High-pressure High-temperature Treatment Research Articles

High-pressure evolution of novel R2CuTiO6 (R = Tb – Lu) perovskite phases

Perovskite oxides of the R2CuTiO6 type with a trivalent rare earth element (R) at the A-cation site and co-occupancy of the B-cation site with the Jahn-Teller d9 Cu2+ and d0 Ti4+ transition metals, provides an interesting playground for various chemical and physical phenomena. However, conventional solid-state synthesis under ambient-pressure (AP) yields the perovskite structure only for the largest R constituents from La to Gd; with the smaller rare earth elements (Y, Tb–Lu) a non-perovskite (hexagonal BaTiO3-type) structure is formed. Here we demonstrate that through high-pressure (HP) high-temperature treatment, the hexagonal AP structure can be converted into a distorted (orthorhombic) perovskite structure for all the smaller R constituents. The critical pressure needed for the conversion increases with decreasing R3+ ion size, up to ca. 6 GPa for R = Lu. Moreover, a novel intermediate phase is found to form for most of the R constituents when pressures lower than the critical pressure are applied. We have employed both X-ray diffraction and UV–vis spectroscopy analyses to systematically follow the phase formation schemes for the different R constituents.

Densification and Surface Carbon Transformation of Diamond Powders under High Pressure and High Temperature.

A new type of poly-diamond plate without a catalyst was produced via the high-pressure high-temperature (HPHT) compression of diamond powders. The densification of diamond powders and sp3 to sp2 carbon on the surface under HPHT compression was investigated through the characterization of the microstructure, Raman spectroscopy analysis and electrical resistance measurement. The densification and sp3-sp2 transformation on the surface are mainly affected by the pressure, temperature and particle size. The quantitative analysis of the diamond sp3 and sp2 carbon amount was performed through the peak fitting of Raman spectra. It was found that finer diamond particles under a higher temperature and a lower pressure tend to produce more sp2 carbon; otherwise, they produce less. In addition, it is interesting to note that the local residual stresses measured using Raman spectra increase with the diamond particle size. The suspected reason is that the increased particle size reduces the number of contact points, resulting in a higher localized pressure at each contact point. The hypothesis was supported by finite element calculation. This study provides detailed and quantitative data about the densification of diamond powders and sp3 to sp2 transformation on the surface under HPHT treatment, which is valuable for the sintering of polycrystalline diamonds (PCDs) and the HPHT treatment of diamonds.

Spectroscopic characterization of yellow gem quality CVD diamond

CVD (chemical vapor deposition) diamond in the gem market today is dominated by colorless and pink material. However yellow CVD material is also found and it is concluded that all the yellow CVD diamonds found in the gem market have been post growth HPHT (high pressure high temperature) processed. This study shows that HPHT and LPHT (low pressure high temperature) treatments are essential for removal of the brown and gray color associated with as grown CVD material. The distinguishing temperatures and conditions of these treatments can be determined from the spectroscopic study of vacancy-related optical centers.

Novel High-Pressure Nanocomposites for Cathode Materials in Sodium Batteries.

A new nanocomposite material was prepared by high pressure processing of starting glass of nominal composition NaFePO4. Thermal, structural, electrical and dielectric properties of the prepared samples were studied by differential thermal analysis (DTA), X-ray diffraction (XRD) and broadband dielectric spectroscopy (BDS). It was demonstrated that high-pressure-high-temperature treatment (HPHT) led to an increase in the electrical conductivity of the initial glasses by two orders of magnitude. It was also shown that the observed effect was stronger than for the lithium analogue of this material studied by us earlier. The observed enhancement of conductivity was explained by Mott's theory of electron hopping, which is more frequent in samples after pressure treatment. The final composite consisted of nanocrystalline NASICON (sodium (Na) Super Ionic CONductor) and alluaudite phases, which are electrochemically active in potential cathode materials for Na batteries. Average dimensions of crystallites estimated from XRD studies were between 40 and 90 nm, depending on the phase. Some new aspects of local dielectric relaxations in studied materials were also discussed. It was shown that a combination of high pressures and BDS method is a powerful method to study relaxation processes and molecular movements in solids. It was also pointed out that high-pressure cathode materials may exhibit higher volumetric capacities compared with commercially used cathodes with carbon additions.

