Jin Ping Li

Study on ZrC-20vol.%SiCw Ultrahigh Temperature Ceramics by Hot Pressing

The ZrC-20vol.%SiCw ultrahigh temperature ceramics with relative density of 99.2% can be prepared by hot pressing at 1900 oC and 30MPa for 60 min. The mechanical properties and microstructure of the composite were studied. The flexural strength and fracture toughness reaches up to 626.17MPa, 5.03 MPa•m1/2, respectively. SEM analysis shows the microstructure of the ZrC-20vol.%SiCw is homogeneous and SiC whiskers are uniformly distributed around the ZrC grains, which inhibit the ZrC grain growth during sintering. The fracture surface of ZrC-20vol.%SiCw reveals a mixture of intergranular and transgranular failure, and SiC whiskers play a remarkable role in improving the strength and toughness of ZrC matrix. The toughening mechanism of the composite is mainly fine-grain strengthening and whisker pull-out.

Properties and Microstructure of an HfB2-HfC-SiC Ultra High Temperature Ceramics

HfB2-HfC-SiC ultrahigh temperature ceramics (UHTCs) were prepared and characterized in this paper. It is showed that the densities of the HfB2-HfC-SiC reach 98.5% of the theory density. The room temperature compressive properties of the HfB2-HfC-SiC are good, while those at high temperature decrease rapidly. The volume expansion ratio monotonously increases (up to 2.35% at 2300°C) with increasing temperature. Furthermore, with increasing temperature, the average linear expansion coefficient hardly changes, while the instant linear expansion coefficient decreases first, and followed by an increase. The minimum value of the instant linear expansion coefficient is 5.65×10-6/K at 900°C and that of the mean linear expansion coefficient is 7.39×10-6/K at 1340°C. HfB2-HfC-SiC were burned with the plasma arc heater. After 8-second ablation, part of the SiC particles melted and spurted from the composites, and holes appeared.

Preparation and Characteristics of ZrO2/ZrW2O8 Composites with Low Thermal Expansion

The ZrO2/ZrW2O8 ceramic matrix composites have been prepared by the two different processes: (1) ZrO2 and ZrW2O8 powders were mixed directly as raw material, then compacted by cold isostatic pressing under 200MPa, and finally, the ceramic matrix composites with low thermal expansion can be prepared by use of heat-pressing sintering or atmospheric sintering at temperature 1215oC. (2) ZrO2 (with excess mass) and WO3 powers were mixed as raw material, then compacted by cold isostatic pressing under 200MPa, and finally, the ZrO2/ZrW2O8 ceramic matrix composites can be made by use of heat-pressing sintering or atmospheric sintering at temperature 1215 oC after ZrW2O8 were synthesized by in-situ reaction of ZrO2 and WO3 powders at the same temperature. The microstructure, density, ZrW2O8 decomposition degree and the thermal expansion coefficient were compared among the sintered samples fabricated by the above two different methods, and affected by the different process parameters. The results show that the ceramic matrix composites with low thermal expansion are really composed of ZrO2, ZrW2O8 and WO3, and their relative densities are all more than 95%. Compared with the composites prepared by in-situ reaction, the densities, ZrW2O8 decomposition degree and the thermal expansion coefficient of the composites made by direct mixing are higher, less and smaller, respectively.

Effect of the Cooling Ways on the Properties and Microstructure of ZrO2/ZrW2O8 Ceramic Composites

ZrO2 has been widely used due to its good physical and chemical properties and the ZrO2/ZrW2O8 ceramic composites with adjustable thermal expansion coefficient can be prepared by mixing and sintering the ZrO2 and ZrW2O8 powders. In this paper, the ZrO2/ZrW2O8 ceramic composites have been prepared by mixing, cold isostatic pressing and atmospheric sintering, and then being cooled in air or in water. We focus on the effect of the cooling ways on the properties and microstructure of the ZrO2/ZrW2O8 ceramic composites. The results show that the relative density of all samples is more than 98%. For the in-situ method, the density and flexural strength of the samples cooled in air and in water are 98.1% and 98.4%, 112.96MPa and 45.23MPa, respectively. The ZrW2O8 content and thermal expansion coefficient of the samples cooled in the air are less and higher than those in the water, respectively. For the direct mixture method, the density and flexural strength of the samples cooled in air and in water are 98.9% and 99.3%, 195.99MPa and 58.71MPa, respectively. For the four groups of the composite samples, they behave better mechanical properties after cooled in air, and lower thermal expansion coefficients after cooled in water.

