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Thermoelectric materials convert heat flux to electricity (or vice versa as Peltier coolers); however, their application to harvest waste heat is limited by challenges in fabrication and materials optimization. Here, cold-spray deposition is used as an additive manufacturing technique to fabricate p- and n-type Bi2Te3, on substrates ranging from quartz to aluminum. The sprayed material has a randomly oriented microstructure largely free from pores (> 99.5% dense), and deposition is achieved without substantial compositional changes. The Seebeck coefficient and thermal conductivity are largely preserved through the spray process, but the defects introduced during deposition significantly increase electrical resistivity. Defects can be removed, and compressive strain relaxed by a post-deposition anneal, which leads to Bi2Te3 blocks with a typical ZT of 0.3 at 100°C. Generators fabricated on sheets or pipes made of copper compare favorably with similar designs constructed using bulk Bi2Te3, displaying a wider operating temperature range. These results demonstrate the power and versatility of cold-spray additive manufacturing and provide a pathway toward fabrication of thermoelectric generators in complex geometries that are inaccessible to generators made by traditional approaches.

The response of rapidly compressed highly oriented pyrolytic graphite (HOPG) normal to its basal plane was investigated at a pressure of ∼80 GPa. Ultrafast x-ray diffraction using ∼100 fs pulses at the Materials Under Extreme Conditions sector of the Linac Coherent Light Source was used to probe the changes in crystal structure resulting from picosecond timescale compression at laser drive energies ranging from 2.5 to 250 mJ. A phase transformation from HOPG to a highly textured hexagonal diamond structure is observed at the highest energy, followed by relaxation to a still highly oriented, but distorted graphite structure following release. We observe the formation of a highly oriented lonsdaleite within 20 ps, subsequent to compression. This suggests that a diffusionless martensitic mechanism may play a fundamental role in phase transition, as speculated in an early work on this system, and more recent static studies of diamonds formed in impact events.

In order to model the fate and transport of particles following a nuclear explosion, there must first be an understanding of individual physical and chemical processes that affect particle formation. One interaction pertinent to fireball chemistry and resultant debris formation is that between uranium and oxygen. In this study, we use laser ablation of uranium metal in different concentrations of oxygen gas, either

Recently acquired high-resolution images of nanosecond laser ablation plumes suggest a strong correlation between the internal plume structure and the type of material being ablated. However, the details of this relation are currently not well understood. In this work, we attempt to explore this correlation using a 2D radiation hydrodynamics model to study the dependence of internal plume structure formation on the ablation material. Spatio-temporal emission maps and plume expansion velocities from experimental measurements are compared with the model predictions, including synthetic emission maps. The shape and expansion rate of an outer air plume region are found to be in good agreement for both carbon and aluminum, as are the inner material plume dynamics for carbon ablation. The largest disagreement is observed in the case of a polished aluminum target, where the chaotic inner plume features seen in the experimental images are not observed in the model. The possible physical mechanisms responsible for this discrepancy are discussed. This effort constitutes a continued development toward a predictive model of ablation plume dynamics and chemistry for various materials in extreme environments.

As theoretically hypothesized for several decades in group IV transition metals, we have discovered a dynamically stabilized body-centered cubic (bcc) intermediate state in Zr under uniaxial loading at sub-nanosecond timescales. Under ultrafast shock wave compression, rather than the transformation from alpha-Zr to the more disordered hex-3 equilibrium omega-Zr phase, in its place we find the formation of a previously unobserved nonequilibrium bcc metastable intermediate. We probe the compression-induced phase transition pathway in zirconium using time-resolved sub-picosecond x-ray diffraction analysis at the Linac Coherent Light Source. We also present molecular dynamics simulations using a potential derived from first-principles methods which independently predict this intermediate phase under ultrafast shock conditions. In contrast with experiments on longer timescale (> 10 ns) where the phase diagram alone is an adequate predictor of the crystalline structure of a material, our recent study highlights the importance of metastability and time dependence in the kinetics of phase transformations.

