Metallurgical Si

Metallurgical Si is manufactured in industrial furnaces from quartzites and carbonaceous reductants containing various intermetallic impurities. These metallic impurities, which are concentrated at grain boundaries, induce precipitates in metallurgical Si during solidification [2], Oxidized iron is a significant source of the oxygen which yields the various siloxanes in DPR. 

Metallurgical Si is produced by reducing quartz by C in an electric arc furnace. Consumable graphite electrodes are used to supply the necessary energy for the reaction. The overall reaction in an idealized form can schematically be written ... 

If we consider metallurgical Si as a starting point for solar grade Si, all other processes can be looked upon as refining process to remove certain impurities. We have several routes or multistep processes where each step removes an impurity or reduces the total level of impurities down to a lower level. In this section, the most usual refining processes for removal of impurities from Si are sketched. 

Solidified metallurgical Si consists of relatively pure Si with most of the impurities present as additional phases between these crystals. The impurities can be removed by crushing and subsequent etching, giving Si grains at approximately 2 mm. If 5% Ca is added to the molten Si, the impurities will be found... 

The reaction is frequently carried out in the presence of scrap iron (with low P and S content) to produce ferrosilicon alloys these are used in the metallurgical industry to deoxidize steel, to manufacture high-Si corrosion-resistant Fe, and Si/steel laminations for electric motors. The scale of operations can be gauged from the 1980 world production figures which were in excess of 5 megatonnes. Consumption of high purity (semiconductor grade) Si leapt from less than 10 tonnes in 1955 to 2800 tonnes in 1980. 

Fig. 12. Transmittance of a Si multiple internal reflection plate before and after H-implantation at room temperature. [From Stein (1975). Reprinted with permission from Journal of Electronic Materials, Vol. 4, p. 159 (1975), a publication of The Metallurgical Society, Warrendale, Pennsylvania.]...

Production of Ions. Several methods are used (11 by bombardment with electrons from a heated filament (2 by application of a strong electrostatic field (field ionization, field desorption) Ot by reaction with an ionized reagent gas (chemical ionization) (4 by direct emission of ions from a solid sample that is deposited on a heated filament (surface ionization) (SI by vaporization from a crucible and subsequent electron bombardment (e.g.. Knudsen cell for high-lcmperalure sludies id solids and (6) by radio-frequency spark bomhardmenl of sample fur parts-per-biliion (ppb) elemental analysis of solids as encountered in metallurgical, semiconductor, ceramics, and geological studies. Ions also are produced by photoion izution and laser ionizalion. 

Various algorithms are also in use to estimate the contribution of mineral dust (MD). With MD is meant all fugitive windblown and mechanically resuspended dust with a composition comparable to the earth s crust. Since chemical analyses of PM samples measure elements directly, the approach here is to sum over those elements known to be present in the earth s crust Al, Si, C03, Ca, Fe, K, Mn, Ti and P [10]. Weights were first recalculated to correct for their oxidised form (e.g. Si is usually present as SiC>2). MD is a parameter difficult to estimate. The use of other algorithms in the estimation of MD results in different values, e.g. the one formulated by Denier van der Gon et al. [11]. Also, local anthropogenic sources may contribute (e.g. metallurgical industry). 

In many chemical measurements one neither knows nor, at the time of measurement, wishes to know the exact composition of the matrix. To give an example, a metallurgical firm will receive ore shipments measured by mass in kilograms. Representative samples in the seller s and receiver s laboratories are measured for quality by the amount of substance of a specified metal element or compound per given mass of ore. It is unnecessary and far too complex to attempt amount-of-sub-stance measurements on all components of the bulk. In exactly the same way, a food laboratory might measure the amount of substance (say lead) in orange juice in milligrams per liter (per cubic decimeter). The charm of the SI system lies in a coherence, which makes it possible to express all measured quantities in a combination of base and derived units [9],... 

In the metallurgical sense, alloys are mixtures of metals with each other or with certain nonmetals, resulting in substances that may be single phase or multiphase. It should be noted that the term alloy, as referring to a macroscopically homogeneous mixture, is also used for nonmetals such as ceramics and semiconductors (e.g. Si-Ge alloys) and polymers (e.g. block copolymers) these alloys, however, are outside the scope of the present article. 

The purity of silicon in this first step is only ca. 98%, and is referred to as metallurgical grade silicon (MG-Si). In order for the silicon to be used for electronics applications, additional steps are necessary to decrease the number of impurities. Reaction of MG-Si with hydrogen chloride gas at a moderate temperature converts the silicon to trichlorosilane gas (Eq. 3). When SiHCl3 is heated to a temperature of ca. 1,150°C, it decomposes into high-purity silicon and gaseous by-products (Eq.4). This reaction is typically performed in a bell-shaped Siemens-type reactor, where Si is deposited onto heated electrodes. 

Generally, it is known that metallurgical impurities in technical-grade silicon can strongly influence both the reactivity of the silicon and the selectivity of the reaction [1 - 5], but up to now no convincing relationships between selectivities and impurity contents have been established. As a continuation of our former work in this field [6], we tried to find out reasons for TCS selectivity losses in the industrial process by experiments with varying Si quality, by varying reaction conditions, and by XPS surface analysis of Si samples before and after the synthesis reaction. 

A distillation process is shown in Fig. P2.59. You are asked to solve for all the values of the stream flows and compositions. How many unknowns are there in the system How many independent material balance equations can you write Explain each answer and show all details whereby you reached your decision. For each stream, the only components that occur are shown below the stream. Metallurgical-grade silicon is purified to electronic grade for use in the semiconductor industry by chemically separating it from its impurities. Si metal reacts in varying degrees with hydrogen chloride gas at 300°C to form several polychlorinated... 

For the solar industry, impurity control and alternative raw materials are of great interest. In the normal metallurgical silicon production furnace, silica is charged into the form of quartz lumps. The carbonaceous materials have many forms like charcoal coke, coal, and wood chips. The charge is carefully composed and the raw materials are selected to give best possible yield with respect to Si and to avoid unwanted impurities. Also, the refractory materials and the consumable electrodes are source of impurities. The Si from the arc... 

The extreme purity Si required in the photovoltaic or electronics applications is obtained by converting metallurgical-grade Si into silanes, which are then distilled and decomposed into pure Si. An overview of the most common pathways to polysilicon production, by the chemical means is given in Fig. 1.2 [4]. 

B is one of the common impurities in the metallurgical grade Si. Since B is difficult to remove by either of directional solidification or evaporation, the oxidative refining processes are frequently used to remove B from liquid Si. 

G.M. Haarberg et al. Electrorefining of metallurgical silicon in molten chloride and fluoride electrolytes, in the 3rd International Workshop on Science and Technology of Crystalline Si Solar Cells, (Trondheim, Norway 2009)... 

For solar cell applications, thin film LPE is economically viable only if it is combined with a low-cost multicrystalline Si substrate (high-throughput silicon ribbons, upgraded metallurgical grade silicon MG-Si) or with a foreign substrate (glass, ceramic, metallic sheet... see section 9-7). 

To compensate the extra cost of the epitaxial growth of the Si thin film, one has to use a low-cost Si substrate like metallurgical grade Si (MG-Si). University of Konstanz [27] developed LPE on upgraded MG-Si (UMG-Si). An efficiency of 10% without surface texturation (Calculated potential n = 14%) was achieved. Thin layer of 30 pm thick was grown from In melt with 0.1 Wt% Ga at 990° C. The saturation of the melt was obtained with the meltback of the UMG-Si wafer It is not necessary to add electronic grade Si to the solution. 

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