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UNT College of Arts and Sciences
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Materials Science and Engineering
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UNT Scholarly Works
- K-shell x-ray-production cross sections in 6C, 8O, 9F, 11Na, 12Mg, and 13Al, by 0.75- to 4.5-MeV protons
- This article discusses K-shell x-ray-production cross sections. Abstract: K-shell x-ray-production cross sections are reported for elements with K-shell x-ray energies between 277 eV (C) and 1487 eV (Al). The x-ray measurements were made with a windowless Si(Li) detector that was calibrated for efficiency by comparing bremsstrahlung spectra from electron bombardment of thin foils of aluminum, silver, and gold with theoretically determined bremsstrahlung spectral distributions. The x-ray-production cross-section measurements are compared to first-order Born and perturbed-stationary-state with energy-loss, Coulomb deflection, and relativistic corrections (ECPSSR) ionization theories using single-hole fluorescence yields. The ECPSSR and first-order Born theoretical predictions are, in general, in close agreement with each other and both generally fit the data quite well. digital.library.unt.edu/ark:/67531/metadc139500/
- Modified embedded atom method study of the mechanical properties of carbon nanotube reinforced nickel composites
- In this article, the authors report an atomistic simulation study of the behavior of nanocomposite materials that are formed by incorporating single-walled carbon nanotubes (SWCNTs), with three different diameters, and a multiwalled carbon nanotube (MWCNT) into a single-crystal nickel matrix. The interactions between carbon and nickel atoms are described by a modified embedded atom method potential. Mechanical properties of these nanocomposite materials are predicted by atomistic calculations and compared with that of fcc nickel and pristine CNTs. The authors' simulations predict that all Ni/CNT composites studied in this work are mechanically stable. Their elastic properties depend on the volume fraction and diameter of embedded CNTs. The single-crystal Young's modulus (E₁₁) of Ni/SWCNT composites exhibit a large increase in the direction of CNTs alignment compared to that of a single-crystal nickel. However, a moderate but gradual decrease is seen for E₂₂ and E₃₃ in the transverse directions with increase in CNT diameters. As a consequence, Ni/SWCNTs show a gradual decrease for the polycrystalline Young's, bulk and shear moduli with the increasing CNT diameters and volume fractions. These reductions, although moderate, suggest that enhancement of mechanical properties for polycrystalline Ni/SWCNT nanocomposites are not achievable at any CNT volume fraction. The Ni/MWCNT composite with high CNT volume fraction shows the highest increase in E₁₁. Unlike the E₂₂ and E₃₃ for Ni/SWCNTs, there is a significant increase in the E₂₂ and the E₃₃ for Ni/MWCNT. As a result, polycrystalline Ni/MWCNT composites show slight increase in the elastic properties. This suggests that nickel nanocomposites with enhanced mechanical properties can be fabricated using large volume fractions of larger diameter MWCNTs. Depending on type, alignment and volume fraction, Ni/CNT composites show varying degrees of elastic anisotropy and Poisson's ration compared to pure Ni. Simulation predicts strong adhesion at the Ni/CNT interface and a significant interfacial stress transfer between CNT and Ni matrix. digital.library.unt.edu/ark:/67531/metadc107769/
- Rapid isothermal annealing of As-, P-, and B-implanted silicon
- This article discusses rapid idothermal annealing of As-, P-, and B-implanted silicon. Single-crystal silicon wafers have been implanted with As, P, and B to doses of 1x1013–1x1016/cm2 and given a transient anneal using a Varian IA-200 Rapid Isothermal Annealer. The system uses infrared radiation to heat the wafers to temperatures in excess of 1000 °C for times on the order of 10 sec. Sheet resistance and Hall measurements have been used to determine the effect of the anneal on the electrical properties of the wafers. Rutherford backscattering and secondary ion mass spectroscopy have been used to measure lattice damage and dopant profiles before and after annealing. As and P are lost during the anneal unless the wafer is capped. Complete activation can be achieved with very little dopant diffusion. Residual damage is minimal in (100) oriented wafers that had been implanted with As. However, for (111) wafers damage is less in (111) wafers implanted to doses ≤5.0x1015/cm2. The diffusion of As during this transient anneal has been modeled using a concentration enhanced diffusion coefficient and the wafer temperature profile obtained from an optical pyrometer. digital.library.unt.edu/ark:/67531/metadc139472/
- Spontaneous coordinated activity in cultured networks: analysis of multiple ignition sites, primary circuits, and burst phase delay distributions
- This articles discusses an analysis of multiple ignition sites, primary circuits, and burst phase delay distributions. All higher order central nervous systems exhibit spontaneous neural activity, though the purpose and mechanistic origin of such activity remains poorly understood. The authors quantitatively analyzed the ignition and spread of collective spontaneous electrophysiological activity in networks of cultured cortical neurons growing in microelectrode arrays. Leader neurons, which form a mono-synaptically connected primary circuit, and initiate a majority of network bursts were found to be a small subset of recorded neurons. Leader/follower firing delay times formed temporally stable positively skewed distributions. Blocking inhibitory synapses usually resulted in shorter delay times with reduced variance. These distributions are characterizations of general aspects of internal network dynamics and provide estimates of pair-wise synaptic distances. The resulting analysis produced specific quantitative constraints and insights into the activation patterns of collective neuronal activity in self-organized cortical networks, which may prove useful for models emulating spontaneously active systems. digital.library.unt.edu/ark:/67531/metadc139463/
- Thermal annealing behavior of an oxide layer under silicon
- This article discusses the thermal annealing behavior of an oxide layer under silicon. High resolution Rutherford backscattering spectrometry and ion channeling have been employed to evaluate the crystallinity of the surface silicon layer in oxygen implanted silicon. The quality of the top surface layer was determined by measuring the minimum yields along 〈110〉 directions in channeling spectra. Single crystal (100) silicon was implanted with 300 keV O2+ to a dose of 1.06 X 10(18) O2+/cm2. Measurements of residual damage of the top layer were made after annealing the samples at 1150 ˚C for times ranging from 10 to 240 min in either Ar or N2. Under the implantation conditions used in this experiment, a uniform oxide layer 0.52 μm thick was buried under a top silicon layer 0.17 μm thick. The buried oxide layer has abrupt silicon to oxide interfaces. The highest quality silicon surface layer was produced after 3-h annealing in an Ar ambient. A lesser quality silicon surface layer was produced by annealing for shorter times or for equivalent times in N2 ambient. digital.library.unt.edu/ark:/67531/metadc139474/