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1.
β-Ga 2O 3 nanocolumns straightened and crossed perpendicularly each other were deposited on MgO (1 0 0) substrate by vapor phase transport method. Growth of the nanocolumns was examined at steps of 1000, 1050, and 1200 °C in elevation of source-boat temperature. We have drawn out the substrate from deposition-tube at each source-boat temperatures of 1000, 1050, and 1200 °C. Scanning electron microscopy of the sample with source-boat temperature of 1200 °C demonstrated that the straightened and elongated nanocolumns are crossing perpendicularly each other. Typical lengths of the nanocolumns were in the range of several hundreds nanometers below 1050 °C, and those of 1200 °C were in the range of ten to fifteen hundreds nanometers. Diameters of the nanocolumns stayed in the range of few hundreds nanometers, notwithstanding variation of the source temperature. X-ray diffraction and transmission electron microscopy with energy dispersive X-ray spectroscopy revealed that the nanocolumns are monoclinic β-Ga 2O 3 crystal, and the (4 0 0) plane of β-Ga 2O 3 nanocolumns is parallel to the (1 0 0) plane of MgO substrate. 相似文献
2.
Cathode material Sm 0.5Sr 0.5CoO 3 (SSC) with perovskite structure for intermediate temperature solid oxide fuel cell was synthesized using glycine-nitrate process (GNP). The phase evolution and the properties of Sm 0.5Sr 0.5CoO 3 were investigated. The single cell performance was also tested using La 0.9Sr 0.1Ga 0.8Mg 0.2O 3−δ (LSGM) as electrolyte and SSC as cathode. The results show that the formation of perovskite phase from synthesized precursor obtained by GNP begins at a calcining temperature of 600 °C. The single perovskite phase is formed completely after sintering at a temperature of 1000 °C. The phase formation temperature for SSC with complete single perovskite phase is from 1000 to 1100 °C. The SrSm 2O 4 phase appeared in the sample sintered at 1200 °C. It is also found that the sample sintered at 1200 °C has a higher conductivity. The electrical conductivity of sample is higher than 1000 S/cm at all temperature examined from 250 to 850 °C, and the highest conductivity reaches 2514 S/cm at 250 °C. The thermal expansion coefficient of sample SSC is 22.8 × 10 −6 K −1 from 30 to 1000 °C in air. The maximum output power density of LSGM electrolyte single cell attains 222 and 293 mW/cm 2 at 800 and 850 °C, respectively. 相似文献
3.
The growth kinetics of WSi 2 coating formed by chemical vapor deposition (CVD) of Si on a W substrate at temperatures between 1000 and 1200 °C using SiCl 4–H 2 gas mixtures was investigated and its isothermal oxidation resistance in 80% Ar–20% O 2 atmosphere was evaluated at temperatures between 800 and 1300 °C. WSi 2 coating grew with a parabolic rate law after an initial incubation period, indicating the diffusion-controlled growth. The activation energy for growth of WSi 2 coating was about 42.5 kcal/mol. The isothermal oxidation rate of WSi 2 coating increased with increasing oxidation temperature but rapidly decreased at 1300 °C. The oxidation product of WSi 2 coating was composed of the WO 3 particles embedded in the amorphous SiO 2 matrix at below 1200 °C but consisted of only SiO 2 phase at 1300 °C. The fast oxidation behavior of WSi 2 coating at below 1200 °C was attributed to the formation of many cracks and pores, i.e. short-circuit diffusion path of oxygen, within the oxide scale, which resulted from the internal stress generated both by the large volume expansion caused by the oxidation reactions of WSi 2 and by the evaporation of WO 3 phase. The slow oxidation behavior of WSi 2 coating at 1300 °C was due to the exclusive formation of a slow-growing continuous SiO 2 scale by the rapid evaporation of WO 3 phase. 相似文献
4.
