Polycrystalline Beryllium Oxide Research Articles

Fracture mechanics of polycrystalline beryllium oxide nanosheets: A theoretical basis

Polycrystalline beryllium-oxide nanosheets (PBeONS) are today key elements of electronic devices due to their large bandgaps. Their mechanical performance can be tuned by adjusting a number of parameters, such as the number of grains, temperature, and defects in their structure. Nevertheless, even theories can hardly predict their fracture behavior. Herein, we employed Molecular Dynamics Simulation (MDS) to visualize for the first time the effect of the number of grains (4, 9, 16, 25, and 36), temperature (200–900 K), and defects (typical cracks and circular notches with different lengths and dimeters of L/12, L/6, L/3, L/2) on the mechanical properties of square-shaped PBeONS with the length of 300 Å (L). It was observed that the failure stress and Young’s modulus of PBeONS with 36 grains dropped from 21.7 GPa and 95.77 GPa to the values of 13.85 GPa (ca. 36%) and 54.14 GPa (ca. 43.5%), respectively, with respect the PBeONS with 4 grains in X direction at 300 K. The higher temperatures and large defects increased interatomic distances and weakened the structure, which deteriorated the mechanical strength. Stress intensity factor dropped against temperature, but rose by crack length enlargement showing lower fracture toughness of small cracks.

Epitaxial Growth of GaN Core and InGaN/GaN Multiple Quantum Well Core/Shell Nanowires on a Thermally Conductive Beryllium Oxide Substrate.

Beryllium oxide (BeO) belongs to a very unique material family that exhibits the divergent properties of high thermal conductivity and high electrical resistivity. BeO has the same crystal structure as GaN, and the absolute difference in the lattice constants is less than 17%. Here, the growth of GaN nanowires (NWs) on the polycrystalline BeO substrate is reported for the first time. The NWs are grown by a vapor–liquid–solid approach using a showerhead-based metal–organic chemical vapor deposition. The growth direction of NWs is along the m-axis on all planes of the substrate, and it is confirmed by transmission electron microscopy (TEM) and selected area electron diffraction (SAED) patterns. The vertical and tilted growth of NWs is due to the different planes of the substrate such as the m-plane, a-plane, and semipolar planes and is confirmed by X-ray diffraction. Subsequently, the GaN shell and InGaN/GaN multiple quantum wells (MQWs) are coaxially grown using a vapor–solid approach in the same reactor. A very high crystal quality is verified by TEM and SAED and is also confirmed by measuring the photoluminescence. The optical emission is tuned for the entire visible spectrum by increasing the indium incorporation in InGaN quantum wells. The conformal growth of InGaN/GaN MQW shells and the defect-free nature of the structure are confirmed from spatially resolved cathodoluminescence. This study will provide a platform for researchers to grow GaN NWs on the BeO substrate for a range of optical and electrical applications.

Optical Spectra of Amorphous and Polycrystalline Beryllium Oxide over a Wide Energy Range of Fundamental Absorption

The complete sets of spectra of the optical fundamental functions for amorphous and polycrystalline beryllium oxides in the energy range 0–40 eV are determined and compared for the first time. The general features of the spectra and their dependence on the long-range order are revealed. The basic parameters of the optical transitions in the range 0–40 eV for amorphous and polycrystalline BeO are obtained for the first time.

Measurement of the directional emittance of polycrystalline beryllium oxide

The spectral emittance of polycrystalline beryllium oxide has been measured at four different angles from 0° to 75°, at a temperature of 150°C. From the spectral measurements the average band emittance in the 8–13 μm window has been calculated. The results show that the band emittance decreases from 0.44 at the angle of 0°, down to 0.23 at an angle 75° off the normal. The low band emittance is caused by a strong reststrahlen band in the wavelength region 9.2–14 μm. This reststrahlen band has been modelled by a Lorentz one-oscillator model. Experiment and calculations show that this band of low emittance increases its width towards shorter wavelengths at inclined angles and covers almost the entire upper atmospheric window 8–13 μm at the angle of 75°.

