Boron In Samples Research Articles

BORON GIVES UP ITS RESISTANCE

SCIENTISTS HAVE MANAGED to strong-arm the superlight element boron into taking on a couple of unnatural attributes: those of both a metal and a superconductor. The new experiments not only confirm theoretical predictions of boron's pressure-induced metallicity but should also help flesh out little-understood periodic trends in superconductivity. Physicist Russell J. Hemley and colleagues at the Carnegie Institution of Washington first squeezed samples of boron under pressures more than a million times that of Earth's atmosphere and then measured the samples' electrical conductivity— a difficult experimental feat. Andrew McMahan, a highpressure physicist at Lawrence Livermore National Laboratory, calls the experiments excellent work, citing the extraordinarily high pressures needed to overcome the strong covalent bonds that hinder metal transformation. The results show that the element becomes metallic at pressures of about 175 gigapascals at room temperature, close to the theoretically predicted pressur...

Low-temperature specific heat of isotopically modified boron single crystals

The measurement of specific heat at low temperature for isotopically modified boron single crystals was performed between 0.5 and 100 K using a calorimetric technique known as the time-constant or relaxation method. The results indicate that the specific heat has obvious divergencies at 1.3 K<T<5 K and the divergencies for four kinds of isotopically modified boron samples vary with different B10 contents. The influence of lattice vibration modes and the isotopic effect on specific heat and thermal conductivity for isotopically modified boron are discussed.

Boron isotope effects on the thermoelectric properties of UB 4 at low temperatures

The thermoelectric power and electrical resistivity of UB 4 with three different boron isotope enrichments were measured in the temperature range from 80 to 300 K. The thermoelectric power of each sample decreased with decreasing temperature, but a slight saturation of the decrease due to phonon-drag effect was observed below 120 K for both boron isotope enriched samples. The resistivity decreased with decreasing temperature and there existed no large difference among the resistivities of natural boron, boron-10 enriched and boron-11 enriched samples. The figure of merit for thermoelectricity of UB 4 was lower by three orders of magnitude than that of well-known thermoelectric material (Bi,Sb) 2(Te,Se) 3.

LTE absorption spectroscopy of an X-ray heated boron plasma

Time-resolved K-shell spectroscopy of an X-ray driven boron sample is reported. A small cavity irradiated by two frequency-doubled Nd:glass laser beams is used as an X-ray heating source, allowing to reach electron temperatures in the 10–25 eV range, in conditions close to local thermodynamic equilibrium. Synthetic spectra obtained from detailed atomic calculations are fitted to the experimental data to determine the electron temperature and the density of plasma. We compare the resulting data to radiative hydrodynamic simulations of the heating and expansion of the sample, and find lower densities and higher temperatures than predicted by the code. The reason for this discrepancy is attributed to difficulties in maintaining the boron sample homogeneity and to take into account properly the expansion of the plasma walls with such small-size cavities.

Laser-triggered ion acceleration and table top isotope production

We have observed deuterons accelerated to energies of about 2 MeV in the interaction of relativistically intense 10 TW, 400 fs laser pulse with a thin layer of deuterated polystyrene deposited on Mylar film. These high-energy deuterons were directed to the boron sample, where they produced ∼105 atoms of positron active isotope C11 from the reaction B10(d,n)11C. The activation results suggest that deuterons were accelerated from the front surface of the target.

Instrumented-Indentation for Mechanical Characterization of Boron Carbide Nano-Composite Coatings

AbstractBoron carbide (BC) is well known as a coating material that is important for a wide range of technological applications. The applicability of boron carbide stems from the fact that it is a very hard material with high lubricity, high elastic modulus, low specific gravity, and good chemical stability. Disadvantages, however, include extreme brittleness and sometimes poor adhesion. Recently, a reactive sputtering involving boron carbide targets and hydrocarbon gases has been used to produce novel nano-composite boron carbide thin films comprised of BC nano-crystals embedded in a matrix of hydrogenated amorphous carbon (DLC). The microstructure of these thin films is similar to that of other metal carbide/DLC nano-composite films. The present paper discusses the results of Vickers indentation experiments carried out on four different samples of boron carbide/DLC coatings that were sputtered deposited onto 52100 steel disks. The four different samples resulted from four different levels of hydrocarbon gas flow during processing. Acoustic emission data was recorded simultaneously with the indentation experiments. The indentations and the associated crack patterns were observed using scanning electron microscopy.

