Activation analysis of high-purity silicon

³²P was separated from the irradiated silicon matrix as ammonium phosphomolybdate and the activity was measured with a GM tube or on a plastic scintillator. The second-order interference was taken into consideration. Research associate of the N.F.W.O.     

Activation analysis of high-purity substances by means of short-lived isotopes

A non-destructive activation analysis method intended for industrial utilization is described, using isotopes with half-lives from 2 sec to 20 min. Expected sensitivity values for 33 elements are listed. The effects on the results of neutron flux variation and other errors of measurement are discussed. Requirements to be met by the electronic spectrometer apparatus are stated.     

Synthesis of high-purity lower silicon alcoholates for new technology

Purification of a number of silicon derivatives (chloride, lower alcoholates) formed in processing of high-purity silicon for electronic and electrical industries was considered. It was found that high-purity substances suitable for use in electronics and optics can be produced from industrial wastes.     

Study of a subfluoride process for production of high-purity silicon

Process was studied in which high-purity silicon (1.07–1.35 ppm) is obtained from powdered technical-grade silicon (99.81–99.86% Si) produced by acid refining to remove impurities by its treatment with gaseous SiF₄ at a temperature of 1200°C to give pure SiF₂ (gas), with its subsequent decomposition at temperatures lower than 800°C into silicon and SiF₄ (gas) circulating in the system.     

Perforated diode neutron detector modules fabricated from high-purity silicon

Compact neutron detectors are being designed and tested for use as low-power real-time personnel dosimeters. The neutron detectors are pin diodes that are mass produced from high-purity Si wafers. Each detector has thousands of circular perforations etched vertically into the device. The perforations are backfilled with ⁶LiF to make the pin diodes sensitive to thermal neutrons. The prototype devices deliver 4.7% thermal neutron detection efficiency while operating on only 15 V, showing a 9% increase in efficiency over identical planar devices.     

Practical analysis of high-purity chemicals-I Preparation and characterization of high-purity EDTA

Advanced laboratories have requirements for high-purity chemicals with less than 500 ppm total impurities (ultrapure chemicals) and with broad analytical definition of each lot. Some economically feasible approaches to the practical analysis of such chemicals, both inorganic and organic, are delineated. Compounds used in the study of lunar samples and in other advanced programmes are noted. EDTA, as the free add, has been prepared by dissolution in water with base and precipitation by addition of add. The product has been broadly characterized. Precision assay is achieved by weight titrimetry, potentiometrically as a triprotic add and photometrically as a chelatmg agent. Other tests applied include elemental analysis, ash, loss on drying, particulatc matter, and tests for nitrilotriacetate, arsenic, and chloride. Boron, silicon, and trace metals are determined by emission spectrography. Many of the procedures are applicable to other high-purity organic chemicals.     

Preparation of high-purity straight silicon nanowires by molten salt electrolysis

Silicon nanowires of high purity and regular morphology are of prime importance to ensure high specific capacities of lithium-ion batteries and reproducible electrode assembly process. Using nickel formate as a metal catalyst precursor, straight silicon nanowires(65–150 nm in diameter) were directly prepared by electrolysis from the Ni/SiO_2 porous pellets with 0.8 wt% nickel content in molten CaCl_2 at900 °C. Benefiting from their straight appearance and high purity, the silicon nanowires therefore offered an initial coulombic efficiency of 90.53% and specific capacity of 3377 m Ah/g. In addition, the silicon nanowire/carbon composite exhibited excellent cycle performance, retaining 90.38% of the initial capacity after 100 cycles. Whilst further study on the charge storage performance is still ongoing, these preliminary results demonstrate that nickel formate is an efficient and effective metal catalyst precursor for catalytic preparation of high purity straight silicon nanowires via the molten salt electrolysis, which is suitable for large-scale production.     

Activation analysis and autoradiography of impurity distributions in silicon multilayer structures

Activation analysis and autoradiography were used to investigate the concentration distribution of contaminants in poly-Si−Si₃N₄−SiO₂−Si substrate multilayer structures (SNOS) on sampling each technological product. Samples were irradiated for 36 hrs at a thermal neutron flux of 4·10¹³n·cm⁻²·sec⁻¹. The thin films of the analysed sample were removed stepwise by selective chemical etching using appropriate masking techniques. Simultaneously autoradiographs were made of the surface of parallel samples activated under the same conditions. The concentration of the technological contaminants (e.g. Na, Cu, Au) increases in the junction interface of the layers as unambiguously shown by the results obtained. Presented at the “4th Symposium on the Recent Developments in Neutron Activation Analysis” Churchill College, Cambridge, 4–7 August, 1975.     