Selective suppression of thermal emission from radomes and materials selection therefor

The possibility of using selectively low emission to reduce the thermal signature of radomes is identified. A material can simultaneously have low radiance in the working range of a detector and cool radiatively to the surrounding atmosphere. If the band of low emission is based on lattice excitation, the signature reduction is compatible with electrically insulating properties and radar transmittance. High-density, polycrystalline beryllium oxide is identified as a material with low emittance in the upper atmospheric window at 8 to 13 μm, owing to a strong reststrahlen band. Radiometer measurements and bulk reflectance spectra are reported for ceramic BeO and are used to calculate the thermal infrared emission factors as well as the radiance for a 50°C BeO surface. The results depend heavily upon the short wavelength threshold of the detector. The calculated values have been compared with radiometer measurements in the 3- to 5-μm and 8- to 13-μm ranges. The possibility of reducing the emittance even further with a second material has been investigated with Fresnel calculations. Very favorable calculated and measured results for a 0.8-μm silicon overlayer on BeO are reported.

Light extinction by bulk voids and surface irregularities in ceramic materials.

Two sources of elastic light scattering, usually associated with scattering by particles, by bulk, polycrystalline samples of partly ionic compounds are demonstrated. In the visible part of the spectrum where the material is transparent, subsurface inhomogeneities cause backscattering which contributes to the total reflectance of the surface. This effect is modeled with a combination of Mie scattering by isolated spherical voids filled with air, and Monte Carlo calculations to account for multiple-scattering events. In the infrared spectral range, these compounds typically exhibit one or more lattice resonances. Such a resonance causes strong dispersion in the optical constants, resulting in a wavelength at which the conditions for the lowest surface-mode resonance are satisfied. It is shown that small inhomogeneities on the surface are sufficient to give a measurable contribution and cause a substructure in the reststrahlen band which has been observed previously but not fully understood. Light-scattering measurements over the wavelength range 0.3--20 \ensuremath{\mu}m for bulk samples of polycrystalline beryllium oxide and 2--20 \ensuremath{\mu}m for silicon carbide powder are reported to verify the interpretation. A coating of polyethylene was applied which shifted the surface resonance to longer wavelengths in agreement with the theory for isolated-particle scattering.

Heat transfer in NMR of conductive samples with radiofrequency decoupling

Under conditions of thermal and fluid mechanical equilibrium, heat transfer in a vortex-free liquid in an NMR tube spinning at about 30 Hz is evidently dominated by conduction, to first approximation. In contrast, heat transfer between the spinning tube and its stationary surroundings occurs primarily by convection. When high-power rf decoupling is used, lossy samples such as solutions of electrolytes or molecules having large electric dipole moments may exhibit large increases in average temperature as well as substantial radial temperature gradients, because of inductive dielectric heating. Equations for conduction of heat through such samples were applied to the case where chemical shift depends linearly on temperature, and the resulting expressions for the lineshape were fitted to 31P spectra of arylphosphine derivatives which were obtained using 100-MHz proton noise decoupling. For samples with ionic conductivity equal to that of 0.05 M aqueous NaCl, radial temperature gradients of 2.1 and 0.3 K were observed in 12- and 5-mm glass tubes, respectively, at 9 W radiated rf power and a maximum sample temperature of 302 K, 11 K above the temperatureof the thermostatted gas stream entering the NMR probe. Twelve-millimeter coaxial-cylinder sample tubes and coaxial inserts machined from polycrystalline beryllium oxide have thermal conductivities 250 times that of glass tubes and equilibrate rapidly in the probe. Heating effects and radial temperature gradients in BeO tubes equipped with coaxial BeO inserts were too small to measure. However, calculations suggest that the values should be close to 0.03 K for 9 W decoupling and other conditions as used for the conductive samples in glass tubes without inserts.

