高压物理学报

Exposure of Metastable Phases by High Pressure

WANG Wen-Kui

1989, 3(4): 257-268 . doi: 10.11858/gywlxb.1989.04.001

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Abstract:
It was proposed ninety years ago, that after condensation of vapor it is necessary for the condensation to pass through possible high temperature phases until the equilibrium phase is reached. However, it was in general impossible for metals and alloys to freeze a structure of liquid or metastable phase for the reasons of technologies at that time. After then, by the other way, high pressure method has been used to synthesize the metastable phase in which diamond is one of the succeeded examples as known is to all, with energy situated between liquid and equilibrium states. During the past decade, the studies on amorphous and other metastable alloy were carried out intensively, because of the improvements of techniques to solidify liquid alloys at large undercooling, such as small droplet processing and liquid quench. In the former case, formation of a metastable phase is dominated by static undercooling. From thermodynamic studies, it was shown that the nucleation of a metastable phase becomes more likely than that of the stable phase. With an increase in undercooling, some metastable phases with lower melting points, which have been exposed at high pressure, were solidified under atmospheric pressure by using small droplet processing. But for alloys with higher melting points the metastable phases have never been prepared in the same way as carrier medium is limited for droplets. In the case of liquid quenching the metstable phases are formed by a kinetic process. Although the quenching rate to freeze liquid into the metallic glasses is usually lower than to transform which into a crystalline metastable phase, the latter is more difficult to exposure owing to its strict quenching condition. Similar to the solidification of liquid, the crystallization of an amorphous alloy may yields some metastable phases before the equilibrium state was formed. However, the metastable phases are not able to discover due to the fast kinetics of crystallization in many cases. Recently, the idea to expose metastable phase kinetically by high pressure was proposed on the basis of the investigations on crystallization processes of the amorphous alloys under high pressure. According to the generd transformation diagram for amorphous alloys heated under pressure, there are three types for transformation mode: a process to decompose multiphases at lower pressure, a process form single phase metallic compound at higher pressure and a process to yield disorder solid solution at ultra pressure. Differences in the mode were attributed to an effect of high pressure on the atomic rearrangements occurred in the interfaces between amorphours and crystalline phases. In general, diffusion for rearranging atomic positions is suppressed by pressure, so the metastable phase accompanied with smaller entropy change and atomic rearrangement during its forming should be preferred to form kinetically.

High-Pressure Raman Spectra and Structure Phase Transition for Molecule Crystals

ZHAO Yong-Nian, ZOU Guang-Tian

1989, 3(4): 269-278 . doi: 10.11858/gywlxb.1989.04.002

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Abstract:
Pressure dependence on the intensities and frequency of Raman bonds, crystal field splitting and mode-Grneisen parameter were discussed. Raman data were taken from our experiments on NH3F, HCCl3, HCBr3, S8 and C3N6H6 et al. Raman spectroscopic evidence for pressure induced structure phase transition of molecule crystal and the methods to determine new phase under high pressure were summed up.

A Three Phase Equation of State for 2024Al

ZHANG Chun-Bin, LI Mao-Sheng, ZHANG Shi-Ze

1989, 3(4): 279-283 . doi: 10.11858/gywlxb.1989.04.003

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Abstract:
A three phase EOS for 2024Al is developed and the effect of the melting entropy on EOS is considered. The ratio of the gaseous temperature Tg to melting temperature Tom is determined from Clapeyron-Clausius equation. When densities less than the nomal density (0), we presented a new form for the cold curve so that the behavior in this region can be described better.

The Generation of 90 GPa Quasi-Hydrostatic Pressures and the Measurements of Pressure Distribution

LIU Zhen-Xian, CUI Qi-Liang, ZOU Guang-Tian

1989, 3(4): 284-289 . doi: 10.11858/gywlxb.1989.04.004

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Abstract:
Quasi-hydrostatic pressures up to 90 GPa were obtained at room temperature in the diamond cell by using solid argon as pressure medium. The pressure distribution was determined by measuring the special shift of the R1 line of ruby at different positions within the sample chamber. Experimental results showed that the pressure differences (p) between the pressures at each point within the chamber and the mean pressure (p) were very small, ratios of p/p were less than 1.5% when below 80 GPa. The shape of ruby R lines at 90 GPa is similar to that at ambient pressure. Thus, quasi-hydrostatic pressure near 100 GPa can be obtained by using solid argon as pressure medium. Moreover, the red shifts with pressures of the peak positions at 14 938 and 14 431 cm-1 in ruby emission spectra, were also examined. It concluded that the line, 14 938 cm-1, can be adopted in the pressure calibration.

