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1.
0.60Ba0.6Sr0.4TiO3(BST)–(0.40− x )Mg2SiO4(MS)– x Al2O3 ( x =0, 0.5, 3, 5wt%) composite ceramics exhibit excellent characteristics suitable for tunable device applications. With increasing amount of Al, the dielectric peak can be quantitatively broadened and suppressed; the "phase transition temperature" T c or ( T m) shifts to a lower temperature. Meanwhile, the tunability is still high in a wider temperature range. Far from T c, pyroelectric effects are observed by using the Byer and Roundy technology and Slim polarization hysteresis loops are observed under high ac dielectric field at 10Hz. These proved the existence of spontaneous polarization in certain possible orientations in a broad temperature range above T c in the paraelectric medium and reveal why 0.60BST–(0.40− x )MS– x Al2O3 have such remarkable dielectric nonlinearity.  相似文献   

2.
A slurry containing YBa2Cu3O7− x particles and a fine YBa2Cu3(OH) x colloid solution was prepared, and a large-scale bulk YBa2Cu3O7− x superconductor (about 50 mm × 35 mm × 2 mm) was produced by plastic forming without high-pressure molding. The samples molded from the slurry were dried and then fired at 1223 K in air. X-ray diffraction data indicated that the samples had the characteristic orthorhombic YBa2Cu3O7− x structure. Measurements of electrical resistance were carried out between 300 and 50 K by the standard four-probe DC electrical measurement. The samples began superconducting at an onset temperature around 92 K, and the full-transition temperature (critical temperature) ( T c) was 88.7±1.4 K. The critical current density ( J c) measured at 77 K was about 440 A/cm2, the value of J c was improved by the heat treatment under an oxygen atmosphere, and J c=1.6 × 103 A/cm2 was observed. Under the magnetic field (B=1 T), the sample held its superconductivity, and demonstrated that this method can be used to produce the magnetic shielding used in magnetic resonance imaging diagnosis.  相似文献   

3.
The tribological properties of Ti2SC were investigated at ambient temperatures and 550°C against Ni-based superalloys Inconel 718 (Inc718) and alumina (Al2O3) counterparts. The tests were performed using a tab-on-disk method at 1 m/s and 3N (≈0.08 MPa). At room temperature, against the superalloy, the coefficient of friction, μ, was ∼0.6, and at ∼8 × 10−4 mm3·(N·m)−1 the specific wear rate (SWRs), was high. However, against Al2O3, at ∼5 × 10−5 mm3·(N·m)−1 and ∼0.3, the SWRs and μ were significantly lower, which was presumably related to more intensive tribo-oxidation at the contact points. At 550°C, the Ti2SC/Inc718 and Al2O3 tribocouples demonstrated comparable μ's of ∼0.35–0.5 and SWRs of ∼7–8 × 10−5 mm3·(N·m)−1. At 550°C, all tribosurfaces were covered by X-ray amorphous oxide tribofilms. At present, Ti2SC is the only member of a family of the layered ternary carbides and nitrides (MAX phases) that can be used as a tribo-partner against Al2O3 in the wide temperature range from ambient to 550°C.  相似文献   

4.
The compositional range for glass formation below 1600°C in the Sm2O3─Al2O3─SiO2 system is (9–25)Sm2O3─(10–35)Al2O3─(40–75)SiO2 (mol%). Selected properties of the Sm2O3─Al2O3─SiO2 (SmAS) glasses were evaluated as a function of composition. The density, refractive index, microhardness, and thermal expansion coefficient increased as the Sm2O3 content increased from 9 to 25 mol%, the values exceeding those for fused silica. The dissolution rate in 1 N HCl and in deionized water increased with increasing Sm2O3 content and with increasing temperature to 70°C. The transformation temperature ( T g ) and dilatometric softening temperature ( T d ) of the SmAS glasses exceeded 800° and 850°C, respectively.  相似文献   

5.
Nanocrystalline β-SiC with additions of 7 wt% Al2O3, 2 wt% Y2O3, and 1 wt% CaO was subjected to tensile deformation to study its microstructural behavior under the dynamic process. The liquid-phase-sintered body had a relative density of >97% and an average grain size of 170 nm. Tension tests were conducted at initial strain rates ranging from 2 × 10−5 to 5 × 10−4 s−1, in the temperature range 1973–2023 K, in both argon and N2 atmospheres. Although grain-boundary liquids formed by the additions vaporized concurrently with the decomposition of SiC and extensive grain growth, the maximum tensile elongation of 48% was achieved in argon. Annealing experiments under the same conditions revealed that vaporization and grain growth were both dependent on experimental time. Therefore, high strain rates suffered less from the hardening effect when cavitation damage was more severe. Testing in an N2 atmosphere brought about crystallization of the grain-boundary phase and prevented severe vaporization; however, fracture occurred at only 8% elongation. Grain-boundary sliding was still the dominant mechanism for deformation.  相似文献   