Trace chemical analysis of high-purity glass

Summary Trace-element impurities in high-purity silica (Corning Code 7940 fused silica and J. T. Baker Ultrex silicon dioxide), 96% silica glass (Corning Code 7913), borosilicate glasses (Corning Code 7740 and Owens-Illinois KG-33) and doped optical waveguide glass have been determined by spark-source mass-spectrometry, optical-emission spectrography, neutron-activation analysis, atomic-absorption and plasma emission spectrometry, spectrophotometry, voltammetry and chemical methods. Particular care was taken to prepare samples and carry out analyses in a clean environment with ultrapure reagents. In most cases some 30 elements as well as water were determined by two or more of the techniques indicated. The impurity level for many elements in the high-purity silica and optical waveguide glass is in the ng/g range and in theg/g range for the other glasses. Spark-source mass-spectrometry and neutron-activation analyses were carried out not only on samples prepared by a hydrofluoric acid dissolution-evaporation procedure but also on undissolved samples for volatile elements such as B, P, S, As and Hg. Results obtained are discussed with respect to application of the materials as well as to the analytical methods developed.

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Zusammenfassung Spurenelement-Verunreinigungen in hochreinem Siliciumdioxid (geschmolzener Quarz, Corning Code 7940, und Ultrex, J. T. Baker), in 96%igem Silicaglas (Corning Code 7913), in Borsilikatgläsern (Corning Code 7740 und Owens-Illinois KG-33) und in optischen, wellenleitenden Gläsern wurden mittels funkenangeregter Massenspektrometrie, Emissionsspektrographie, Neutronenaktivierungsanalyse, Atomarabsorption, Plasmaemissionsspektrometrie, Spektralphotometrie, Voltammetrie und mittels chemischer Methoden bestimmt. Besonders geachtet wurde hierbei darauf, die Probenherstellung und deren Analyse in reiner Umgebung mit ultrareinen Reagenzien durchzuführen. In den meisten Fällen wurden etliche 30 Elemente sowie der Wassergehalt mit zwei oder mehr der erwähnten Verfahren bestimmt. Das AusmaS der Verunreinigung hochreinen Siliciumdioxids und optischer wellenleitender Gläser mit vielen Elementen liegt in der Größenordnung ng/g, bei den anderen Glassorten in der Größenordnungg/g. Funkenangeregte Massenspektrometrie und Neutronenaktivierungsanalyse wurden nicht nur auf Proben angewendet, die durch Flußsäurelösung und -verdampfung hergestellt worden waren, sondern auch auf ungelöste Proben zur Bestimmung flüchtiger Elemente wie B, P, S, As und Hg. Die Ergebnisse wurden im Hinblick auf die Verwendung der untersuchten Materialien sowie auf die entwickelten analytischen Verfahren diskutiert.

     

Trace element analysis of high-purity copper

Summary In copper samples trace impurities of the elements As, Bi, Cr, Fe, Pb, Sb, Se and Te in the low g/g range were coprecipitated with lanthanum hydroxide and determined by flame AAS (Fe) and electrothermal AAS (other elements). Reduction of the sample weight results in a considerable saving of time compared to elder procedures and allows to apply centrifugation instead of filtration as separation technique, by which the risk of contamination is decreased. The completeness of separation was examined by analyzing standard solutions. Additionally radio tracer experiments were employed with Sb and Se. Based on recovery data, a classification scheme was set up with regard to adsorption behaviour. High-purity copper samples were analyzed by the procedure described; INAA and ICP-MS were used as comparison methods. Parameters affecting the precision were examined; the main influence was caused by the separation step. The procedure is well suitable for the determination of 2–10 g/g As, Cr and Pb and 5–50 g/g Fe in high-purity copper.Dedicated to Professor Dr. Wilhelm Fresenius on the occasion of his 80th birthday     

A radioreagent method for determination of traces of phosphorus in high-purity silicon

The present study explores the feasibility of the determination of phosphorus at the extreme trace levels in high-purity silicon by radioreagent method. After silicon dissolution with hydrofluoric and nitric acids and matrix volatilization, 12-molybdophosphoric acid (12-MPA) is formed by the addition of the radioreagent,⁹⁹MoO ₄ ^2– , in nitric acid medium and then extracted into isobutyl acetate. By plotting the phosphorus content against the radioactivity of⁹⁹Mo in the organic phase, a linear relationship persisting down to 5 ng is obtained. Special effort has been made to the elimination of the unreacted⁹⁹MoO ₄ ^2– reagent and the optimal control of phosphorus blank introduced through the multistage analytical procedure in order to ensure reliable determination of phosphorus at the ppb level.     

