热处理高压烧结AlN陶瓷的显微组织和热导率

在流动N2保护下,对高压烧结制备的AlN(Y2O3)陶瓷进行了热处理,研究了热处理对AlN陶瓷显微组织及导热性能的影响.结果表明:在970℃热处理2 h后的AlN陶瓷材料与未热处理的试样相比,晶粒尺寸显著增大,晶粒形状越发规整,析出相均位于晶界处或者三角晶界区域,热导率从77.3 W/(m·K)提高到了156.7 W/(m·K).但是,将热处理时间延长到4 h,AlN陶瓷的气孔增大,出现了反致密化现象,热导率也降低到92.6 W/(m·K).     

The thermal conductivity of metallic ceramics

Transition metal carbides, nitrides, and borides can be called metallic ceramics because they are electronically conductive and extremely hard. Their various applications include cutting and grinding tools, thermal-barrier coatings, diffusion-resistant thin films, interconnects, and superconductivity devices. In each case, the ability of the material to resist or permit heat flow is important. Because of the high concentration of nonmetal atom vacancies in the carbides and nitrides, the carriers of heat—conduction electrons and phonons (the quanta of lattice waves)—are severely scattered, and the thermal conductivity, K, is strongly affected, although differently in high- and low-temperature regions. Measurements of both the electrical and thermal conductivity of single-crystal metallic ceramics at low temperatures and the application of the Callaway formalism help explain the puzzling temperature dependence of K. The finding of a large peak in K of NbC just below its superconducting transition temperature confirms phonon-electron scattering and could lead to a thermal switch. The single-crystal thermal conductivity behavior of TiC and WC is used to interpret the measured K values for cemented carbides TiC/Ni-Mo and WC/Co through a broad temperature range. Wendell S. Williams earned his Ph.D. in physics at Cornell University in 1956. He was a research physicist with Union Carbide Corporation, a professor of physics and ceramic engineering at the University of Illinois, and department chair of materials science and engineering at Case Western Reserve University. Dr. Williams, now retired, is a member of TMS.     

Sintering behavior and thermal conductivity of Y2O3 fully stabilized HfO2 ceramics

A series of Y2O3 fully stabilized HfO2 ceramics(Hf1-xYxO2-0.5x,x = 0.20,0.24,0.28,0.32,0.36 and 0.40)were synthesized by solid-state reaction at 1500℃.The phase...     

Effect of lattice defects on thermal conductivity of Ti-doped,Y2O3-stabilized ZrO2

The evolution of lattice structure and thermal conductivity has been studied systematically for a range of Ti-doped, Y₂O₃-stabilized ZrO₂ (YSZ) solid solutions. The mechanism of reducing the thermal conductivity by Ti doping has been determined. Ti⁴⁺ mainly substitutes for Zr⁴⁺ below a critical composition factor (x ? 0.08), above which the interstitial Ti⁴⁺ need to be considered separately. The effect of lattice defects caused by mass and radius differences between Ti⁴⁺ and Zr⁴⁺ ions on the phonon scattering coefficient was discussed quantitatively. And the reduction of oxygen vacancy by interstitial Ti⁴⁺ ions which increases the thermal conductivity at high Ti doping content was also determined. Concerning the integrated phase stability and thermo-mechanical properties, Ti-doped YSZ is believed to be a promising candidate for thermal barrier coatings at higher temperature.     

Effect of starch filler content and sintering temperature on the processing of porous 3Y–ZrO2 ceramics

A direct casting process was used to produce porous 3Y–ZrO₂ ceramics using starch as a fugitive filler and binder. The compositions with low additions of starch had higher porosity than the volume fraction of starch initially in the green body (X_st), whereas, the compositions with high amounts of starch produced lower porosity than the predicted value. The well ordered structure consisted of spherical pores of 8–10 μm diameter, retained from the original starch particles, connected by channels. The interconnection between pores was dependent on the volume fraction of starch incorporated, as well as on the sintering temperature. Pore interconnection was observed for all the compositions sintered at 1000–1300 °C. Increasing the sintering temperature to 1400–1500 °C produced a marked dependence of the open to total porosity ratio on X_st. For a high porosity material, a bimodal channel size distribution was found at 1400 and 1500 °C. The primary pore channel diameter was 0.7 μm and the secondary one was close to 4 μm. As the sintering temperature increased, the volume of the connecting channels decreased; at 1500 °C only a minor volume of the larger channels was found.     