Fracture Energy of Polycrystalline Beryllium Oxide

Journal of the American Ceramic SocietyVolume 51, Issue 10 p. 602-602 Fracture Energy of Polycrystalline Beryllium Oxide PAUL L. GUTSHALL, PAUL L. GUTSHALLSearch for more papers by this authorGORDON E. GROSS, GORDON E. GROSSSearch for more papers by this author PAUL L. GUTSHALL, PAUL L. GUTSHALLSearch for more papers by this authorGORDON E. GROSS, GORDON E. GROSSSearch for more papers by this author First published: October 1968 https://doi.org/10.1111/j.1151-2916.1968.tb13332.xCitations: 5 Supported in part by the Naval Air Systems Command, Department of the Navy under Contract N00019–68–C-0127. At the time the work was done the writers were, respectively, senior physicist and manager, Physics Branch, Midwest Research institute, Kansas City, Missouri 64110. P. L. Gutshall is now rcsearch physicist, Physics Department, University of Missouri, Kansas City, Missouri 64110. AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article.Citing Literature Volume51, Issue10October 1968Pages 602-602 RelatedInformation

Self-Diffusion of Beryllium in Polycrystalline Beryllium Oxide

The self-diffusion of beryllium in polycrystalline beryllium oxide was measured from 1330° to 2135°C with the 7Be tracer. Results are compared with the findings of other investigators, and it is concluded that the differences reported are due to variations in impurity and porosity contents. Diffusion has been shown to be via a vacancy mechanism, with such impurities as aluminum and silicon being the principal contributors of vacancies. A roll-over in the plot of log D versus 1/T, where D is the diffusion coefficient and T is absolute temperature, was found in this study as in some others. Above about 1500°C the activation energy is no higher than 62 kcal, whereas it is 92 kcal at lower temperatures. It is suggested that this roll-over is due to aluminum impurity. There appears to be a temperature-independent anisotropy of diffusion, with the rate in the 〈c〉 direction being 30% higher than that in the 〈a〉 direction. It is concluded that for this and other investigations the diffusion coefficient of beryllium in beryllium oxide above the rollover temperature is governed by impurities and in this region it is given by the formula where Fp is the fraction of porosity and Nv is the molar fraction of vacancies introduced by impurities. There is sufficient scatter in individual 〈a〉 and 〈c〉 diffusion values so that there is no value in discriminating between directions in this formula.

Cation self-diffusion in fast neutron-irradiated beryllium oxide

Abstract A considerable enhancement of beryllium self-diffusion rates in irradiated polycrystalline beryllium oxide has been observed which is strongly time-dependent above 800°C. Amongst the possible mechanisms, increased point defect concentrations and long-range disorder due to lattice expansion could account for the observed enhancement.

High‐Temperature Creep and Dislocation Etch Pits in Polycrystalline Beryllium Oxide

Compressive creep of high‐density polycrystalline beryllium oxide was investigated in the range 1850° to 2050°C. Creep rate was dependent on the applied stress to the 2.5 power, and the apparent activation energy for creep was 145 kcal/mole. Etch pit studies showed that the dislocation density in tested specimens was two orders of magnitude greater than that in assintered material. The diffusion process controlling creep was ascribed to volume diffusion of the anion. The deformation behavior was governed by dislocation motion.

Cation Self-Diffusion and Electrical Conductivity in Polycrystalline Beryllium Oxide

Beryllium self-diffusion coefficients were determined in high-density polycrystalline BeO in the temperature range 800° to 1795°C. A precision lapping method using Be as a radiotracer was used in the interval above 1180°C, and electrical conductivity between 800° and 1250°C. Volume diffusion coefficients determined by both methods correlate well between 1100° and 1250°C. The results can be expressed over the temperature range 1100° to 1800°C by the relation D=3.2×10−3 exp(−63.0×103/RT). Above 1100°C, the agreement between the results obtained by the two techniques supports an ionic drift mechanism for the electrical conductivity of BeO. Below this temperature, however, the transport behavior appears to be considerably influenced by the history of the sample as well as the nature and concentrations of impurities. Classical volume diffusion was observed in a region near the surface of the specimens; at greater depths, deviations occurred and the magnitude of the deviations increased with decreasing grain size of the BeO. These discrepancies have been explained on the basis of a simultaneous grain-boundary diffusion mechanism.

