A15 Nb 3 Si is, until now, the only ‘high’ temperature superconductor produced at high pressure (∼110 GPa) that has been successfully brought back to room pressure conditions in a metastable condition. Based on the current great interest in trying to create metastable-at-room-pressure high temperature superconductors produced at high pressure, we have restudied explosively compressed A15 Nb 3 Si and its production from tetragonal Nb 3 Si. First, diamond anvil cell pressure measurements up to 88 GPa were performed on explosively compressed A15 Nb 3 Si material to trace T c as a function of pressure. T c is suppressed to ∼5.2 K at 88 GPa. Then, using these T c ( P ) data for A15 Nb 3 Si, pressures up to 92 GPa were applied at room temperature (which increased to 120 GPa at 5 K) on tetragonal Nb 3 Si. Measurements of the resistivity gave no indication of any A15 structure production, i.e. no indications of the superconductivity characteristic of A15 Nb 3 Si. This is in contrast to the explosive compression (up to P ∼ 110 GPa) of tetragonal Nb 3 Si, which produced 50%–70% A15 material, T c = 18 K at ambient pressure, in a 1981 Los Alamos National Laboratory experiment. This implies that the accompanying high temperature (1000 °C) caused by explosive compression is necessary to successfully drive the reaction kinetics of the tetragonal → A15 Nb 3 Si structural transformation. Our theoretical calculations show that A15 Nb 3 Si has an enthalpy vs the tetragonal structure that is 70 meV atom −1 smaller at 100 GPa, while at ambient pressure the tetragonal phase enthalpy is lower than that of the A15 phase by 90 meV atom −1 . The fact that ‘annealing’ the A15 explosively compressed material at room temperature for 39 years has no effect shows that slow kinetics can stabilize high pressure metastable phases at ambient conditions over long times even for large driving forces of 90 meV atom −1 .

Voids in high explosives (HE) are the most effective way of producing sensitizing "hot spots"', areas of shock-induced energy localization involved in detonation. Thus it can be important to know whether a charge is close to or at Theoretical Maximum Density (TMD). For example, PBX 9502 has a nominal density of 1.890 g/cc with a void fraction of 1.8%. When the sample density is increased to 1.907 g/cc, the void fraction decreases to 0.9%, producing a measurable effect on the sensitivity and detonation properties. Immersion density measurements have shown that density gradients of this amount or more (from tenths to a few percent) can appear across pressed HE parts. Immersion methods can measure bulk density with very high accuracy. However, to measure density variation in a part requires it to be cut into pieces (>= 13 mm or 0.5"), which is time consuming and destructive. Thus the method can't be used to correlate observed gradients to performance, or applied to every part being made. A non-destructive imaging method such as x-ray computed tomography (CT) would be ideal to evaluate the density variation in intact parts with better spatial resolution. However, getting a quantitative measurement of such small variations in density is challenging for x-ray imaging. Artifacts due to beam attenuation and scattering can often obscure the desired features of interest in thick parts. In this work, we use x-ray CT to study a variety of pressed HE parts. We show that obtaining radiographs of materials of density bracketing that of the HE can be used to gain a quantitative measure of density. 1% changes in density between pressed HE parts can be resolved. We also show the successful use of a beam hardening algorithm to help correct one of the major artifacts, "cupping", that can obscure the radial density gradients within an HE part. During our scans of pressed HE parts we also observed macroscopic features such as areas of relatively higher density that may be inclusions of other material or undissolved material from the manufacturing process. CT methods such as dual energy to further improve the sensitivity of measuring density gradients and help identify these inclusions are presented.

The insensitive explosive PBX 9502 contains 95wt-% of TATB crystals and a plastic bonding agent (Kel-F). The TATB crystals have plate-like morphology, similar to that of graphite or boron nitride. We have used X-ray diffraction to measure the preferred orientation (texture) of the TATB crystals in parts fabricated by pressing PBX 9502 powder. Independently, we have used finite-element calculations to derive the direction and magnitude of the shear imposed during the consolidation of this composite material. Based on our results, we propose that the texture develops because the applied shear causes the TATB crystals to rotate such that their (002) basal planes are parallel to shear planes. The texture predicted by this model agrees qualitatively with that measured at various locations within the PBX 9502 compact. Further validation of this model is obtained by the measurement of the thermal expansion coefficient of PBX 9502, which is highly anisotropic.