In this study, the influence of the glass addition and sintering parameters on the densification and mechanical properties of tetragonal zirconia polycrystals (3Y-TZP) ceramics were evaluated. High-purity tetragonal ZrO 2 powder and La 2O 3-rich glass were used as starting powders. Two compositions based on ZrO 2 and containing 5 wt.% and 10 wt.% of La 2O 3-rich glass were studied in this work. The starting powders were mixed/milled by planetary milling, dried at 90 °C for 24 h, sieved through a 60 mesh screen and uniaxially cold pressed under 80 MPa. The samples were sintered in air at 1200 °C, 1300 °C, 1400 °C for 60 min and at 1450 °C for 120 min, with heating and cooling rates of 10 °C/min. Sintered samples were characterized by relative density, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Hardness and fracture toughness were obtained by Vickers indentation method. Dense sintered samples were obtained for all conditions. Furthermore, only tetragonal-ZrO 2 was identified as crystalline phase in sintered samples, independently of the conditions studied. Samples sintered at 1300 °C for 60 min presented the optimal mechanical properties with hardness and fracture toughness values near to 12 GPa and 8.5 MPa m 1/2, respectively. 相似文献
5.
Nano-crystalline SrAl 2O 4 with spinel structure was successfully prepared at 700 °C using amorphous SrAl 2(diethylenetriaminepentaacetic acid (DTPA) 1.6)(H 2O) 4 as precursor. The precursor was synthesized by a simple inorganic reaction and decomposed into SrAl 2O 4 at temperatures above 500 °C, which was proved by DTA–TGA and X-ray photoelectron spectroscopy (XPS) analysis. X-ray diffraction (XRD) results illustrated that a crystalline SrAl 2O 4 phase can form at 700 °C, which is about 600 °C lower than that used in the traditional method. The crystalline SrAl 2O 4 prepared at 900 °C for 2 h had a crystal size of about 28 nm and a grain size of about 80 nm, and its BET surface area can reach 28.056 m 2/g. Calcination temperature and time had a weak effect on crystal size. 相似文献
6.
The ionic conductivity and thermo-Raman spectra of anhydrous sodium pyrophosphate Na 4P 2O 7 were measured dynamically in the temperature range from 25 to 600 °C with a heating rate of 2 °C min −1 to understand the structural evolution and phase transformation involved. The DSC thermogram was also measured in the same thermal process for the phase transformation investigation. The spectral variations observed in the thermo-Raman investigation indicated the transformation of Na 4P 2O 7 from low temperature phase ( ) to high temperature phase ( ) proceeded through pre-transitional region from 75 to 410 °C before the major orientational disorder at 420 °C and minor structural modifications at 511, 540 and 560 °C. The activation energies and enthalpies of the proposed phase transformations were determined. The possible mechanism for temperature dependent conductivity in Na 4P 2O 7 was discussed with the available data. 相似文献
7.
Since ultra-fine Ti(C, N) has large surface and high activity, preparation of high performance cermets using ultra-fine Ti(C, N) powders is very difficult at the present. In the paper, deoxidation process of ultra-fine TiC 0.7N 0.3 powder is carried out firstly, and the oxygen content of ultra-fine TiC 0.7N 0.3 powder can be decreased from more than 1 wt% to 0.06 wt%; milling technology of ultra-fine TiC 0.7N 0.3-based cermet is studied in the paper, the results show that the optimum milling time is 45 h and the ball to powder weight ratio is 15:1, and the dispersant helps to achieve a homogeneous distribution of the ultra-fine powder; during vacuum sintering of ultra-fine cermet, pores tend to form, hence NT6B shows relatively lower properties than NT6A. After HIP process (1350 °C, 90 min, 70 MPa), the porosity can be largely decreased. The prepared ultra-fine cermet has typical core–rim microstructure, finer grain size and enhanced properties. 相似文献
8.
