首页 | 官方网站   微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 24 毫秒
1.
The medium-entropy carbide (W,Ti,V)C0.8 ceramics were prepared by sparking plasma sintering at temperatures between 1400 and 1700°C. The effects of sintering temperature on the microstructure and mechanical properties of the medium-entropy carbide (W,Ti,V)C0.8 ceramics were investigated. X-ray diffraction, scanning electron microscope, and energy dispersive spectrometer were used to confirm the formation of single-phase face-centered cubic (FCC) solid solution of the medium-entropy carbide (W,Ti,V)C0.8 ceramics prepared at a sintering temperature of 1600°C. It was found that the mechanical properties of the material were improved by solid solution strengthening during the formation of single-phase FCC solid solution, and the best overall performance of the medium-entropy carbide (W,Ti,V)C0.8 ceramics was achieved at 1600°C, when the hardness value was 22.3 ± 1.8 GPa, the fracture toughness was 5.7 ± 0.8 MPa·m1/2, the flexural strength was 605 ± 4 MPa, and the compressive strength was 1.84 GPa. Most importantly, the addition of TiC0.4 promoted the diffusion among the elements of the medium-entropy carbide (W,Ti,V)C0.8 ceramics, which contributed to the formation of single-phase FCC solid solution and significantly reduced the sintering temperature of the medium-entropy carbide (W,Ti,V)C0.8 ceramics due to the effect of vacancies. This study provides a new idea for the preparation of medium-entropy carbide ceramics.  相似文献   

2.
Yttrium tantalate ceramics with ferroelasticity are potential candidates for thermal barrier coating (TBC) ceramics. During the phase transition process, there are three main phases with monoclinic (I2/a), monoclinic-prime (P2/a), and tetragonal structures (I41/a), and a comprehensive understanding of their thermophysical properties is required. In this study, the thermal and mechanical properties of polymorphous yttrium tantalate (YTaO4) ceramics are systematically investigated under finite temperature by performing first-principles calculations combined with quasi-harmonic approximation. The first-principle study results show that the volume change from M' to T phase is 12.85 Å3 to 12.95 Å3 per atom, whereas the T to M is 12.95 Å3 to 12.84 Å3 per atom, and the change is less than 1%, showing that this process produces almost no volume change. However, the thermal expansion coefficients (TECs) and Young's modulus vary greatly, the TECs value of M YTaO4 is about 11.13 × 10−6 K−1, which is smaller than T YTaO4 as the value 12.01 × 10−6 K−1, and the Young's modulus values of M, M', and T phases are 140.34, 156.68, and 123.29 GPa, respectively. Lastly, the calculated O–Ta bond is stronger than the O–O and O–Y bonds according to the mean bond population and average bond length, resulting in a higher modulus. This work will not only expand the internal mechanism of the thermophysical properties of YTaO4, but also provides support for the design and application of TBC systems.  相似文献   

3.
(Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C high-entropy ceramics (HEC) with a submicron grain size of 400 to 600 nm were fabricated by spark plasma sintering using a two-step sintering process. Both X-ray and neutron diffractions confirmed the formation of single-phase with rock salt structure in the as-fabricated (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C samples. The effect of submicron grain size on the thermal stability and mechanical properties of HEC was investigated. The grain growth kinetics in the fine-grained HEC was small at 1300 and 1600°C, suggesting high thermal stability that was possibly related to the compositional complexity and sluggish diffusion in HEC. Compared to the coarse-grain HEC with a grain size of 16.5 µm, the bending strength and fracture toughness of fine-grained HEC were 25% and 20% higher respectively. The improvement of mechanical properties in fine-grained HEC may be attributed to micromechanistic mechanisms such as crack deflection.  相似文献   

