Pyrolitic Boron Nitride Research Articles

A new cell for high temperature EXAFS measurements in molten rare earth fluorides

A new cell with simple design has been developed for high temperature X-rays absorption measurements in both solid and molten lanthanide fluorides. Two plates of pyrolitic boron nitride are fixed hermetically together around the samples in order to avoid any evaporation and atmosphere interaction. EXAFS spectra of molten mixtures of LiF–LaF3 measured at the La LIII absorption edge are reported up to 900 °C, and show the ability of this cell to keep the salt and to perform long time acquisition improving the signal to noise ratio.

C 60 bonding to graphite and boron nitride surfaces

The present study focuses on the interaction of C60 with the surfaces of highly oriented pyrolitic graphite (HOPG) and sp2-bonded boron nitride (BN). The nanocrystalline BN film was deposited by mass selected ion beams and features an sp2-bonded surface layer, which covers a cubic phase BN film. The first part of the experiment is the sequential deposition of C60, which is monitored by photoelectron spectroscopy in the x-ray (XPS) and ultraviolet (UPS) regime. The growth of the C60 layer on HOPG is close to a layer-by-layer growth mode, but on the BN surface island growth is favored. No charge transfer or chemical reaction (e.g., carbide formation) between the fullerene layer, and the underlying substrate is observed in either case. In the second part of the experiment the samples are heated at a rate of 10 K/min while simultaneously recording the UPS VB spectra. The complete desorption of C60 from the HOPG surface occurs in a small temperature interval between 510–530 K. For the sp2 BN surface the majority of C60 desorbs around 493 K, about half a monolayer (ML) remains, and the C60 concentration decreases gradually with increasing temperature; less than a tenth of a ML can be detected even at 1000 K. The first desorption event at 493 K is attributable to the multilayer desorption from islands. The remaining C60 directly in contact with the BN surface is then removed in a large temperature interval between 500 and 1000 K which indicates the presence of a multitude of adsorption sites. The presence of C60 on the BN film surface also induces a band bending and related B 1s and N 1s core level shifts. An upward band bending is present in the C60 overlayer, which indicates that defects are responsible for the pinning of the Fermi level at the interface.

Optical and electrical phenomena in dielectric materials under irradiation

Optical and acoustic properties of the materials based on Al 2O 3, SiO 2 and BN under 8 MeV proton irradiation (<10 4 Gy/s) have been measured. Electric charge partitioning has been shown to result in charging the microscopic regions in the bulk of the dielectrics under irradiation, which is due to different mobility of free electrons and holes (sapphire), concentration inhomogeneity in the system of charge carrier traps (alumina), or thermodynamic instability of the homogeneous distribution of the filled traps (silica glasses). Prevalent charge carrier recombination in the grain boundaries causes re-crystallization of pyrolitic boron nitride under irradiation, which shows up as simultaneous decrease of the intensity of radiation-induced luminescence (RIL) of the centres in the grain boundaries and the BN. The local charging results in optical inhomogeneity of the silica glasses which is sustained by the optical loss spectra of the irradiated glasses, features of kinetics of bleaching, RIL kinetics and dose dependence of the ultrasonic vibration decrement.

Bridgman growth of detached GeSi crystals

Ge 1− x Si x (0< x<0.12) has been grown by the vertical Bridgman technique using adjustments in the applied temperature profile to control the pressure difference between the bottom and top of the melt. Using this technique, a pressure difference is created by decreasing the temperature in the gas volume above the melt while the sample is molten but prior to growth. A maximum pressure difference approximately equal to the hydrostatic pressure of the molten sample can thus be obtained. Several GeSi crystals were grown in pyrolitic boron nitride ampoules. When a pressure difference was applied, samples were reproducibly grown mostly detached. For comparison, samples were also grown in a configuration in which gas could flow freely between the gap below the melt and the volume above the melt and no pressure difference could be established. These samples were initially attached. Existence of detachment was determined both by measuring the surface roughness of the samples with a profilometer and by observations of the sample surfaces with optical and electron microscopy.