Effects of particle distribution level on recrystallization were investigated in a Fe 3Al-based alloy containing coarse κ-Fe 3AlC precipitate particles. Volume fraction of 10–12% of rod-like κ particles with different size and interparticle spacing was introduced within the Fe 3Al matrix by changing the cooling rate from 1200 °C, which is above the precipitation temperature of the κ phase. These samples were warm rolled at 700 °C to a total reduction of 75%. Annealing of the warm rolled samples produced complete recrystallized structures. The average recrystallized grain size against interparticle spacing showed a valley-shaped curve with a minimum size of 20 μm. Orientation analyses of the warm rolled samples with high resolution EBSD method revealed that the valley shape of the curve may be explained by the particle stimulated nucleation density of recrystallization around κ particles, dependent on the particle distribution. 相似文献
9.
The oxidation behavior of Ti 3SiC 2-based material in air has been studied from 900°C to 1200°C. The present work showed that the growth of the oxide scale on Ti 3SiC 2-based material obeyed a parabolic law from 900°C to 1100°C, while at 1200°C it followed a linear rule. The oxide scale was generally composed of an outer layer of coarse-grained TiO 2 (rutile) and an inner layer of fine-grained TiO 2 and SiO 2 (tridymite) above 1000°C. A discontinuous coarse-grained SiO 2 layer was observed within the outer coarse-grained TiO 2 layer on the samples oxidized at 1100°C and 1200°C. Marker experiments showed that the oxidation process was controlled by the inward diffusion of oxygen, outward diffusion of titanium and CO or SiO, and that internal oxidation predominated. The TiC content in Ti 3SiC 2 was deleterious to the oxidation resistance of Ti 3SiC 2. 相似文献
10.
In this study we present the results on complex structural changes of the Co 70Fe 5Si 10B 15 amorphous alloy induced during heating in the temperature range between 20 and 1000 °C. The structural and phase transformation changes were correlated with DTA, XRD and SEM properties. It is shown that initial Co 70Fe 5Si 10B 15 alloy during heating undergoes complex crystallochemical changes. In the range between ambient temperature and near 400 °C, investigated alloy retains the solid-state amorphous properties. Prolonged heating induces complete transformation to crystalline solid state. The solid–solid amorphous to crystalline state transformation process is completed at 500 °C, when two nanocrystalline phase alloy systems are formed. Prolonged thermal treatment between 600 and 1000 °C, influenced further elemental segregation and phase transition. At 1000 °C, the composite material consisting of two FCC cobalt-rich alloys and a hexagonal unidentified alloy are formed. 相似文献
11.
Two-phase Fe-rich Fe–Al–Zr alloys have been prepared consisting of binary Fe–Al with a very low solubility for Zr and the ternary Laves phase Zr(Fe,Al) 2 or τ 1 phase Zr(Fe,Al) 12. Yield stress, flexural fracture strain, and oxidation behaviour of these alloys have been studied in the temperature range between room temperature and 1200 °C. Both the Laves phase and the τ 1 phase act as strengthening phases increasing significantly the yield stress as well as the brittle-to-ductile transition temperature. Alloys containing disordered A2+ ordered D0 3 Fe–Al show strongly increased yield stresses compared to alloys with only A2 or D0 3 Fe–Al. The binary and ternary alloys with about 40at.% Al and 0 or 0.8at.% Zr show the effect of vacancy hardening at low temperatures which can be eliminated by heat treatments at 400 °C. At higher Zr contents this effect is lost and instead an increase of low-temperature strength is observed after the heat treatment. The increase of the high-temperature yield strength of Fe-40at.% Al by adding Zr is much stronger than by other ternary additions such as Ti, Nb, or Mo. Tests on the oxidation resistance at temperatures up to 1200 °C indicate a detrimental effect of Zr already for additions of 0.1at.%. 相似文献
12.