Two new cubic perovskite oxides Ba3CoSb2O9 and Ba2SrCoSb2O9: Syntheses, crystal structures and magnetic properties

Two new cubic perovskite oxides Ba3CoSb2O9 and Ba2SrCoSb2O9 were prepared and characterized. Ba3CoSb2O9 was prepared in polycrystalline form by high-pressure (HP) high-temperature treatment of the hexagonal 6H polytype of Ba3CoSb2O9. Polycrystalline samples of Ba2SrCoSb2O9 were obtained at ambient pressure by a solid-state reaction method. Combined Rietveld refinements of X-ray and neutron diffraction patterns indicated that HP Ba3CoSb2O9 and Ba2SrCoSb2O9 both crystallize in a Fm3¯m cubic double perovskite A2BB’O6 structure with formulae Ba2(Sb0.83Co0.17)(Co0.5Sb0.5)O6 and (Ba1.33Sr0.67)Sb(Co0.7Sb0.3)O6, respectively. Electron diffraction results obtained for Ba2SrCoSb2O9 showed no sign of additional periodicity. Weiss temperatures extracted from high-temperature magnetic susceptibility data were θ ​≈ ​−113 ​K and −118 ​K for HP Ba3CoSb2O9 and Ba2SrCoSb2O9, respectively. HP Ba3CoSb2O9 displays a weak ZFC-FC magnetic irreversibilty below 8 ​K. Ba2SrCoSb2O9 exhibits a spin freezing at Tf ​≈ ​6 ​K, and μSR and 121Sb NMR data revealed a magnetic transition towards a disordered ground state.

High-pressure stabilisation of R = Y member of R2CuTiO6 double perovskite series

Double perovskite oxides of the A2B’B″O6 type with Jahn-Teller active Cu2+ as the B′ constituent have gained considerable research interest in recent years. For fundamental studies, the rare earth element (R) based systems such as R2CuTiO6 form an intriguing research platform as they allow systematic chemical-pressure studies by simply controlling the size of the R constituent. However, for the R2CuTiO6 compounds conventional ambient-pressure high-temperature synthesis yields an orthorhombic (Pnma) double perovskite structure for the largest R constituents (La–Gd) only, while the compounds with the smaller R:s adopt a hexagonal structure. Here we demonstrate a hexagonal-to-perovskite structure conversion for the R ​= ​Y compound achieved through a high-pressure (HP) high-temperature treatment at 4 ​GPa and 1000 ​°C. Structural details of the thus stabilized new double perovskite phase of Y2CuTiO6 are addressed through a combined DFT simulation and Rietveld refinement study, revealing signs towards the rare layered-type ordering of the B-site (Cu and Ti) cations. Similar to the previously reported R2CuTiO6 perovskite phases with R ​= ​La, Pr, and Nd, the R ​= ​Y member is found paramagnetic throughout the measured temperature range of 5–300 ​K. From UV–vis absorption measurements the optical bandgap is estimated to be ca 3.4 ​eV.

Effect of high pressure-high temperature treatment on the microstructure and dielectric properties of cobalt doped СaСu3Тi4О12

The results of studying the effect of high pressures and high temperatures (8 GPa, 1273 K) used in the process of thermobaric treatment of CaCu3-xCoxTi4O12 ceramics (x = 0; 0.4; 0.6), initially prepared by solid-phase synthesis, on the structure and electrical characteristics are presented. The dielectric parameters were evaluated in a temperature interval (293–1000) K in a frequency range 1 Hz–32 MHz. It was established that the value of dielectric permittivity ε of samples upon thermobaric treatment is an order of magnitude higher than ε of samples prepared by solid-phase synthesis. The main reasons for the high values of dielectric permittivity are both the intragrain effects related, among other things, to hopping mechanism of polaron motion, and the polarization effects provided by the presence microstructure inhomogeneities. The latter are nonconducting boundaries of conducting grains and, in some examined materials, microscopic-scale surfaces of inhomogeneities, which appeared due to deformation during thermobaric treatment. At high temperatures, oxygen vacancies make the main contribution to the observed values of dielectric permittivity.

Study on Structural Evolution of Synthetic Graphite Derived from Lignite Prepared by High Temperature–High Pressure Method

Abundant and cheap lignite is regarded as inferior fuel due to its disadvantages such as low heat and high moisture. In order to realize the high value and clean, non-fuel utilization of lignite, we used lignite as precursor to prepare high-quality synthetic graphite through graphitization treatment using the high temperature–high pressure (HTHP) method, and afterwards characterized the samples by XRD, Raman, FTIR, SEM-EDS, and HRTEM, and systematically analyzed the effect of P-T conditions on the structure evolution of lignite. Our results demonstrated that temperature dominates the graphitization of lignite, and high pressure accelerates the graphitization process of lignite. Under HTHP treatment, the aromatic layer of lignite accelerates directional rearrangement; the ordered domain increases rapidly, and the ductility and stacking degree of the carbon sheets of the lignite sample are greatly enhanced. Compared with the traditional high-temperature graphitization method, the HTHP method greatly reduces the graphitization temperature and shortens the graphitization time. Remarkably, the as-prepared graphite with a graphitization degree of 91.87% superior to graphite fabricated by the conventional high-temperature processing were rapidly prepared from lignite at 6 GPa and 1300 °C in 20 min. This study demonstrates that the HTHP method is a feasible and effective method to realize the green, efficient, and high value utilization of lignite.