The predictive models that describe the fate and transport of radioactive materials in the atmosphere following a nuclear incident (explosion or reactor accident) assume that uranium-bearing particulates would attain chemical equilibrium during vapor condensation. In this study, we show that kinetically driven processes in a system of rapidly decreasing temperature can result in substantial deviations from chemical equilibrium. This can cause uranium to condense out in oxidation states (e.g., UO3 vs UO2) that have different vapor pressures, significantly affecting uranium transport. To demonstrate this, we synthesized uranium oxide nanoparticles using a flow reactor under controlled conditions of temperature, pressure, and oxygen concentration. The atomized chemical reactants passing through an inductively coupled plasma cool from ∼5000 to 1000 K within milliseconds and form nanoparticles inside a flow reactor. The ex situ analysis of particulates by transmission electron microscopy revealed 2-10 nm crystallites of fcc-UO2 or α-UO3 depending on the amount of oxygen in the system. α-UO3 is the least thermodynamically preferred polymorph of UO3. The absence of stable uranium oxides with intermediate stoichiometries (e.g., U3O8) and sensitivity of the uranium oxidation states to local redox conditions highlight the importance of in situ measurements at high temperatures. Therefore, we developed a laser-based diagnostic to detect uranium oxide particles as they are formed inside the flow reactor. Our in situ measurements allowed us to quantify the changes in the number densities of the uranium oxide nanoparticles (e.g., UO3) as a function of oxygen gas concentration. Our results indicate that uranium can prefer to be in metastable crystal forms (i.e., α-UO3) that have higher vapor pressures than the refractory form (i.e., UO2) depending on the oxygen abundance in the surrounding environment. This demonstrates that the equilibrium processes may not dominate during rapid condensation processes, and thus kinetic models are required to fully describe uranium transport subsequent to nuclear incidents.

Ultrafast (130-fs) x-ray diffraction at the Linac Coherent Light Source has been applied to observe shockmelting, which is driven by a rapid (120-ps) laser pulse impinging on a thin (few micrometers) bilayer ofaluminum/zirconium. At a pressure of 100 GPa in the aluminum (130 GPa in the zirconium), there is rapidmelting of both metals and the recrystallization of zirconium into the bcc β phase. We observe the solidificationof the melt starting a few hundred picoseconds following the shock melting, out to 50 ns when the zirconiumis fully crystallized into the bcc β phase at a residual temperature of approximately 2000 K. The pressure isobtained directly from the early time x-ray data, whereas the additional information from the x-ray line widthand intensity at longer times inform a model of crystal nucleation and growth.

We use a recently developed plasma-flow reactor to experimentally investigate the formation of oxide nanoparticles from gas phase metal atoms during oxidation, homogeneous nucleation, condensation, and agglomeration processes. Gas phase uranium, aluminum, and iron atoms were cooled from 5000 K to 1000 K over short-time scales (∆t In-situ emission spectroscopy is used to measure the variation in monoxide/atomic emission intensity ratios as a function of temperature and oxygen fugacity. Condensed oxide nanoparticles are collected inside the reactor for ex-situ analyses using scanning and transmission electron microscopy (SEM, TEM) to determine their structural compositions and sizes. A chemical kinetics model is also developed to describe the gas phase reactions of iron and aluminum metals. The resulting sizes and forms of the crystalline nanoparticles (FeO-wustite, eta-Al2O3, UO2, and alpha-UO3) depend on the thermodynamic properties, kinetically-limited gas phase chemical reactions, and local redox conditions. This work shows the nucleation and growth of metal oxide particles in rapidly-cooling gas is closely coupled to the kinetically-controlled chemical pathways for vapor-phase oxide formation.

The shock response of polymers is important for a number of commercial and defense-related applications, but it is difficult to obtain empirical shock response data over the wide range of preparations and aging conditions typically found in such applications. Ultrafast compression is useful to characterize polymer shock response over a wide range of polymer initial conditions due to the high throughput of this method. To establish greater confidence in ultrafast compression experiments and to characterize the detailed shock response of several variations in a single base polymer, the results of sub-nanosecond shock compression experiments in ∼5 μm thick layers of the polydimethylsiloxane (PDMS)-based elastomeric rubbers Sylgard-184, SE1700, and an unfilled, end-linked model PDMS network are presented. The results of conventional ultrafast shock etalon measurements to time-of-flight measurements for similar thickness layers of irradiated and unirradiated SE1700 are compared. Good agreement between the shock response measured by these two ultrafast shock methods, as well as consistency between ultrafast data and long time scale gas gun data is found. From measurements across a variety of PDMS formulations, a statistically significant variation in the shock response with the quasistatic elastic modulus is presented. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018.

