Qiwei Hu

Diamond-cBN alloy: A universal cutting material

Diamond and cubic boron nitride (cBN) as conventional superhard materials have found widespread industrial applications, but both have inherent limitations. Diamond is not suitable for high-speed cutting of ferrous materials due to its poor chemical inertness, while cBN is only about half as hard as diamond. Because of their affinity in structural lattices and covalent bonding character, diamond and cBN could form alloys that can potentially fill the performance gap. However, the idea has never been demonstrated because samples obtained in the previous studies were too small to be tested for their practical performance. Here, we report the synthesis and characterization of transparent bulk diamond-cBN alloy compacts whose diameters (3 mm) are sufficiently large for them to be processed into cutting tools. The testing results show that the diamond-cBN alloy has superior chemical inertness over polycrystalline diamond and higher hardness than single crystal cBN. High-speed cutting tests on hardened steel and granite suggest that diamond-cBN alloy is indeed a universal cutting material

Stress control of heterogeneous nanocrystalline diamond sphere through pressure-temperature tuning

The hollow nanocrystalline diamond (NCD) sphere, a promising ablator material for inertial confinement fusion capsule, is generally fabricated by the chemical vapor deposition method. Herein, we report on a method to transform hydrogenated tetrahedral amorphous carbon coatings on spherical molybdenum (Mo) substrates into nanocrystalline diamond films via a designed high pressure high temperature (HPHT) treatment that balances the mismatch in the thermal expansion coefficient between a diamond coating and the Mo substrate through the difference in the bulk modulus. The results show that the density and strength of the diamond shell increase significantly and the residual stress is eliminated as well. The methodology of the designed HPHT treatment can not only provide an alternative way to fabricate NCD spheres but also can apply to other heterogeneous material stress control applications.

Micro-stress dominant displacive reconstructive transition in lithium aluminate

It is supposed that diffusive reconstructive transitions usually take place under hydrostatic pressure or low stresses, and displacive reconstructive phase transitions easily occur at nonhydrostatic pressure. Here, by in-situ high pressure synchrotron X-ray diffraction and single-crystal Raman scattering studies on lithium aluminate at room temperature, we show that the reconstructive transition mechanism is dependent on the internal microscopic stresses rather than the macroscopic stresses. In this case, even hydrostatic pressure can favor the displacive transition if the compressibility of crystal is anisotropic. During hydrostatic compression, γ-LiAlO2 transforms to δ-LiAlO2 at about 4 GPa, which is much lower than that in previous nonhydrostatic experiments (above 9 GPa). In the region where both phases coexist, there are enormous microscopic stresses stemming from the lattice mismatch, suggesting that this transition is displacive. Furthermore, the atomic picture is drawn with the help of the shear R...

In situ high-pressure measurement of crystal solubility by using neutron diffraction

Crystal solubility is one of the most important thermo-physical properties and plays a key role in industrial applications, fundamental science, and geoscientific research. However, high-pressure in situ measurements of crystal solubility remain very challenging. Here, we present a method involving high-pressure neutron diffraction for making high-precision in situ measurements of crystal solubility as a function of pressure over a wide range of pressures. For these experiments, we designed a piston-cylinder cell with a large chamber volume for high-pressure neutron diffraction. The solution pressures are continuously monitored in situ based on the equation of state of the sample crystal. The solubility at a high pressure can be obtained by applying a Rietveld quantitative multiphase analysis. To evaluate the proposed method, we measured the high-pressure solubility of NaCl in water up to 610 MPa. At a low pressure, the results are consistent with the previous results measured ex situ. At a higher pressure, more reliable data could be provided by using an in situ high-pressure neutron diffraction method.

Strengthening effects of interstitial nitrogen on rhenium

Introducing interstitial nitrogen atoms into the rhenium lattices is accepted for enhancing the mechanical and thermal properties of the metal. Here, we present an experimental study on the strengthening effects of interstitial nitrogen in rhenium on bulk modulus B0 and elastic shear constant C44 by comparing the difference in the compression and vibration behaviors between Re3N and Re in the in situ high pressure angle dispersive X-ray diffraction and high-pressure Raman scattering. The incorporation of interstitial N atoms introduces the covalent bonding and the special Z-type structure along the c-axis, which caused the bulk modulus B0 and elastic shear constant C44 increase by 14% and 15%, and the incompressibility of the c-axis is obviously enhanced. In addition, this work well explains the strengthening effects of interstitial N atoms on mechanical and vibrational properties.Introducing interstitial nitrogen atoms into the rhenium lattices is accepted for enhancing the mechanical and thermal properties of the metal. Here, we present an experimental study on the strengthening effects of interstitial nitrogen in rhenium on bulk modulus B0 and elastic shear constant C44 by comparing the difference in the compression and vibration behaviors between Re3N and Re in the in situ high pressure angle dispersive X-ray diffraction and high-pressure Raman scattering. The incorporation of interstitial N atoms introduces the covalent bonding and the special Z-type structure along the c-axis, which caused the bulk modulus B0 and elastic shear constant C44 increase by 14% and 15%, and the incompressibility of the c-axis is obviously enhanced. In addition, this work well explains the strengthening effects of interstitial N atoms on mechanical and vibrational properties.