This study compares the shock initiation behavior of PBX 9502 pressed to less than nominal density (nominal density is 1.890 {+-} 0.005 g/cm{sup 3}) with PBX 9502 pressed to nominal density and then ''ratchet grown'' to low density. PBX 9502 is an insensitive plastic bonded explosive consisting of 95 weight % dry-aminated tri-amino-tri-nitro-benzene (TATB) and 5 weight % Kel-F 800 plastic binder. ''Ratchet growth'' - an irreversible increase in specific volume - occurs when an explosive based on TATB is temperature cycled. The design of our study is as follows: PBX 9502, all from the same lot, received the following four treatments. Samples in the first group were pressed to less than nominal density. These were not ratchet grown and used as a baseline. Samples in the second group were pressed to nominal density and then ratchet grown by temperature cycling 30 times between -54 C and +80 C. Samples in the final group were pressed to nominal density and cut into 100 mm by 25.4 mm diameter cylinders. During thermal cycling the cylinders were axially constrained by a 100 psi load. Samples for shock initiation experiments were cut perpendicular (disks) and parallel (slabs) to the axial load. The fourmore » sample groups can be summarized with the terms pressed low, ratchet grown/no load, axial load/disks, and axial load/slabs. All samples were shock initiated with nearly identical inputs in plate impact experiments carried out on a gas gun. Wave profiles were measured after propagation through 3, 4, 5, and 6 mm of explosive. Side by side comparison of wave profiles from different samples is used as a measure of relative sensitivity. All reduced density samples were more shock sensitive than nominal density PBX 9502. Differences in shock sensitivity between ratchet grown and pressed to low density PBX 9502 were small, but the low density pressings are slightly more sensitive than the ratchet grown samples.« less

PBX 9502 is a plastic bonded explosive that contains 95 wt % TATB a graphitic structured high explosive known to undergo "ratchet growth,' i e irreversible volume change that accompanies temperature excursions Earlier studies have reported changes in TATB based composites as a function of thermal cycling and density change however a clear distinction between density and ratchet growth effects has not been made In the work reported here, an "as pressed density' baseline for the mechanical response of recycled PBX 9502 is established over a density range of Interest, then high density specimens are thermally cycled between 55 and 80 C to achieve "ratchet grown parts in the same low density region As pressed and ratchet grown specimens with identical densities are then analyzed using microX ray computed tomography and USANS techniques to obtain information about pore size distributions Data show that after ratchet growth PBX 9502 specimens contain in general more numerous and smaller voids than specimens that were pressed with lower compaction pressures to match the same density The mechanical response of the ratchet grown material is consistent with damage, showing lower tensile stress and modulus lower compressive modulus and higher tensile and compressive strain, than as pressed specimens of the same density

PBX 9502 is a plastic-bonded high explosive (PBX) containing 95 weight% TATB (triaminotrinitrobenzene) crystals in a polymer binder. TATB crystals are graphitic in nature, with a sheet-like structure and anisotropic CTE. Although the mechanism is not understood, solid-pressed TATB composites have been observed to undergo irreversible volume change ('ratchet growth') upon thermal cycling . This phenomenon has been studied but many aspects remain elusive and uncharacterized. Engineering or performance changes associated with ratchet growth have often been attributed to changes in density alone. We propose that the density changes which accompany ratchet growth involve a unique form of micro-damage distinguishable from the pore structure associated with low-pressed density. We have performed ratchet growth studies on Recycled PBX 9502 between -54 to 80{sup o}C with density changes of about 1.5%. Specimens of the same density were obtained using a lower pressure in the manufacturing process. Comparative measurements were made using quasi-static uniaxial tension tests, as well as micro x-ray computed tomography and ultra-small angle neutron scattering experiments. Through these measurements we have shown that ratchet grown PBX 9502 has properties quite different from predictions based on density alone. The pore size distribution of ratchet grown specimens is unique and easily distinguishedmore » from parts pressed to an equivalent density.« less

Dense solid high explosives are made by compacting plastic-bonded explosive molding powders with high pressures and temperatures for extended periods of time. The density is influenced by manufacturing processes of the powders, compaction temperature, the magnitude of compaction pressure, pressure duration, and number of repeated applications of pressure. The internal density variation of compacted explosives depends on method of compaction and the material being compacted.

Bulk sound speed measurements, isothermal volume compression/X‐ray diffraction experiments and shock loading experiments (maximum pressure ≊20 GPa) have been performed for high initial density (≥94% TMD) ammonium nitrate (AN) and ammonium perchlorate (AP). The experimental data, and full density Hugoniots calculated from that data, suggest the presence of low pressure, shock induced phase transitions in both the AN and AP. The AP phase transition occurs at ≊4 GPa, and exhibits characteristics of a high density to low density phase transition, but the present data are not conclusive. The AN phase change occurs at a shock pressure of less than 3.5 GPa, but the associated volume change is relatively large, indicating the presence of a previously unidentified high pressure, high density phase.

