon frequencies of α-LiAlO2 at gamma point for the infrared and Raman modes are assigned which to distinguish the α-LiAlO2 and γ-LiAlO2 in ITER and in MCFC.The physical properties including the structural, electronic, mechanical, lattice dynamical and thermodynamic properties of α-LiAlO2 are investigated using first-principles calculation. It is found that α-LiAlO2 is an insulator with an indirect gap of 6.319 eV according to band structure and density of states. The elastic constants are obtained and the results indicate that α-LiAlO2 is mechanically stable. The mechanical properties including bulk modulus (B), shear modulus (G), Young’s modulus (E), Poisson’s ratio (υ) are predicted with the value of 147.0 GPa, 105.2 GPa, 254.8 GPa and 0.211, respectively. The phonon dispersion curves and the phonon density of states are also calculated. The calculated phonon frequencies for the Raman-active and the infrared-active modes considering the LO-TO splitting are assigned. The two Raman active frequencies are 407.0 cm−1 of Eg mode and 628.8 cm−1 of A1g mode, and show satisfactory agreement with experiment. The thermodynamic functions such as ΔF, ΔE, CV and S is predicted by using the phonon density of states. These results provide valuable information for further insight into the properties of α-LiAlO2 in atomic scales, which is strategically important in ITER and in molten carbonate fuel cells (MCFC).

λ-Ti3O5 was a newly discovered material with intriguing phase transition characteristics, which exhibits huge potential in the application of memory and tunable optoelectronic devices. However, the fabrication of λ-Ti3O5 still presents a great challenge and its application needs further investigation. In this work, we developed a novel method to fabricate nanocrystalline λ-Ti3O5 by carbothermal reduction of nano-TiO2, and explored its terahertz transmission properties through a temperature induced phase transition. A second phase was introduced to inhibit the grain growth of titanium oxide during the carbothermal reduction, by performing a surface modification of the precursor nano-TiO2 particles with Al2O3. This process was proved to be critical for the formation of nanocrystalline λ-Ti3O5. An in situ XRD analysis combined with a first-principles calculation based on plane wave DFT indicated that the nanocrystalline λ-Ti3O5 exhibited a semimetallic λ phase to metallic α phase transition across a large temperature range. The phase transition was accompanied by continuous, slow and reversible tuning of the terahertz transmission amplitude. This work provides considerable insights into the synthesis of λ-Ti3O5 and opens up studies on the applications of λ-Ti3O5 in the THz range, such as but not limited to sensors and smart windows.