High pressure destruction kinetics of Clostridium botulinum (Group I, strain PA9508B) spores in milk at elevated temperatures

Milk inoculated with Clostridium botulinum spores (PA9508B) (107 CFU/mL) was subjected to high-pressure high-temperature treatment (HPHT) (700–900 MPa; 90–110 °C for 0–24 min) using an insulated sample chamber, as well as in a water bath (90–100 °C) (conventional) and surviving spores were enumerated. Destruction kinetics were well described by a first order kinetic model. Conventional heating D values ranged from 156 to 12 min with a z value of 7.8 °C. During HPHT, higher pressures and temperatures resulted in accelerated spore destruction rates. At 900 MPa, the associated D values were 14.5, 1.8 and 0.35 min at 90, 100 and 110 °C, respectively, much lower than the conventional D values. The pressure dependent ZT values were 11.2, 12.3, 12.4 °C at 700, 800, 900 MPa respectively, increasing with pressure. The temperature dependent ZP values were 470, 630, and 800 MPa at 90, 100, and 110 °C, respectively. By comparing ZT with ZP values, it was clear the spores were relatively more sensitive at higher temperatures than at higher pressures in this temperature-pressure range. D value trends beyond 120 °C indicated thermal inactivation was the principal mode of spore destruction and high pressure can protect spores under these conditions.

Studying the effects of high pressure–temperature treatment on the structure and immunoreactivity of β-lactoglobulin using experimental and computational methods

The effects of high hydrostatic pressure (HHP) on the conformation and immunoreactivity of bovine β-lactoglobulin (BLG) were studied. BLG was treated under 100–600 MPa at the temperature of 20–60 °C. The immunoglobulin E (IgE) binding ability of BLG decreased when the pressure increased from 0.1 to 200 MPa. However, the IgE binding increased with the increase in pressure from 200 to 400 MPa, followed by a gradual decrease until a pressure of 600 MPa. The IgE binding ability continuously decreased with an increase in pressure at 60 °C. The conformation of HHP-treated BLG was evaluated using fluorescence spectroscopy, circular dichroism spectroscopy and molecular dynamics (MD) simulation. Increasing the temperature and pressure promoted the unfolding of BLG, causing the disappearance of some α-helixes and some β-sheets. Based on ELISA analysis, it was revealed that HHP-termperature treatment altered the immunoreactivity of BLG by altering the structures and conformational states of BLG.

Preparation of CoGe2-type NiSn2 at 10 GPa

Abstract An unprecedented NiSn2 intermetallic with CoGe2-type crystal structure has been recovered (at ambient conditions) after high-pressure high-temperature treatment of a Ni33Sn67 precursor alloy at 10 GPa and 400 °C. The orthorhombic structure with Aeam space group symmetry is pseudotetragonal. Based on the evaluation of powder X-ray diffraction data, lattice parameters of a = b = 6.2818 Å and c = 11.8960 Å have been determined. Complicated line broadening and results of a further microstructure analysis, however, imply a defective character of the crystal structure. First-principles calculations with different model structures and a comparison with structural trends in the literature suggest that at the high-pressure high-temperature conditions a CuAl2-type crystal structure might be stable, which transforms to the recovered CoGe2-type crystal structure upon cooling or the release of pressure.

Spatial distribution of defects in a plastically deformed natural brown diamond

Photoluminescence, Raman mapping, cathodoluminescence and transmission electron microscopy (TEM) have been carried out on a “zebra” diamond, containing both brown and colourless bands. The stone was cut into two and one part was given high-pressure high temperature (HPHT) treatment, removing the brown colouration. The parts were then cut into (110) sections. In the untreated stone the morphology of brown stripes is consistent with that of slip bands formed during plastic deformation and Raman mapping shows they are under strong compressive stress. Photoluminescence from N3 and H3 centres, as well as lines at 406.3 nm, 491.3 nm and 535.9 nm, are correlated with brown bands in the untreated sample, while cathodoluminescence shows that band-A luminescence is anticorrelated. HPHT treatment reduces internal stress, and eliminates or reduces correlated luminescence. TEM reveals long straight dislocations and dislocation dipoles in the brown bands, consistent with deformation by slip and concurrent intrinsic point defect production, while clear bands have curved and tangled dislocation networks. We postulate that vacancies produced by plastic deformation aggregate into clusters responsible both for the brown colouration and an increase in volume that results in compressive stress. The 535.9 nm line has characteristics of an interstitial-type defect and may be formed by the trapping of interstitials generated during plastic deformation.

