Concentration Of Metal Impurities Research Articles

Preparation of extrapure potassium nitrate

This paper examines isothermal potassium nitrate crystallization from aqueous solutions and the influence of metallic impurity concentration, percent crystallization, and washing of the crystals on the impurity distribution. We have obtained potassium nitrate with a content of each impurity in the range 5 × 10−7 to 1 × 10−5 wt %.

Purification of metallurgical silicon through directional solidification in a large cold crucible

A large cold crucible has been used to purify metallurgical silicon in an induction heating furnace. Melt and directional solidification of more than 200kg metallurgical silicon melt are realized by controlling down speed of the cold crucible relative to an induction coil. Metal impurity content in metallurgical silicon is reduced to a very low level, typically below 0.05ppmw for transition metals like iron. The total processing time is within 30h, and the mass ratio is higher than 70%. A numerical model is set up to reveal the electromagnetic field, the temperature distribution and the flow field. This purifying method could remove metal impurities effectively and has advantages of low contamination and low cost by avoiding use of graphite and quartz crucibles.

Fabrication of glasses in the TeO2-WO3 system using plasma chemical oxidation of tellurium and tungsten chlorides

The possibility of producing tungsten-telluride glasses using plasma chemical vapor phase deposition using corresponding chlorides as source components has been experimentally demonstrated. Homogeneous layers in the entire vitrification range of the TeO2-WO3 system have been obtained. The concentration of metal impurities in the deposited layers does not exceed n × 10−4 wt % that is comparable with their concentration in the source chlorides. Using the techniques of differential scanning calorimetry and Raman scattering, it has been shown that, at the melting stage, the layers obtained are vitreous and comparable in their characteristics with the glasses fabricated using traditional techniques.

Optimization of CVD growth of CNT-based hybrids using the Taguchi method

Taguchi method, extensively applied for the optimization of multifactor processes in the most diverse fields, is for the first time applied to the synthesis of hybrids based on C nanotubes by iron-catalyzed chemical vapor deposition in 1:1 i-C4H10+H2 atmosphere. For this purpose, assumed synthesis-temperature (500°C, 600°C or 700°C), support-material (alumina, magnesia or Na+-exchanged K10 montmorillonite), calcination (450°C, 600°C or 750°C) and reduction (500°C, 600°C or 700°C) temperature of the iron catalysts as the four factors of importance in the process, nine suitably designed experiments are conducted and the influence is evaluated of the four three-level factors on the issue of the process in terms of selectivity toward nanotubes, catalytic yield and content of carbonaceous and metallic impurities in the C nanotubes. By this procedure, the configurations giving optimal results are predicted, and tested by carrying out new experiments.

다이아몬드 코팅 와이어로 가공된 태양전지용 실리콘 웨이퍼의 표면 특성에 관한 연구

현재 결정질 태양전지에서 웨이퍼 가공은 대부분 슬러리 분사 방식의 다중 와이어를 이용한 방법이 사용되고 있다. 이와 같은 슬러리 분사 방식의 웨이퍼 가공은 가공속도가 낮아 생산성이 떨어지는 단점이 있을 뿐만 아니라 금속 재질의 와이어와 실리콘 블록의 직접적인 마찰에 의하여 웨이퍼 표면의 금속 불순물에 의한 오염이 발생되는 단점이 있다. 뿐만 아니라 와이어와 실리콘 블록간의 직접적인 마찰로 인하여 와이어가 빨리 마모되며, 이로 인하여 일회성의 와이어를 사용하게 되면서 제조원가는 상승하게 된다. 반면에 다이아몬드 입자가 코팅된 와이어를 이용하여 실리콘 웨이퍼를 가공하게 되면, 가공속도가 기존 슬러리 분사방식보다 빠르며, 공정진행에 따른 와이어의 마모율이 적어 와이어의 재사용에 의한 제조원가 절감이 가능하다. 따라서 이와 같은 다이아몬드 입자가 코팅된 와이어를 이용하여 가공하는 기술은 슬러리 분사방식에 비하여 더 효율적이라 할 수 있다. 본 연구에서는 슬러리 분사방식으로 가공된 웨이퍼와 다이아몬드 코팅된 와이어로 가공된 웨이퍼의 표면특성에 대하여 분석하고 셀 공정에 영향을 미치는 것에 대하여 설명하고자 한다. 또한, 다이아몬드 와이어로 가공된 웨이퍼를 활용하기 위한 셀 공정의 개선방향에 대하여 제안하고자 한다. Most of the silicon cutting methods using the multi-wire with the slurry injection have been used for wafers of the crystalline solar cell. But the productivity of slurry injection cutting type falls due to low cutting speeds. Also, the direct contact with the metal wire and silicon block increases the concentration of metallic impurities in the wafer's surface. In addition, the abrasive silicon carbide (SiC) generates pollutants. And production costs are rising because it does not re-use the worn wire. On the other hand, the productivity of the cutting method using the diamond coated wire is about 2 times faster than the slurry injection cutting type. Also, the continuous cutting using the used wire of low wear is possible. And this is a big advantage for reduced production costs. Therefore, the cutting method of the diamond coated wire is more efficient than the slurry injection cutting technique. In this study, each cutting type is analyzed using the surface characteristics of the solar wafer and will describe the effects of the manufacturing process of the solar cell. Finally, we will suggest improvement methods of the solar cell process for using the diamond cutting type wafer.