6.
Phase-pure perovskite Pb(Zn x Mg1– x )1/3Nb2/3O3 solid solution (PZ x M1– x N) is obtained for x ≦ 0.7 by heating a milled stoichiometric mixture of PbO, Mg(OH)2, Nb2O5, and 2ZnCO3·3Zn(OH)2·H2O at 1100°C for 1 h. Percent perovskite ( f P) with respect to total crystalline phase decreases with increasing temperature of subsequent heating then increases to 900°C for the mixtures where x ≦ 0.8 and milled for 3 h. For mixtures with x = 0.9 and x = 1, f P decreases monotonically. Curie temperature increases almost linearly with increasing x up to x = 0.7. The maximum dielectric constant at 1 kHz is 2×104 and 1.7×104 for the mixture with x = 0.4 and x = 0.7, respectively. The stabilization mechanism of strained perovskite is discussed.  相似文献   

7.
The monolithic glass-forming region of the low phonon and low softening point antimony glasses containing high Sb2O3 (40–75 mol%) in the novel quaternary K2O–B2O3–Sb2O3–ZnO system has been found with the help of X-ray diffraction (XRD) analysis. The structure of a series of glasses with the general composition of (mol%) 15K2O–15B2O3–(70− x )Sb2O3– x ZnO (where x =5–25) has been evaluated by infrared reflection spectral (FT-IRRS) analyses. All the glasses are found to possess a low phonon energy of around 600 cm−1, as revealed by FT-IRRS. Their softening point ( T s), glass transition temperature ( T g), and coefficient of thermal expansion (CTE) have been found to vary in the ranges of 351°–379°C, 252°–273°C, and 195–218 × 10−7 K−1, respectively. These properties are found to be controlled by their fundamental property, like the covalent character of the glasses, which is found to increase with an increase in Sb2O3 content. In addition, the devitrified glasses have been characterized by XRD and field emission scanning electron microscopy, which manifests the presence of nanozinc antimony oxide crystals with sizes of 21–43 nm. The exhibited properties have revealed that they are a new class of versatile materials.  相似文献   

8.
Nanocrystalline La0.9Sr0.1Al0.85Co0.05Mg0.1O3 oxide powder was synthesized by a citrate–nitrate auto-ignition process and characterized by thermal analysis, X-ray diffraction, and impedance spectroscopy measurements. Nanocrystalline (50–100 nm) powder with perovskite structure could be produced at 900°C by this process. The powder could be sintered to a density more than 96% of the theoretical density at 1550°C. Impedance measurements on the sintered samples unequivocally established the potential of this process in developing nanostructured lanthanum aluminate-based oxides. The sintered La0.9Sr0.1Al0.85Co0.05Mg0.1O3 sample exhibited a conductivity of 2.40 × 10−2 S/cm in air at 1000°C compared with 4.9 × 10−3 S/cm exhibited by La0.9Sr0.1Al0.85Mg0.15O3.  相似文献   

9.
Glasses with compositions 50Bi2O3– x Sb2O3–10B2O3–(40– x ) SiO2 ( x =0, 1, 3, 5, 8, 10) have been prepared by conventional melt quench technique. Substitution of Sb2O3 for SiO2 exerted an obvious effect on properties of glasses, especially, increased glass transition temperature ( T g) and crystalline temperature ( T c) greatly. Results of infrared transmission spectra attributed the effect to the formation of new bridging bonds of Sb–O–B and Sb–O–Si in glass network.  相似文献   

10.
Porous glass-ceramics with a skeleton of the fast-lithium-conducting crystal Li1+ x Ti2− x Al x (PO4)3 (where x = 0.3–0.5) were prepared by crystallization of glasses in the Li2O─CaO─TiO2─Al2O3–P2O5 system and subsequent acid leaching of the resulting dense glass-ceramics composed of the interlocking of Li1+ x Ti2− x Al x (PO4)3 and β-Ca3(PO4)2 phases. The median pore diameter and surface area of the resulting porous Li1+ x Ti2− x Al x (PO4)3 glass-ceramics were approximately 0.2 μm and 50 m2/g, respectively. The electrical conductivity of the porous glass-ceramics after heating in LiNO3 aqueous solution was 8 × 10−5 S/cm at 300 K or 2 × 10−2 S/cm at 600 K.  相似文献   