A filter based analyzer for studies of X-ray Raman scattering

Non-resonant X-ray Raman scattering (XRS) with hard X-rays holds the potential for measuring local structure and local electronic properties around low- Z atoms in environments where traditional soft X-ray techniques are inapplicable. However, the small cross-section for XRS requires that experiments must simultaneously achieve high detection efficiency, large collection solid angles, and good energy resolution. We report here that a simple X-ray analyzer consisting of an absorber and a point-focusing spatial filter can be used to study some X-ray Raman near-edge features. This apparatus has greater than 10% detection efficiency, has an energy resolution of 8 eV, and can be readily extended to collection angles of more than 1 sr. We present preliminary measurements of the XRS from the nitrogen 1 s shell in pyrolitic boron nitride.

A new radiative microfurnace for X-ray single-crystal diffractometry

Microfurnaces available today for X-ray single-crystal investigation (radiative, gas-flame, gas-flow) are generally temperature-monitored by a thermocouple placed in a different site with respect to the crystal. The new F1 microfurnace aims at the following goals: fast crystal mounting; access to a large portion of reciprocal lattice; low absorption of direct beam; efficient in situ temperature measurement; fast power supply regulation; high temperature stability; controlled atmosphere. In the F1, the crystal is directly glued to a thermocouple with refractory cement. The heating body, fixed on the χ and radially shiftable along the ϕ axis circle, hosts the Pt winding and connections of both gas flow and power supply, and is enclosed in a thin pyrolitic boron nitride (PBN) shield. Temperature stability is within ± 1°C. Calibration of the F1 was carried out by collecting X-ray data from a single-crystal synthetic periclase at tem- peratures of 28, 150, 300, 450, 600, 700, 900 and 1000°C, and yielded the equation: a = 0.0000625 (7) (T(K) - 273) + 4.2083 (4), based on the ratio between cell edge and temperature. Structural refinements gave structural parame- ters and thermal expansion values at the investigated temperature. The mean coefficient of linear expansion up to 1000°C was 14.3 × 10-6 °C.

Vertical Bridgman growth of Al xGa 1− xSb single crystals

Al x Ga 1− x Sb crystals were grown by vertical Bridgman method with (1 1 1)B oriented GaSb seeds in a two-zone vertical gradient furnace. The growth was conducted using a two-step growth process, which consists of the growth under a constant temperature period and the growth in a vertical gradient freeze mode. During the period of the constant-temperature profile, the growth was realized by the transport of Al to the solid–liquid interface. Al x Ga 1− x Sb single crystals with a diameter of 14.5 mm and a length of more than 27 mm were successfully obtained in pyrolitic boron nitride (PBN) crucibles. The longitudinal cross sections were investigated by using a Nomarski differential interference contrast microscope (N-DICM) and it was found that strong striations were induced due to the convection on the ground. The AlSb distribution was measured by spatially resolved photoluminescence (SRPL) and it was found that AlSb composition reached 10.4%.

Continuum optical radiation produced by H + and He + ion bombardment of pyrolytic boron nitride

An optical emission phenomenon was observed for pyrolitic boron nitride (PBN) which was excited via bombardments by 1.0 MeV H + and 1.8 MeV He + ions. From blue to near-ultraviolet photons were observed with a continuum spectra from 280 to 600 nm having two broad peaks at 350 and 430 nm. It is suggested that the two broad peaks correspond to luminescence through two different deep recombination centers at 3.6 eV below the conduction band and at 3.0 eV above the valence band. No band-to-band luminescent recombination was observed near 250 nm (∼ 5 eV).