Effect of trimethylsilane pre-capping on monomeric C18 stationary phases made from high-purity type-B silica substrates: Efficiency, retention, and stability

Silica-based monomeric C18 stationary phases are the most widely used in reversed-phase liquid chromatography (RPLC), and methods to improve the efficiency and chemical stability of such phases are always being investigated. Previous work showed that stationary phases made by pre-treating the silica surface with small amounts of trimethylsilane (TMS) reagents prior to C18 silanization showed vast improvements in the chromatographic efficiencies of both polar and non-polar analytes. It was concluded that this "pre-capping" step improved efficiency by selectively neutralizing the most reactive highly acidic silanol sites, producing a more energetically homogenous surface prior to exhaustive derivatization that subsequently yielded a more evenly distributed alkyl bonding arrangement. These previous studies were performed on Type-A silica, an older variety of silica gel material that contains higher levels of metal impurities than the Type-B silica used today. The purpose of the work presented here is to investigate the effects of trimethylsilane pre-capping on monomeric C18 stationary phases made from high-purity Type-B silica substrates. The results show little or no efficiency improvements for non-polar compounds, but a more noticeable improvement was observed for some drug compounds and bases under buffered conditions, with the magnitude of the improved efficiencies correlating with metal impurity content and physical parameters of the silica substrate. The results lend supporting evidence that metal impurities are the primary source of highly acidic silanols, but they also suggest a means to improve efficiencies for compounds of interest. Pre-capping also resulted in a decreased retention due to a decrease in carbon loading. In addition, hydrolytic stability of the stationary phases was investigated at high pH and elevated temperature, with results indicating a slight decrease in hydrolytic stability with increased pre-capping.

Influence of defects and impurities on the deteriorated border region in multicrystalline silicon ingots

AbstractThe origin of the lower minority carrier lifetime in the border region of multicrystalline silicon (mc‐Si) ingots was investigated. Based on minority carrier lifetime measurements and chemical analyses, distributions of defects and impurities were studied. It is found that the content of oxygen, carbon and other metal impurities in the border region were almost at the same level as that in the bulk, while the concentration of Fe‐B pairs is significantly higher. More interestingly, the dislocation density in the border region was observed to be much lower than that in the adjacent part. It is elucidated that the lower dislocation density in the border region leads to more iron existing in the form of Fe‐B pairs, which could be responsible for the low lifetime. In addition, it is shown that the low minority carrier lifetime in the border region can be improved effectively by phosphorous gettering. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Comparison Studies between Hydrogenation and Oxidation of MWNTs Followed by Acid Treatment

MWNTs obtained using iron catalyst and ethylene as a carbon source were submitted to a purification procedure. To purify these materials from amorphous carbon two parallel methods--based on either hydrogen or air treatment--were applied. At the second purification stage iron particles were removed using 1 M or 5 M nitric or hydrochloric acids. The phase composition of the samples was determined using X-ray diffraction method. The sample morphology was characterized using a high resolution transmission electron microscopy. The relative fraction of impurities in the samples was estimated by Raman spectroscopy and the quantitative analysis of metal impurity content was validated by means of thermogravimetric analysis.