Study on the solid solubility of transition metals in high-purity silicon by instrumental neutron activation analysis and anticompton-spectrometry

The solid solubility of the 3d metals Cr, Mn, Fe, Co and Ni in high-purity silicon was studied by INAA and electron paramagnetic resonance techniques in the temperature range of ≊600°C–1250°C. The solubility increases with atomic number from Cr to Mn and more distinctly from Co to Ni. For Mn, Fe and Co the solubilities are nearly the same within the experimental errors. An enthalpy of formation (ΔH) of about 2.8 eV was determined for Cr, Mn, Fe and Co, whereas for Ni 1.7 eV was derived. Relatively large diffusion coefficients were estimated from the rather short times, in which saturation of the solid solution was reached. From these observations and from the results of the EPR measurements it is concluded that the 3d metals occupy predominantly interstitial sites in the silicon lattice in thermal equilibrium.     

Atomic-absorption methods for analysis of high-purity substances

A review is given of the use of atomic-absorption methods for determining the trace impurities in high-purity metals.     

Mass spectrometric analysis of high-purity carbon dioxide

A high-vacuum, low-temperature, continuous separation technique has been used in conjunction with a mass spectrometer for the analysis of carbon dioxide containing vpm amounts of H(2), He, CH(4), Ne, N(2), CO, O(2) and Ar. The method relies on the condensation of carbon dioxide on the walls of a glass U-tube, cooled in liquid nitrogen, connected between an inlet and the ion source. A high-pressure carbon dioxide sample thus enters the inlet leak but only the impurities pass through the U-tube and reach the ion source, resulting in considerable gain in sensitivity and elimination of interference from carbon dioxide. The sensitivity of the method is several orders of magnitude better than the normal mass spectrometric method.     

Trace analysis of high-purity graphite by LA-ICP-MS

Laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has been established as a very efficient and sensitive technique for the direct analysis of solids. In this work the capability of LA-ICP-MS was investigated for determination of trace elements in high-purity graphite. Synthetic laboratory standards with a graphite matrix were prepared for the purpose of quantifying the analytical results. Doped trace elements, concentration 0.5 microg g(-1), in a laboratory standard were determined with an accuracy of 1% to +/- 7% and a relative standard deviation (RSD) of 2-13%. Solution-based calibration was also used for quantitative analysis of high-purity graphite. It was found that such calibration led to analytical results for trace-element determination in graphite with accuracy similar to that obtained by use of synthetic laboratory standards for quantification of analytical results. Results from quantitative determination of trace impurities in a real reactor-graphite sample, using both quantification approaches, were in good agreement. Detection limits for all elements of interest were determined in the low ng g(-1) concentration range. Improvement of detection limits by a factor of 10 was achieved for analyses of high-purity graphite with LA-ICP-MS under wet plasma conditions, because the lower background signal and increased element sensitivity.     

Spectrographic determination of ultramicro amounts of boron in high-purity silicon tetrachloride and trichlorosilane

A new sensitive method is proposed for the determination of boron in highpurity silicon tetra chloride and trichlorosilane. The method consists of complexing boron with triphenylchloromethane, removal of excess silicon tetrachloride, hydrolysis of the complex, and improved spectrographic excitation of liberated boric acid.The best sensitivity attainable is 0.03μg B, which corresponds to 0.06 p.p.b. B for a 500-g sample.     

Activation analysis: The most important method for control of purity of semiconductor silicon

NAA and CPAA are the most sensitive analytical tools for the determination of residual impurities in semiconductor silicon. Extremely small detection limits are found by measuring the activity in an underground laboratory. The accuracy and reliability of the methods were checked by several comparisons: interlaboratory comparison on NAA, another CPAA with IR-spectrometry as well as comparison of NAA-results, obtained instrumentally and by chemical separation.     

The analysis of solid and liquid high-purity substances

This paper reviews the problems of atomic-spectrometry, mass-spectrometry and activation methods in elemental analysis of high-purity substances. The specific techniques of sample preparation, preconcentration of impurities and the determination of gas-forming trace elements are discussed.