摘要:

Cadmium sulfide crystals were shocked along the c-axis to peak instantaneous stresses ranging from 27 to 53 kbar, above and below the phase transition stress of 32 kbar. Particle velocity measurements, using a VISAR system with nanosecond temporal resolution, were used to determine the time dependent mechanical response at the impact surface at higher time resolutions than those reported by Tang and Gupta, and to complement the recent electronic spectroscopy measurements. For experiments with longitudinal stresses above 32 kbar, but below 53 kbar, the instantaneous impact stress of the initial wurtzite phase is maintained for significant times before relaxing to a metastable state, and then further relaxing to the final rock salt phase. Transition rates and metastable state lifetimes are observed to be stress dependent. At 53.5 kbar impact stress, transition to a metastable state appears to be complete within 3 ns of impact, and the full relaxation to the rock salt phase indicates that the phase transition may proceed through multiple metastable states. Epoxy bonds with VISAR windows degrade the ability to accurately measure fast, stress-relaxing phenomena. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1303432

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

Shock polarization and changes in conductivity have been observed in previous electrical investigations of the shock behavior of crystalline materials. For this reason a differential system has been designed by LLNL in their investigations of the conductivity under shock to separate these effects. The measurement removes voltages produced in the shock electrical field allowing determination of those induced by resistance changes. Potassium chloride has been studied extensively and experiments are reported in which it is shocked to various stresses in order to observe electrical effects. The behavior observed when single crystal and pressed targets are shocked above and below the B1:B2 phase transition is presented. The effects of loading to a stress in a single or in multiple steps are discussed. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1303433

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

Real time x-ray diffraction measurements were used to examine the polymorphic phase transformation in KCl shocked along the [100] direction. Shock wave continuum data, obtained previously by Hayes, were used to design the experiments and to predict diffraction from KCl shocked to different peak stresses. Here, we present the results obtained below the transition stress: between 1.4 and 2 GPa. Diffraction data obtained were quantitatively related to macroscopic compression. Interplanar spacing measurements revealed isotropic compression of the unit cell in contrast to previously reported results. Above the transition stress, descriptions of the atomic arrangement with respect to shock propagation (not available in the literature) are required for setting up the detection system. Hence, continuum results in combination with various crystallographic considerations were utilized to obtain data above the transition stress. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1303434

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

Thorough analysis of theUsvs.Upprincipal Hugoniot data for liquidCCl4shows that our values along with all the data reported by other groups except the low pressure results of Dick can be described by the single smooth function proposed by Woolfolk et al. with the following coefficients:Us/C0=(2.08±0.07)+(1.34±0.04)Up/C0−(1.08±0.07)exp((−1.2±0.2)Up/C0).This satisfies the boundary conditionUs=C0=926&hthinsp;m/satUp=0and gives the same asymptotic linear relationship as the best-fit expression reported by Dick at shock pressures above ∼16 GPa. On-Hugoniot temperatures calculated using the EOS of Cowperthwaite and Shaw and the revisedUsvs.Upare in good agreement with the results of our infrared (IR) measurements and reported optical pyrometry data. The maximum discrepancy does not exceed 5&percent; in the 3.3 to 7 GPa and 10 to 15 GPa shock pressure ranges and is less than 10&percent; for other pressures below 20 GPa. Within the accuracy of our data, the behavior of single-shockedCCl4does not contradict to the predictions of the constant(∂P/∂T)vmodel, at least between 3.3 and 7 GPa. All reliable experimental results suggest only two singularities along the principal Hugoniot ofCCl4:the threshold of chemical reactions at 7 GPa and complete decomposition at 20 GPa. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1303435

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

We present a new data on pyrometry measurements ofSiO2-aerogel under shock loading. Optically transparent samples with initial densities 0.36, 0.27 and 0.008 g/cc was shocked up to 65 kJ/g total energy of schock compression; temperatures up to 25000 K was measured. The analysis of data is described. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1303436

