High-Performance Ceramics IV

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Authors: Zheng Zhang, Jiang Hong Wu, Guang Shu Si
Abstract: The hybrid electric vehicle (HEV) equipped both engine and generator is a transitional type from the internal-combustion engine vehicle to electric vehicle, but is a self-existent type, too. Applying a thermoelectric generator to recovery the waste heat of its engine, could not only improve the energy saving, but also the discharge / charge performance of the vehicle. For this purpose, the applicable generator must be a high-power and high-density one, which exchanges energy between the waste heat flow field and the conversion electric field; moreover, the temperature field is the coupling field with them. One of the methods to increase power-density of the generator is to strengthen the conversion intensity among above fields. In this paper, firstly a novel internal-axial-netted thermoelectric generator is presented, which includes following basic ideas: (1) using the stereo-electrodes to improve the integration of thermo-elements; (2) let the thermocouples to do heat convection directly with heat gas flow in pipe, to increase the heat flow density and electric current density, reduce the thermal resistance on solid conducting; (3) using the compensating wire to connect thermoelectric circuit and elongate the distance between hot source and cold source, so a higher temperature difference and a stronger cooling effect can be made. The analyses show though this generator used existing thermoelectric materials, its out-power can be increased and volume can be compacted. After this, secondly, the application scheme and key technologies of the novel high-intensity thermoelectric generator (HTG) as a main electrical source to supply electric power in HEV are chiefly discussed also.
Authors: Ke Feng Cai, L.C. Zhang, Q. Lei
Abstract: Sr or Ba filled type-I Ge clathrate single crystals were prepared using high pure elemental Ga, Ge, and Sr or Ba as the starting materials, and Ga as a flux. Powder X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), differential scanning calorimeter (DSC) were used to characterize the crystals, respectively. The results show that large and well-crystallized single crystals can be prepared under optimum synthesis condition. The composition of the crystals is in good agreement with the nominal composition (M8Ga16Ge30, M=Sr, Ba). Raman spectra of the single crystals were studied at room temperature and several of the Raman active vibrational modes in the crystals have been identified.
Authors: Gui Ying Xu
Abstract: In order to improve conversion efficiency of thermoelectric materials, a designing model named Combined Model of Dense and Hollow Quantum Structures (CMDHQS) and a concept named Combined Effect of Dense and Hollow Quantum Structures (CEDHQS) have been proposed. A proof-of-concept experiment has been carried out for fabrication of porous silicon wafer with CMDHQS by physical vapor deposition and selective chemical erosion methods. By the investigation on the relation of microstructure, micro-composition and thermoelectric properties, it is found that porous silicon wafers with CMDHQS can be obtained by these two methods and have much higher thermoelectric properties than normal silicon materials, that is 3 times higher than the highest results reported, which make porous silicon become a candidate as thermoelectric materials.
Authors: George C. Wei
Abstract: Ceramics contributes to progression of civilization. Yet advancement of ceramic science and technology benefits from the improved infrastructure and productivity. This paper assesses the development of structural and optical ceramics in the last quarter century. For example, structural ceramics such as silicon carbide whisker reinforced alumina matrix and titanium carbide dispersed silicon carbide matrix composites have made possible high-speed, wear-resistant, specialty tools. Designing of the structural materials involved consideration of thermodynamic compatibility of phases during fabrication, and consideration of the initiation and propagation of fracture. Mechanisms of the observed toughening have been proposed on a scale ranging from continuum to microstructure to atomistics. Optical ceramics including translucent polycrystalline alumina have facilitated the construction of high-pressure sodium and ceramic metal halide lamps. The microstructure and properties such as transmittance and sodium resistance of polycrystalline alumina have improved. The starting powders improved in purity, particle size, and de-agglomeration. Sintering of alumina advanced through optimization of dopants and sintering atmosphere. Densification involves grain-boundary diffusion; retardation of grain growth is due to solute drag. The solid solubility of magnesia sintering aid in alumina is a function of grain size. During grain growth and sintering of magnesia-doped alumina, both the enriched dopant level at grain boundaries and the equilibrium dopant content in the lattice alter, resulting in boundary pinning and pore annihilation. Oxygen vacancies when in motion significantly influence the boundary transport, and when stationary are important to optical properties of the sintered alumina. Further development of improved and new functional ceramics involves consideration of energy and environmental renewability. Prior achievements and outstanding challenges will be discussed.