The structures and phase transitions of N2O were studied by powder x-ray diffraction in a tungsten–carbide anvil device from about 100 to 300 K and 2 to 12 GPa. Two solid phases, α-N2O and β-N2O, were observed. The α pattern is consistent with the known low-pressure low-temperature ordered cubic form, space group Pa3, up to 4.8 GPa where transition to a new β solid occurs. From refinements using photographic x-ray intensities, the β-N2O structure was determined to be orthorhombic Cmca. There are four molecules in a unit cell with a=4.954 Å, b=4.497 Å, and c=6.201 Å at 5.81 GPa and 298 K. The molecular axes lie parallel to the bc plane and are tipped at an angle of 37.2° to the b axis. Random head-to-tail orientation is probable in both solids. The phase diagram and values of the molar volume for N2O are compared with published data for the isoelectronic molecule CO2. Recent theoretical calculations correctly predict the β-N2O structure.

The structures and phase transitions of N2 and CO were studied by powder x-ray diffraction from 100 to 300 K and 4 to 13 GPa. Three solid phases, β, δ, and ε, were observed in each material. The known β and δ solids were confirmed to have hexagonal space group P63/mmc and cubic space group Pm3n, respectively. From refinements using photographic x-ray intensities, the new ε-N2 and ε-CO structures were determined to be rhombohedral R3̄c. There are eight ordered molecules in the ε-N2 unit cell with a=5.928 Å and α=85.14° at 110 K and 7.8 GPa, and eight ordered molecules in the ε-CO unit cell with a=6.059 Å and α=85.73° at 100 K and 5.5 GPa. The CO molecules are randomly oriented head to tail. The δ–ε transition takes place through an ordering and small displacement of the N2 and CO molecules, accompanied by a slight extension of the lattice along a cube diagonal. Molar volumes are presented over an expanded P-T region. Recent theoretical calculations using lattice energies, molecular dynamics, and symmetry correlations correctly predict features in the N2 and CO phase diagrams.

The measurement of the isothermal compression of solid nitromethane to 15 GPa at 298 K using high pressure x-ray diffraction techniques is described. The compression data are fit to a model from which bulk moduli are calculated. Most interesting are the linear compression data. The a axis direction, along which the C–N bonds are aligned, shows little increase in repulsion with increasing pressure above 5 GPa. This indicates that the nitro and methyl groups of neighboring molecules may be interacting.

AbstractXRD‐Daten für 6‐ I3 GPa zeigten fünf verschiedene 02‐Phasen, von denen zwei bislang nicht nachgewiesen worden waren.

We have found that explosive compression similar to that used to create A15 ${\mathrm{Nb}}_{3}$Si seriously degrades superconductivity in A15 ${\mathrm{V}}_{3}$Si which, before compression, was typical of well-ordered material. Specifically, the midpoint of the bulk superconducting transition is depressed by 1.8 K, the bulk transition width is increased by a factor of 3, and the specific heat \ensuremath{\gamma} is decreased by more than 20% compared to the starting material. Implications of these results for the ultimate achievable transition temperature in A15 ${\mathrm{Nb}}_{3}$Si are discussed.

Alpha-Si3N4 powders were explosively shock loaded at levels of 260 and 570 kbar pressure. Specimens exhibited densification without additives into the 93 to 98% dense range as a result of shock-induced consolidation. X-ray line broadening investigations indicated that significant residual, internal strain levels of the order of 0.4% were developed in the densified material. Hardness and indentation fracture toughness values for shock-densified regions were nearly equivalent to those for ultra-high-pressure hot-pressed Si3N4 without densification aids.

Alpha-Si3N4 powders in the form of cold-pressed (57% dense) pellets 3.18 mm in diameter and 6.35 mm in length were explosively shocked at levels of 260 and 570 Kbar pressure using a recently developed experimental approach which allows routine specimen recovery at shock pressures as high as 1000 kbar. X-ray line broadening studies indicated that significant internal strain levels of the order of 0.4% were developed. Specimens subjected to 260 Kbar were densified without additives into the 93-98% range. Hardness and indentation fracture toughness values in densified regions were observed to be nearly equivalent to those for ultra-high-pressure hot-pressed Si3N4 without densification aids.

The Raman scattering of solid carbon monoxide was studied in a diamond cell from 15 to 297 K at pressures from 1.0 to 5.8 GPa. At low temperature a transition occurs near 3.4 GPa from the known ..cap alpha..-phase (space group P2/sub 1/3) to a new element of-phase (structure unknown), rather than to the predicted ..gamma..-phase (P4/sub 2//mnm). A transformation from ..beta..-CO (space group P6/sub 3//mmc) into a new sigma-phase (Pm3n.) was found near 5.2 GPa at room temperature. Above about 4.6 GPa and 80 K, CO reacts photochemically when irradiated with visible laser light. The photoreactivity may be associated with the formation of a yellow polymer, which can be recovered at zero pressure. 31 references, 3 figures.