High-pressure High-temperature Technique Research Articles

Basic characterization of three kinds of bio-asphalt and research on the low-temperature performance of bio-asphalt mixtures

Bio-oil is a by-product of waste biomass treatment using high-pressure liquefaction, bio-enzymatic, or high-temperature thermal cracking techniques, and can alleviate the shortage of petroleum asphalt resources. This study investigated the production of bio-oil from selected raw materials, including corn stover, castor oil from castor beans, and epoxy vegetable oil from soybeans for the preparation of bio-asphalt. The properties and mechanisms of modification of the three modified bio-asphalts were explored by assessment of basic physical properties, gel permeation chromatography (GPC), temperature scanning (DSR), and thermogravimetric analysis, as well as Semi-Circular Bending (SCB), low-temperature bending, and fatigue tests. Bio-oil was found to improve the low-temperature cracking resistance of the asphalt mixtures, with the improvement further enhanced by changing the blending amount. The bio-oil was observed to negatively affect the low-temperature fatigue performance of the mixes, and the decrease in the n-value indicated that the bio-oil could make the asphalt mixtures less stress-sensitive under repeated loading, with improved anti-fatigue performance of bio-asphalt mixtures observed when the stress ratio was 0.5. Finally, the low-temperature cracking performance index of the corn stover bio-asphalt mixtures was correlated with the gray entropy. The use of the SCB fracture index was found to be more appropriate when using the semicircular bending test to characterize the low-temperature cracking performance of bio-asphalt mixtures.

Ternary B–C–N compounds layered materials with regulated electronic properties and ultrawide bandgaps

The exploration of novel ultrawide bandgap (UWBG) semiconductors is becoming a challenging and compelling research focus on semiconductor physics, materials, and device applications. Ternary B–C–N compounds have attracted much attention because their electronic structure and semiconductor properties are quite different depending on the chemical composition and atomic arrangement of boron, carbon, and nitrogen elements in the lattice. However, the lack of well-controlled high-quality B–C–N crystals has limited their potential as UWBG devices. In this study, B–C–N compounds are synthesized in bulks from graphite and hexagonal boron nitride (h-BN) using ball milling and high-pressure high temperature technique. The synthesized B–C–N compounds produced are highly crystallized layered-materials with intercalated graphene layers in C-doped h-BN layers. The doped carbon atoms occupy boron sites and nitrogen sites of the h-BN layers unbalanced, giving rise to the n-type conductivity of the B-C-N layered compounds. The measured optical bandgaps range from 3.4 to 6.0 eV, which can be regulated by the carbon content. Their electronic properties are also tunable. Our work is expected to initiate potential applications of the B–C–N material as UWBG semiconductors.

Improving the Performance of HPHT-Diamond Detectors for Pulsed X-Ray Dosimetry Using the Synchronous Detection Technique

A single-crystal diamond sample grown by high-pressure high-temperature (HPHT) technique was used for the fabrication of a metal–semiconductor–metal (MSM) photoconductor. The sample quality was evaluated by means of spectral photocurrent measurements highlighting the presence of a significant density of defect states within the diamond bandgap, responsible for trap-mediated conduction mechanisms. The photoconductor was fully characterized under 6 MeV pulsed X-rays, sourced by a medical linear accelerator (LINAC), in the 0.05–20 Gy and 0.017–0.100 Gy/s dose (D) and dose-rate (DR) ranges, respectively. Photocurrent measurements performed with a conventional precision electrometer showed that the detector performance is strongly affected by charge-trapping phenomena, resulting in a sublinear dependence with the DR (power-law dependence with an exponent of 0.86). Measurements were repeated in the same experimental conditions by coupling the detector to a specifically developed gated integrator, allowing for a synchronous integration of photocurrent pulses (limited to 40 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <inline-formula> <tex-math notation="LaTeX">$\mu$</tex-math> </inline-formula> </i> s) centered on each impinging X-ray pulse. In this case, the detector photoresponse showed an excellent linearity with both the D and the DR (power-law dependence with 1.0009 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pm$</tex-math> </inline-formula> 0.0004 and 1.009 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pm$</tex-math> </inline-formula> 0.005 exponents, respectively). Significantly, the proposed synchronous integration technique is then able to mitigate the detrimental effect that defects have on detector performance. The proposed method paves, therefore, the way to the exploitation of HPHT diamond as a low-cost alternative to single-crystal chemical vapor deposition (CVD)-diamond for the fabrication of accurate and linear X-ray dosimeters.