Variabilidade espacial de micronutrientes em solo sob pivô central no sul do Estado de Mato Grosso

Este estudo foi desenvolvido para identificar, caracterizar e comparar a estrutura da dependência espacial dos micronutrientes boro, cobre, ferro, manganês e zinco solúveis em um Latossolo Vermelho-Escuro sob pivô central após 14 anos de uso intensivo, no sul do Estado de Mato Grosso. O esquema de amostragem consistiu de coletas de 132 amostras com espaçamento regular de 167 m, especialmente idealizado para determinar a variabilidade espacial em distância de até 1 m. Com exceção do zinco, o uso intensivo propiciou um aumento significativo nas concentrações desses nutrientes na camada mais afetada pelo manejo (0-20 cm), mesmo assim insuficientes para atingir o nível crítico estabelecido para a região. Cerca de 95% das amostras de boro, 75% das amostras de cobre, 95% das amostras de manganês e 1,5% das amostras de zinco apresentaram valores abaixo do nível crítico, distribuídos diferentemente pelos quadrantes, o que mostra que as práticas de fertilização e/ou as operações de preparo de solo não foram eficientes na distribuição e homogeneização dos fertilizantes.

Oxygen precipitation and secondary defects in silicon by high energy ion implantation and two-step annealing

Intrinsic gettering is usually used to improve wafer quality which is an important factor for reliable ultralarge scale integration devices. The two-step annealing method was adopted to investigate the oxygen precipitates and secondary defects in the aspect of various nucleations and growths for 1.5 MeV, 1×1015 As/cm2 implanted wafers. After two-step annealing, the wafers were cleaved and etched with Wright etchant. Oxygen precipitation density observed by optical microscope in lightly boron doped samples was very low. However, in heavily boron doped samples, the density of oxygen precipitation was the largest at 600 °C in the first annealing, and decreased abruptly until 800 °C. But it increased slightly from 800 to 1000 °C and was independent with the implantation. The location and type of secondary defects were inspected by high resolution transmission electron microscopy. The aligned dislocation was located in the vicinity of the amorphous/crystalline interface in the case of the first annealing temperature of 600 °C. But the dislocations climbed to the surface for the first annealing temperature of 700–1000 °C. The difference indicated that the nucleation might be occurring at the first annealing of 600 °C but growth of defects occurred at 700 °C or above.

Fast radiochemical extraction of carbon in photoactivation analysis of pure boron

Special fast procedure for radiochemical extraction of carbon impurities from boron samples was developed. The procedure consists of alloying the sample in a mixture of KNO3+KOH at 600°C, dissolving of the cooled melt in water, and subsequent distillation of CO2 from the boiling acidic solution of HNO3. Time span of the whole procedure equals 25 min and provides more than one thousand-fold extraction of initial carbon impurity from the boron sample. Kinetics of the carbon extraction was measured with carbon-11 radiotracers which were induced in the investigated samples by the irradiation with 26 MeV bremsstrahlung.

