Recension of boron nitride phase diagram based on high-pressure and high-temperature experiments

Cubic boron nitride and hexagonal boron nitride are the two predominant crystalline structures of boron nitride.They can interconvert under varying pressure and temperature conditions.However,this transformation requires overcoming significant potential barriers in dynamics,which poses great difficulty in determining the c-BN/h-BN phase boundary.This study used high-pressure in situ differential thermal measurements to ascertain the temperature of h-BN/c-BN conversion within the commonly used pressure range(3-6 GPa)for the industrial synthesis of c-BN to constrain the P-T phase boundary of h-BN/c-BN in the pressure-temperature range as much as possible.Based on the analysis of the experimental data,it is determined that the relationship between pressure and temperature conforms to the following equation:P=a+1/bT.Here,P denotes the pressure(GPa)and T is the temperature(K).The coefficients are a=-3.8±0.8 GPa and b=229.8±17.1 GPa/K.These findings call into question existing high-pressure and high-temperature phase diagrams of boron nitride,which seem to overstate the phase boundary temperature between c-BN and h-BN.The BN phase diagram obtained from this study can provide critical temperature and pressure condition guidance for the industrial synthesis of c-BN,thus optimizing synthesis efficiency and product performance.

Gas encapsulation technology for large volume press

For samples in the gaseous state at room temperature and ambient pressure,mature technology has been developed to encapsulate them in a diamond anvil cell(DAC).However,the large volume press(LVP)can only treat samples with starting materials in solid or liquid form.We have achieved stable encapsulation and reaction treatment of carbon dioxide in a centimeter sized sample chamber for a long time(over 10 min)under conditions of temperature higher than 1200C and pressure over 5 GPa through the use of integrated low-temperature freezing and rapid compression sealing method for LVP cell assemblies.This technology can also be applied to the packaging of other gaseous or liquid samples,such as ammonia,sulfur dioxide,water,etc.in LVP devices.

Strength enhancement of nanocrystalline tungsten under high pressure

Three tungsten powder samples—one coarse grained(c-W;grain size:1μm–3μm)and two nanocrystalline(n-W;average grain sizes:10nm and 50 nm)—are investigated under nonhydrostatic compression in a diamond anvil cell in separate experiments,and their in situ X-ray diffraction patterns are recorded.The maximum microscopic deviatoric stress in each tungsten sample,a measure of the yield strength,is determined by analyzing the diffraction line width.Over the entire pressure range,the strength of tungsten increases noticeably as the grain size is decreased from 1μm–3μmto 10 nm.The results show that the yield strength of tungsten with an average crystal size of 10nmis around 3.5 times that of the sample with a grain size of 1μm–3μm.

Pressure-induced disordering of site occupation in iron–nickel nitrides

Controlled disordering of substitutional and interstitial site occupation at high pressure can lead to important changes in the structural and physical properties of iron–nickel nitrides.Despite important progress that has been achieved,structural characterization of ternary Fe–Ni–N compounds remains an open problem owing to the considerable technical challenges faced by current synthetic and structural approaches for fabrication of bulk ternary nitrides.Here,iron–nickel nitride samples are synthesized as spherical-like bulk materials through a novel highpressure solid-state metathesis reaction.By employing a wide array of techniques,namely,neutron powder diffraction,Rietveld refinement methods combined with synchrotron radiation angle-dispersive x-ray diffraction,scanning electron microscopy/energy dispersive x-ray spectroscopy,and transmission electron microscopy,we demonstrate that high-temperature and high-pressure confinement conditions favor substitutional and interstitial site disordering in ternary iron–nickel nitrides.In addition,the effects of interstitial nitrogen atoms and disorderly substituted nickel atoms on the elastic properties of the materials are discussed.

Equal compressibility structural phase transition of molybdenum at high pressure

We have studied the high-pressure compression behavior of molybdenum up to 60 GPa by synchrotron radial x-ray diffraction(RXRD)in a diamond anvil cell(DAC).It is found that all diffraction peaks of molybdenum undergo a split at around 27 GPa,and we believe that a phase transition from a body-centered cubic structure to a rhombohedral structure at room pressure has occurred.The slope of pressure–volume curve shows continuity before and after this phase transition,when fitting the pressure–volume curves of the body-centered cubic structure at low pressure and the rhombohedral structure at high pressure.A bulk modulus of 261.3(2.7)GPa and a first-order derivative of the bulk modulus of 4.15(0.14)are obtained by using the nonhydrostatic compression data at the angleψ=54.7°between the diffracting plane normal and stress axis.

Evidence for a High-Pressure Isostructural Transition in Nitrogen

We observed an isostructural phase transition in the solid nitrogen λ-N_(2) at approximately 50 GPa accompanied by anomalies in lattice parameters,atomic volume and Raman vibron modes.The anomalies are ascribed to a slight reorientation of the nitrogen molecules,which does not seem to affect the monoclinic symmetry(space group P2_(1)/c).Our ab initio calculations further confirm the phenomena,and suggest an optimized structure for the λ-N_(2) phase.In addition,a new high-pressure amorphous phase of η′-N_(2) was also discovered by a detailed investigation of the pressure-temperature phase diagram of nitrogen with the aim of probing the phase stability of λ-N_(2).Our result may provide helpful information about the crystallographic nature of dissociation transitions in diatomic molecular crystals(H_(2),O_(2),N_(2),etc).

Synthesis of novel superhard materials under ultrahigh pressure

Superhard materials are solids whose Vickers hardness is beyond 40 GPa. They have wide applications in industry such as cutting and polishing tools, wear-resistant coatings. Most preparations of superhard materials are conducted under extreme pressure and temperature conditions, not only for scientific investigations, but also for the practical applications. In this paper, we would introduce the recent progress on the design and preparations of novel superhard materials, mainly on nanopolycrystalline diamond, B–C–N superhard solid solutions, and cubic-Si3N4/diamond nanocomposites prepared under ultrahigh pressure and high temperature(HPHT), using multi-anvil apparatus based on the hinged-type cubic press. Bulk materials of all these superhard phases have been successfully synthesized and are systematically tested. We emphasize that ultra-HPHT method plays an important role in the scientific research and industrial production of superhard materials. It provides the driving forces for the light elements forming novel superhard phases as well as the way for sintering high-density nanosuperhard materials.

Is the hardness of material harder than diamond reliable?

With the development of new synthesis methods and chemistries,a number of new superhard materials have been reported to be harder than diamond.While such materials are highly desirable due to their wide-ranging applications,there are some inherent uncertainties in the methods utilized to determine and define the hardness of such materials.In this paper,we employed the standard Vickers diamond indenter and substitute indenters with the same shape to measure the hardness of nine ceramics and superhard materials within well-defined criteria and methodology,for the assessment of consistency in the hardness testing.The findings and the developed testing method in the current study have broad implications in characterizing new and emerging superhard materials,leading to new discoveries.