Authors: Jiang Tao Li, Yun Yang, Hai Bo Jin
Abstract: The progress on the combustion synthesis of Si3N4 powders during the past decades was summarized with the emphasis on the recently developed mechano-chemically activated combustion synthesis (MACS) method. The effects of processing parameters such as the addition of diluent and ammonium salts into the green mixtures, the variation of nitrogen pressure as well as the mechanical activation treatment on the degree of Si to α-Si3N4 conversion was evaluated. The combination of mechanical activation and chemical stimulation was effective in enhancing the reactivity of Si powder reactants, which was responsible for the extension of the minimum nitrogen pressure normally required for the combustion synthesis of Si3N4. This breakthrough indicates that nitriding combustion of silicon in pressurized nitrogen could be promoted by activating the solid reactants instead of by increasing the pre-exerted nitrogen pressure. The MACS process was successfully applied to the industrial production of Si3N4 powders, the regularities for the large-scale synthesis were reported, and the as-synthesized Si3N4 powder products were systematically characterized.
Authors: Fu Xiang Wang, Guo Sheng Gai, Cheng Chang Jia
Abstract: This paper describes a dry composite process, in which fine α-Al2O3 ceramic guest particles with a mean size of 150 nm were mechanically compounded with metal Cu host particles with a mean size of 25 μm by embedding and curling the guest particles on the surface and inside of host particles using mechanical alloying (MA) and micron/nano particles composite system (PCS). The optimum rotor speed and processing time were determined according to the treatment effect under different conditions. Particles size and other features were investigated. Particles size decreased quickly, and bulk density and tap density of the composite particles increased noticeably under appropriate process conditions. Al2O3/Cu composite material was fabricated via using hot-press sintering technique in argon shield. Microstructure of the composite material was studied by SEM, and the results have shown that a-Al2O3 particles distribute evenly in Al2O3/Cu composite material, which also proved that Al2O3/Cu composite particles could be fabricated perfectly by MA & PCS.
Authors: Sang Geun Lee, Yoon Sik Jang, Seong Soo Park, Byung Young Moon, Beom Soo Kang, Hee Chan Park
Abstract: Submicron silica powders were prepared by w/o emulsion method using inexpensive sodium silicate, ammonium sulfate and Triton N-57 as SiO2 source, precipitant and emulsifier, respectively. Nano-sized silica powders with narrow size distribution were prepared at a low temperature of 25°C for 1h by ultrasonic irradiation of the reactants using a commercial ultrasonic cleaner operating at a frequency of 47kHz, whereas silica powders with larger size and broad size distribution were synthesized using the same reactants at the same temperature and for the same time by the conventional process without ultrasounds. The particle size and size distribution of the silica powders obtained by the ultrasonic process were smaller and narrower than those by the conventional process, indicating that the application of ultrasound in the synthesis of silica powders by the w/o emulsion method is an efficient way to have powders with smaller and narrower particle size distribution.
Authors: Bo Quan Zhu, Xue Dong Li, Rui Hao, Hou Zhi Wang
Abstract: Mullite powder was prepared through reaction of aluminum sulfate and silica in molten sodium sulfate media. The phase composition and microstructure of mullite powder were investigated. The results showed that high purity mullite is prepared by this method. The mullite powder exhibits needle-like whisker crystal with 50~100 nm in diameter and 3~8 #m in length. It is found that by this method mullite starts to form at 900oC, the quartz phase is disappeared about 1000oC, and mullite begin to decompose when the temperature is over 1100oC. Thus, the reasonable synthesizing temperature is about 1000oC. This synthetic method is characterized by process simplicity, the temperature of mullite formation is lower than 200-400oC compared with other conventional methods.
Authors: Hai Jun Zhang, Hai Tang Zhang, Jin Qi Miao, Zi Liang Wang, Quan Li Jia, X.L. Jia
Abstract: Ultrafine β-sialon powder is synthesized by citrate sol-gel and carbothermal reduction and nitridation (CRN) process. The presence of small amounts of β-sialon as crystal seed can obviously accelerate the formation of final β-sialon product and lower its formation temperature. The effects of nitriding temperature on the formation of the final β-sialon are investigated. The mean particle size of the prepared β-sialon powder is 100 ~ 150 nm. The β-sialon precursor gel and the ultrafine β-sialon powder are characterized by XRD, TG-DTA and SEM. The isoelectric point of ultrafine β-sialon is pH 2.46.
Authors: G.H. Liu, Ke Xin Chen, He Ping Zhou, Ke Gang Ren, C. Pereira, José Maria F. Ferreira
Abstract: In this paper, α-SiAlON powders co-doped with (Ca+M) (M=Mg, Yb, Sr) were prepared by combustion synthesis. The effect of Ca incorporation on the phase composition and grain morphology of reaction products was discussed. The experimental results showed that Ca incorporation promoted the growth of rod-like α-SiAlON crystals in (Ca+Mg) and (Ca+Yb) systems. For (Ca+Sr) system, the addition of Ca assisted Sr into α-SiAlON crystal lattice and increased the relative content of α-SiAlON in reaction product.

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