The Fabrication of Co-Continuous SiC/P20 Steel Composites

The network SiC ceramics was fabricated by combining organic foam impregnation and presureless sintering method. The co-continuous interpenetrating SiC/P20 steel composites were fabricated by means of pressureless infiltration technique. The results showed that coarse surface of SiC ceramics with three-dimensional network structure was obtained and observed, which was halpful for the reaction of metal alloy during the impregnation process. The continuous interface could be detected in the SiC/P20 steel composites.

Influence of Reaction Temperature on the Densification Behavior of the SiC/B4C Composites

In this investigation, the rare-earth oxide La2O3 combined with Al2O3 served as sintering additives and commercial α-SiC and B4C powder were applied to fabricate SiC/B4C composites by pressureless sintering. The results proved that combination of Al2O3 and La2O3 sinter additives were effective for densification of SiC/B4C composites. The influence of sinter temperature on the phase constitution, micro-structure and densification behavior of SiC/B4C composites was investigated.

Theoretical Calculation of Crystal Cohesive Energy of (ZrTi)B2 Solid-Solutions

The two kinds of the Ti-rich (Ti0.8Zr0.2)B2 and Zr-rich (Zr0.8Ti0.2)B2 permutation solid solutions were formed when hot pressed ZrB2 and TiB2 ceramics. On the base of the empirical electron theory (EET) of solids and molecules, the crystal bond energy of the two solid-solutions was calculated by use of Average Atom Model, Average Cell Model and Real Cell Model. In Real Cell Model, the crystal cell parameters were assumed to be unchanged and changed. The calculation results were compared among the three models. The results showed that the general trend about the deviation electrons of B element is consistent except Real Cell Model when the lattice constant is unchanged. Namely, the number of the deviation electrons in TiB2 is more than that in ZrB2.The maximum error of crystal bond energy calculated by the three models is 38KJ/mol and the relative error is less than 2%.The general trend of the crystal bond energy calculated by the three models is monotonically increased similar to the trend of melting point of these materials. Relatively speaking, the calculation of crystal bond energy calculated by Average Atom Cell Model is simpler and more reasonable.

Design, Implementation and Performance Testing of Vector Control for a Flexible Biomimetic Octopus Arm

According to the principle of muscular hydrostats, a flexible biomimetic octopus arm is put forward firstly. The method of vector control for the flexible biomimetic octopus arm is presented and analysed in detail. Based on the combination of ARM (advanced RISC microprocessor) and FPGA (Field Programmable Gate Arry), a control system for the flexibie biomiometic octopus arm is discussed, and the functions of every components is analysed. To verify the feasibility and reliability of the method of vector control, experimets are carried out. The experimemtal results demonstrate that this method applied in the control of the biomimetic octopus arm has a good performance.

Microscope Measurement of Jet-Pan Based on CCD

The jet-pan is a key part of jet-pan servo valve. Required precision of jet-pan should be guaranteed in process. CCD microscope measurement is used to deals with the measurement of jet-pan in the paper. The worepiece is enlarged by the optical microscope, and a series of image processing algorithms is used to process the image. The basic dimensions and geometrical tolerance of jet-pan are measured. And the stability error is less than ±0.5μm.

Design and Control of Planar Parallel Mechanisms with High Speed and High Precision

In order to satisfy the requirement of advanced manufacturing equipments with high speed and high precision, two planar parallel mechanisms have been developed. Based on these mechanisms, firstly, in consideration with the velocity and the precision of the end-effector together, the dimension optimization design is performed based on conditioning index and the precision characteristics. Then a disturbance observer is designed for the purpose of restraining load disturbance in the direct-drive system, and the experimental results show that load disturbance can be effectively restrained by the disturbance observer.