J. P. Petitet

Thermodynamic properties of quartz, cristobalite and amorphous SiO2: drop calorimetry measurements between 1000 and 1800 K and a review from 0 to 2000 K

We report relative enthalpy measurements on quartz, cristobalite and amorphous SiO2 between 1000 and 1800 K. We have observed a glass transition around 1480 K for amorphous SiO2. From our results and available Cp, relative enthalpy, and enthalpy of solution data we have derived a consistent set of thermodynamic data for these phases. Our calculated enthalpies of fusion are 8.9 ± 1.0 kJ mole−1 for cristobalite at 1999 K and 9.4 ± 1.0 kJ mole−1 at 1700 K for quartz.

Phase transitions and chemical transformations of benzene up to 550 °C and 30 GPa

The phase diagram of benzene has been investigated up to 550 °C and 30 GPa in a diamond anvil cell by Raman scattering. A new triple point at 400 °C and 5 GPa and a new phase of solid benzene, called phase IV, have been located at high temperature. The boundaries have been determined between the following solid phases: (i) benzene II and benzene III; (ii) benzene III and benzene III’. The irreversible chemical transformation line of benzene has been determined and can be divided in three parts leading to three different recovered compounds. The P–T diagram obtained by our experimental results is discussed in comparison with published works.

Phase transitions and chemical transformations of nitromethane up to 350 °C and 35 GPa

Nitromethane has been studied as a model of the energetic nitro compounds. The phase diagram has been determined by Raman scattering in the pressure and temperature ranges of 0–35 GPa and 20–350 °C, respectively. Three new solid phases of nitromethane called III, IV, V, and their domain of stability have been located. A first chemical transformation is observed by the disappearance of nitromethane Raman modes and by the irreversible formation of a transparent solid called compound I (CI). A second chemical transformation [compound I–compound II (CII)], at higher temperature than the first one, is observed by the sudden darkening of the sample.

Structural transformations in laser shock-loaded quartz

The kinetics of the phase transitions occurring in solid materials under strong compression are an issue of considerable interest, which can be studied using dynamic loading processes. Here, we investigate the phase transformation behavior of quartz under laser-induced shock compression of a very short duration (nanosecond order). Time-resolved measurements, coupled with simple computer simulations, have been used to characterize the pressure pulse induced in the samples. Recovery shots have been performed for postshock spectroscopy, that has revealed permanent structural changes of various types, depending on the experimental conditions (loading pressure, sample thickness, and initial temperature). The results, which differ notably from observations reported under quasi-static compression or longer pulsed loads (microsecond order), suggest a partial amorphization of the shocked samples, as well as the formation of a structure that can be compared to known orthorhombic high-pressure phases. They provide a...

Thermodynamic study of alkali sulphates: Calculation of enthalpy, entropy and Gibbs free energy

Abstract Joined with measures of specific heat at low temperature , a calorimetric study of alkali sulphates from ordinary temperature to 1500 K has allowed the calculation of enthalpy, entropy and Gibbs free energy of these salts. The value of entropy at melting point is compared with the value assessed by an acoustical method.

High-pressure Raman scattering in dimethyl phenyl silanol

Raman scattering of dimethyl phenyl silanol has been studied in a diamond anvil cell at room temperature and pressure up to 24 GPa. The liquid‐solid transition is found near 5 GPa. Above 15 GPa, an irreversible transformation occurs to a translucent polymerlike solid which can be recovered at zero pressure with permanent internal stresses.

Shock-induced irreversible transition from α-quartz to CaCl2-like silica

Previous analyses of quartz samples recovered after being submitted to laser shocks of very short duration (nanosecond order) have shown the presence of CaCl2-like silica [T. de Resseguier, P. Berterretche, M. Hallouin, and J. P. Petitet, J. Appl. Phys. 94, 2123 (2003)]. To date, this transition has never been observed under shocks of longer duration (microsecond order) generated by explosives or plate impacts. While this phase is produced from stishovite under static compression at very high pressure (above 50GPa) and disappears on pressure release, it is observed after low pressure laser shocks (below 5GPa) and it is quenched to ambient conditions. The origins of these differences are still unclear. This paper presents complementary laser shock experiments involving setups to provide additional information on the influence of various shock parameters. The results suggest a direct transition from α-quartz to CaCl2-type silica following a diffusionless mechanism involving high shear strains. They also sho...

Raman spectroscopy study of nitromethane in a shear diamond anvil cell

In addition to pressure and temperature effects, shear strain has been proposed to play an important role in the initiation and decomposition mechanisms of energetic materials. To study the effect of shear strain in more detail, a shear diamond anvil cell has been developed. In this paper, we present a Raman spectroscopy study of the effects of shear strain on the high-pressure behaviour of nitromethane (NM). Two major effects are observed in our experiments. The first one is a lowering of the pressures at which the different structural modifications of NM occur. The second one is observed at 28 GPa, where the sample decomposes suddenly just after shear application. Examination of the black residue showed that it is composed of carbon, as indicated by the Raman spectrum.

Influence of the pressure on the activity of PbO in an equimolar molten PbOSiO2 mixture

Abstract The PbO activity in molten PbSiO 3 was determined with e.m.f. measurements in galvanic cells in the range 30–205 MPa and up to 1273 K. We notice a decrease of the PbO activity at 1198 K from α PbO =0.150 at 30 MPa to α μ bO =0.140 at 205 MPa.

Structural changes and phase stability of graphitelike BC3 under explosive shock-wave loading

The response of graphitelike BC3 phases (t-BC3) to shock-wave loading has been studied using two types of high explosives, in order to investigate the possible routes to synthesize via dynamic compression superhard materials in the form of high-pressure phases such as the B-doped diamond produced recently under high static pressures and temperatures. The loading conditions resulting from wave propagation in the shock recovery setup have been determined from theoretical predictions confirmed by numerical simulations and velocity measurements. Over the explored range of shock pressure (from 10 to 30 GPa), no detectable diamond phase could be quenched, probably because of insufficient temperature, but Raman and x-ray diffraction studies of the recovered samples indicate permanent structural changes that have been compared to those observed after shorter, laser driven shock compression. These changes include local phase segregation of t-BC3 and the production of highly disordered phases.