Conventional High-Temperature Superconductivity in Metallic, Covalently Bonded, Binary-Guest C-B Clathrates.

Inspired by the synthesis of XB3C3 (X = Sr, La) compounds in the bipartite sodalite clathrate structure, density functional theory (DFT) calculations are performed on members of this family containing up to two different metal atoms. A DFT-chemical pressure analysis on systems with X = Mg, Ca, Sr, Ba reveals that the size of the metal cation, which can be tuned to stabilize the B-C framework, is key for their ambient-pressure dynamic stability. High-throughput density functional theory calculations on 105 Pm3̅ symmetry XYB6C6 binary-guest compounds (where X, Y are electropositive metal atoms) find 22 that are dynamically stable at 1 atm, expanding the number of potentially synthesizable phases by 19 (18 metals and 1 insulator). The density of states at the Fermi level and superconducting critical temperature, Tc, can be tuned by changing the average oxidation state of the metal atoms, with Tc being highest for an average valence of +1.5. KPbB6C6, with an ambient-pressure Eliashberg Tc of 88 K, is predicted to possess the highest Tc among the studied Pm3̅n XB3C3 or Pm3̅ XYB6C6 phases, and calculations suggest it may be synthesized using high-pressure high-temperature techniques and then quenched to ambient conditions.

Preparation and characterization of sintered polycrystalline diamond (PCD) with 15 wt% Nb binder

This study aimed to obtain and characterize a polycrystalline diamond body (PCD) sintered with 15 wt% Nb binder, through the high-pressure high-temperature (HPHT) sintering techniques at three different temperatures of 1650 °C, 1750 °C, and 1850 °C, with a pressure of 7.7 GPa. Characterizations on the starting powders were performed before and after high energy ball milling, and then the obtained mixture was sintered. The results showed that niobium could perform as a good binder for the sintering PCD at the specified concentration, achieving relative densities in the range of 96% to 100% for all three sintering temperatures. Also, analyses of the electron micrographs and the Raman spectrograms showed that there was no graphitization, a common problem in this type of material, and there was uniform dispersion of the niobium at the interfaces of the diamond particles, and no cracking.

Deep-ultraviolet near band-edge emissions from nano-polycrystalline diamond

ABSTRACTDeep-ultraviolet continuous-wave photoluminescence spectroscopy is performed for nano-polycrystalline diamond (NPD) synthesized by a high pressure high-temperature technique. NPD exhibits clear deep-ultraviolet emissions, which originate from intrinsic excitonic transitions assisted by a momentum-conserving phonon with a photon. Surprisingly, the peak emission energy is about 30 meV higher than that of the single-crystalline diamond. Raman scattering spectroscopy indicates that the energy difference should originate from the excitonic properties of the NPD and not the phonon. Hence, NPD has a large bandgap compared to single-crystalline diamond.

Thermal conductivity of high purity synthetic single crystal diamonds

Thermal conductivity of three high purity synthetic single crystalline diamonds has been measured with high accuracy at temperatures from 6 to 410 K. The crystals grown by chemical vapor deposition and by high-pressure high-temperature technique demonstrate almost identical temperature dependencies $\ensuremath{\kappa}(T)$ and high values of thermal conductivity, up to 24 $\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ at room temperature. At conductivity maximum near 63 K, the magnitude of thermal conductivity reaches 285 $\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$, the highest value ever measured for diamonds with the natural carbon isotope composition. Experimental data were fitted with the classical Callaway model for the lattice thermal conductivity. A set of expressions for the anharmonic phonon scattering processes (normal and umklapp) has been proposed which gives an excellent fit to the experimental $\ensuremath{\kappa}(T)$ data over almost the whole temperature range explored. The model provides the strong isotope effect, nearly 45%, and the high thermal conductivity ($&gt;24$ $\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$) for the defect-free diamond with the natural isotopic abundance at room temperature.