The use of MFM for investigating domain structures in modern permanent magnet materials

Magnetic force microscopy occupies a special niche in the array of techniques which are currently available for imaging magnetic structures of high energy permanent magnet materials, yielding high resolution data under ambient conditions on bulk samples. The high anisotropies of permanent magnet samples mean that to a good approximation the tip stray field does not modify the magnetic state of the sample. However strong stray fields, of the order of 1 Tesla, may be experienced by a tip in close proximity to the sample. These stray fields are known to perturb the magnetic state of the tip considerably. As a result, contrast may be generated between neighboring domains, or at domain boundary walls or a combination of both, depending on the degree of perturbation of the tip. Examples demonstrating this variation with a series of batch-fabricated tips with various magnetic coatings ranging in coercivity from ∼1400 Oe to <1 Oe are presented. The origin of the different types of observed contrast is discussed, and it is shown that very low coercivity tips coated with Fe nanoparticles embedded in a SiO 2 matrix provide the most readily understood contrast, and have an additional benefit in that they are `self-focusing'. That is, the active magnetic volume scales with the characteristic dimension of the sample domain structure. Image interpretation is also discussed in the context of specific MFM images from samples of neodymium iron boron (sintered, melt-quenched and nanocomposite) and barium ferrite.

Petrology and Geochemistry of Subbituminous Coals from the Red Deer River Valley, Alberta Plains, Canada

Coals and associated carbonaceous strata along the Red Deer River Valley in Alberta have a wide variation in boron concentrations (10–628 ppm). Boron concentrations decrease from bottom to top of the coal-bearing succession, closely reflecting changes in depositional environment conditions. These changes range from subaquatic deposition in the delta plain area and influence of brackish waters due to a marine transgression, to deposition in areas removed from any brackish water influence. There is good agreement between boron variation and depositional environment as interpreted from regional geology. However, the relation between boron and sulfur is not clear; samples with high boron concentrations are high in sulfur while low boron samples also have high sulfur. Compared to mean concentrations in world coals, some of the highest elemental concentrations measured in coals of this study are: As (38.0 ppm), Ba (2800 ppm), Cr (91.0 ppm), Mn (232 ppm), and V (209 ppm). There is a similarity in the vertical variation of the elements Th and Hf while bentonite layers are enriched in Ba and Sr, pointing to the presence of gorceixite. No enrichment of REEs was observed in the bentonite layers. A comparison of elemental concentration to world mean values for coab shows that the RDRV coals are elevated in As (up to 4×x), Cr (up to 12×), Mn (up to 4×), Mo (up to 12×), Th (up to 5×), V (up to 12×), and Zn (up to 5×). Compared to coals from the eastern Alberta Plains of similar rank, age, and depositional environment, the RDRV coals have higher or similar concentrations of all elements of environmental significance, with the exception of Be and Pb. When compared to coals of similar rank in the western Alberta plains deposited under freshwater conditions, the RDRV coals have higher concentration of As, B, Ba, Sb, and Sr, similar concentrations of Be, Co, Cu, Mn, Pb, Se, Th, and U, and lower concentrations of Ni.

Experimental test of the possible violation of the Pauli exclusion principle by photo-activation analysis of carbon content in pure boron

Experiments which search for chemically boron-like carbon nuclei in samples of pure boron were carried out by a photo-activation technique. Use was made of the 12C( γ,n) 11C reaction with irradiation by bremsstrahlung photons of below 30 MeV. The resulting positron active isotope carbon-11 has a half-life of 20 min, and there are no radioactive nuclides formed by boron. These features of carbon-11 allow use of the rather sensitive and selective γ– γ coincidence technique to detect carbon nuclei. Special radiochemical procedures were developed for purification of the samples from initial carbon and metal impurities in the samples. Experimental results provided an estimate of the lifetime of atomic electrons relative to the Pauli exclusion principle of T>10 21 yr.