Refractive index spectral dependence, Raman spectra, and transmission spectra of high-purity Si28, Si29, Si30, and Sinat single crystals

Precise measurement of the refractive index of stable silicon isotopes $^{28}$Si, $^{29}$Si, $^{30}$Si single crystals with enrichments above 99.9 at.% and a silicon single crystal $^{nat}$Si of natural isotopic composition is performed with the Fourier-transform interference refractometry method from 1.06 to more than 80 mkm with 0.1 cm$^{-1}$ resolution and accuracy of $2 \times 10^{-5} ... 1 \times 10^{-4}$. The oxygen and carbon concentrations in all crystals are within $5 \times 10^{15}$ cm$^{-3}$ and the content of metal impurities is $10^{-5} ... 10^{-6}$ at.%. The peculiar changes of the refractive index in the phonon absorption region of all silicon single crystals are shown. The coefficients of generalized Cauchy dispersion function approximating the experimental refractive index values all over the measuring range are given. The transmission and Raman spectra are also studied.

Purified Silicon Film Formation from Metallurgical-Grade Silicon by Hydrogen-Plasma-Induced Chemical Transport

A purified Si film is prepared directly from metallurgical-grade (MG) Si by chemical transport using sub-atmospheric pressure H2 plasma. The purification mechanism is based on the selective etching of Si using atomic H. It is demonstrated that the concentrations of most metal impurities (e.g., Fe, Cr, Ni, Ti, and Mn) in the prepared Si film are in the acceptable range for solar-grade Si material, or below the determination limit of the several impurity measuring methods employed in this study. From the infrared absorption measurements of the etching product produced by the reaction between H2 plasma and MG-Si, it is found that the main etching product is SiH4. Therefore, a remote-type chemical transport process is developed to produce SiH4 gas directly from MG-Si. Using other purifying principles (such as a pyrolysis filter in combination with this process), it is demonstrated that purified Si films about B, P and metal atoms can be produced from metallurgical-grade Si (<98% purity).

The role of surface functionalization in the solar light-induced production of reactive oxygen species by single-walled carbon nanotubes in water

Risk assessment of engineered nanomaterials necessitates the need for information on reactivity and persistence of these materials in the environment. To this end, we report on reactivity of single-walled carbon nanotubes (SWCNTs) that: (i) have been functionalized by acid oxidation to contain carboxylic acid groups, (ii) have been further functionalized by esterification with polyethylene oxide groups, or (iii) were unmodified (i.e., unfunctionalized), with all materials used as received from the manufacturer. Aqueous colloidal dispersions of both types of functionalized SWCNTs generated reactive oxygen species (ROS) including singlet oxygen ( 1O 2), superoxide anion ( O 2 - ) , and hydroxyl radicals ( OH) in light within the solar spectrum ( λ = 300–410 nm) under oxic conditions. Under similar experimental conditions (i.e., several days under light near solar intensity) but with a surfactant added as a dispersing agent, aqueous dispersions of unfunctionalized SWCNTs exhibited no measurable ROS production. Defects in the surface caused by functionalization, as well as differences in amorphous carbon and metal impurity content within different SWCNT preparations, may facilitate some ROS production. Adding β-nicotinamide adenine dinucleotide to dispersions of carboxylated SWCNTs resulted in O 2 - generation in the absence of light, suggesting dark reaction electron transfer as a potential mechanism of toxicity.