Role of Zr(OH)4 hard agglomerates in fabricating porous ZrO2 ceramics and the reinforcing mechanisms

Porous ZrO₂ ceramics were fabricated by adding Zr(OH)₄ hard agglomerates to ZrO₂ powder, followed by pressureless sintering. The mechanical properties of porous ceramics sintered from pure ZrO₂ powder were poor. The addition of Zr(OH)₄ increased the strength and fracture toughness of the porous ZrO₂ ceramics for sintered specimens containing lower porosity. However, the Young’s modulus had little change so that the strain to failure of porous ZrO₂ ceramics increased with the incorporation of Zr(OH)₄. Scanning electron microscopy (SEM) observations revealed that microstructures of the green compacts prepared from pure ZrO₂ powder were nonuniform due to the ZrO₂ soft agglomeration, which resulted in a localized nonuniform shrinkage during densification. The localized nonuniform shrinkage led to a weaker grain bonding and degraded the mechanical properties of porous ZrO₂ ceramics. In this work, we found that this microstructure nonuniformity could be eliminated by the addition of Zr(OH)₄, because the bimodal particle size distribution confined the formation of ZrO₂ soft agglomerates due to a space constraint and an internal friction between the Zr(OH)₄ hard agglomerates during compaction. As Zr(OH)₄ decomposed into ZrO₂ grains during heating, the Zr(OH)₄ hard agglomerates disappeared before sintering occurred. The present study indicates that Zr(OH)₄ hard agglomerate is a unique agent to improve the mechanical properties of porous ZrO₂ ceramics.     

Microstructure and electrical properties of porous PZT ceramics derived from different pore-forming agents

Porous piezoceramics have promising applications in underwater sonar detectors or medical ultrasonic imaging. We report the electrical and acoustic properties based on different pore microstructures of porous lead zirconate titanate (PZT) ceramics, fabricated using stearic acid (SA) and polymethylmethacrylate (PMMA) as pore-forming agents. The corresponding ferroelectric and piezoelectric properties decreased with increasing porosity due to the decrease in volume fraction of PZT phase, and were in good agreement with a modified cubes model in the case of isolated porosity. The corresponding acoustic impedance decreased from 16 to 8 MRayls (10⁶ kg/m² s), with increasing porosity from 3% to 43%, due to low acoustic impedance of pore phase. The electrical properties of porous PZT ceramics were closely associated with porosity and the interconnection of pores, but only slightly associated with the shape of pores. However, the acoustic impedance was only connected to porosity, not to either shape or interconnection of pores.     

Ti/ZrO2和TiAl/ZrO2反应的差热分析

借助SEM和差热分析研究Ti-ZrO2（CaO）和TiAl-ZrO2（CaO）体系在不同加热速率时的界面显微组织和反应温度,并利用Kissinger方程对反应动力学参数进行计算。结果表明：随着加热速率的提高,Ti-ZrO2（CaO）和T认1．ZrO2（CaO）体系的化学反应逐渐减弱,初始反应温度逐渐升高;Al元素的加入使Ti和ZrO22（CaO）的初始反应温度提高近170℃,从而弱化Ti与ZrO2（CaO）的界面反应;Ti-ZrO2（CaO）体系和TiAl-ZrO2（CaO）体系反应活化能分别为l348kJ／mol和l675kJ／mol,反应级数分别为1．51和1．59。     

Fabrication and mechanical properties of TiB2/ZrO2 functionally graded ceramics

Five layers were considered for the present TiB₂/ZrO₂ functionally graded ceramics and TiB₂/ZrO₂ functionally graded ceramics were prepared by hot-pressing. The first layer, marked as L₁, was composed of TiB₂–15vol%SiC. The L₂ layer was composed of TiB₂–15vol%SiC–10vol%ZrO₂. The L₃ layer was composed of TiB₂–25vol%ZrO₂ without addition of SiC, because 25vol%ZrO₂ was enough to densify TiB₂. The last two layers, L₄ and L₅, were composed of TiB₂–35vol%ZrO₂ and TiB₂–45vol%ZrO₂, respectively. Denser graded microstructures as well as the stronger interfaces were achieved for TiB₂/ZrO₂ functionally graded ceramics hot-pressed at 1900 °C. Due to the phase transformation of ZrO₂, the toughness of TiB₂/ZrO₂ functionally graded ceramics increased from ~ 4.3 MPa m^1/2 for the TiB₂-rich layer to ~ 8.9 MPa m^1/2 for the ZrO₂-rich layer. The effect of hot-pressing temperature on the toughness of TiB₂/ZrO₂ functionally graded ceramics was further investigated in detail.     