Effects of High-Pressure High-Temperature Sintering on the Band Gap and Thermoelectric Properties of PbSe

In this study, PbSe bulk samples were prepared by a high-pressure high-temperature (HPHT) sintering technique, and the phase compositions, band gaps and thermoelectric properties of the samples were systematically investigated. The sintering pressure exerts a significant influence on the preferential orientation, band gap and thermoelectric properties of PbSe. With increasing pressure, the preferential orientation decreases, mainly due to the decreased crystallinity, while the band gap first decreases and then increases. The electrical conductivity and power factor decrease gradually with increasing pressure, mainly attributed to the decreased carrier concentration and mobility. Consequently, the sample prepared by 2 GPa shows the highest thermoelectric figure-of-merit, ZT, of 0.55 at ∼ 475 K. The ZT of the HPHT-sintered PbSe could be further improved by properly doping or optimizing the HPHT parameters. This study further demonstrates that the sintering pressure could be another degree of freedom to manipulate the band structure and thermoelectric properties of materials.

Vacancy–impurity centers in diamond: prospects for synthesis and applications

The bright luminescence of impurity–vacancy complexes, combined with high chemical and radiation resistance, makes diamond an attractive platform for the production of single-photon emitters and luminescent biomarkers for applications in nanoelectronics and medicine. Two representatives of this kind of defects in diamond, silicon-vacancy (SiV) and germanium-vacancy (GeV) centers, are discussed in this review; their similarities and differences are demonstrated in terms of the more thoroughly studied nitrogen-vacancy (NV) complexes. The recent discovery of GeV luminescent centers opens a unique opportunity for the controlled synthesis of single-photon emitters in nanodiamonds. We demonstrate prospects for the high-pressure high-temperature (HPHT) technique to create single-photon emitters, not only as an auxiliary to chemical vapor deposition (CVD) and ion-implantation methods but also as a primary synthesis tool for producing color centers in nanodiamonds. Besides practical applications, comparative studies of these two complexes, which belong to the same structural class of defects, have a fundamental importance for deeper understanding of shelving levels, the electronic structure, and optical properties of these centers. In conclusion, we discuss several open problems regarding the structure, charge state, and practical application of these centers, which still require a solution.

Disorder and defects are not intrinsic to boron carbide.

A unique combination of useful properties in boron-carbide, such as extreme hardness, excellent fracture toughness, a low density, a high melting point, thermoelectricity, semi-conducting behavior, catalytic activity and a remarkably good chemical stability, makes it an ideal material for a wide range of technological applications. Explaining these properties in terms of chemical bonding has remained a major challenge in boron chemistry. Here we report the synthesis of fully ordered, stoichiometric boron-carbide B13C2 by high-pressure–high-temperature techniques. Our experimental electron-density study using high-resolution single-crystal synchrotron X-ray diffraction data conclusively demonstrates that disorder and defects are not intrinsic to boron carbide, contrary to what was hitherto supposed. A detailed analysis of the electron density distribution reveals charge transfer between structural units in B13C2 and a new type of electron-deficient bond with formally unpaired electrons on the C–B–C group in B13C2. Unprecedented bonding features contribute to the fundamental chemistry and materials science of boron compounds that is of great interest for understanding structure-property relationships and development of novel functional materials.

Parabolic single-crystal diamond lenses for coherent x-ray imaging

We demonstrate parabolic single-crystal diamond compound refractive lenses designed for coherent x-ray imaging resilient to extreme thermal and radiation loading expected from next generation light sources. To ensure the preservation of coherence and resilience, the lenses are manufactured from the highest-quality single-crystalline synthetic diamond material grown by a high-pressure high-temperature technique. Picosecond laser milling is applied to machine lenses to parabolic shapes with a ≃1 μm precision and surface roughness. A compound refractive lens comprised of six lenses with a radius of curvature R=200 μm at the vertex of the parabola and a geometrical aperture A=900 μm focuses 10 keV x-ray photons from an undulator source at the Advanced Photon Source facility to a focal spot size of ≃20×90 μm2 with a gain factor of ≃50−100.

Y3Pt4Ge13: A superconductor with a noncentrosymmetric crystal structure

The intermetallic compound Y${}_{3}$Pt${}_{4}$Ge${}_{13}$ has been synthesized by high-pressure high-temperature technique. It crystalizes with its own noncentrosymmetric type of crystal structure in the monoclinic space group $Cc$ and is superconducting below ${T}_{c}=4.5$ K. Thermodynamic properties as well as transport measurements were performed down to 0.4 K. The upper critical field is ${\ensuremath{\mu}}_{0}{H}_{c2}=3.8$ T, the lower one only ${\ensuremath{\mu}}_{0}{H}_{c1}=5$ mT. All observations are consistent with an $s$-wave symmetry single-energy gap for the superconducting phase.

