Caustic fusion of columbite-tantalite concentrates with subsequent separation of niobium and tantalum

A procedure for the production of spectrographically pure niobium and tantalum oxides from columbite-tantalite concentrates was developed in this study. The procedure involves a caustic fusion process and the separation of tantalum and niobium by liquid-liquid extraction

Electron-Hole Generation and Recombination Rates for Coulomb Scattering in Graphene

We calculate electron-hole generation and recombination rates for Coulomb scattering (Auger recombination and impact ionization) in Graphene. The conduction and valence band dispersion relation in Graphene together with energy and momentum conservation requirements restrict the phase space for Coulomb scattering so that electron-hole recombination times can be much longer than 1 ps for electron-hole densities smaller than $10^{12}$ cm$^{-2}$.Comment: 13 pages, 7 figure

Theoretical study of isolated dangling bonds, dangling bond wires and dangling bond clusters on H:Si(100)-(2×\times1) surface

We theoretically study the electronic band structure of isolated unpaired and paired dangling bonds (DB), DB wires and DB clusters on H:Si(100)-(2$\times$1) surface using Extended H\"uckel Theory (EHT) and report their effect on the Si band gap. An isolated unpaired DB introduces a near-midgap state, whereas a paired DB leads to $\pi$ and $\pi^*$ states, similar to those introduced by an unpassivated asymmetric dimer (AD) Si(100)-(2$\times$1) surface. Such induced states have very small dispersion due to their isolation from the other states, which reside in conduction and valence band. On the other hand, the surface state induced due to an unpaired DB wire in the direction along the dimer row (referred to as $[\bar{1}10]$), has large dispersion due to the strong coupling between the adjacent DBs, being 3.84$\AA$ apart. However, in the direction perpendicular to the dimer row (referred to as [110]), due to the reduced coupling between the DBs being 7.68$\AA$ apart, the dispersion in the surface state is similar to that of an isolated unpaired DB. Apart from this, a paired DB wire in $[\bar{1}10]$ direction introduces $\pi$ and $\pi^*$ states similar to those of an AD surface and a paired DB wire in [110] direction exhibits surface states similar to those of an isolated paired DB, as expected. Besides this, we report the electronic structure of different DB clusters, which exhibit states inside the band gap that can be interpreted as superpositions of states due to unpaired and paired DBs.Comment: 7 pages, 10 figure, 1 tabl

Measurements of Carrier Generation-Recombination Parameters in Silicon Solar Cell Material Using MOS Techniques

Modified and new measurement techniques were developed for determining the carrier generation-recombination (G-R) parameters in silicon solar cell material under carrier deficit and low-level carrier excess conditions using MOS-based test structures. The structures mainly consisted of ring-dot MOS Capacitors (MOS-C) and Schottky-Drained Gate-Controlled Diodes (SGCD). Sample G-R parameters were extracted from n-type high quality silicon solar cell material. Additional measurements were also performed on low-quality ntype silicon substrates for comparison purposes. The photoaccelerated MOS-C Capacitance-time (C-t) transient measurement technique, modified from the standard C-t method, allows one to drastically reduce the observation time in deducing the carrier generation lifetime (Tg) by simply illuminating the test structure during the transient. In applying the technique to MOS-C’s (which exhibited generation lifetime on the order of I msec) the observation time was reduced by approximately an order of magnitude. This is important in dealing with solar cell material because of typically long generation lifetimes. The SGCD structure, which consisted of an extended Schottky diode located next to an MOS-C, was developed and utilized for extracting the surface generation velocity (sg). The measurement is based on recording two C-t transients at Vd = 0 and at Vd = V t , respectively. The structure has a distinct advantage over the conventional PN junction GCD in that it is only slightly more complicated to fabricate and interrogate than a simple MOS-C. It was also demonstrated that steady-state deep-depletion C-V characteristics can be obtained using the SGCD structure. An MOS-C photo/forward-sweep measurement technique was primarily developed to extract the recombination lifetime (rp for n-type substrates) under low-level carrier excess conditions. The new technique is based on the change in inversion capacitance in response to a set of illumination and forward-sweep voltages applied to the MOS-C. The technique conveniently allows one to extract the recombination lifetime under room temperature conditions and was successfully applied to MOS-C’s fabricated on high quality silicon solar cell substrates

Zinc Oxide-on-Silicon Surface Acoustic Wave Devices

A monolithic ZnO-on-silicon surface acoustic wave (SAW) memory correlator has been fabricated which utilizes induced junctions separated by ion implanted regions to store a reference signal. The performance characteristics of this device have been investigated including storage time, dynamic range, and degenerate convolution efficiency. Verification of the existence of charge storage regions is possible prior to completed device fabrication. A theory explaining the charge storage process is developed and applied to the implant-isolated storage correlator. The implant-isolated correlator theory is applied to related structures which employ slightly different storage mechanisms. The ion implanted correlator is used to determine the wave potential associated with a propagating SAW. Characteristics of ZnO-on-Si SAW resonators with sputtered ZnO films limited to the interdigital transducer (IDT) regions are investigated. Upper limits on propagation loss for surface waves on silicon substrates are determined by employing externally coupled limited ZnO SAW resonators. Resonator Q-values are enhanced by restricting the lossy ZnO area and predictions are made as to achievable Q-values for resonators fabricated in the externally coupled configuration. Experimental results for limited ZnO, internally coupled ZnO-on-Si resonators are also given. A complete theory for the mode conversion resonator is presented which predicts the array separation for proper device operation. The theory also gives way to a special condition for spatial ndependence of resonator output with respect to IDT placement. Mode conversion resonators are fabricated which experimentally verify these predictions

Burkholderia pseudomallei in a lowland rice paddy: seasonal changes and influence of soil depth and physico-chemical properties.

Melioidosis, a severe infection with the environmental bacterium Burkholderia pseudomallei, is being recognised increasingly frequently. What determines its uneven distribution within endemic areas is poorly understood. We cultured soil from a rice field in Laos for B. pseudomallei at different depths on 4 occasions over a 13-month period. We also measured physical and chemical parameters in order to identify associated characteristics. Overall, 195 of 653 samples (29.7%) yielded B. pseudomallei. A higher prevalence of B. pseudomallei was found at soil depths greater than the 30?cm currently recommended for B. pseudomallei environmental sampling. B. pseudomallei was associated with a high soil water content and low total nitrogen, carbon and organic matter content. Our results suggested that a sampling grid of 25 five metre square quadrats (i.e. 25?×?25?m) should be sufficient to detect B. pseudomallei at a given location if samples are taken at a soil depth of at least 60?cm. However, culture of B. pseudomallei in environmental samples is difficult and liable to variation. Future studies should both rely on molecular approaches and address the micro-heterogeneity of soil when investigating physico-chemical associations with the presence of B. pseudomallei