Temperature Dependence of Electrical Resistivity (4–300 K) in Aluminum‐ and Boron‐Doped SiC Ceramics

Al‐ and B‐doped 3C–SiC ceramics were prepared by hot‐pressing powder compacts containing submicrometer‐sized β‐SiC, precursors of 5 wt% nanosized β‐SiC, and an optional additive (Al or B) in an Ar atmosphere. Electron probe microanalysis (EPMA) investigation on the obtained specimens revealed that a portion of the doped Al and B atoms substituted the zinc blende lattice sites. The temperature‐dependent electrical resistivity data of the Al‐ and B‐doped SiC specimens were measured in the 4–300 K range and compared with those of an undoped specimen. The Al‐ and B‐doped SiC specimens exhibited resistivities that were as high as ~10³ Ω cm at room temperature and ~10⁵ and ~10⁴ Ω cm, respectively, below 100 K. These values are larger than those of the undoped SiC specimen by a factor of ~10⁴. Such high resistivities of the impurity‐doped specimens are attributable to the carrier compensation by the Al‐ and B‐derived acceptors located well above the valence‐band edge of 3C–SiC. Photoluminescence investigation revealed that the Al‐ and B‐doped specimens exhibited emission profile below 2 eV, implying the existence of the acceptors.     

Electrical and Thermal Properties of SiC Ceramics Sintered with Yttria and Nitrides

The electrical and thermal properties of SiC ceramics containing 1 vol% nitrides (BN, AlN or TiN) were investigated with 2 vol% Y₂O₃ addition as a sintering additive. The AlN‐added SiC specimen exhibited an electrical resistivity (3.8 × 10¹ Ω·cm) that is larger by a factor of ~10² compared to that (1.3 × 10^?1 Ω·cm) of a baseline specimen sintered with Y₂O₃ only. On the other hand, BN‐ or TiN‐added SiC specimens exhibited resistivity that is lower than that of the baseline specimen by a factor of 10^?1. The addition of 1 vol% BN or AlN led to a decrease in the thermal conductivity of SiC from 178 W/m·K (baseline) to 99 W/m·K or 133 W/m·K, respectively. The electrical resistivity and thermal conductivity of the TiN‐added SiC specimen were 1.6 × 10^?2 Ω·cm and 211 W/m·K at room temperature, respectively. The present results suggest that the electrical and thermal properties of SiC ceramics are controllable by adding a small amount of nitrides.     

Ti3SiC2/4SiC复相陶瓷的高温力学性能研究

为了进一步了解Ti3SiC2/nSiC复合材料优良的综合性能,特别是其高温力学性能,本文以热等静压原位合成技术制备的Ti3SiC2/4SiC复相陶瓷为试验材料,对其高温拉伸和高温弯曲行为进行研究。结果表明:Ti3SiC2/4SiC复相陶瓷的高温抗拉强度比室温抗拉强度高;Ti3SiC2/4SiC复相陶瓷的高温抗弯强度在900℃出现一极大值,1000℃后具有好的高温塑性。     

Thermal Shock Resistance of an AlN–BN–SiC Ceramic

Mechanical and thermal properties of AlN–BN–SiC (ABS) ceramics were used to calculate the R, R ', and R "" thermal shock parameters. The R parameter values ranged from ∼400° to 450°C. Specimens were thermal shocked by water quenching and the critical quench temperatures (Δ T _C) were compared with those of a baseline SiC composition. The behavior of the ABS was predicted by R parameter calculations while the behavior of the baseline material was predicted by the R ' calculations due to its higher thermal conductivity (87 W·(m·K)^-1) as compared with the ABS materials (∼30 W·(m·K)⁻¹). The highest critical quench temperature for ABS was ∼415°C with the lowest at 360°C, while the critical quench temperature for the baseline material was 450°C. Using temperature dependent data over an appropriate temperature range (room temperature to the predicted Δ T _C), the R parameters of the ABS materials were within 15°C of predictions. The baseline material was ∼1.7 times higher than predicted and this was attributed to the high-thermal conductivity of the material resulting in soft thermal shock during quench testing.     

纳米晶钛酸钡陶瓷铁电性能和介电性能的温度依赖性(英文)

研究50nm BaTiO3陶瓷的铁电性能、介电性能的温度依赖性,其介温谱和介电损耗谱具有明显的弥散相变特征,当频率为1kHz时计算得到弥散指数γ为1.60。不同温度下的介电常数–电场强度(ε–E)曲线显示,介电异常发生在110～120℃的温度范围内,110℃时的介电可调性为20.2%,介电损耗小于0.02。压电位移曲线计算得到50nm BaTiO3陶瓷的压电系数d33为45pm/V。     

Thermal Conductivity, Electrical Resistivity, and Seebeck Coefficient of Uranium Mononitride