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

Ab initio atomistic simulations can be used to predict the mechanical properties of materials subjected to shock waves, using a minimum of calibration data. The equation of state and elasticity can be calculated from quantum mechanical treatments of the electrons and lattice vibrations. Typically, the only experimental input is a pressure correction to match the ambient density. The theoretical treatment is being extended to include dynamic contributions to polymorphic phase changes and a microstructural model of plasticity. Equilibrium polymorphic equations of state were calculated for silicon; these suggest that shocks with pressures between about 15 and 80 GPa should split into two waves with different speeds. Transient X-ray diffraction results from laser-driven shocks appear to agree with the calculated behavior. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1303437

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

Foams, porous solids and granular materials have a characteristic Hugoniot locus that for weak shocks is concave in the (particle velocity, shock velocity)-plane. An equation of state (EOS) that has this property can be constructed implicitly from a Helmholtz free energy of the form&PSgr;(V,T,&fgr;)=&PSgr;s(V,T)+B(&fgr;)where the equilibrium volume fraction&fgr;eqis determined by minimizing &PSgr;,i.e., the condition∂&fgr;&PSgr;=0.For many cases, a Hayes EOS for the pure solid&PSgr;s(V,T)is adequate. This provides a thermodynamically consistent framework for theP-&agr;model. For this form of EOS the volume fraction has a similar effect to an endothermic reaction in that the partial Hugoniot loci with fixed &fgr; are shifted to the left in the(V,P)-plane with increasing &fgr;. The equilibrium volume fraction can then be chosen to match the concavity of the principal Hugoniot locus. An example is presented for the polymer estane. A small porosity of only 1.4 percent is required to match the experimental concavity in the Hugoniot data. This type of EOS can also be used to obtain the so-called “universal” Hugoniot for liquids. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1303438

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

Mixture theories are used to model reacting porous solids such as granular explosives. Volume fraction variables are incorporated into these theories because the solid and gaseous reaction products occupy physically distinct volumes within the mixture. Volume fraction is often treated as a kinetic state variable. However, a simple decomposition of the motion demonstrates that it is not a kinematical variable. Indeed, this decomposition yields another variable, the distention, which is kinematical. By using distention as a state variable we suggest a fundamental and important alteration of the constitutive description of reacting porous solids. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1303439

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

An equation of state for porous mixtures is described based on the mixing properties of the Gibbs potential. For a mixture, neglecting surface energy, the Gibbs potential is simply the sum of the Gibbs potentials of the components. Nitrogen is added to account for porosity, with the mole number proportional to the porosity. The Gibbs potential for fluid species is calculated by means of a perturbation theory originated by Weeks, Chandler, and Anderson. A semi-empirical Debye-Gruneisen equation of state with a Murnaghan form for the zero degree isotherm is used to describe solids. Due to the complexity of the Weeks-Chandler-Anderson fluid equation of state, the possibility of using an ideal gas representation for nitrogen was investigated. A very good match to the shock Hugoniot for porous copper results from this approach. The model is also used to calculate the Hugoniot for an Al-TEFLON mixture and compared to data obtained by Miller and Lindfors and by Holt and Mock. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1303440

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

Recently, a first-order phase transition is predicted to occur in liquid carbon using atomistic simulation and Brenner’s bond order potential. There are also experimental data suggesting a possibility for a first-order phase transition. In light of this, a thermochemical equilibrium code (CHEQ) is used to provide guidance to experiments to find a liquid-liquid phase change in carbon foam and carbon-rich aerogel, resorcinol formaldehyde. Isotherms and Hugoniots are computed using the model of carbon by van Thiel and Ree. The present calculations predict the liquid-liquid-graphite triple point to be at 5000 K and 5.2 GPa and its critical point to be at 6000 K and 8.8 GPa. The Hugoniot calculations suggest that the liquid-liquid phase transition may be detected by performing a shock experiment with initial density of approximately 0.15 g/cm3. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1303441

出版商:AIP    

年代:1900

数据来源: AIP