Studies related to U gas-phase oxidation through plasma- and thermo-chemistry are important for many fields, including environmental monitoring, forensic analysis, debris analysis in a weapon detonation event, and nucleation physics. Recently, significant efforts have been made to understand the chemical pathways involved in the progression from U atoms to diatoms (UO) and polyatomic molecules (UxOy), employing optical spectroscopy tools and computational modeling. In many studies, laser ablation of U or a U-containing flow reactor are used as a highly resource-efficient, repeatable, tunable, and lab-scale testbed for studying gas-phase oxidation in U plasmas. The spectroscopic analysis of high-temperature gas-phase oxidation of U is challenging due to the congested U spectra, resolution limitations of instrumentation, and the numerous chemical reaction pathways possible. This article focuses on the current understanding and challenges related to studying U plasma chemistry, specifically U gas-phase oxidation and molecular formation, via optical spectroscopy of plasmas and associated computational and spectral modeling. The physical and chemical processes involved in the evolution from U atoms to U oxide molecules to nanoparticles and agglomerates (i.e., debris) are discussed in the context of optical spectroscopic studies. The article concludes by highlighting opportunities for future research efforts based on existing knowledge published in the literature.

We have performed a detailed x-ray diffraction structural study of Zr under pressure and unambiguously identify the existence of a first-order isostructural bcc-to-bcc phase transition near 58 GPa. First-principles quantum molecular dynamics calculations support the existence of this phase transition, in excellent agreement with experimental results, triggered by anharmonic effects. Our results highlight the potential ubiquity of anharmonically driven isostructural transitions within the periodic table under pressure and call for follow-up experimental and theoretical studies.

The availability of pulsed x rays on short timescales has opened up new avenues of research in the physics and chemistry of shocked materials. The continued installation of shock platforms such as gas guns and high power lasers placed at beamline x-ray facilities has advanced our knowledge of materials shocked to extreme conditions of pressure and temperature. In addition, theoretical advancements have made direct correspondence with high-pressure x-ray experiments more viable, increasing the predictive capability of these models. In this paper, we discuss both recent experimental results and the theory and modeling that has been developed to treat these complex situations. Finally, we discuss the impact that new platforms and increased beam time may have on the future direction of this field.

The first-order magneto-structural transformation that occurs in Gd5Si2Ge2 near room temperature makes it a strong candidate for many energy harvesting applications. Understanding the single crystal properties is crucial for allowing simulations of device performance. In this study, magnetically and thermally induced transformation strains were measured in a single crystal of Gd5Si2.05Ge1.95 as it transforms from a high-temperature monoclinic paramagnet to a lower-temperature orthorhombic ferromagnet. Thermally induced transformation strains of −8500 ppm, +960 ppm and +1800 ppm, and magnetically induced transformation strains of −8500 ppm, +900 ppm and +2300 ppm were measured along the a, b and c axes, respectively. Using experimental data coupled with general thermodynamic considerations, a universal phase diagram was constructed showing the transition from the monoclinic to the orthorhombic phase as a function of temperature and magnetic field.

The early-time kinetic behavior of species in a plume produced from laser ablation of U3Si2 in an environment containing 2% O2 is characterized using time-resolved absorption spectroscopy. The UO band around 593.55 nm and the SiO band around 230 nm is observed as well as various atomic and ionic uranium transitions. Temperatures and concentrations of these species are tracked and reported as well.

High-temperature chemistry in laser ablation plumes leads to vapor-phase speciation, which can induce chemical fractionation during condensation. Using emission spectroscopy acquired after ablation of a SrZrO3 target, we have experimentally observed the formation of multiple molecular species (ZrO and SrO) as a function of time as the laser ablation plume evolves. Although the stable oxides SrO and ZrO2 are both refractory, we observed emission from the ZrO intermediate at earlier times than SrO. We deduced the time-scale of oxygen entrainment into the laser ablation plume using an 18O2 environment by observing the in-growth of Zr18O in the emission spectra relative to Zr16O, which was formed by reaction of Zr with 16O from the target itself. Using temporally resolved plume-imaging, we determined that ZrO formed more readily at early times, volumetrically in the plume, while SrO formed later in time, around the periphery. Using a simple temperature-dependent reaction model, we have illustrated that the fo...

We report results from ultrafast compression experiments conducted on β-HMX single crystals. Results consist of nominally 12 picosecond time-resolved wave profile data, (ultrafast time domain interferometry –TDI measurements), that were analyzed to determine high-velocity wave speeds as a function of piston velocity. TDI results are used to validate calculations of anisotropic stress-strain behavior of shocked loaded energetic materials. Our previous results derived using a 350 ps duration compression drive revealed anisotropic elastic wave response in single crystal β-HMX from (110) and (010) impact planes. Here we present results using a 1.05 ns duration compression drive with a 950 ps interferometry window to extend knowledge of the anisotropic dynamic response of β-HMX within eight microns of the initial impact plane. We observe two distinct wave profiles from (010) and three wave profiles from (010) impact planes. The (110) impact plane wave speeds typically exceed (010) impact plane wave speeds at t...