Measurements are reported for the thermal conductivity, λ(80° to 400°K), electrical resistivity, ρ(4.2° to 400°K), and absolute Seebeck coefficient, Q(6° to 400°K), of pressed and sintered uranium mononitride. The measurements between 77° and 400°K were made using an absolute longitudinal heat flow apparatus. These data and literature values for the thermal conductivity and electrical resistivity at higher temperatures were used to separate the electronic and lattice portions of the thermal conductivity. The results indicate that the lattice conductivity peaks in the range 250° to 300°K and that the high-temperature limit of the Lorenz function may be greater than the Sommerfeld value of 2.443 × 10^-8 (V^2°K^-2). The electrical resistivity and the absolute Seebeck coefficient exhibit sharp slope changes near the Néel temperature, T _N(∼50° to 60°K). The Seebeck coefficient has a minimum at 34°K and then rises to a local maximum at 10°K. This low-temperature peak is probably due to magnon drag. The temperature dependence of the electrical resistivity is dominated by the magnetic contribution which increases as T ^2.38±0.08 between 10° and 52°K. The magnetic contribution is constant at high temperatures with an estimated value of 142 μΩ-cm.     

Enhancing Electrical Properties in NBT–KBT Lead-Free Piezoelectric Ceramics by Optimizing Sintering Temperature

Conventional sintering of (Na_{1− x} K _x )_0.5Bi_0.5TiO₃ (abbreviated as NKBT x , x =18–22 mol%) lead-free piezoelectric ceramics was investigated to clarify the optimal sintering temperature for densification and electrical properties. Both sintered density and electrical properties were sensitive to sintering temperature; particularly, the piezoelectric properties deteriorated when the ceramics were sintered above the optimum temperature. The NKBT20 and NKBT22 ceramics synthesized at 1110°–1170°C showed a phase transition from tetragonal to rhombohedral symmetry, which was similar to the morphotropic phase boundary (MPB). Because of such MPB-like behavior, the highest piezoelectric constant ( d ₃₃) of about 192 pC/N with a high electromechanical coupling factor ( k _p) of about 32% were obtained in the NKBT22 ceramics sintered at 1150°C.     

Fabrication and Electrical Properties of Mn–Ni–Co–Cu–Si Oxides Negative Temperature Coefficient Thermistors

The microstructure and electrical properties of Mn–Ni–Co–Cu–Si oxides negative temperature coefficient (NTC) thermistors were studied. The as-sintered (Mn_1.62Ni_0.72Co_{0.57− x} Cu _x Si_0.09)O₄ (0≤ x ≤0.12) and (Mn_1.2Ni_0.78Co_{0.87− x} Cu_0.15Si _x )O₄ (0≤ x ≤ 0.15) ceramics showed the solid solutions of Mn–Ni–Co–Cu–Si oxides with a cubic spinel structure. The addition of SiO₂ led to an increase in the temperature coefficient of resistivity. This demonstrates that the SiO₂ addition is desirable for developing highly sensitive NTC thermistors. In addition, the resistivity and the temperature coefficient of resistivity for (Mn_1.62Ni_0.72Co_{0.57− x} Cu _x Si_0.09)O₄ and (Mn_1.2Ni_0.78Co_{0.87− x} Cu_0.15Si _x )O₄ NTC thermistors were controlled by changing the composition.     

Temperature Dependence of Hardness of Alumina-Based Ceramics

Hardness was measured as a function of temperature (20° to 1000°C) for several Al₂O₃ ceramics, including single-crystal sapphire and polycrystalline aluminas containing different amounts of second phase. Hardness decreased steadily with increasing temperature for all materials tested, in accordance with a semiempirical relation of the form H=H₀ (1 – T/T₀). This behavior conformed with a thermally activated slip process, limited by Peierls stresses. At lower temperatures, the hardness values for debased aluminas were less (smaller H₀) than for the pure materials, consistent with a reduction in shear modulus resulting from the "soft" phase. However, at higher temperatures the hardness values for all the aluminas converged (identical T₀, i.e., material-invariant activation energy). The latter behavior indicated that the temperature dependence of the indentation deformation was controlled predominantly by the Al₂O₃ component.     

Dielectric and Electrical Conductivity Properties of PMS–PZT Ceramics

The dielectric and electrical conductivity properties of x Pb(Mn_1/3Sb_2/3)O₃–(1− x )Pb(Zr_0.52Ti_0.48)O₃ (PMS–PZT) ceramics were studied. X-ray diffraction indicated that all samples exhibit a single-phase perovskite structure. Dielectric study revealed that the dielectric relaxor behavior was induced by co-doping Mn²⁺ and Sb⁵⁺ into Pb(Zr_0.52Ti_0.48)O₃ ceramics. Electrical conductivity measurements showed that the concentration of carriers are increased with the increase in PMS contents. After annealing in an oxygen atmosphere for 30 h, the direct current conductivity of PMSZ15 was much lower than that of the as-sintered sample. The reason why this phenomenon occurs may be the reduction of oxygen-vacancy concentration.     

Temperature Dependence of Luminescence in Beryllium Ceramics

A relationship is established between x-ray luminescence and variation of the elementary cell parameters of BeO. The lattice deformation and pyroelectric fields lead to the formation of point defects (which are luminescence centers), on which excitons, as well as holes and conduction electrons, are presumably localized.     