Experimental Study of the Effect of Shear Stress on Phase Transition in c-Axis CdS Single Crystal under Dynamic Loading

TANG Zhi-Ping, Gupta Y M

1989, 3(4): 290-297 . doi: 10.11858/gywlxb.1989.04.005

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Abstract:
For a long time, the problem whether shear stress affects the phase transition initial pressure is not well solved. Duvall and Graham suggested that cadmium sulfide (CdS) crystal could be used to study the effect of shear stress on the initial pressure of phase transition in c-axis CdS single crystal specimens under high velocity impact systematically. The axial stress of initial phase transition measured is T=(3.250.1) GPa, corresponding to a mean pressure pT=(2.290.07) GPa, which agrees the value 2.3 GPa of static results quite well within the experimental error. The shear stress in this case, T=0.72 GPa, is as high as 31.5% of the mean pressure. This result shows that the mechanism of phase transition may be assumed only to relate to a critical mean pressure or critical thermodynamic state, and the effect of shear stress can be ignored.

Behavior of Two Oxide Superconductors under High Pressure

ZHANG Jin-Long, LIU Yong, CUI Chang-Geng, YANG Qian-Sheng

1989, 3(4): 298-301 . doi: 10.11858/gywlxb.1989.04.006

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Abstract:
The high-pressure observations at low, normal and high temperature have been performed on two single-phased superconducting oxides, NdBa2Cu3Ox and (Bi,Pb)2Sr2Ca2Cu3Oy, respectively. The superconducting transition temperatures were all raised with increasing pressure. It was noted that the normal state resistances increased abruptly (then down again) near 1.0 GPa in both of the superconductors corresponding to a gentle step of the increasing Tc. The high-pressure DTA for NdBa2Cu3Ox showed that the transition from high temperature tetragonal phase to superconducting orthogonal phase is advanced, which indicates that the high pressure is advantageous to the stability of the superconducting phase structure.

Notes on the Relation between K1c and DF

DONG Lian-Ke, WANG Xiao-Wei

1989, 3(4): 302-307 . doi: 10.11858/gywlxb.1989.04.007

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Abstract:
In this paper, we analyze theoretically the quantitative relationship of contrary change between K1c and DF based on the different structure of plastic domain near a crack tip and the different mechanism of fracture. It suggests that the analysis and the calculation of dissipation energy play an important role when fractal is applied to study dissipation system.

Numerical Simulation of the Performance of Electromagnetic Railguns

ZHOU Zhi-Kui, CHEN Qiu-Hua

1989, 3(4): 308-314 . doi: 10.11858/gywlxb.1989.04.008

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Abstract:
A numerical calculation model simulating the main performance of electro-magnetic railguns is described in this paper. The model takes into account the increase in plasma armature masses due to the wall ablation and the drags caused by various effects such as the friction between projectile and wall, the plasma viscosity and the air resistance etc. We characterize the drags by an experimentally determined coefficient and consider the plasma armature as a time-dependent resistance while neglecting the detail related with the plasma properties. The agreement between the numerical and the experimental results is rather good.

Influence of the Vacuum Treatment on Wearing Capacity of Sintered Polycrystalline Diamond (PCD)

WANG De-Xin, JIAO Qing-Yu, WANG Fu-Quan, ZHENG Zhen-Kai

1989, 3(4): 315-320 . doi: 10.11858/gywlxb.1989.04.009

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Abstract:
The surface state of the diamond grains greatly affects the working capability of the PCD sintered under super pressure and high temperature. Usually the diamond grains are treated by vacuum or cation bombardment so that its surfaces are in a purer state than those sintered only under super pressure and high temperature, such diamond grains are more intense. This paper discusses the effects of the vacuum treatment method with different binders and grain sizes. First the raw material are treated for an hour in the condition of 10-3 Pa and 600 ℃, next sintered them by high temperature in a type DS-029B 5.884106 N/35.304106 N (600/3 600 T) super pressure apparatus, lastly we test their wearing capacity. We choose three kinds of binders: TiSi-B, CrSi-B and NiSi-B, in which the weight percentages of Si, B and metals all keep the same. The weight proportions of all the binder to diamond are 1∶9. When the raw material were not treated by vacuum, the wearing capacity sequence of the binding PCD was: TiSi-BCrSi-BNiSi-B. After treatment in vacuum, their wearing capacity are all increased and their sequence is changed to TiSi-BCrSi-BNiSi-B. Obviously, the wearing capacity of TiSi-B increased greatly. Using TiSi-B type binder and choosing two diamond grain sizes, one was coarse and another was fine, the weight proportions of binder to diamond were the same as above. We compared the effects of vacuum treatment and grain sizes. When it was not treated by vacuum, the wearing capacity of the coarse grain was higher than that for fine grain. After treatment, the wearing capacities of both cases are increased, but that of the fine grain increases 4.8 times and the coarse one only 2.2 times, the sequence order is reversed. So the authors suggest that if using diamond of fine grain size and binders of transition element with loss in more d-electrons, treated in vacuum, it is possible to get high wearing capacity PCD.

1989 Vol. 3, No. 4

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