Boron dusts produced by arc vaporization using boron rods

Elemental boron can be obtained by reducing boron oxide with Mg or by chemical vapor deposition (CVD) of the gas mixture of boron halide and hydrogen [1±3]. The solid boron appears in at least three crystal forms, a-rhombohedral, â-rhombohedral and tetragonal, which are based on icosahedron B12 unit. In addition to these modi®cations, there exists amorphous boron, which is also composed of a random arrangement of B12 icosahedrons [4±7]. The arc vaporization method is frequently used for producing the particles and ®lms of high-melting materials, such as carbon, W and Mo [8, 9], but it has not been applied to boron because it is an insulator at room temperature. This letter reports the experimental results on arc vaporization of boron using a pair of boron electrodes and is concerned with the conditions for generating an arc discharge and with the nature of the resultant materials. The boron electrodes used in this study were prepared by machining sintered boron produced by hot-pressing. The raw material for the boron was amorphous with a small amount of â-rhombohedral phase (Herman Stark y300 mesh 99% B), containing oxygen and Mg impurities, which was hotpressed at a temperature of 1850 8C under a pressure of 15 MPa in an atmosphere of argon, as reported elsewhere [10]. The sintered boron bodies had a strength suf®cient for subsequent handling, despite their low densities of around 1.0 g=cm. It is dif®cult to generate the arc discharge at room temperature because the electrical conductivity of boron is very low. The conductivity of the rectangular-shaped boron samples from the sintered body were measured, and Fig. 1 shows the electrical conductivity as a function of 1=T. The conductivity increased rapidly with the temperature reaching 0.1 ohmy1 cmy1 at 700 8C. This required that the boron electrodes be pre-heated to a temperature at least above 600 8C to make it possible to generate the discharge. Fig. 2 shows the apparatus used in this study. This consists of a quartz reaction tube for generating the arc discharge and an electric furnace for heating the tube. A pair of the boron electrodes, 7 mm in diameter, between two carbon joints that were connected to an a.c. electric source, were held along the axis of the quartz tube, which had an inner diameter of 25 mm. The sealed quartz tube was evacuated to remove air before admitting the desired gas ow and heated to a temperature of 600 8C. The electrodes were contacted manually with each other, which caused them to become red-hot by resistive self-heating. When the electrodes were separated to a distance of 3±5 mm, the arc discharge occurred with a violent emission of light, accompanied with the vaporization of boron. The experiments were performed at a voltage of 100 V and a current of about 10 A. It appeared that the occurrence and stability of the discharge depended on the magnitudes of the electric voltage and the current. Voltages below 70 V made it dif®cult to generate the discharge, while higher currents induced an excess heating of the electrodes, resulting in the softening of the boron. Crystallographic measurements were carried out using an MXP3VA-BR X-ray diffractometer (XRD, Mac Science) with a graphite monochromator.

Opacity measurements of a radiatively heated boron sample

A preliminary measurement of the opacity of a thin boron foil radiatively heated at moderate temperatures (5–20 eV) in conditions near local-thennodynamic-equilibrium (LTE) are reported. The opacity of a 0.37 μm thick boron sample has been measured in the K-shell range (40–60 Å) using a 5000 l mm −1 transmission grating. The sample is heated by the X-rays emitted at the rear side of two laser-irradiated 0.1 μm thick gold foils. Ab-initio atomic structure calculations are used to fit the experimental spectrum and deduce the ionization charge distribution. Using a non-LTE kinetic code, the heated sample temperature and density ranges are determined. The plasma conditions are predicted with a radiative hydrodynamic code and compared with experimental data. This preliminary analysis shows that the numerical tools are ready for the analysis of future experiments which will be performed with cleaner experimental conditions.

The influence of boron doping on the structure and characteristics of diamond thin films

Abstract Diamond thin films doped with various boron concentrations were grown by hot-filament-assisted chemical vapor deposition. The charge carrier density in the films ranges from 3.4 × 10 17 to 1.8 × 10 21 cm −3 . TEM studies show that all films are of cubic diamond structure with no impurity phase. As the boron concentration increases, the average grain size of the film decreases from 10 to 1 μm. A significant decrease in plane defect density and a slight increase in dislocation density were also observed as boron content increases. Penetration twin, caused by secondary nucleation, was found to be a common feature in films with high boron contents. It is considered that the small grain size in heavily boron doped samples is mainly caused by the formation of these twins. Significant differences were observed from the Raman spectra of the films with different boron content. As the boron concentration increases, the zone center optical phonon line at 1332 cm −1 downshifts and weakens while broad peaks around 465 and 1220 cm −1 appear and their intensities increases. This is considered to be the result of the breakdown of the k = 0 selection rule due to the high boron content.

