Pavel V. Zinin

Phase transition in BCx system under high-pressure and high-temperature: Synthesis of cubic dense BC3 nanostructured phase

We synthesized a cubic BC3 (c-BC3) phase, by direct transformation from graphitic phases at a pressure of 39 GPa and temperature of 2200 K in a laser-heated diamond anvil cell. A combination of x-ray diffraction, electron diffraction, transmission electron microscopy (TEM) imaging, and electron energy loss spectroscopy (EELS) measurements lead us to conclude that the obtained phase is hetero-nano-diamond, c-BC3. High-resolution TEM imaging of the c-BC3 specimen recovered at ambient conditions demonstrates that the c-BC3 is a single, uniform, nanocrystalline phase with a grain size of about 3–5 nm. The EELS measurements show that the atoms inside the cubic structure are bonded by sp3 bonds. The zero-pressure lattice parameter of the c-BC3 calculated from diffraction peaks was found to be a = 3.589 ± 0.007 A. The composition of the c-BC3 is determined from EELS measurements. The ratio of carbon to boron, C/B, is approximately 3 (2.8 ± 0.7).

Imaging of spheres with the confocal scanning optical microscope.

The scalar theory of image formation for a strongly spherical object in ref lection confocal optical microscopy is considered. The image contrast is derived from the far-field scattering amplitude. Experimentally obtained images of the sphere are seen to be in good agreement with the theoretical prediction.

A cubic phase of C3N4 synthesized in the diamond-anvil cell

A cubic phase of C3N4 was discovered. It was recovered at ambient conditions from the graphite-like C3N4 (g-C3N4) phase subjected to pressures between 21 and 38 GPa in a diamond-anvil cell, laser heated to temperatures between 1600 and 3000 K. The x-ray-diffraction data of the phase are best explained by a cubic unit cell with the lattice parameters a=3.878±0.001A. With an assumption of 1molecule∕unit cell (Z=1) for the cubic phase, the molar volume of the cubic phase is 35.126cm3∕mol and the density is 2.62g∕cm3. The density of the cubic phase is less than that which was predicted for the high-pressure phases but is 12% denser than the low-pressure graphitic phase (ρ=2.336g∕cm3). The cubic phase has not been predicted theoretically and represents an unknown structure in C3N4.

Elastic properties of nc-TiN∕a-Si3N4 and nc-TiN∕a-BN nanocomposite films by surface Brillouin scattering

The hardness of nanocomposite (nc) films developed recently appears to reach the hardness of diamond. High hardness is commonly attributed to the granular structure of nanocomposites (Hall–Petch effect) [E. O. Hall, Proc. Phys. Soc. Lond. B 64, 747 (1951); N. J. Petch, J. Iron Steel Inst. 174, 25 (1953)]. However, grain size in nanocomposites is generally small (5–15nm) and falls in the region where the Hall–Petch effect does not apply. The objective of the present study is to report the elastic properties of the superhard nanocomposites determined by means of surface Brillouin scattering (SBS), and to compare the results with those obtained by nanoindentation. Two types of nanocomposite films were studied: nc-TiN∕a-Si3N4 and nc-TiN∕a-BN. The SBS measurements presented yield values of Young’s modulus significantly larger than those obtained from the slope of unloading indentation curve. This discrepancy is attributed to the lack of the validity of the assumptions behind the Sneddon’s derivation of the for...

Laser generation and detection of longitudinal and shear acoustic waves in a diamond anvil cell

Laser ultrasonics in a point-source-point-receiver configuration is applied for the evaluation of elastic properties of nontransparent materials in a diamond anvil cell at high pressures. Measurement of both longitudinal and shear acoustic wave velocities in an iron foil at pressures up to 23 GPa does not require any information in addition to the one obtained by all-optical pump-probe technique.