The effect of sequential and continuous high-energy impact mode in the magneto-mill Uni-Ball-Mill 5 on the mechano-chemical synthesis of nanostructured ternary complex hydride Mg 2FeH 6 was studied by controlled reactive mechanical alloying (CRMA). In the sequential mode the milling vial was periodically opened under a protective gas and samples of the milled powder were extracted for microstructural examination whereas during continuous CRMA the vial was never opened up to 270 h duration. MgO was detected by XRD in sequentially milled powders while no MgO was detected in the continuously milled powder. The abundance of the nanostructured ternary complex hydride Mg 2FeH 6, produced during sequential milling, and estimated from DSC reached 44 wt.% after 188 h, and afterwards it slightly decreased to 42 wt.% after 210 and 270 h. In contrast, the DSC yield of Mg 2FeH 6 after continuous CRMA for 270 h was 57 wt.%. Much higher yield after continuous milling is attributed to the absence of MgO. This behavior provides strong evidence that MgO is a primary factor suppressing formation of Mg 2FeH 6. The DSC hydrogen desorption onset temperatures are close to 200 °C while the desorption peak temperatures for all powders are below 300 °C and the desorption process is completed within the range 10–20 min. Within the investigated nanograin size range of 5–13 nm, the DSC desorption onset and peak temperatures of β-MgH 2 and Mg 2FeH 6 do not exhibit any trend with nanograin (crystallite) size of hydrides. TPD hydrogen desorption peaks from the powders containing a single ternary complex hydride Mg 2FeH 6, are very narrow, which indicates the presence of small but well-crystallized hydride particles. Their narrowness provides good evidence that the phase composition, bulk hydrogen distribution and hydride particle size distribution are very homogeneous. The overall amount of hydrogen desorbed in TPD from single-hydride Mg 2FeH 6 powders is somewhat higher than that observed in DSC and TGA desorption. The powder milled sequentially for 270 h and desorbed in a Sieverts-type apparatus at 250 and 290 °C, yielded about a half of the hydrogen content obtained during DSC and TGA tests. No desorption of hydrogen was detected in a Sieverts-type apparatus at 250 and 290 °C after 128 and 70 min, respectively, from the powder continuously milled for 270 h. The latter easily desorbed 3.13 and 2.83 wt.% hydrogen in DSC and TGA tests, respectively. 相似文献
13.
In Na–U(IV)–Mo–O system, two quaternary compounds Na 2U(MoO 4) 3 and Na 4U(MoO 4) 4 were prepared by solid state reactions of Na 2MoO 4, UMoO 5 and MoO 3 in the required stoichiometric ratio at 500 °C in evacuated sealed quartz ampoules. The crystal structure of both the compounds were derived from X-ray powder diffraction data in the tetragonal system by Rietveld profile method. Na 2U(MoO 4) 3 has scheelite structure, whereas Na 4U(MoO 4) 4 has scheelite superlattice structure. TG curves of Na2U(MoO4)3 and Na4U(MoO4)4 did not show any significant weight change up to 750 °C in an inert atmosphere. During the heating cycle in an inert atmosphere, DTA curves of Na2U(MoO4)3 and Na4U(MoO4)4 showed endothermic peaks due to the melting of the compounds at 740 °C and 730 °C, respectively. Na2U(MoO4)3 and Na4U(MoO4)4, when heated in air atmosphere at 1200 °C, decomposed to form Na2U2O7 which was confirmed by weight loss calculation and XRD. 相似文献
14.
