Relationship Between Electric Properties and Temperature of ZrB_2-SiC Composite Ceramic Heating Element

ZrB2 -SiC composite ceramic has been successfully introduced as heating element in super high temperature .field. This paper further investigated the microstructure of ZrB2 - SiC composite ceramic heating element and the relationship between electric properties and temperature. SEM photos show that the heating element consists of SiC grains and ZrBz grains smaller than 10 μm. The voltage and current gradually increase and the furnace tempera- ture rises lineally with heating time prolonging. The electric resistance increases linearly with the temperature rising. The service temperatltre of the heating element can reach 1 800 ℃ and 2 150 ℃ in air and argon at- mosphere, respectively.

Recent Research Progress on ZrB_2-and HfB_2-based Ceramics

The strong covalent bonding characters of zirconium diboride（ Zr B2） and hafnium diboride（ Hf B2） with natural hexagonal Al B2-type crystal structure give them high melting points（ 〉 3 200 ℃）, high bending strength, high elastic modulus and high hardness.Zr B2- and Hf B2-based ceramics are promising candidates for the thermal protection components of propulsion systems,rocket nozzles,sharp leading edges and nose cones. This paper introduces the recent research progress on Zr B2- and Hf B2-based ceramics,including the synthesis methods of the boride powders,the sintering methods of the boride-based ceramics and their properties.The advantages of different synthesis methods and sintering methods are compared. The influence factors on the mechanical properties,oxidation resistance and thermal shock resistance are summarized.

Experimental Research on Grinding Surface-Quality of Dental Zirconia

The properties and cutting performance of Y_2O_3-stabilized tetragonal zirconia polycrystal( Y-TZP) used widely for dental restoration have been introduced. The surface roughness of denture made by the ceramics plays an important role in product quality. In order to enhance the surface quality of denture processing,the effect of grinding parameters such as spindle speed,feed rate and the depth of cut with different levels on denture surface roughness were deeply studied to look for reasonable grinding parameters. The experiment was arranged in the orthogonal method. A new dental bur used only for dentistry was employed to carry out the experiment in a high accuracy machining center which was developed by Nanjing University of Aeronautics and Astronautics. The experimental results demonstrated that the depth of cut was the major parameter on surface roughness. With scanning electron microscope( SEM),it was found that the ceramics debris and its surfaces damage were mainly affected by depth of cut. The surface formation mechanism of zirconia ceramics was revealed by the morphology of machined surface. The topographies of the grinded ceramics surfaces demonstrated that zirconia ceramics was removed by its brittle fracture and plastic deformation. The plastic deformation region at grinded ceramic surface was larger with the smaller grinding depth.Finally, an example for grinding molar crown using zirconia ceramics was provided. After processing,the surface morphology of the molar crown was measured. The results showed that its quality was high and met the requirements of dental restoration. The research results of this paper provide useful and theoretical basis for denture processing.