11.
Sintering and microstructural evolution were studied in Fe3O4 as a model system for spinel ferrites. Fe3O4 powder, purified by the salt-crystallization method, was sintered to ∼99.5% density in a CO-CO2 atmosphere. The p O2 Of the sintering atmosphere drastically affects the microstructure (grain size) of sintered Fe3O4 without significantly affecting density. The measured grain-boundary mobilities, M , of Fe3O4 fit the equation M=M 0( T ) p O2−1/2 with M 0( T ) = 2.5×105 exp[-(609kJ·mol-1/ RT ](m/s)(N/m2)−l. The grain-boundary migration process appeared to be pore-drag controlled, with lattice diffusion of oxygen as the most likely rate-limiting step.  相似文献   

12.
Thermal expansion of the low-temperature form of BaB2O4 (β-BaB2O4) crystal has been measured along the principal crystallographic directions over a temperature range of 9° to 874°C by means of high-temperature X-ray powder diffraction. This crystal belongs to the trigonal system and exhibits strongly anisotropic thermal expansions. The expansion along the c axis is from 12.720 to 13.214 Å (1.2720 to 1.3214 nm), whereas it is from 12.531 to 12.578 Å (1.2531 to 1.2578 nm) along the a axis. The expansions are nonlinear. The coefficients A, B , and C in the expansion formula L t = L 0(1 + At + Bt 2+ Ct 3) are given as follows: a axis, A = 1.535 × 10−7, B = 6.047 × 10−9, C = -1.261 × 10−12; c axis, A = 3.256 × 10−5, B = 1.341 × 10−8, C = -1.954 × 10−12; and cell volume V, A = 3.107 × 10−5, B = 3.406 × 10−8, C = -1.197 × 10−11. Based on α t = (d L t /d t )/ L 0, the thermal expansion coefficients are also given as a function of temperature for the crystallographic axes a , c , and cell volume V.  相似文献   

13.
In order to evaluate the crystallization tendency of glasses, the ratio of the crystallization temperature to the liquidus temperature ( T c/ T L) was obtained by DTA measurement for the Na2O–B2O3 and Na2O–B2O3–Al2O3 systems. The critical cooling rate for glass formation ( Q *) was also measured. The measurements were performed in the composition range of (100 − x )Na2O–( x )B2O3, ( x = 25–35 and 60–100 mol%), and (100 − y )0.5Na2O·0.5B2O3−( y )Al2O3, ( y = 6–34 mol%). The relationship between T c/ T L and Q * was discussed. A linear relationship between T c/ T L and log Q * for these systems was found. Furthermore, the relationship between T c/ T L and Q * was verified by computer simulation based on the crystallization kinetics of glass or supercooled liquid.  相似文献   

14.
The crystal structure of YAl3C3 was refined from laboratory X-ray powder diffraction data (Cu K α1) using the Rietveld method. The crystal structure is hexagonal (space group P 63 mc , Z =2) with lattice dimensions a =0.342157(4) nm, c =1.72820(1) nm, and V =0.175217(3) nm3. The final reliability indices were R wp=9.94% ( R wp/ R e=1.18), R p=7.36%, R B=1.77%, and R F=1.03%. The compound shows an intergrowth structure with electroconductive [YC2] thin slabs separated by Al4C3-type [AlC] layers. This material had thermoelectric properties superior to those of the layered carbides Zr2[Al3.56Si0.44]C5, Zr2Al3C4, and Zr3Al3C5 in the temperature range of 500– 1073 K, with a maximal power-factor value of 1.96 × 10−4 W·(m·K2)−1 at 1073 K.  相似文献   

15.
The effects of substituting Nb5+ with Ta5+ on the microwave dielectric properties of the ZnNb2O6 ceramics were investigated in this study. The forming of Zn(Nb1− x Ta x )2O6 ( x =0–0.09) solid solution was confirmed by the measured lattice parameters and the EDX analysis. By increasing x , not only could the Q × f of the Zn(Nb1− x Ta x )2O6 ( x =0–0.09) solid solution be tremendously boosted from 83 600 GHz at x =0 to a maximum 152 000 GHz at x =0.05, the highest ɛr∼24.6 could also be achieved simultaneously. It was mainly due to the uniform grain morphology and the highest relative density of the specimen. A fine combination of microwave dielectric properties (ɛr∼24.6, Q × f ∼152 000 GHz at 8.83 GHz, τf∼–71.1 ppm/°C) was achieved for Zn(Nb0.95Ta0.05)2O6 solid solution sintered at 1175°C for 2 h.  相似文献   