BaTi 0.6Zr 0.4O 3 powder was prepared from barium oxalate hydrate, zirconium oxy-hydroxide and titanium dioxide precursors. Barium oxalate hydrate and zirconium oxy-hydroxide were precipitated from nitrate solution onto the surface of suspended TiO 2. Phase formation behaviour of the materials was extensively studied using XRD. BaTiO 3 (BT) and BaZrO 3 (BZ) start forming separately in the system upon calcinations in the temperature range 600–700 °C. BT–BZ solid solution then forms by diffusion of BT into BZ from 1050 °C onwards. The precursor completely transforms into BaTi 0.6Zr 0.4O 3 (BTZ) at 1200 °C for 2 h calcination. The activation energy (AE) of BT (134 kJ mol −1) formation was found to be less than that of BZ (167.5 kJ mol −1) formation. BTZ formation requires 503.6 kJ mol −1 of energy. The sintering kinetics of the powder was studied using thermal analyzer. The mean activation energy for sintering was found to be 550 kJ mol −1. 相似文献
15.
The La 0.9Sr 0.1Ga 0.8Mg 0.2O 3−δ (LSGM) powders for intermediate temperature SOFC electrolyte have been synthesized by glycine-nitrate combustion process. The as-synthesized powders show almost pure perovskite phase. And then, the as-synthesized powders were sintered by SPS at 1300 °C to prepare electrolyte. The SEM, XRD and AC impedance were employed to characterize the microstructure, phase and electrical conductivities. Results show that the grain size is very fine, less than 1 μm, and the relative density of the pellet after sintering by SPS is about 94.7%. There is very little amount of secondary phases after SPS and the grain boundary and secondary phase resistance is very small. The electrolyte sintered by SPS shows higher conductivities than that sintered by conventional method at the same temperature. The activation energy at lower temperatures (400–700 °C) and higher temperatures (700–800 °C) is about 0.94 and 0.49 eV, respectively. Spark plasma sintering is a promising and effective method to sinter the LSGM electrolyte. 相似文献
16.
In this work, two composite compositions—one with 30% (v/v) SiC, the other with 30% (v/v) TiC, balance Ti 3SiC 2—were synthesized and characterized. Fully dense samples were fabricated by hot isostatically pressing Ti, SiC and C powders for 8 h at 1500 or 1600 °C and a pressure of 200 MPa. Both TiC and SiC lower grain boundary mobility in Ti 3SiC 2. Coarsening of the SiC particles was also observed. At comparable grain sizes, all composites tested were weaker in flexure than the unreinforced Ti 3SiC 2 matrix, with the reduction in strength being the worst for the SiC composites. This reduction in strength is most probably due to thermal expansion mismatches between the matrix and reinforcement phases. The composite samples were exceptionally damage tolerant; in one case a 100 N Vickers indentation (in a 1.5-mm thick bar) did not reduce the flexural strength as compared to an unindented or as-fabricated samples. The same is true for thermal shock resistance; quenching samples from 1400 °C in room temperature water, resulted in strength reductions that were 12% at best and 50% at worst. In the 25–1000 °C temperature range, the thermal expansion coefficients of the two composites were indistinguishable at 8.2×10 −6 K −1. The Vickers hardness values depended on load; at 100 N, the hardnesses were ≈15 GPa; at 300 N, they asymptote to 7–8 GPa. For the most part, very few cracks emanate from the corners of the Vickers indents even at loads as high as 500 N. In the few cases where cracks did initiate, fracture toughness values were crudely estimated to lie in the 5–7.5 MPa √m range. 相似文献
17.
In this paper we present a transmission electron microscopic study on the effect of annealing on the microstructure of a cold-rolled Ni 50.2Ti 49.8 ribbon. Transmission electron microscopy of the as-received sample shows the presence of alternating amorphous and crystalline bands. The crystalline bands have widths of the order of a few microns and contain amorphous nanopockets and B2 nanograins, the latter at around 20 nm diameter and preferentially oriented with their normal along the 111 direction and perpendicular to the strip surface. As-received samples were annealed for 30 min at different temperatures up to 800 °C. Crystallization starts in the amorphous bands at around 350 °C and finally ends up with the coarsening of the grains in the entire sample. Annealing of the samples at 450 °C entirely transforms the amorphous bands into crystalline bands. At 800 °C the grain size increases to 30–50 μm with a formation of a tweed kind of morphology inside the grains when observed at room temperature. Diffraction patterns from such grains reveal the presence of diffuse intensity around 1/3110 * indicating the formation of the R-phase. NiTi 2 precipitates form at 450 °C while annealing at 600 °C and higher yields Ni 3Ti 2 precipitates. For samples annealed at 500 °C for a longer time, Ni 4Ti 3 precipitates have been observed along with the austenite to martensite transformation in the grains. 相似文献
18.