Here we review the scaling of pulse energy with duration for sub-ns laser-driven dynamic compression experiments, which suggests that extreme pressures (multiple Mbar) might be achieved in rapidly equilibrating materials with substantially lower energy than used in traditional experiments. For instance, conventional scaling of pressure with laser intensity indicates that pressures well into the multiple Mbar range should be accessible by compressing with a hundreds of picosecond duration drive pulse with some tens of mJ of energy – orders of magnitude less than required for conventional experiments. Via a related scaling argument, we also show that the throughput of time-resolved pulsed x-ray dynamic compression experiments (such as those performed at x-ray free electron lasers) varies as the inverse square of the time scale of the experiment. The strong variation of throughput with the scale of the experiment should be a significant consideration in the design of such experiments – to obtain high through...

Measuring the volume of a material compressed quasi-statically to a high-pressure typically involves the determination of a primitive crystal cell volume using x-ray diffraction (XRD) techniques. The accumulation of pressure dependent volumes leads to an understanding of the material’s equation of state, (EOS); however, in the case of low-symmetry (e.g. triclinic) materials with twining features, and large primitive cells, conventional XRD approaches can be technically problematic and lead to ambiguous or mathematically under-determined lattice constants. To resolve this long-standing issue, we examined the utility of a “direct” approach toward determining a material’s volume by measuring surface area and sample height using optical microscopy and interferometry respectively. To test the validity of our proposed approach, we first compared our results from pressurized Triamino-Trinitrobenzene (TATB, SG P-1) with a published EOS determined by synchrotron XRD measurements of Stevens et al.. A near-perfect m...

We report theoretical and experimental evidence for a new class of high-nitrogen content energetic material compounds consisting of molecular pentazoles, which are stabilized in the crystal phase upon introduction of elemental cesium. First-principles structural predictions show that the material with composition CsN5 is thermodynamically stable above 15 GPa. Indexing of the measured X-ray diffraction spectra indicate the synthesis of this material at 60 GPa as well its stability upon decompression down to 24 GPa.

Efforts to synthesize, characterize, and recover novel polynitrogen energetic materials have driven attempts to subject high nitrogen content precursor materials (in particular, metal and nonmetal azides) to elevated pressures. Here we present a combined theoretical and experimental study of the high-pressure behavior of ammonium azide (NH4N3). Using density functional theory, we have considered the relative thermodynamic stability of the material with respect to two other crystal phases, namely, trans-tetrazene (TTZ), and also a novel hydronitrogen solid (HNS) of the form (NH)4, that was recently predicted to become relatively stable under high pressure. Experimentally, we have measured the Raman spectra of NH4N3 up to 71 GPa at room temperature. Our calculations demonstrate that the HNS becomes stable only at pressures much higher (89.4 GPa) than previously predicted (36 GPa). Our Raman spectra are consistent with previous reports up to lower pressures and at higher pressures, while some additional subtle behavior is observed (e.g., mode splitting), there is again no evidence of a phase transition to either TTZ or the HNS.

We will present results from a steady-state inductively coupled plasma-based flow reactor designed to investigate chemical kinetics for combustion, fusion energy, and nuclear forensics applications with high signal to noise. Article not available.

We describe a 100%-efficient moving-wire interface (MWI) for online coupling of high-performance liquid chromatography (HPLC) that transmits 100% of the carbon in nonvolatile analytes to a CO2-gas-accepting ion source. This interface accepts a flow of analyte in solvent, evaporates the solvent, combusts the remaining analyte, and directs the combustion products to the instrument of choice. Effluent is transferred to a periodically indented wire by a coherent jet to increase efficiency and maintain peak resolution. The combustion oven is plumbed such that gaseous combustion products are completely directed to an exit capillary, avoiding the loss of combustion products to the atmosphere. This system achieves almost-complete transfer of the analyte at HPLC flow rates up to 125 μL/min at a wire speed of 6 cm/s. This represents a 30× increase in efficiency and an 8× increase in maximum wire loading, compared to the spray transfer technique used in earlier MWIs.

The evaluation of optical components in various laser systems, with regard to their resistance to laser-induced damage, has often relied on measuring damage threshold fluences. For large-aperture laser systems a small amount of damage in optics does not impede performance. This necessitates the development of damage testing instrumentation that can directly provide information regarding beam obscuration. The number and size of damage scattering sites for a specific laser fluence, wavelength, and pulse duration determine overall beam losses due to damage. We present a design for rapid quantitative characterization of bulk damage performance of optical materials for use in large-aperture laser systems.