The effect of neutron irradiation on beryllium oxide

Results are presented of the effect of neutron irradiation to doses up to 1.4 × 10 21 nvt at temperatures of 75–700 °C on polycrystalline beryllium oxide fabricated by various routes to give a range of grain size and densities. The onset of microcracking due to anisotropic growth is shown to be very dependent on microstructure; fine grain-size, cold pressed and sintered, material has been found to withstand much higher doses without microcracking than coarse grained material. The macroscopic growth in the absence of microcracking is found to be closely related to the X-ray growth at temperatures up to 600° C. The available information on the defect structure of beryllium oxide is reviewed and related to observed property changes. It is concluded that the property changes (at temperatures up to 700° C) are due to displaced atoms and that effects due to helium are not significant. It is shown that the available data on the variation of the damage rate with irradiation temperature can be adequately described analytically by assuming competition between defect production and annealing processes with a spectrum of activation energies. Using this treatment to extrapolate available data, predictions have been made of the property changes which are expected to occur in beryllium oxide under reactor conditions.

Compressive Creep of Polycrystalline Beryllium Oxide

Compressive creep of polycrystalline beryllium oxide was studied in the temperature range 2500° to 2800°F. The apparent activation energy for creep was 96.0 kcal per mole and the creep rate was linearly dependent on the applied stress. Results were consistent with the Nabarro‐Herring creep mechanism. Experimental evidence showed that extrapolation of data to 3200°F was possible.

The scattering of long wavelength neutrons by irradiated beryllium oxide

Abstract The scattering of long wavelength neutrons by polycrystalline beryllium oxide irradiated to integrated fission neutron doses between 2·5 x 1019 and 6·8 x 1020 at temperatures of 75–100°c has been investigated. A considerable amount of extra scattering is introduced by irradiation and this scattering has a strong wavelength dependence. The extra scattering is interpreted in terms of scattering from defects. A model in which half the defects are present in the form of clusters containing a minimum of 10–20 interstitials or vacancies and the other half are randomly distributed, describes very closely the experimentally observed wavelength dependence of the scattering. Based on this model, calculations have been made of the total number of defects present in the irradiated material and it is shown that each primary knock-on results in about 10–20 displaced atoms. The effect on the proposed model of a dilatation around the defects is discussed and it is shown that provided a reasonable figure is chosen for the dilatation, the results are not significantly affected. Annealing studies show that the defects anneal in the temperature range 800–1400°c.

Inelastic scattering of cold neutrons by polycrystalline beryllium oxide in the temperature range from 100°K to 1000°K

The total cross-section of beryllium oxide for cold neutrons of mean wavelength 5.55 Å has been measured in the temperature range from 100 to 1000°K. The results are satisfactorily interpreted in terms of the incoherent approximation using the Placzek expansion. The values of the Debye temperature obtained from these measurements in the low and high temperature ranges are compared with those got by specific heat measurements. A method of choosing the effective mass and effective neutron cross-section of a molecule in a diatomic lattice is suggested.

Electron Spin Resonance in Neutron-irradiated Polycrystalline Beryllium Oxide

Electron spin resonance, presumably from electrons or holes bound to appropriate vacancies, has been detected in neutron-irradiated polycrystalline BeO. A preliminary study of the behaviour of the intensity and line-shape of the electron spin resonance signal with increasing radiation dose, and on annealing, has been made. Considered as an anisotropic line, the electron spin resonance signal prior to annealing has the following g values (approximate only): gII = 2.0036 ± 0.001, g⊥ = 2.0110 ± 0.001. However, the change of line-shape of the electron spin resonance signal after annealing tends to indicate that the initial signal may come from more than one type of spin centre.