Core@shell nanocomposites Fe7C3 / FexOy /C obtained by high pressure-high temperature treatment of ferrocene Fe(C5H5)2

The core@shell nanostructures were obtained in the process of transformation of ferrocene Fe(C5H5)2 at high pressure (HP) of 8 GPa and high temperature (HT) of 900 °C with an isothermal exposure time t varying from 10 to 10000 s. At t > 300 s, the iron carbide o-Fe7C3 nanoparticles with an orthorhombic crystal structure (sp.gr. Pnma) can be created, which are dispersed in highly defective carbon matrix. After opening the high-pressure cell, a series of redox reactions occurs, leading to a formation of iron oxides on the surface of the iron carbide core. When the size of Fe7C3 nanoparticle is less than critical one the nanoparticle is fully oxidized, while in the larger particle an amorphous iron oxide shell is formed. A sequential increase in t initiates crystallization processes both in the iron carbide subsystem and in the carbon subsystem, resulting in the formation of core@shell Fe7C3/FexOy/C structures. Iron oxides with a cubic spinel-type structure (Fe3O4/γ-Fe2O3) appear in the shell. However, under oxygen reduction, part of magnetite can be transformed into wüstite FeO. The magnetic properties of magnetite and wüstite are radically different, and by varying the thickness of these layers, structures with the desired functional properties can be obtained.

Monoclinic semimetal IrSi synthesized under high pressure above 25 GPa: Crystal structure, electronic, and magnetic properties

Transition metal monosilicides exhibit diverse physical properties depending on their crystal structures. Regarding IrSi, only the MnP-type orthorhombic form has been known so far. In this paper, the authors report a successful synthesis of a monoclinic phase through high-pressure high-temperature treatment above 25 GPa, while the theoretically predicted B20-type cubic structure has not been identified up to 48 GPa. Magneto-transport measurements reveal a semimetallic nature of the monoclinic form with a low carrier density of ~1019 /cm${}^{3}$. Combined with large Rashba spin splitting of surface electronic bands, monoclinic IrSi may offer a potential material platform for versatile spintronic applications in the two-dimensional limit.

Effect of HPHT Treatment on Spectroscopic Features of Natural Type Ib-IaA Diamonds Containing Y Centers

In this paper, we report a spectroscopic study of natural type Ib-IaA diamonds containing Y centers subjected to high-pressure high-temperature treatment at 7–7.5 GPa and 1700–2200 °C. Diamond samples showing the Y centers as the dominant absorption feature in the infrared spectra were selected from a collection of natural diamonds from alluvial placers of the northeastern Siberian Platform. The samples were investigated by spectroscopic techniques before and after each annealing stage. It was found that upon annealing at temperatures higher than 2000°C, the defect-induced one-phonon spectra changed from the Y centers to a new form with a characteristic band peaking at 1060 cm−1. Photoluminescence spectra of the samples were modified after each annealing stage starting from 1700 °C. The most significant changes in photoluminescence occurred at temperatures higher than 2000 °C and were associated with a sharp increase of the intensity of an emission band peaking at about 690 nm. A comparison with natural red-luminescing diamonds from Yakutian kimberlite pipes was performed. It was concluded that the observed 1060 cm−1 IR band and the 690 nm red emission band are genetically related to the Y centers and that defects or impurities responsible for the Y centers appear quite widespread in natural diamonds from various deposits worldwide.