Fluorescence Quenching of Dyes Covalently Attached to Single-Walled Carbon Nanotubes

The development of chromophore-carbon nanotube hybrids requires efficient and accurate methods to investigate their photophysical properties. Using the ability of the fluorescence labeling of surface species (FLOSS) technique to determine the density of covalently attached dyes to the surface of single-walled carbon nanotubes (SWCNTs), the luminescence of dye-SWCNT hybrids was quantitatively studied with two chromophores: dansyl hydrazine (DH) and panacyl bromide (PB). The fluorescence intensity of PB-SWCNT hybrids was reduced by 20-80% compared to that of free PB. A strong positive correlation between the degree of quenching and the residual metal impurity content in the SWCNT sample suggests that quenching of fluorescence of PB in PB-SWCNTs may be caused by the metal impurities and not by SWCNTs. On the contrary, the intensity of fluorescence of DH-SWCNT hybrids was reduced by almost 2 orders of magnitude compared to free DH, independent of the residual metal content in the SWCNT sample, suggesting that quenching of fluorescence in DH-SWCNT hybrids might occur via charge transfer from DH chromophores to SWCNTs, and revealing the potential of DH-SWCNT hybrids for solar light harvesting applications.

Study of Dislocation Generation and Growth Orientations in Upgraded Metallurgical Grade Multicrystalline Silicon

We used high-purity multicrystalline silicon prepared by metallurgical method for the study of directional solidification. The optical microscope was used to observe the etch pits on the surface of silicon wafers, and we calculated their dislocation density. The result showed the space distribution of dislocation density presented “V” shape for each ingot produced at different drop-down rates. The dislocation density of slicon ingots followed the order 10&lt;20&lt;30&lt;40μm/s. The high-resolution glow discharge mass spectroscopy was used to analyze the concentration of transition metal impurities. The macro-morphology of vertical-section of silicon ingots growth at different drop-down rates was observed. The x-ray diffraction measurement was performed to analyze the crystallographic orientations of the silicon ingot growth at 20μm/s, which was a better drop-down rate for producing high-quality multicrystalline silicon.

Effects of high temperature annealing on the dislocation density and electrical properties of upgraded metallurgical grade multicrystalline silicon

High temperature annealing was performed on upgraded metallurgical grade multicrystalline silicon (UMG multi-Si) wafers with a purity of 99.999%. The samples were mechanically polished and chemically etched, and then the microstructures were observed by a scanning electron microscope (SEM). The minority carrier lifetime and resistivity of the samples were measured using microwave photoconductance decay and four-point probe techniques, respectively. The results show that the electrical properties of the samples decrease rather than increase as the annealing temperature increases, while the number of dislocations in bulk Si reduced or even disappeared after annealing for 6 hours at 1100–1400°C. It is considered that the structural microdefects induced by the high concentration of metal impurities (including interstitial or substitutional impurities and nanoscale precipitates) determine the minority carrier recombination activity and thus the electrical properties of UMG multi-Si wafers rather than dislocations in bulk Si.

Synthesis and analysis of multi-walled carbon nanotubes/oxides hybrid materials for polymer composite applications

Nanotubes-based nanocomposites to be used as polymer reinforcing/flame-retardant additives are synthesized by decomposition of isobutane at 600 °C. Catalytic chemical vapor deposition (CCVD) is carried out over 17 wt%Fe-catalysts supported on various oxides (Al 2O 3, MgO, CaO, SrO or BaO) reduced at 600 °C. Catalysts utilized and carbonaceous deposits obtained are systematically characterized by the use of several analysis techniques, in order to investigate the influence of catalyst specifics on reaction yield, selectivity and characteristics (crystallinity and purity) of the grown nanotubes. The results show that the support greatly influences the catalyst performance. The lack of metallic iron renders Fe/SrO- and Fe/BaO-catalysts inactive. Fe/Al 2O 3 catalysts exhibit the highest catalytic activity, but give rise to scarce selectivity and large metallic impurity contents. Contrarily, using Fe/MgO and Fe/CaO catalysts leads to lower yields, but allows reducing impurities and remarkably improving selectivity and nanotube crystallinity.

Kinetic characteristics of concentration and isolation of metal impurities from solutions and industrial wastewater by ion flotation

The process of isolation of ferrous and nonferrous metals from diluted solutions in the form of simple hydrated cations and anions of complexes, as well as the dispersed phase of hydroxides and poorly soluble sublates, respectively, in the modes of “foam fractionation” and ion (“foam”) flotation, is studied. The flotation rate constant and the concentration coefficient of metals upon ion flotation are ascertained to be several times higher than those upon foam fractionation. A technological scheme for finish purification of industrial wastewater by flotation is proposed. The prevented environmental and economic damage from the contamination of the environment by wastewater is calculated.