Nickel microspheres were produced by the spark erosion technique under both liquid nitrogen and water conditions. Density measurements and Scanning electron microscope analysis revealed that a significant portion of the Ni particles produced under the liquid nitrogen conditions are hollow spheres with a density of 6.67(4)g∕cm3 while the particles produced in water are primarily solid spheres with a density of 8.40(1)g∕cm3, close to the bulk nickel value of 8.90g∕cm3. Nickel∕polymer composites incorporating the hollow and solid nickel microspheres were manufactured with volume fractions of 25% and 36%, respectively. The hollow and solid nickel composites exhibited saturation magnetostrictions of −24 and −28ppm, respectively. In addition, small quantities of Terfenol-D (Tb0.3Dy0.7Fe2) were spark eroded under liquid argon conditions with ∼10% by volume of the spark eroded particles being solid microspheres. Calculations indicate that aligned composites that incorporate these Terfenol-D microspheres could rea...

Ni particles were prepared by spark erosion in a fixed-gap apparatus, and in the usual “shaker-pot” assembly, in an investigation of the influence of various processing parameters on the particles’ properties. The sizes of the particles were studied as functions of spark energies ranging from 10 μJ to 1 J, and a scaling relation derived from a simple model was verified. Several different static and rotating electrode configurations were compared with respect to their suitability for producing significant yields of small particles. The advantages of stirring the dielectric with the fixed-gap apparatus and of rotating the electrodes were demonstrated. Water, kerosene, and liquid argon and nitrogen were used as dielectric liquids. When compounds were formed, the reaction with the dielectric proceeded inversely with particle size. Spark erosion in kerosene at low spark energies, followed by annealing, proved to be an effective method to produce fine nickel particles.

Optical absorption and electron paramagnetic resonance (EPR) techniques have been used to characterize the production and thermal decay of point defects in undoped single crystals of KD2PO4 grown at Lawrence Livermore National Laboratory. A crystal was irradiated at 77 K with x rays, and then warmed to room temperature. Immediately after irradiation, broad optical absorption bands were observed to peak near 230, 390, and 550 nm. During warming, these absorption bands thermally decayed in the 80–140 K range. Another absorption band peaking near 450 nm appeared as the three bands disappeared. This last band decayed between 140 and 240 K. Correlations with EPR data suggest that the band at 230 nm is associated with interstitial deuterium atoms, the two bands at 390 and 550 nm are associated with self-trapped holes, and the band at 450 nm is associated with holes trapped adjacent to deuterium vacancies. Additional EPR spectra from several oxygen-vacancy centers and a silicon-associated hole center were observ...

Employing ab initio calculations we have determined the structural parameters of the paraelectric and ferroelectric phases of KH2PO4, which are in good agreement with experiment. The calculations reveal that the O-O bond length and the coordinated motion of the P and K atoms have a large effect on the double-well potential for H, controlling the distance, δ, between the two equilibrium positions of the H along the O-O bond in the paraelectric phase. The calculations provide new evidence that the ferroelectric phase transition in H-bonded crystals has also a displacive feature.

We present the development of a steady state plasma flow reactor to investigate gas phase physical and chemical processes that occur at high temperature (1000 < T < 5000 K) and atmospheric pressure. The reactor consists of a glass tube that is attached to an inductively coupled argon plasma generator via an adaptor (ring flow injector). We have modeled the system using computational fluid dynamics simulations that are bounded by measured temperatures. In situ line-of-sight optical emission and absorption spectroscopy have been used to determine the structures and concentrations of molecules formed during rapid cooling of reactants after they pass through the plasma. Emission spectroscopy also enables us to determine the temperatures at which these dynamic processes occur. A sample collection probe inserted from the open end of the reactor is used to collect condensed materials and analyze them ex situ using electron microscopy. The preliminary results of two separate investigations involving the condensation of metal oxides and chemical kinetics of high-temperature gas reactions are discussed.

We have measured vibronic emission spectra of an oxide of uranium formed after laser ablation of the metal in gaseous oxygen. Specifically, we have measured the time-dependent relative intensity of a band located at approximately 593.6 nm in 16O2. This band grew in intensity relative to neighboring atomic features as a function time in an oxygen environment but was relatively invariant with time in argon. In addition, we have measured the spectral shift of this band in an 18O2 atmosphere. Based on this shift, and by comparison with earlier results obtained from free-jet expansion and laser excitation, we can confirm that the oxide in question is UO, consistent with recent reports based on laser ablation in 16O2 only.