Effect of high temperature-high pressure treatment on microstructure and mechanical properties of Cu-Cr alloy

The microstructure of the Cu-Cr alloy prepared by infiltration before and after different pressures treatment at 900 °C for 10 min was observed by metallographic microscope, scanning electron microscope (SEM), and transmission electron microscopy (TEM). And the mechanical properties were measured by nanoindenter, hardness tester and tensile testing machine. According to the experimental results, the effects of high pressure treatment on the mechanical properties were discussed. The results showed that the hardness of Cu matrix and Cr phase were 114Hv and 194Hv under the pressure of 3 GPa, which were 18.75% and 10.23% higher than those of the infiltrated alloy. Also, the hardness and compression yield strength of Cu-Cr alloy were 134 HB and 241 MPa after 3 GPa pressure treatment, respectively, 11.67%, 19.31% higher than those of the infiltrated alloy. On the basis of analysis, the high pressure treatment can improve the compactness of Cu–Cr alloys, promote the precipitation of nanometer Cr particles and increase the hardness and elastic modulus of Cu matrix and Cr phase, resulting in the improvement of hardness and compression yield strength in Cu–Cr alloys. In the range of 1 ∼ 6 GPa, the hardness and compression yield strength of alloys increase with the increase of pressure. However, these mechanical properties of Cu-Cr alloy increases slowly when the pressure exceeds 1 GPa.

The melting curve of cobalt under high pressure

The new melting curve of cobalt (Co) was determined to be 12 GPa using the in situ high-pressure temperature measurement method (HPTM) in a high-volume cubic press using two melting criteria: first, plateaus in the temperature vs power functions in situ experiments, and second, the texture changes in the samples before and after high-pressure high-temperature (HP-HT) treatments with a scanning electron microscope (SEM). The slope of the melting point with increasing pressure was approximately 33 K/GPa in our experimental pressure range and progressively decreased under further compression. Our new melting curve of Co under high pressure is reasonably consistent with the simulation results using one-phase and two-phase calculations and has a similar trend with the reported melting curves of Fe and Ni.

Spectroscopic features of natural and HPHT-treated yellow diamonds

High pressure high temperature (HPHT) treatment has long been applied in the gem trade for changing the body colour of diamonds. The identification of HPHT-treated diamonds is a field of on-going research in gemological laboratories, as different parameters of treatment will result in either the creation or the destruction of a variety of lattice defects in diamonds. Some features that exist in treated diamonds can also be found in natural diamonds, and consequently must not be employed for the separation of treated and natural diamonds. In this research, we investigated the properties of 11 natural yellow diamonds (directly obtained from the Ekati Diamond Mine to ensure that they are untreated) before and after HPHT treatment, conducted at a temperature of 2100 °C and a pressure of 6 GPa for 10 min. We report spectroscopic data and fluorescence characteristics, collected using PL mapping, FTIR mapping and fluorescence imaging showing the distribution of lattice defects and internal growth structures. PL mapping indicates SiV defects exist in one of the nitrogen-rich natural diamonds prior to treatment. Silicon-related defects can also be created by HPHT treatment, and they seem to show a relationship with pre-existing NV− centres. SIMS analysis was conducted to confirm the presence of silicon in these diamonds. The increase in the hydrogen-related infrared absorption peak at 3107 cm−1 (VN3H) is very strong in some diamonds that do not form B-centres during treatment. NVH was observed in our HPHT-treated natural diamonds, so it is possible that this strong increase in VN3H suppresses the aggregation of A- to B-centres as the newly formed A-centres were captured by NVH lattice defects to form VN3H. HPHT-altered and HPHT-induced platelet peaks are different from their natural counterparts in peak width and shape. Strong green fluorescence over a large area of a diamond, which is linked to relatively high concentration of H3 centres, was produced after HPHT treatment. We are confident that the unusual platelet peaks and strong emission of H3 centres are reliable indicators for HPHT-treated diamonds as they are not observed in untreated natural diamonds.

Room temperature optical thermometry based on the luminescence of the SiV defects in diamond-=SUP=-*-=/SUP=-

AbstractDiamond microcrystals containing silicon-vacancy (SiV) defects were synthesized by using a high-pressure high-temperature treatment of a mixture of pertinent organic-inorganic precursors. Photoluminescence of the SiV defects and its temperature dependence (80–400 K) was studied. A strong sharp zero-phonon line (ZPL) at 738 nm was recorded at all temperatures under 488 nm laser excitation. In particular the thermally induced shift of the ZPL was found promising for optical temperature sensing in the near infrared spectral range at biomedically relevant temperatures.

Room Temperature Optical Thermometry Based on the Luminescence of the SiV Defects in Diamond

Diamond microcrystals containing silicon-vacancy (SiV) defects were synthesized by using a high-pressure high-temperature treatment of a mixture of pertinent organic-inorganic precursors. Photoluminescence of the SiV defects and its temperature dependence (80–400 K) was studied. A strong sharp zero-phonon line (ZPL) at 738 nm was recorded at all temperatures under 488 nm laser excitation. In particular the thermally induced shift of the ZPL was found promising for optical temperature sensing in the near infrared spectral range at biomedically relevant temperatures.