The crystal structure of aluminum doped β-rhombohedral boron

A crystal structure of aluminum doped β-rhombohedral boron was studied by single-crystal X-ray diffraction at 80K. The crystals were synthesized using high-pressure high temperature technique at 3GPa and 2100K. The structure is based on three-dimensional framework made of B12 icosahedra with voids occupied by the B28–B–B28 units, it has the R-3m space group with a=10.9014(3), c=23.7225(7) Å lattice dimensions in hexagonal setting. Aluminum atoms are located in A1 and D special positions of the β-B structure with occupancies of 82.7(6)% and 11.3(4)%, respectively. Additional boron atoms are located near the D-site. Their possible distribution is discussed. Finally we have found two appropriate structural models whose refinement suggests two possible chemical compositions, AlB44.8(5) and AlB37.8(5), which are in a good agreement with the chemical analysis data obtained from EDX. The crystal structure of AlB44.8(5) is described in detail.

Magnetic and Thermal Behavior of Ru0.9Sr2YCu2.1O7.9 Magneto-Superconductor Synthesized by High-Pressure High-Temperature Technique

The structural, physical, and thermal property details of Ru0.9Sr2YCu2.1O7.9 (Y/Ru-1212) superconducting material synthesized through high pressure (6 GPa) and a high temperature (1400 °C) (HPHT) route are reported here. (Y/Ru-1212) crystallizes in P4/mmm tetragonal structure and is found free of any detectable impurities through X-ray diffractometry. Ru-spins are ordered magnetically above 145 K, with a sizeable ferromagnetic component at 5 K. Further clear diamagnetic transitions are seen in both zero-field-cooled (ZFC) and field-cooled (FC) magnetic susceptibility measurements and exhibiting superconductivity below 50 K. Both the thermoelectric power (S) and thermal conductivity (κ) measurements show superconductivity onset below 50 K with S=0 at 30 K and a broad hump in heat capacity Cp(T) below 30 K. Heat capacity (Cp) measurements also exhibit the magnetic ordering temperature as a hump below 145 K. The appearance of a hump in Cp(T), instead of a clear transition, is indicative of short range magnetic correlations like spin glass (SG). Neither the high (145 K) nor the low (30 K) temperature humps of Cp(T) could be analyzed quantitatively because of short magnetic correlations in former and mixing of both superconductivity and FM components in a later case. The observed data is compared with various reported Ru-1212 systems synthesized under normal pressure conditions. It is concluded that HPHT synthesized Y/Ru-1212 is a bulk superconductor below 30 K with a substantial FM component.

Superconductivity in diamond

We survey methods of synthesis of boron doped diamond with high pressure-high temperature techniques. New route is proposed for synthesis of relatively large heavily boron doped diamond single crystals, which exhibit superconductivity. Superconducting boron-doped diamond samples were synthesized with isotopes of 10B, 11B, 13C and 12C. We claim the presence of a carbon isotope effect on the superconducting transition temperature, which supports the “diamond–carbon”-related nature of superconductivity and the importance of the electron–phonon interaction as the mechanism of superconductivity in diamond. Isotope substitution permits us to relate almost all bands in the Raman spectra of heavily boron-doped diamond to the vibrations of carbon atoms. The 500 cm − 1 Raman band shifts with either carbon or boron isotope substitution and may be associated with vibrations of paired or clustered boron.

Growth and Characterization of Bismuth Magnesium Titanate Bi(Mg&lt;sub&gt;1/2&lt;/sub&gt;Ti&lt;sub&gt;1/2&lt;/sub&gt;)O&lt;sub&gt;3&lt;/sub&gt;

A new oxide, Bi(Mg1/2Ti1/2)O3 was prepared using a high-pressure high-temperature technique at 8 GPa and 1000°C. As-sintered Bi(Mg1/2Ti1/2)O3 has the BiFeO3-type perovskite structure with the rhombohedral R3c. After ground, crystal structure changed to tetragonal P4mm. Structure parameters of tetragonal Bi(Mg1/2Ti1/2)O3 were refined from laboratory X-ray powder diffraction data (a = 4.033(10) Å, and c = 3.938(10) Å). Bi(Mg1/2Ti1/2)O3 does not decompress at ambient pressure, but the structure changed from rhombohedral to tetragonal on heating above 800°C. This phase transition was nonreversible. Bi(Mg1/2Ti1/2)O3 is expected to be one of the end members of lead-free ferroelectric materials.

