The influence of high-voltage electrical discharge on dispersion and structure of B<Subscript>4</Subscript>C powder

The influence of high-voltage electrical discharge on the distilled water–boron carbide powder dispersion system has been studied, making it possible to clarify the relationship between the discharge parameters and the powder dispersion and structure. It is shown that the necessary conditions for effective dispersion of the boron carbide powder are the compression wave pressure amplitude at least ca. 100 MPa and the specific processing energy about 20 MJ/kg.     

Mechanical properties of the monocrystals of lanthanum hexaboride

We find the modulus of elasticity, microhardness, crack resistance, and specific resistance of the monocrystals of lanthanum hexaboride with perfect structure that can be used as reference data. The energy of covalent bond between the B–B atoms and the total energy of binding of LaB₆ and diborides of transition metals are computed. These data are used to explain high mechanical characteristics of lanthanum hexaboride.     

Effect of the crystallization kinetic parameters on the structure and properties of a eutectic alloy of the LaB6–TiB2 system

The effect has been studied of the cooling rate and excess of boron on the formation of microstructures of LaB₆–TiB₂ reinforced ceramic composites during centrifugal sputtering in the argon atmosphere and during sputtering of the alloy melt onto a copper substrate in the helium atmosphere. It has been shown that the increase of the cooling rate to 10⁶ deg/s results in a decrease of diameters of composite material fibers to 0.1–0.15 μm. The decrease of the boron concentration from 2 to 1 wt % has been found to lead to an increase of diameters and decrease of the number of the titanium diboride fibers by a factor of 2–3.     

Formulation and Solution of the Problem of Drying of a Layer of Combustible Forest Materials

The drying of combustible forest materials is the most important and least studied stage of the multiphase process of their burning under natural conditions. Physical and mathematical modeling of the drying of a layer of combustible forest materials is performed in a conjugate formulation by solving the equations of a binary boundary layer and the equations of heat and mass transfer in a layer of combustible forest materials with corresponding boundary and initial conditions. Solutions of this problem for diurnal and seasonal changes in environmental temperature are obtained for three scenarios of development of weather conditions. The data obtained are compared with experimental data on drying of needles of pine and some other coniferous trees. A rigorous physicomathematical basis for prediction of forest fires is given.     

Mechanical properties of HfB2 whiskers

Nanoindentation has been used to study mechanical behavior of HfB₂ whiskers 10–20μm in diameter in a directionally reinforced ceramics. For comparison a bulk (0001) HfB₂ single crystal 5 mm in diameter has been tested. For both the samples a pop-in due to the nucleation of dislocations in a previously dislocation-free region under the indent has been observed. It has been shown that for a HfB₂ whisker in reinforced ceramics the critical load of the elastoplastic transition is twice as high as for a bulk HfB₂ single crystal and, the maximum shear stress, at which the nucleation of the first dislocation loop in a HfB₂ whisker occurs, approaches to the theoretical shear strength value. The observed effect has been caused by the higher structural perfection of whiskers as compared with a bulk sample. Hardness and elastic modulus of the HfB₂ whisker are higher than that of the bulk crystal by 10 and 3%, respectively.     

Reconstructing disturbance history using satellite-based assessment of the distribution of land cover in the Russian Far East

Russian boreal forests are the largest forested zone on Earth and a tremendous pool of organic carbon. Current limited records on forest structure, composition, successional stage and disturbances contribute to large uncertainties in estimates of carbon stocks and fluxes in this zone. Our ability to monitor ongoing changes in forest cover has improved with the influx of remotely sensed data products since 2000 from multiple satellite platforms. Here we present a method aimed at reconstructing disturbance history from a known distribution of land cover. We developed and tested the method over a biologically and topographically diverse region of the Russian Far East. This method explores capabilities introduced through fusion of the long-term but spatially limited Landsat data archive and the spatially continuous but temporally limited 2000-present data record from the Moderate Resolution Spectroradiometer (MODIS). Landsat data from 1972 to 2002 were used to develop a reference disturbance dataset to train and validate a MODIS-based decision tree classification. The results showed a reliable differentiation of disturbed and mature forests with an overall accuracy of 88% (Kappa 0.73). Individual disturbances by type and decade were estimated with an overall accuracy of 70% (Kappa 0.64).     

Quantitative Comparison Between Dislocation Densities in Offcut 4H-SiC Wafers Measured Using Synchrotron X-ray Topography and Molten KOH Etching

Molten KOH etching and x-ray topography have been well established as two of the major characterization techniques used for observing as well as analyzing the various crystallographic defects in both substrates and homoepitaxial layers of silicon carbide. Regarding assessment of dislocation density in commercial wafers, though the two techniques show good consistency in threading dislocation density analysis, significant discrepancy is found in the case of basal plane dislocations (BPDs). In this paper we compare measurements of BPD densities in 4-inch 4H-SiC commercial wafers assessed using both etching and topography methods. The ratio of the BPD density calculated from topographic images to that from etch pits is estimated to be larger than 1/sinθ, where θ is the offcut angle of the wafer. Based on the orientations of the defects in the wafers, a theoretical model is put forward to explain this disparity and two main sources of errors in assessing the BPD density using chemical etching are discussed.