Al2 O3与ZrO2复合粉涂层的组织形貌分析

利用大气等离子喷涂技术（ASP）在20号锅炉钢（20G）基体上制备Al2O3和ZrO2复合粉热障涂层（TBC）。通过X射线衍射分析（XRD）、扫描电镜（SEM）等观察分析了复合粉涂层表面形貌，分析了表面微裂纹和孔隙的形成过程。结果表明，涂层表面主要由α-Al2O3和t-ZrO2高温稳定相组成。复合粉涂层面层组织致密，Al2O3和ZrO2颗粒互熔在一起，从而提高了涂层的结合强度和致密度。     

前驱体浸渍ZrO2陶瓷的烧结性能及显微结构

采用以ZrOCl2·8H2O为原料制备的前驱体溶胶浸渍ZrO2陶瓷.结果表明纳米ZrO2前驱体溶胶浸渍后的ZrO2陶瓷烧结性能提高,显气孔率从原先的20%降至11%;表面结晶完好,大孔(10 μm左右)减少,微孔(100nm左右)明显增多;浸渍前后的分形维数变化不大,也即浸渍不会显著影响陶瓷表面的光滑度.     

ttao相结构对ZrO2陶瓷透光度的影响

研究了相结构对ZrO₂陶瓷透光度的影响。本文制备了3 mol%Y₂O₃稳定ZrO₂(3YSZ)和5 mol%Y₂O₃稳定ZrO₂(5YSZ),分别在1450℃、1500℃、1550℃下烧结,通过XRD、SEM、EMPA等研究了3YSZ和5YSZ相结构和相含量,用UV-Vis分光光度计测试其在可见光范围内的透光率,探讨相结构和透光度的关系。结果表明,3YSZ和5YSZ均为稳定四方相 (t相)和亚稳定四方相 (t′相)的两相复合物,其中3YSZ以t相为主,且t相含量随烧结温度的升高而增加,透光性较差;5YSZ以t′为主,且t′相含量随烧结温度的升高而增加,透光性较好。得出结论,t相透光性较差,t′相透光性较好,并对此进行解释。     

ZrO2+SiC颗粒强韧化MoSi2复合材料的显微组织和性能

通过显微组织观察和力学性能测试 ,初步探讨了ZrO2 SiC颗粒对MoSi2 基体材料的强韧化效果和机制。结果表明 ,材料复合具有较好的强韧化协同效应 ,复合材料中ZrO2 相和少量SiC颗粒在基体的间层作用 ,可抑制MoSi2 晶粒长大 ;断口呈现晶粒细小、裂纹扩展曲折和沿晶与穿晶混合型断裂等特征 ;ZrO2 SiC颗粒通过弥散强化和细化晶粒使复合材料强度提高 ,通过晶粒细化、裂纹偏转和分支、微裂纹形成等机制的综合作用使复合材料增韧     

Microstructure and PTCR characteristics of porous BaTiO3-based ceramics prepared by adding activated carbon

As a pore precursor, activated carbon of different contents (0 to 30 vol%) was added to (Ba,Sr)TiO₃ powder. Porous (Ba,Sr)TiO₃ ceramics were prepared by pressureless sintering at 1350 °C for 1 h in air. The effects of the activated carbon content on the microstructure and positive temperature coefficient of resistivity (PTCR) characteristics in porous (Ba,Sr)TiO₃ ceramics were investigated. The porosity of the porous (Ba,Sr)TiO₃ ceramics increased from 6.5% to 33.5%. The PTCR jump of the porous (Ba,Sr)TiO₃ ceramics prepared with activated carbon was > 10⁵ and increased slightly with increasing activated carbon concentration. These results correspond to the Heywang model of the PTCR effect in (Ba,Sr)TiO₃ ceramics, suggesting that activated carbon is an effective additive for preparing porous BaTiO₃-based ceramics. The newly prepared (Ba,Sr)TiO₃ ceramics can be used as a gas sensor.     

ZrO2热障涂层残余应力分析

用扫描电子显微镜(SEM)观察了等离子喷涂ZrO2涂层的显微结构,并用X射线衍射法(XRD)测试了ZrO2涂层相的组成和残余应力.同时测试了激光冲击处理和激光热处理后的ZrO2涂层残余应力分布,分析了热障涂层残余应力形成机理.结果表明,等离子喷涂ZrO2涂层表面残余应力均为拉应力,其均值为179.2MPa,经激光表面处理后的ZrO2涂层残余应力均表现为压应力;热应力对涂层残余应力贡献最大,当热应力超过涂层结合强度时,涂层脱落,通过控制涂层残余应力可以提高涂层界面结合强度.