4.
The Orowan-Petch relation is a famous model to describe the strength of polycrystalline ceramics covering a wide range of grain sizes. However, it becomes difficult to explain the strength trend when the grain size decreases to the sub-microscale or nanoscale. This is because some microstructural parameters (such as grain size, grain boundary fracture energy, and grain boundary defects) vary with different processing technologies, and their coupling effects on mechanical properties are still unclear. In this study, a finite element method (FEM) was applied to investigate the dependence of mechanical properties, such as strength and damage resistance, on the abovementioned microstructural parameters on example of alumina. The numerical results show that the grain boundary energy is weakly coupled with the grain size and grain boundary defects. The grain size and grain boundary are intercoupling, which affects mechanical properties. The mechanical properties could be improved by increasing the grain boundary fracture energy and decreasing the grain size and the grain boundary defect density.  相似文献   

5.
The heat capacity Cp, thermal diffusivity χT, and lattice thermal conductivity κlatt of ceramic solid solutions of sesquisulfides Gd3‐xVGd,xS4 (0 < x < 0.33) in the temperature range 300‐700 K has been studied. Changing the real structure, namely the concentrations of vacancies (NV) and deformation (NDc) centers of polycrystals, significantly decreases κlatt. A deviation of composition from the stoichiometry 2:3 is accompanied by an increase in the specific area of the crystallite boundaries per unit volume, and, hence, the concentration of deformation centers DC increases. This observation was confirmed by examining the short‐range order disturbance of the lattice and symmetry environment of the Gd3+ and S2? environment by Raman spectroscopy and the magnetic susceptibility Faraday method. Therefore the thermal diffusivity of gadolinium sesquisulfide is reduced because of the mean free path of phonons decrease. As a result, the thermal conductivity of the polycrystalline samples is reduced by 10%.  相似文献   

6.
To improve the properties of BaTiO3 piezoelectric ceramics fabricated by 3D printing, effects of particle size were investigated on the properties of ceramic slurries and the electrical properties of BaTiO3 fabricated by Digital light processing (DLP) 3D printing method. It was found that the curing ability of the slurries decreased significantly when the particle size is close to the ultraviolet wavelength, while the viscosity kept decreasing with the increase of particle size. When the particle size in a range of submicron (d50<1 μm), the grain size of sintered ceramics decreased from 13.27 to 6.84 μm as particle size increasing. Moreover, the piezoelectric constant and relative permittivity of sintered ceramics were measured, and it turns out to reach 168.1 pC/N and 1512, respectively, while using the BaTiO3 powder with particle size of 993 nm. Finally, a cellular structural BaTiO3 ceramics was fabricated by using optimized powder and process parameters and packaged as a piezoelectric sensor, showing a good function of force-electricity conversion. These results demonstrate the feasibility of fabricating high-quality functional ceramics with designed geometry by DLP.  相似文献   

7.
This study prepared textured (Ti1/3Zr1/3Hf1/3)B2 medium-entropy ceramics for the first time that maintain enhanced flexural strength up to 1800°C using single-phase (Ti1/3Zr1/3Hf1/3)B2 powders, slip casting under a strong magnetic field, and hot-pressed sintering methods. Effects of WC additive and strong magnetic field direction on the phase compositions, orientation degree, microstructure evolution, and high-temperature flexural strength of (Ti1/3Zr1/3Hf1/3)B2 were investigated. (Ti1/3Zr1/3Hf1/3)B2 grain grows along the a,b-axes, resulting in a platelet-like morphology. Pressure parallel and perpendicular to the magnetic field direction can promote the orientation degree and hinder the texture structure formation, respectively. Reaction products of W(B,C) and (Ti,Zr,Hf)C between (Ti1/3Zr1/3Hf1/3)B2 and WC additive can efficiently refine the (Ti1/3Zr1/3Hf1/3)B2 grain size and promote grain orientation. (Ti1/3Zr1/3Hf1/3)B2 ceramics doped with 5 vol.% WC yielded a Lotgering orientation factor of 0.74 through slip casting under a strong magnetic field (12 T) and hot-pressed sintering at 1900°C. Furthermore, cleaning the boundary by W(B,C) and introducing texture can enhance the grain-boundary strength and improve its high-temperature flexural strength. The four-point flexural strength of textured (Ti1/3Zr1/3Hf1/3)B2-5 vol.% WC ceramics was 770 ± 59 MPa at 1600°C and 638 ± 117 MPa at 1800°C.  相似文献   