Novel Micro-Cavity Substrates for Improving the Raman Signal from Submicrometer Size Materials

A novel and simple method for improving the detection limit of conventional Raman spectra using a micro-Raman system and picoliter volumes is presented. A micro-cavity in a reflecting metal substrate uses various mechanisms that collectively improve the entire Raman spectrum from the sample. A micro-cavity with a radius of several micrometers acts as a very effective device that provides multiple excitation of the sample with the laser and couples the forward-scattered Raman photons toward the collection optics in the back-scattered Raman geometry. One of the important features of the micro-cavity substrate is that it enhances the entire Raman spectrum of the molecules under investigation and maintains the relative intensity ratios of the various Raman bands. This feature of maintaining the overall integrity of the Raman features during signal enhancement makes the micro-cavity substrate ideal for forensic science applications for chemical detection of residual traces and other applications requiring low sample concentrations. The spectra measured in these cavities are also observed to be highly reproducible and reliable. A simple method for fabricating micro-cavity substrates with precise sizes and shapes is described. It is further shown that micro-cavities coated with nanofilms of gold take advantage of both surface-enhanced Raman scattering (SERS) and micro-cavity methods and also significantly improve sample detection limits.

DETERMINATION OF DENSITY AND ELASTIC CONSTANTS OF A THIN PHOSPHORIC ACID-ANODIZED OXIDE FILM BY ACOUSTIC MICROSCOPY

Quantitative acoustic measurements have been carried out on phosphoric acid-anodized (PAA) aluminum. This process creates a porous oxide layer which varied from 1 to 22 μm thick in the different specimens used here. The elastic properties of the PAA oxide layer were determined by measurements of the dispersion curves in the complex plane [speed of the surface acoustic wave (SAW) and its attenuation] alone. This was possible because two modes were observed on samples with thicker oxide layers. It has also been shown that the dependence of the attenuation on the oxide layer thickness around the cutoff can be used for the determination of the density of the layer. The shape of the attenuation curve measured by the acoustic microscope near the cutoff was smoother than predicted by the theory, and this phenomenon has been discussed.

Anomalous behaviour of leaky surface waves for stiffening layer near cutoff

The propagation of surface acoustic waves has been investigated for a layered system (water-isotropic layer-isotropic substrate) where the layer is faster than the substrate. It has been found that when the velocity of the surface wave reaches the velocity of the shear wave of the substrate (cutoff) the behaviour of the attenuation has an anomalous character. Attenuation drops down towards zero below cutoff and increases abruptly beyond cutoff. Acoustic microscopy measurements on aluminum coated with oxalic oxide film were in coincidence with theory.

The theory of three-dimensional imaging of strong scatterers in scanning acoustic microscopy

Abstract The angular spectrum approach to three-dimensional image formation of strong scatterers in scanning acoustic microscopy is developed. It is shown that in the case of strong scatterers the image can be represented as a twofold two-dimensional Fourier transform of a far field scattering amplitude of the object. The proposed theory provides the opportunity to investigate the imaging process for objects having an arbitrary shape. The common features of images are demonstrated by using a disk and an elastic sphere as examples. It is shown that contrast in reflection microscopy is mainly due to scattering from the boundary of the object and can be described as a thin layer whose medium surface is in coincidence with the boundary of the objects. The thickness of the layer is equal to the lateral resolution of the microscope. For transmission acoustic microscopes the contrast is connected with the contour of the object.

Pressure- and temperature-induced phase transition in the B–C system

A direct transformation from the g-BC1.6 phase to a diamondlike BC1.6 phase was obtained in a diamond-anvil cell at high temperature, 2230±140K, and high pressure, 45GPa. Such a transition has not been observed in the B–C system before. The recovered samples were examined using both synchrotron-based x-ray diffraction and confocal micro-Raman spectroscopy at ambient conditions. The lattice parameter of the diamondlike phase (3.5745A) is close to that of diamond. The measured visible laser-excited Raman spectra of a diamondlike BC1.6 phase reveal a peak at 1315cm−1 which is attributed to the LO mode. It is also found that the orthorhombic/hexagonal phase was formed under high pressure in the areas where heating was not enough to make transformation in the cubic phase.