Influence of Magnetic Field and Bias Voltage on the Thermal Conductivity and Seebeck Coefficient of AA-Stacked Bilayer SiC

Silicon - Thermoelectric properties of AA-stacked bilayer SiC in presence of a magnetic field and a bias voltage were studied using the tight binding model. Green function method was applied to...     

Piezoresistivity in Semiconducting Positive Temperature Coefficient Ceramics

The experimental and theoretical aspects of piezoresistivity in semiconducting, positive temperature coefficient (PTC) of resistivity ceramics will be reviewed. Emphasis will be placed on their potential application in sensor technology. Future material and modeling challenges are highlighted.     

碳化硅泡沫陶瓷烧结温度和烧结机理的研究

将制备碳化硅泡沫陶瓷的浆料通过烘干、制粉、干压成型、烧结来探讨烧结温度对制品性能的影响。试验结果表明样品的最高抗弯强度出现在1400℃、保温2h的工艺条件下，而不是更高的烧结温度1450℃。主要原因在于过高的烧结温度导致碳化硅氧化严重，生成了大量的方石英，方石英在随后的冷却过程中出现微裂纹所致。而碳化硅泡沫陶瓷的烧结机理主要是玻璃相对碳化硅颗粒的包覆、连接作用和新相莫来石的生成。     

烧成温度对SiC多孔陶瓷的影响

选用SiC作为骨料,低熔点陶瓷结合剂和活性C作为成孔剂,对不同的烧成温度下多孔陶瓷的基本性能进行了研究.温度的提高使SiC多孔陶瓷的气孔率增大,气孔形状逐渐呈不规则变化,晶界玻璃相对SiC颗粒的润湿以及在SiC颗粒表面的扩展作用增强,提高了对骨料颗粒的粘结作用,使瓷体强度提高,但晶界玻璃相自身结合强度降低;气孔通道在1240℃烧成温度下多为贯通型,1280℃呈网状分布,1320℃下多为贯通型且存在大量交联通道;大于1300℃烧成时,由于SiC的高温氧化产物参与晶界相反应,使局部界面结合强度大大提高,出现SiC颗粒拔出断裂现象.     

Synthesis and Properties of MoSi2–MoB–SiC Ceramics

MoB and SiC particulate reinforced MoSi₂ matrix composites were synthesized in situ from Mo, Si, and B₄C powder mixtures by self‐propagating high‐temperature synthesis (SHS). The SHS MoSi₂–MoB–SiC products were vacuum hot‐pressed (HPed) at 1400°C for 90 min to fabricate high‐density (> 97.5% relative density) bulk composites. Microstructure refinement and improvements in the Vickers hardness and fracture toughness of the HPed composites were observed with increasing B₄C content in the reaction mixture. The HPed composite of composition MoSi₂–0.4MoB–0.1SiC exhibited grain size of 1–5 μm, Vickers hardness of 12.5 GPa, bending strength of 537 MPa, and fracture toughness of 3.8 MPa.m^1/2. These excellent mechanical properties indicate that MoB and SiC particulate reinforced MoSi₂ composites could be promising candidates for structural applications.     

Reactive Hot Pressing of ZrC–SiC Ceramics at Low Temperature

Composites of ZrC–SiC with relative densities in excess of 98% were prepared by reactive hot pressing of ZrC and Si at temperature as low as 1600°C. The reaction between ZrC and Si resulted in the formation of ZrC_1?x, SiC, and ZrSi. Low‐temperature densification of ZrC?SiC ceramics is attributed to the formed nonstoichiometric ZrC_1?x and Zr–Si liquid phase. Adding 5 wt% Si to ZrC, the three‐point bending strength of formed ZrC_0.8–13.4 vol%SiC ceramics reached 819 ± 102 MPa with hardness and toughness being 20.5 GPa and 3.3 MPa·m^1/2, respectively.     

Magnetic Properties of Al‐doped B4C and SiC Ceramics

Al‐doped B₄C and Al‐doped SiC ceramic compounds were synthesized by high‐temperature solid‐state sintering. Ferromagnetic (FM) ordering was observed in the doped semiconducting ceramics. The changes in the FM ordering with doping level were different in the two doped systems although they were prepared by similar experimental condition. Our results indicated that some of the C in B₄C were replaced by the Al atoms, and these displaced C atoms existed in the form of graphite. The synthesized SiC samples, undoped or Al doped, were found to be a mixture consisting of the magnetic and nonmagnetic SiC polytypes. The mechanisms in the generation of the FM ordering due to the dopants in the two ceramics followed different routes.     

Electrical and Magnetic Properties of Iron-Nickel Sulfide Ceramics

Solid solutions of (Fe,Ni)S were prepared by the metal oxide-carbon disulfide reaction sintering method. These ceramics have the hexagonal NiAs structure, and their lattice parameters depend on the Fe-Ni ratio. The magnetization of these ceramics reduces with decreasing Fe content between 100/0 (Fe/Ni) and 50/50, increases between 50/50 and 40/60, and decreases again between 40/60 and 0/100. The electrical resistivity reduces uniformly with decreasing Fe content of the ceramics.