8.
Hexagonal boron nitride (h-BN) with low dielectric loss and high temperature resistance opens up new opportunities for the preparation of polymer-derived SiCN ceramics (PDCs-SiCN ceramics) with excellent mechanical and dielectric properties. BN-containing polymer-derived SiCN composite ceramics (PDCs-SiCN(BN) composite ceramics) with different BN content were prepared via a pyrolysis process of ball-milling-blended Polyvinylsilazane/boron nitride (PVSZ/BN) precursors. BN is stably embedded in the SiCN tissue and tightly bound with it. The appropriate content of BN greatly improves the mechanical properties of PDCs-SiCN ceramics, as BN reduces the number of pores and prevents crack expansion. Additionally, BN is also beneficial in lowering the dielectric loss of PDCs-SiCN ceramics because of the weakened polarization relaxation behavior. PDCs-SiCN (BN) composite ceramics have optimal mechanical and dielectric properties when the BN content is 1 wt%. The flexural strength, flexural modulus and compression strength of PDCs-SiCN(BN) composite ceramics with 1 wt% BN doping content were 189.37 MPa, 46.38 GPa, and 399.02 MPa, respectively. Its average dielectric loss (tanδε) at 12.4-18 GHz is 0.0049.  相似文献   

9.
A series of dense barium titanate (BaTiO3, BTO) ceramics with different grain sizes (GS) were prepared by two-step sintering method. The effect of GS on piezoelectric coefficient (d33) and planar electromechanical coupling factor (kp) displayed a trend similar to that on relative permittivity (ɛ′). The values of d33, kp, and ɛ′ increased significantly with decreasing GS, reaching maximum values (ɛ = 6079, d33 = 519 pC/N and kp = 39.5%) at approximately 1 μm, and then decreased rapidly with further decreasing GS. The results revealed that high-performance BTO ceramics could be effectively prepared by controlling GS. Polarization–electric field hysteresis loops and temperature dependence of ɛ′ were also investigated.  相似文献   

10.
By using three-point bending tests and numerical simulation, influence of polarization on the electromechanical properties of GaN piezoelectric semiconductive ceramics (PSCs) were investigated in this paper. The results show that the piezoelectricity of GaN PSCs can be attained through a special polarization treatment. For polarized samples under loading, because piezoelectric polarization charges and the electric field are concentrated at high-strain positions, their bending strength increases by 7%. Polarization results in a nearly 55% improvement of the electrical current transport capacity. Due to piezoelectricity, the electric displacement of polarized samples is also largely changed. It is shown that there is a strong correlation between polarization and electromechanical properties of PSCs. These findings highlight the influence of polarization on the electromechanical performance of PSCs, and also imply some potentials for their applications.  相似文献   

11.
《Ceramics International》2017,43(7):5505-5508
The effects of secondary phases on ferroelectric properties of Bi0.5Na0.5TiO3 (BNT) have been studied. Ceramic powders were prepared by solid state reaction employing different sintering temperatures and characterized by X-ray diffraction (XRD), Scanning Electron Microscopy and impedance spectroscopy. The perovskite structure was detected by XRD; together with small peaks corresponding to a secondary phase assigned to the Na2Ti6O13-based phase in calcined powders. In addition, morphology and the content of the secondary phase were modified by the sintering temperatures, affecting the ferroelectric properties, and ac and dc conductivities. We believe that our results can benefit not only the understanding of BNT ceramics, but also expand the range of applications.  相似文献   

12.
Colored zirconia ceramic samples were prepared via pressureless sintering with yttria-stabilized zirconia as a raw material and aluminum chromium slag (ACS) as an additive. Then, the effects of added ACS (0-15.0 wt%) on the microstructure, phase composition, and mechanical properties of the ceramic were investigated. The addition of ACS changed the apparent color of zirconia ceramics from white to pink, and the color deepened as the ACS content increased. In addition, more pores appeared in the sintered ceramic substrate as the content of ACS increased, and the relative density of samples declined from 97.7% to 91.1% with an increase in ACS content. However, the microhardness and bending strength each reached their maximum values (1887.2 HV and 433.5 MPa, respectively) when the content of ACS was 5.0 wt%. Fracture surface analysis of the samples showed that intergranular fractures were the main features of sintered samples with no added ACS, whereas numerous transgranular fractures occurred in sintered samples to which ACS had been added. The XRD results revealed that the prepared mainly composite ceramics were composed of t-ZrO2, m-ZrO2, and chromium-corundum, and the content of the t-ZrO2 gradually increased as the ACS content increased.  相似文献   