Electron paramagnetic resonance (EPR) has been used to characterize a hole trap and several electron traps in single crystals of potassium dihydrogen phosphate (KH2PO4 or KDP). The paramagnetic charge states of these centers are produced by ionizing radiation (e.g., x rays or a 266 nm beam from a pulsed Nd:YAG laser) and are stable for days and even weeks at room temperature. One center consists of a hole trapped on an oxygen ion adjacent to a silicon impurity located on a phosphorus site. This defect has a small, but easily observed, hyperfine interaction with the adjacent substitutional proton. The other centers are formed when an electron is trapped at an oxygen vacancy. These latter defects are best described as (PO3)2− molecular ions, where the primary phosphorus nucleus is responsible for a large hyperfine splitting (500–800 G in magnitude). Five EPR spectra representing variations of these oxygen vacancy centers are observed, with the differences being attributed to the relative position of a nearb...

Photoexcited defect clusters in the bulk of KH2PO4 crystals are investigated using a microscopic fluorescence imaging system with 1 μm spatial resolution. The observed defect cluster concentration is approximately 104–106 per mm3 depending on the crystal growth method and sector of the crystal. The intensity of the emission clusters varies widely within the image field while a nearly uniformly distributed background is present. Spectroscopic measurements provided information on the emission characteristics of the observed defect population.

Inelastic neutron scattering experiments were performed on to determine the magnetic ground state of the Pr ions. In this compound, superconductivity is suppressed by the presence of Pr ions. The magnetic ground state is found to be very similar to that in and . However, the high energy spectrum has a stronger resemblance to that of , where the Pr ions see a very similar local structure. Therefore, a similar set of higher order crystalline-electric-field parameters and an inversion of can explain qualitatively the neutron spectra. The suppression of superconductivity and the quasi-triplet magnetic ground state support models which correlate the unusual magnetic properties with suppression by Pr.

The structural parameters, phase purity, and oxygen stoichiometry of the (Pr 1.5 Ce 0.5 )Sr 2 Cu 2 TaO 10-δ and (R 1.5 - x Pr x Ce 0.5 ) Sr 2 Cu 2 NbO 10-δ , R = Nd, Sm, Eu, compounds are investigated with powder X-ray diffraction (XRD), Rietveld refinements, and thermogravimetric analysis (TGA). The XRD data indicate that all the (R 1.5-x Pr x Ce 0.5 )Sr 2 Cu 2 NbO 10-δ compounds are isostructural and typically 98% pure. Rietveld refinements reveal a contraction of the c lattice parameter and features in the rare earth-planar oxygen bond length (as functions of Pr doping) that are correlated with the unusual electronic and magnetic transitions induced by the Pr ion in these materials. Results for deoxygenated samples show that the lattice expands upon deoxygenation and suggest that the oxygen is removed primarily from the SrO layers. TGA results reveal reliable techniques for determining oxygen stoichiometries and deoxygenating these compounds and that these compounds are oxygen deficient with δ's of typically 0.045. The structural features of these compounds are compared to similar high T c cuprates, and related to the electronic and magnetic transitions and the suppression of superconductivity induced by the Pr ion in these materials. We discuss our results in terms of the current understanding of high Tc cuprates and models for the suppression of superconductivity by the Pr ion in the related (R 1-x Pr x )Ba 2 Cu 3 O 7 system.

We report on the electronic and magnetic phase diagram for ${(R}_{1.5\ensuremath{-}x}{\mathrm{Pr}}_{x}{\mathrm{Ce}}_{0.5}){\mathrm{Sr}}_{2}{\mathrm{Cu}}_{2}{\mathrm{NbO}}_{10\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ $(R=\mathrm{N}\mathrm{d},\mathrm{S}\mathrm{m},\mathrm{E}\mathrm{u}).$ The results indicate that Pr suppresses superconductivity in these materials in the same manner as in the ${(R}_{1\ensuremath{-}x}{\mathrm{Pr}}_{x}){\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$ compounds and that this suppression is correlated with anomalous Pr magnetism, a reduced Pr effective moment, a metal-to-insulator transition, and an ion size effect. The significance of these results in terms of a generalization of this phenomenon, the criteria they establish for modeling these materials, and the role of $f$-electron hybridization is discussed.