Behaviors of Zn2GeO4 under high pressure and high temperature

The structural stability of Zn2GeO4 was investigated by in-situ synchrotron radiation angle dispersive x-ray diffraction. The pressure-induced amorphization is observed up to 10 GPa at room temperature. The high-pressure and high-temperature sintering experiments and the Raman spectrum measurement firstly were performed to suggest that the amorphization is caused by insufficient thermal energy and tilting Zn–O–Ge and Ge–O–Ge bond angles with increasing pressure, respectively. The calculated bulk modulus of Zn2GeO4 is 117.8 GPa from the pressure-volume data. In general, insights into the mechanical behavior and structure evolution of Zn2GeO4 will shed light on the micro-mechanism of the materials variation under high pressure and high temperature.

Effects of substitution, pressure, and temperature on the phonon mode in layered-rocksalt-type Li(1−x)/2Ga(1−x)/2ZnxO (x = 0.036–0.515) alloys

ZnO-based semiconductor alloys, Li(1−x)/2Ga(1−x)/2ZnxO (x = 0.036–0.515) with a layered-rocksalt-type structure, have been prepared under high pressure. The composition, pressure, and temperature dependence of phonons have been studied by Raman spectroscopy. We observe two disorder-activated Raman (DAR) modes when the Zn composition x increases: a broad Raman peak at ca. 400 cm−1 and a left-shoulder peak at ca. 530 cm−1 on the low-frequency side of A1g mode at ca. 580 cm−1, which can be explained by reference to the phonon density of states for rocksalt-type ZnO. With the increase of the pressure and temperature, the left-shoulder DAR mode induced by substitution does not change at the same pace with the A1g mode at Brillouin-zone center. We find that ion substitution can be seen as a kind of chemical pressure, and the chemical pressure caused by internal substitution and the physical pressure caused by external compression have equivalent effects on the shortening of correlation length, the distortion of...

Experimental study on the pressure-generation efficiency and pressure-seal mechanism for large volume cubic press

Measuring the pressure of a gasket (Pg) and cell (Pc) in situ is the key point to understanding the mechanism of pressure-generation and pressure-seal for the widely used large volume cubic press. However, it is a challenge to measure Pg due to the large deformation in the gasket zone and the complex rheological behavior of the pressure transmitting medium. Herein, a method of in situ electric resistance measurement has been developed to measure Pg. The open circuit failure in electric resistance measurement was avoided by using powder electrodes which could match the mould-pressed pyrophyllite cube in rheological behavior during compression. The relationships between press-load vs. Pc and press-load vs. Pg were obtained through in situ electric resistance measurements of bismuth, thallium, barium, and manganin. It was found that Pg exceeded Pc at around 5 GPa and Pc generated in the large volume cubic press was limited to the rapid rise of Pg above 5 GPa. Furthermore, the maximum ΔP (ΔP = Pc - Pg) above 0.9 GPa has been observed when Pc was released to a pressure range of 3-4 GPa, and this also leads to a large probability of high pressure cavity seal failure.

Neutron powder diffraction and high-pressure synchrotron x-ray diffraction study of tantalum nitrides*

Tantalum nitride (TaN) compact with a Vickers hardness of 26 GPa is prepared by a high-pressure and hightemperature (HPHT) method. The crystal structure and atom occupations of WC-type TaN have been investigated by neutron powder diffraction, and the compressibility of WC-type TaN has been investigated by using in-situ high-pressure synchrotron x-ray diffraction. The third-order Birch–Murnaghan equation of state fitted to the x-ray diffraction pressure–volume (P – V) sets of data, collected up to 41 GPa, yields ambient pressure isothermal bulk moduli of B 0 = 369(2) GPa with pressure derivatives of for the WC-type TaN. The bulk modulus of WC-type TaN is not in good agreement with the previous result (B 0 = 351 GPa), which is close to the recent theoretical calculation result (B 0 = 378 GPa). An analysis of the experiment results shows that crystal structure of WC-type TaN can be viewed as alternate stacking of Ta and N layers along the c direction, and the covalent Ta–N bonds between Ta and N layers along the c axis in the crystal structure play an important role in the incompressibility and hardness of WC-type TaN.

Anomalous compression behavior of ∼12 nm nanocrystalline TiO2

When the grain size decreases, there inevitably exists a critical size (dc) where the contribution of surface atoms to the physical properties is competitive with that of the interior atoms, giving rise to a wide variety of new phenomena. The behavior of granular materials near dc is particularly interesting because of the crossover, a continuous transition from one type of mechanism to another. In situ high-pressure x-ray diffraction experiments showed that the compression curve of nanocrystalline anatase TiO2 with grain size near dc reached a platform after about 5%–6% of deformation under hydrostatic compression. Eventually, the unit cell volume of anatase expanded at ∼14–16 GPa. We propose that the anomalous compression behavior is attributed to the formation and thickening of the stiff high density amorphous shell under high pressure, giving rise to a great arching effect at the grain boundary at the nanolevel. This process results in a remarkable difference in stress between inside and outside of th...