silicon for the chemical and solar industry XIII

Silicon for the Chemical and Solar Industry XIII

Kristiansand, Norway, June 13 – 16, 2016

A new and fast method for determination of boron, phosphorus

and other trace elements in metallurgical grade silicon

A. Rietig, J. Acker

Brandenburg University of Technology Cottbus-Senftenberg, Faculty 2 - Department

of Physical Chemistry, 01968 Senftenberg, Germany

Abstract

A new method for accurate and precise determination of non-metallic and metallic

impurities in silicon was developed and statistically validated. The first step is the fast

dissolution of silicon in a microwave-assisted high pressure system to minimize a loss

of phosphorus. The essential innovation is the use of the concentrated digestion

solution for ICP-OES measurements. This approach avoids the common removal of

the silicon and acid matrix by volatilization, which can cause considerable losses of

boron. Finally, for the ICP-OES measurements in such high-silicon matrices the

optimum measuring conditions were determined and a careful selection of emission

lines with respect to selectivity, spectral and non-spectral inferences and matrix

effects was performed. The method of matrix matched calibration (MMC) is used for

quantification of the impurities’ contents. For Al, Mg, Ca, Ti, Cr, Mn, Fe, Ni, Cu, Zr

and P the validation was performed against certified reference materials (IPT134,

IPT135, NIST57b). To validate the determination of boron 9 silicon samples of

different boron contents from three interlaboratory comparisons were used. The new

procedure allows the determination of impurities of 4N-silicon (12 elements) with

high precision and accuracy.

Introduction

Metallurgical silicon (MG-Si) is an important raw material in silicon industry.

Both metallic and non-metallic impurities significantly influence the reaction

behaviour of silicon as known from the hydrochlorination or the Müller-Rochow

reaction. All industries that use silicon need to determine the amount of impurities

with high precision and high accuracy in the shortest possible time.

Most crucial for the chemical analysis are the digestion and the subsequent sample

preparation. After the dissolution of MG-Si in a mixture of HF/HNO

the matrix

components, i.e. the high amount of dissolved silicon (H

SiF

) and the excess of the

acids HF and HNO

, have to be removed by evaporation since they produce massive

interferences during the measurements by ICP-OES (optical emission spectrometry