Removal of metallic impurities in metallurgical grade silicon by directional solidification

Multicrystalline silicon ingot of w 130 × 120 mm, with characteristics of large and well directional columnar crystals, is obtained by Bridgman directional solidification (DS). Metallic impurities mainly form impurity particles instead of depositing in the grain boundaries. Solid/liquid interface is changed from concave to convex during DS process. Removal efficiencies of Al, Fe, Ca, Ti and Cu reach 96·4, 90·5, 96·6, 95·7 and 96·3% respectively, at the edge of ingot with the height of 32·5 mm. Contents of metallic impurities in solar grade silicon can be satisfied by DS twice through theoretical calculation.

Towards solar grade silicon: Challenges and benefits for low cost photovoltaics

It is well known that silicon in its various structural configurations (single crystal, multicrystalline, amorphous, micro-nanocrystalline) supplies almost 90% of the substrates used in the photovoltaic industry. It is also known, since years, that the photovoltaic (PV) industry shows a marked growth trend, which demanded and demands a continuous, huge increase of the bulk silicon supply in the order of 30%/yr. In order to fulfill their today- and future needs, many companies worldwide took the decision to start the installation of many thousand tons/year plants, most of them using the Siemens process, some of them using the MG route, to produce the so called solar grade (SG) silicon. The advantages of the Siemens process are well known, as it provides ultrapure silicon, directly usable for growing either single crystalline Czochralski ingots or multicrystalline ingots using the directional solidification (DS) technique. The disadvantages are its high energetic cost (a minimum of 120 kWH/kg) and the possible losses of chlorinated gases in the atmosphere, with possible severe environmental problems. The advantages of the MG route are still potential, as there is no commercially available production of solar silicon as yet, and rely on its reduced energetic costs (a maximum of 25–30 kWh/kg) for a feedstock directly usable for growing multicrystalline ingots using the DS technique. The drawbacks of silicon of MG origin are its larger concentration of metallic impurities, as compared with the Siemens one, the higher B and P content, and the potentially high carbon content. The aim of this paper is to deal with some of the problems encountered so far with the silicon of MG origin with respect to the metallic and non-metallic impurities content, as well as to propose technologically feasible solar grade feedstock specifications.

Chemical Transport Deposition of Purified Poly-Si Films from Metallurgical-grade Si Using Subatmospheric-pressure H2 Plasma

AbstractPurified Si film is prepared directly from metallurgical-grade Si (MG-Si) by chemical transport using subatmospheric-pressure H2 plasma. The purification mechanism is based on the selective etching of Si by atomic H. Since most metals are not etched by H, this process is efficient to reduce metal impurities in Si films. It is demonstrated that the concentrations of most metal impurities (Fe, Mn, Ti, Co, Cr, Ni, etc.) in the prepared Si film are in the acceptable range for applying it to solar-grade Si (SOG-Si) material, or below the determination limit of the present measurements. On the other hand B and P atoms, which make volatile hydrogen compounds such as B2H6 and PH3, are difficult to eliminate by the present principle. From the infrared absorption measurements of the etching product produced by the reaction between H2 plasma and MG-Si, it is found that the main etching product is SiH4. Therefore, a remote-type chemical transport process is possible to produce SiH4 gas directly from MG-Si. Combining other purifying principle (such as a pyrolysis filter), this process may have an advantage to eliminate B2H6 and PH3 from the produced SiH4 gas.

Purified Si film formation from metallurgical-grade Si by hydrogen plasma induced chemical transport

Purified Si film is prepared directly from metallurgical-grade Si (MG-Si) by using hydrogen plasma induced chemical transport at subatmospheric pressure. The purification mechanism is based on the different hydrogenation behaviors of the various impurity elements in MG-Si. The prepared Si films clearly had fewer typical metal impurities (Fe, Al, Ti, Cr, Mn, etc.) than those in the MG-Si. In particular, the Fe concentration was drastically reduced from 6900 mass ppm to less than 0.1 mass ppm by one time chemical transport. Furthermore, metal impurity concentrations were further reduced by repeating chemical transport deposition.