13.
Terbium aluminum garnet (Tb3Al5O12, TAG) ceramics have become a promising magneto-optical material owing to the outstanding comprehensive performance, including the magneto-optical, thermal, and mechanical properties. Fine-grained TAG ceramics with high optical quality and mechanical properties have attracted much attention. In this study, TAG ceramics with fine grains and high optical quality are fabricated successfully by a two-step sintering method from co-precipitated nano-powders. After pre-sintered at 1525°C in vacuum and hot isostatic pressed at 1600°C, the in-line transmittance of TAG ceramics reaches 81.8% at 1064 nm, and the average grain size is 7.1 μm. The Verdet constant of TAG ceramics is −179.6 ± 4.8 rad T−1 m−1 at 633 nm and −52.1 ± 1.9 rad T−1 m−1 at 1064 nm, higher than that of commercial Tb3Ga5O12 crystals. The thermal conductivity of TAG ceramics is determined from 25 to 450°C, and the result is 5.12 W m−1 K−1 at 25°C and 3.61 W m−1 K−1 at 450°C. A comparison of mechanical properties between large- and fine-grained TAG ceramics fabricated under different conditions is conducted. The fine-grained TAG ceramics possess a bending strength of 226.3 ± 16.4 MPa, which is 9.7% higher than that of the large-grained ceramics. These results indicate that reducing the grain size on the premise of high optical quality helps improve the comprehensive performance of TAG ceramics.  相似文献   

14.
15.
The influence of Ca-doping on the microstructure development and electrical characteristics was studied in the novel, ZnO-Cr2O3-based, varistor ceramics with the nominal composition (99.47-x) mol% ZnO + 0.1 mol% Cr2O3 + 0.33 mol% Co3O4 + 0.1 mol% La2O3 + x mol% CaCO3, with x ranging from 0 to 6, sintered at 1200 °C. The results showed that, within the limits of the solid solubility of CaO in ZnO, the addition of CaO greatly enhanced the diffusion processes and hence the grain growth, thus broaden the range of breakdown voltages. Moreover, the Ca-doping simultaneously increases the height of the electrostatic Schottky barrier, resulting in a higher nonlinear coefficient of 40 and a lower leakage current of 2.8 μA/cm2. The results are important for the potential application of these novel ZnO-Cr2O3-based varistors, having different nominal voltages and yet suitable dimensions, in overvoltage protection across a broad range of voltages.  相似文献   

16.
In order to investigate the relationship between pore structure and thermal conductivity as well as mechanical strength, porous alumina ceramics (PAC) with various pore structures were fabricated, using starch as the pore‐forming agent. Fractal theory was employed to characterize the pore size distribution more accurately than ever used parameters. The results show that the increase in starch content in PAC leads to an enhanced porosity, a higher mean pore size, and reduced fracture dimension, thermal conductivity and strength. The fractal analysis indicated that the fractal dimension decreases gradually and reaches its minimum value with increasing the starch content up to 25 wt%, but the further incorporation results in an opposite trend. It is suggested from micro‐pore fractographic analysis that the optimization of both thermal insulation performance and mechanical strength are positively correlated with the increase in the mean pore size and proportion of 2‐14 μm pores but negatively corrected with the porosity. These results provide a new perspective and a deeper understanding for fabrication of PAC with both excellent thermal insulation and mechanical performance.  相似文献   