We report a joint experimental and theoretical study of the dynamical and structural properties of lead fluoride (PbF{sub 2}). Specifically, we have performed detailed measurements of the room-temperature Raman-active phonon modes to 310 kbar. We verify the transition at about 4 kbar from the cubic ({beta}) to the orthorhombic ({alpha}) phase and the recovery of the {alpha} phase at ambient conditions. At approximately 147 kbar, we observe a transformation from the {alpha} phase to a modification that we label {gamma}. The data indicate that the {gamma} phase is structurally similar to the orthorhombic {alpha} phase. Moreover, we have carried out first-principles calculations which support the existence of a transition in this pressure range, although the crystal structure of the new phase remains undetermined. We have also calculated the equations of state for the two low-pressure phases, {beta} and {alpha}. In the case of the {alpha} phase, we have made a complete structural determination by calculating the pressure-dependent values of all six internal structural parameters as well as the axial ratios, c/a and b/a, for pressures ranging from zero up to 2 Mbar. Using the calculated {alpha}-phase equation of state, we also extract Gr{umlt u}neisen parameters from the data for several more » optical phonon modes. {copyright} {ital 1997} {ital The American Physical Society} « less

The magnetic properties of nonsuperconducting (${\mathrm{Pr}}_{1.5}$ ${\mathrm{Ce}}_{0.5}$ )${\mathrm{Sr}}_{2}$ ${\mathrm{Cu}}_{2}$ ${\mathrm{MO}}_{10\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ with M=Nb, Ta are characterized with dc magnetization, specific-heat, and neutron-diffraction experiments. Data for (${\mathrm{Pr}}_{1.5}$ ${\mathrm{Ce}}_{0.5}$ )${\mathrm{Sr}}_{2}$ ${\mathrm{Cu}}_{2}$ ${\mathrm{NbO}}_{10\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ reveal complex Cu magnetism marked by antiferromagnetic order below 200 K, spin structure transitions at 130 and 57 K, both collinear and noncollinear antiferromagnetic spin structures, and weak ferromagnetic behavior below 130 K. The data also indicate an anomalous ordering of the Pr spins near 10 K, a large linear contribution to the low-temperature specific heat, and a Pr 4f crystal-field ground state similar to that found in ${\mathrm{PrBa}}_{2}$ ${\mathrm{Cu}}_{3}$ ${\mathrm{O}}_{7}$ . Furthermore, there is evidence that the weak ferromagnetic behavior couples to the Pr ordering near 10 K. Identical Pr magnetism and similar Cu magnetism are found in (${\mathrm{Pr}}_{1.5}$ ${\mathrm{Ce}}_{0.5}$ )${\mathrm{Sr}}_{2}$ ${\mathrm{Cu}}_{2}$ ${\mathrm{TaO}}_{10\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ , deoxygenated (${\mathrm{Pr}}_{1.5}$ ${\mathrm{Ce}}_{0.5}$ )${\mathrm{Sr}}_{2}$ ${\mathrm{Cu}}_{2}$ ${\mathrm{NbO}}_{10\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ , and deoxygenated (${\mathrm{Pr}}_{1.5}$ ${\mathrm{Ce}}_{0.5}$ )${\mathrm{Sr}}_{2}$ ${\mathrm{Cu}}_{2}$ ${\mathrm{TaO}}_{10\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ . These results indicate that superconductivity is suppressed in these compounds in the same phenomenological manner as in ${\mathrm{PrBa}}_{2}$ ${\mathrm{Cu}}_{3}$ ${\mathrm{O}}_{7}$ . We interpret this as evidence that superconductivity is suppressed by the same mechanism in both structures and propose that a general correlation exists between anomalous Pr magnetism and a lack of superconductivity in these Pr-based high-${\mathrm{T}}_{\mathrm{C}}$ cuprates. The significance of these results and analyses to understanding and modeling the suppression of superconductivity by Pr in high-${\mathrm{T}}_{\mathrm{C}}$ cuprates is discussed.

An experimental campaign to tune the initial shock compression sequence of capsule implosions on the National Ignition Facility (NIF) was initiated in late 2010. The experiments use a NIF ignition-scale hohlraum and capsule that employs a re-entrant cone to provide optical access to the shocks as they propagate in the liquid deuterium-filled capsule interior. The strength and timing of the shock sequence is diagnosed with velocity interferometry that provides target performance data used to set the pulse shape for ignition capsule implosions that follow. From the start, these measurements yielded significant new information on target performance, leading to improvements in the target design. We describe the results and interpretation of the initial tuning experiments.