Cross section of the B(α,p)C nuclear reaction for analytical applications

Cross sections (cs) of the 10B(α,p 0) 13C, 10B(α,p 1) 13C and 11B(α,p 0) 14C reactions for NRA applications were measured for α-particle energies between 4 and 5 MeV and a laboratory scattering angle of 135° at the 7 MV Van de Graaff accelerator of IRMM by irradiation of enriched boron-10 and natural boron targets. Absolute cs values were derived by using the simultaneously measured RBS spectra of the α-projectiles on the Ta backings of the boron samples. The cs errors are less than 10%.

SIMS Depth Profiling of Delta-doped Layers in Silicon

Boron and germanium 6-doped silicon samples were studied using SIMS depth profiling on a Cameca IMS-4F instrument with O 2 + , N 2 + and Cs + primary beams at various energies and incidence angles. The depth resolution characteristics were compared. We show that, with experimental conditions being the same, the N 2 + primary beam provides for better decay lengths, while the leading edges of the profiles turn out to be significantly broadened because of the beam-induced roughness which develops at an early stage of silicon bombardment by N 2 + ions. The influence of ripple-like roughness on the SIMS profile shape is considered within the present simple analytical model. The effect of a modified layer on the SIMS profile depth was experimentally investigated. For profiles obtained with an N 2 + primary beam, it was shown that swelling of the modified layer produced by the implantation and trapping of primary particles shifts the profile as a whole toward the surface.

Phase identification of boron nitride thin films by polarized infrared reflection spectroscopy

Six different types of boron nitride films were investigated by polarized infrared reflection spectroscopy. Films with a highly cubic, mixed cubic and noncubic, and exclusively noncubic phase composition were synthesized using ion beam assisted deposition. Additionally, postdeposition argon ion irradiated cubic and noncubic boron nitride films as well as a nitrogen implanted boron sample were analyzed. Using this technique, besides the cubic phase, two different noncubic modifications, layered anisotropic and amorphous, could be distinguished. A preferential orientation of the normal axis of the sp2-bonded basal planes parallel to the substrate surface was observed.

H-[B]-ZSM-5 as catalyst for methanol reactions

Hydrothermally synthesized silicalite-I, [Al]-ZSM-5 and borosilicates with the MFI structure were subjected to several characterization techniques (SEM, IR, BET, XRD, NMR, ICP and ion exchange measurements). It was confirmed that boron had entered the framework in the boron samples. In the protonated from the borosilicate catalyzed the alkylation of toluene by methanol and also the dehydration of methanol to hydrocarbons, though it was much less active than H-[Al]-ZSM-5 and showed markedly different selectivities. It was observed that whereas 98–99% of the methanol was converted to C 2+ hydrocarbons over H-[Al]-ZSM-5 the corresponding conversion over H-[B]-ZSM-5 was less than 0.2%.

Optical properties of amorphous boron

Amorphous boron samples of different origin were investigated by X-ray diffraction, scanning electron microscopy, i.r. and Raman spectroscopy. Traces of a crystalline β-rhombohedral boron phase (less than 5–6%) were found in one specimen, while the other was completely amorphous. Amorphous boron is known to consist of B 12 icosahedra statistically bonded. This situation is different from that in really statistically arranged atoms, where the symmetry selection rules of phonons do not hold, and both the i.r.-active and the Raman-active phonon spectra are expected to yield the phonon density distribution. In amorphous boron there is a certain similarity, but neither a far-reaching agreement of these spectra with one another nor with the phonon density determined by neutron scattering by Medwick et al. or Delaplane et al. The marked short-range and medium-range order of amorphous boron are indicated by pronounced maxima in the ranges of the intra-icosahedral vibrations and in particular of the two-center B-B bonds known from crystalline boron-rich solids. This confirms that the external bonds of the icosahedra are largely covalently saturated, like in many crystals. Strong optical excitation of amorphous boron leads to an increasing order, probably caused by electron-phonon interaction.