Phase structure, microstructure, dielectric and piezoelectric properties of 0.4 wt% CeO 2 doped 0.94(Bi 0.5Na 0.5)TiO 3–0.06BaTiO 3 (Ce-BNT6BT) ceramics sintered in the temperature range from 1120 to 1200 °C have been investigated as a candidate for lead-free piezoelectric ceramics. Tetragonal phase played an important role in improvement of electrical properties and the density of the ceramics. Dielectric constant decreased slightly with the increase of sintering temperature in ferroelectric region but a reverse phenomenon occurred in antiferroelectric and paraelectric regions, suggesting that interfacial polarizations were improved with the increase of sintering temperature and domain walls of ferroelectricity became active after depolarization. At room temperature, Ce-BNT6BT ceramics sintered at 1180 °C showed good performances: dielectric constant was 914 at 1 kHz, thick coupling factor kt was 0.52, and the ratio of kt/ kp was 2.3. The ceramics were suitable for narrowband filters and ultrasonic transducers in commercial applications. 相似文献
19.
This study investigates the microstructures and mechanical properties of Ti 50Al 50 alloys prepared via mechanical alloying (MA) starting from elemental powders. The process of the spark plasma sintering (SPS) has also been studied. It is found that the nanocrystallization process of the Ti–Al alloy proceeds and the sintering temperature can control the microstructure of alloy. The sintering of the compacts is carried out at the temperatures of 1100–1200 °C with a compaction pressure of 30 MPa and a heating rate of 30 °C min −1. Specimens with high densities and approaching the equilibrium state can be obtained in short time by spark sintering than conventional sintering. Such shorter high temperature is important to prevent grain growth. 相似文献
20.
Copper matrix was reinforced with Al 2O 3 particles of different size and amount by internal oxidation and mechanical alloying accomplished using high-energy ball milling in air. The inert gas-atomised prealloyed copper powder containing 1 wt.% Al as well as a mixture of electrolytic copper powder and 3 wt.% commercial Al 2O 3 powder served as starting materials. Milling of Cu-1 wt.% Al prealloyed powder promoted formation of fine dispersed particles (1.9 wt.% Al 2O 3, approximately 100 nm in size) by internal oxidation. During milling of Cu-3 wt.% Al 2O 3 powder mixture the uniform distribution of commercial Al 2O 3 particles has been obtained. Following milling, powders were treated in hydrogen at 400 °C for 1 h in order to eliminate copper oxides formed at the surface during milling. Compaction was executed by hot-pressing. Compacts processed from 5 to 20 h-milled powders were additionally subjected to high-temperature exposure at 800 °C in order to examine their thermal stability and electrical conductivity. Compacts of Cu-1 wt.% Al prealloyed powders with finer Al 2O 3 particles and smaller grain size exhibited higher microhardness than compacts of Cu-3 wt.% Al 2O 3 powder mixture. This indicates that nano-sized Al 2O 3 particles act as a stronger reinforcing parameter of the copper matrix than micro-sized commercial Al 2O 3 particles. Improved thermal stability of Cu-1 wt.% Al compacts compared to Cu-3 wt.% Al 2O 3 compacts implies that nano-sized Al 2O 3 particles act more efficiently as barriers obstructing grain growth than micro-sized particles. Contrary, the lower electrical conductivity of Cu-1 wt.% Al compacts is the result of higher electron scatter caused by nano-sized Al 2O 3 particles. 相似文献
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