16.
The deviation from stoichiometry, δ, in Cr2−δO3 was measured by a tensivolumetric method in the high pO2 range of ≊104 to 104 Pa at 1100°C. The value of δ, or chromium vacancy concentration, was≊9×10−5 mol/mol Cr2O3 in air for Cr2O3 with 99.999% purity. The chemical diffusion coefficient, DT, determined from equilibration data was ≊4.6× cm2·s−1 at 1100°C for pO2= 2.2 ×101 Pa. The self-diffusion coefficient of Cr ions was calculated from and δ and found to be≊1.6×10-17 cm2-s−1, in good agreement with recently measured values.  相似文献   

17.
An investigation of the properties of high-purity (>99 wt%) tantalum tungstates (Ta22W4O67, Ta, WO8, and Ta16W18O94) included determination of density (bulk and theoretical), refined lattice constants, maximum use temperatures, micro-hardness, heat capacity, thermal expansion (contraction) and diffusivity, calculated thermal conductivity, and electrical resistivity. Usable to ∼ 1700 K in air or inert atmospheres, these tantalum tungstates have theoretical densities of 7.3 to 8.5 g/cm3, are relatively soft (120 to 655 kg/mm2 hardnesses), and are electrical insulators (6× 103 to 2× 108Ω.cm resistivities). The distinguishing properties of the materials are their thermal expansion (average CTE values from + 0.6×10−8/K to −5.1× 10−6/K at 293 to 1273 K), thermal expansion hysteresis with minimal observable microcracking, and thermal diffusivity  相似文献   

18.
Pb(Zn1/3Nb2/3)0.20(Zr0.50Ti0.50)0.80O3 ceramics of pure perovskite structure were prepared by the two-stage method with the addition of 0–3.0 wt% MnO2 and their piezoelectric properties were investigated systematically. The MnO2 addition influences in a pronounced way both the crystal structure and the microstructure of the materials. The materials are transformed from the tetragonal to the rhombohedral structure, and the grain size is enhanced when manganese cations are added. The distortion of crystal structure for samples with MnO2 addition can be explained by the Jahn–Teller effect. The values of electromechanical coupling factor ( k p) and dielectric loss (tan δ) are optimized for 0.5-wt%-MnO2-doped samples ( k p= 0.60, tan δ= 0.2%) and the mechanical quality factor ( Q m) is maximized for 1.0-wt%-MnO2-doped samples ( Q m= 1041), which suggests that oxygen vacancies formed by substituting Mn3+ and Mn2+ ions for B-site ions (e.g., Ti4+ and Zr4+ ions) in the perovskite structure partially inhibited polarization reversal in the ferroelectrics. The ceramics with 0.50–1.0 wt% MnO2 addition show great promise as practical materials for piezoelectric applications.  相似文献   

19.
Some K2O-Nb2O5-GeO2 glasses are prepared, and their crystallization behaviors are examined. 25K2O·25Nb2O5·50GeO2 glass with the glass transition temperature T g= 622°3C and crystallization onset temperature T x= 668°3C shows a prominent nanocrystallization. The crystalline phase is K3,8Nb5Ge3O20,4 with an orthorhombic structure. The sizes of crystals in the crystallized glasses heat-treated at 630° and 720°3C for 1 h are °10 and 20–30 nm, respectively, and the crystallized glasses obtained by heat treatments at 620°-850°3C for 1 h maintain good transparency. The density of crystallized glasses increases gradually with increasing heat-treatment temperature, and the volume fraction of crystals in the sample heat-treated at 630°3C for 1 h is estimated to be ∼35%. The usual Vickers hardness and Martens hardness (estimated by nanoindentation) of 25K2O·25Nb2O5·50GeO2 glass change steeply by heat treatment at T g, i.e., at around 35% volume fraction of nanocrystals. The present study demonstrates that the composite of nanocrystals and the glassy phase has a strong resistance against deformation during Vickers indenter loading in crystallized glasses.  相似文献   

20.
An approach to select appropriate network modifiers to tailor the thermal and dielectric properties of zinc borosilicate (ZBS) glass has been explored to apply the glass composition to a dielectric layer of plasma display panels. Based on ionic field strength of the modifiers and the ionic polarizability of the corresponding crystalline form, Bi2O3 and Na2O modifiers have been selected to modify the thermal and dielectric properties of the glass toward the required properties for the application (440< T g<460°C, 7.5 × 10−6−6/K, and 10< K <15). Bi2O3 addition to ZBS could meet all the required properties simultaneously at a given addition range (8.5–14 mol%), while the addition of Na2O could not meet all the properties at a single composition range.  相似文献   

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