Ignition implosions on the National Ignition Facility (NIF) [Lindl et al., Phys. Plasmas 11 , 339 (2004)] are driven with a very carefully tailored sequence of four shock waves that must be timed to very high precision in order to keep the fuel on a low adiabat. The first series of precision tuning experiments on NIF have been performed. These experiments use optical diagnostics to directly measure the strength and timing of all four shocks inside the hohlraum-driven, cryogenic deuterium-filled capsule interior. The results of these experiments are presented demonstrating a significant decrease in the fuel adiabat over previously un-tuned implosions. The impact of the improved adiabat on fuel compression is confirmed in related deuterium-tritium (DT) layered capsule implosions by measurement of fuel areal density (ρR), which show the highest fuel compression (ρR ∼ 1.0 g/cm2 ) measured to date.

Rotational magnetization-vector measurements were carried out on a superconducting polycrystalline disk of EuCSCNO. The results at 4.2 K and low fields resemble recent results for YBa 2 Cu 3 O 7−δ (YBCO) and Ba 0.57 K 0.43 BiO 3 (BKBO) in revealing a broad distribution in the strength of the vortex pinning torques. This resemblance extends to higher fields, where it was found that the average pinning torque per vortex ( τ p ), which is directly related to the critical current density, decreases steadily with increasing field in a manner consistent with vortex bundling. Quantitatively, however, the values of τ p for EuCSCNO are smaller by an order of magnitude than those for YBCO but are only moderately smaller than those for BKBO. This comparison supports the notion that the vortex pinning may derive in part from deviations from oxygen stoichiometry, which are normally present in YBCO but are essentially absent in EuCSCNO and BKBO. Moreover, as in YBCO and BKBO, the τ p in EuCSCNO at fixed field is found to diminish rapidly as the temperature rises towards T c .

The issue of the valence state of praseodymium (Pr) in Pr-containing cuprates is addressed using electron-energy-loss spectrometry (EELS) in a transmission electron microscope (TEM) on (${\mathrm{Eu}}_{1.5\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Pr}}_{\mathit{x}}$${\mathrm{Ce}}_{0.5}$)${\mathrm{Sr}}_{2}$${\mathrm{Cu}}_{2}$${\mathrm{NbO}}_{10}$ compounds. EELS analysis, in reference to Ce ${\mathit{M}}_{4,5}$ edges, clearly indicates that the energy onset, ${\mathit{M}}_{5}$/${\mathit{M}}_{4}$ intensity ratio and the fine structure of the ${\mathit{M}}_{5}$ and ${\mathit{M}}_{4}$ edges of a Pr are consistent with Pr valence of +3. These results eliminate theoretical descriptions on these compounds based upon a Pr +4 valence state and a suppression of carrier states. Localized holes, magnetic interactions, and f-electron hybridization remain viable candidates to explain the lack of superconductivity in Pr-based cuprates. \textcopyright{} 1996 The American Physical Society.

The ${\mathrm{Pr}}_{1.5}$${\mathrm{Ce}}_{0.5}$${\mathrm{Sr}}_{2}$${\mathrm{Cu}}_{2}$${\mathrm{Nb}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ta}}_{\mathit{x}}$${\mathrm{O}}_{10}$ (0\ensuremath{\le}x\ensuremath{\le}1) compounds display an in-plane weak ferromagnetic component similar to that observed in the ${\mathrm{Nd}}_{2}$${\mathrm{CuO}}_{4}$-type T\ensuremath{'} structure. Zero-field-cooled and field-cooled magnetization spectra indicate that the magnetic peak, previously ascribed by neutron diffraction in the ${\mathrm{Pr}}_{1.5}$${\mathrm{Ce}}_{0.5}$${\mathrm{Sr}}_{2}$${\mathrm{Cu}}_{2}$${\mathrm{NbO}}_{10}$ system as originating from an interlayer Cu spin reorientation, shifts towards lower temperature as the content of Ta increases. In addition, the effective Pr ion moment obtained by Curie-Weiss fits to the magnetization data taken at fields sufficiently high to saturate the weak ferromagnetic component indicate that the long-range antiferromagnetic ordering of the Pr ions in these compounds (\ensuremath{\sim}2.8${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$) is not affected by the substitution of Ta in for Nb. The nature of the complex magnetic behavior of these compounds originates from a thermal competition between magnetocrystalline anisotropy and magnetic exchange interactions. \textcopyright{} 1996 The American Physical Society.

Hollow spherical particles of Ni, CoFe, the ferromagnetic shape memory alloy Ni49Mn30Ga21, and the giant magnetostrictive alloy Fe83Ga17, with diameters up to several tens of microns were produced by spark erosion, using liquid nitrogen as the dielectric liquid. In contrast, the particles were primarily solid when the dielectric liquid was argon. The wall thicknesses of the hollow particles depended on the elemental composition. Different models are considered to account for the formation of the spark-eroded hollow spheres, and some of the potential benefits to be derived from their use are described.