Vertical seeded melt growth of GaAs

The growth of GaAs single crystals, 37, 50 and 62.5 mm in diameter, by the dynamic gradient freeze technique is described. Both quartz and pyrolitic boron nitride crucibles have been used successfully. Characterization of the crystals, 37 mm in diameter, grown in quartz is presented. Using very shallow temperature gradients over the melt and slow growth rates, the crystal-melt interface can be maintained flat over about 75% of the diameter. Through control of stoichiometry and with a thermal environment designed to reduce thermal stresses, about 50% of the crystals have a dislocation density of less than 1000 cm -2. Finally, we present the results from post-growth annealing.

Growth of ZnSe crystals from the melt under Zn partial pressure

This paper describes melt growth of ZnSe crystals by a modified Bridgman method under argon pressure. Stoichiometric deviations of the ZnSe melt are controlled by Zn partial pressure. A crucible and a Zn reservoir are placed in a pyrolitic boron nitride (pBN) vessel, which is covered with a deep, closed pBN cap to minimize loss of Zn. A definite Zn partial pressure over the melt during growth is maintained by controlling the temperature of the Zn reservoir. The melt composition is analyzed as a function of the temperature of the Zn reservoir. The stoichiometric melt is obtained under Zn partial pressure. Low-resistivity n-type crystals of 0.2-0.9 Ω cm are grown from Zn-rich melts in a one-step process.

Characterisation of deep electron states in LEC grown GaAs material

Deep electron states in undoped GaAs grown by the liquid-encapsulated Czochralski (LEC) method using a pyrolitic boron nitride (PBN) crucible were investigated by DLTS and Hall measurements. Six electron traps with activation energies ranging from 0.14 to 0.82 eV were detected. One of these traps, with an energy of 0.27 eV (EL6), was reduced in concentrations by more than an order of magnitude by an 800 degrees C heat treatment for 1 h. Temperature-dependent Hall measurement on as-grown material indicated that the 0.27 eV state plays an important role in the decrease of the resistivity of undoped crystals that have a low concentration of carbon impurity. The effect of gamma irradiation on the annealing properties of the traps was also demonstrated.

Initial stage in the transformation of pyrolitic graphitelike boron nitride as a result of shock compression

The influence is investigated of the orientation of the hexoganal planes relative to the shock front on the change in its properties. Specimens oriented with hexagonal layers perpendicular or parallel to a shock front were subjected to shock compression in a plane loading geometry. In order to detect the possible presence of a dense-phase, IR spectra were recorded of the shock-compressed specimens on an IKS-29 spectrometer in the 400-4000 cm/sup -1/ range. For the transformation of well-crystalized graphite or h-BN under shock compression, it is sufficient to produce the necessary pressure and temperature in the stability domain of the dense phase. The authors note that the presence of transition processes consisting of separate ''seams'' between the layers and occuring in the earliest stages of dense phase formation, is reliably recorded successfully just by a magnetometric method using a supersensitive quantum magnetometer.

Optical properties of graphite and boron nitride

The energy bands of graphite and boron nitride have been calculated using the tight-binding approximation, with atomic potentials obtained by the SCF CNDO method. On the basis of the calculated bands some optical properties of the above materials are discussed. The optical absorption of the hot-pressed pyrolitic boron nitride was measured in the wavelength region from 25000 to 50000 cm-1 and the width of the forbidden gap was found to be 4.3 eV. Comparisons with the previous calculations have also been made.

Some properties of pyrolitic boron nitride

Pyrolitic boron nitride is a pore-free, vacuum tight material possessing low dielectric permeability, moderate dielectric loss, and a very low (the lowest of all known materials) coefficient of secondary electron emission. Owing to this, pyrolitic boron nitride is a promising material for use in electronics technology. The high thermal conductivity, and especially the structure, determine the excellent resistance to thermal shock of the articles made from pyrolitic boron nitride. The relatively high density, the excellent resistance to oxidation, the chemical inertness to most reagents, the structural features, and especially the strength at elevated temperatures opens up wide possibilities for employing pyrolitic boron nitride in chemical and metallurgical industries, in semiconductor techniques, and other regions of technology.

Frictional properties of pyrolitic boron nitride and graphite

Frictional properties of pyrolitic boron nitride and graphite