17.
TiO2 alloying effect is applied to optimize the thermophysical properties of fluorite-type Sm3TaO7 ceramics synthesized via solid-state reaction, and the influence of TiO2 alloying effect on the optical properties and elastic modulus is determined. According to the decreasing unit cell volume calculated by the X-ray diffraction and the broadening Raman peak, Ti4+ substitutes the same number of Sm3+ and Ta5+ ions of Sm3TaO7 ceramics at the same time. As Ti4+ substitutes Sm3+ and Ta5+ ions, the band gap of TiO2-Sm3TaO7 ceramics decreases from 4.71 to 4.11 eV. The phase transition of Sm3TaO7 ceramics is eliminated by TiO2 alloying effect and the coefficient of thermal expansion is increased. Via TiO2 alloying effect, two different phonon scattering mechanisms: (a) the misfit of atomic weight and ionic radius among Ti4+, Sm3+, and Ta5+ ions; (b) the rattling Ti4+ ions are introduced in Sm3TaO7 ceramics. The lowest thermal conductivity of TiO2-Sm3TaO7 ceramics reaches 1.37 W K−1 m−1 (800°C, 9 mol% TiO2-Sm3TaO7), which is much lower than 7YSZ and Sm2Zr2O7 ceramics. Accordingly, it is believed that TiO2-Sm3TaO7 ceramics are promising thermal barrier coatings.  相似文献   

18.
《Ceramics International》2022,48(15):21600-21609
Stereolithography (SL) shows advantages for preparing alumina-based ceramics with complex structures. The effects of the particle size distribution, which strongly influence the sintering properties in ceramic SL, have not been systematically explored until now. Herein, the influence of the particle size distribution on SL-manufactured alumina ceramics was investigated, including bending strength at room temperature, post-sintering shrinkage, porosity, and microstructural morphology. Seven particle size distributions of alumina ceramics were studied (in μm/μm: 30/5, 20/3, 10/2, 5/2, 5/0.8, 3/0.5, and 2/0.3); a coarse:fine particle ratio of 6:4 was maintained. At the same sintering temperature, the degree of sintering was greater for finer particle sizes. The particle size distribution had a larger influence on flexural strength, porosity and shrinkage than sintering temperature when the particle size distribution difference reached 10-fold but was weaker for 10 μm/2 μm, 5 μm/2 μm and 5 μm/0.8 μm. The sintering shrinkage characteristics of cuboid samples with different particle sizes were studied. The use of coarse particles influenced the accuracy of small-scale samples. When the particle size was comparable to the sample width, such as 30 μm/5 μm and 5 mm, the width shrinkage was consistent with the height shrinkage. When the particle size was much smaller than the sample width, such as 2 μm/0.3 μm and 5 mm, the width shrinkage was consistent with the length shrinkage. The results of this study provide meaningful guidance for future research on applications of SL and precise control of alumina ceramics through particle gradation.  相似文献   

19.
詹水清  周孑民  吴烨  李远  梁艳南  杨莺 《化工学报》2012,63(8):2341-2347
针对目前高温熔盐相变蓄热材料热物性参数测定方法的不足,在借鉴Stefan模型和Lamvik模型的基础上,利用相变材料相变时相界面移动速率与液固相热物性参数之间的关系,设计了一套高温熔盐熔点附近热物性的动态测定装置,并提出了一种基于数值模拟的测定误差修正方法。利用该装置对已知热物性的熔盐硝酸钠、硝酸钾及溴化锌进行了动态实验测定,并利用数值模拟对测定结果进行了修正。修正后的结果和参考值之间的误差比修正前要显著减小,硝酸钠和硝酸钾的热导率、热扩散率误差均小于5%。表明该修正方法可以有效地降低测定过程中由于非一维导热、边界条件不稳定所造成的测定误差,将动态测定与该修正方法结合起来能提高测试精度,为其他熔盐热物性的深入研究提供了基础数据和理论依据。  相似文献   

20.
离子液体(ILs)由于其优越的性能目前已成为多个领域的研究热点,但热力学基础数据的缺乏是其应用的障碍之一,除实验测定外,基团贡献法也为设计和筛选ILs提供了重要的性质预测方法。概述了预测纯ILs的熔点、黏度、密度、热容、电导率、声速、生态毒性、界面性质、临界性质、传递性质等一系列热物理性质的基团贡献方法,同时对含有ILs混合体系相行为的基团贡献估算模型进行了评述,展望了基团贡献法今后需进一步努力的方向。  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司    京ICP备09084417号-23

京公网安备 11010802026262号