Homologous Series of SrCoO(3n−1)/n Perovskites Obtained Through Br2 Oxygenation of SrCoO2.5

Starting from brownmillerite SrCoO2.50, a series of SrCoO3−δ samples with different oxygen contents up to 3 − δ = 2.87 was successfully prepared using Br2 in CH3CN/H2O medium as an oxidant. Six different phases, including the precursor phase, were identified depending on the oxygen content. Four of the phases have been known from earlier studies, that is, SrCoO2.50, SrCoO2.75, SrCoO2.80, and SrCoO2.87 (the latter three obtained through electrochemical and high-pressure high-temperature oxidation techniques) representing, respectively, the n = 2, 4, 5, and 8 members of the SrCoO(3n−1)/n homologous series, whereas signatures for the n = 6 and 7 members were revealed here for the first time.

High-pressure Synthesis and Physical Characterization of Y-based 1222-type Niobio–Cuprate NbSr2(Y1.5Ce0.5)Cu2O10

We have successfully synthesized a new 1222-type Y-based niobio-cuprate for the first time with the chemical composition, NbSr2(Y1.5Ce0.5)Cu2O10 by a high-pressure and high-temperature (HPHT) technique under different synthesis conditions. Good quality (nearly single-phase) Nb1222Y polycrystalline sample was synthesized at 1,450 ○C and 6 GPa for 2 h. It crystallizes in a tetragonal symmetry of space group I4/mmm with lattice parameters a=b=3.863(2) A and c=28.687(5) A, which is isostructural with the corresponding Ru1222Y (RuSr2(Y1.5Ce0.5)Cu2O10) phase. The dc-magnetic susceptibility measurements show a mixed magnetic state at ∼29 K, which is apparently associated with magneto-superconductivity. The magnetic moment (M) versus field (H) curve of Nb1222Y sample measured at 1.8 K looks like a hybrid and it normally ensembles for both superconducting and a ferromagnetic curve. As the field increases, a low field diamagnetic signal emerges. However, temperature dependence of electrical resistivity of Nb1222Y did not show superconductivity, but semiconducting nature with a relatively large magnetoresistance (∼6% at almost entire region). No specific heat anomaly was observed at the vicinity of the transition temperature.

Properties of graphene produced by the high pressure–high temperature growth process

The authors report on a new method for the synthesis of graphene, a mono-layer of carbon atoms arranged in a honey comb lattice, and the assessment of the properties of obtained graphene layers using micro-Raman characterisation. Graphene was produced by a high pressure–high temperature (HPHT) growth process from the natural graphitic source material by utilising the molten Fe–Ni catalysts for dissolution of carbon. The resulting large-area graphene flakes were transferred to the silicon–silicon oxide substrates for the spectroscopic micro-Raman and scanning electron microscopy inspection. The analysis of the G peak, D, T+D and 2D bands in the Raman spectra under the 488 nm laser excitation indicate that the HPHT technique is capable of producing high-quality large-area single-layer graphene with a low defect density. The disorder-induced D peak ∼1359 cm−1 while very strong in the initial graphitic material is completely absent in the graphene layers. The proposed method may lead to a more reliable graphene synthesis and facilitate its purification and chemical doping.

High-pressure high-temperature synthesis and magnetic properties of ordered perovskite Sr2Cu(Re0.69−xWxCa0.31)O6 (0⩽x⩽0.6)

We report the synthesis and magnetic properties of polycrystalline Sr2Cu(Re0.69−xWxCa0.31)O6 samples with x=0, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6, which were prepared by high-pressure high-temperature synthesis technique. These phases had ordered cubic perovskite structures. The pristine compound (x=0) underwent a ferromagnetic (ferrimagnetic) transition with TC∼460K. The ferromagnetism was suppressed by the W substitution and disappears almost completely for x=0.6. Mechanism of the suppression of the magnetism is discussed assuming a localized electron system.