340 PHILlPS TECHNICAL REVIEW VOLUME26

A silicon carbide mortarW. F. Knippenberg, G. Verspui and J. Visser

If a solid substance has to be chemically analysed,it is often first necessary to grind it to powder in amortar - either so that it will dissolve quickly inthe case of a wet analysis, or in order to make certainspectral analyses possible.It makes a difference here whether one wishes to

analyse relatively large amounts, or only traces. Fortrace analysis, the chance of the introduetion of im-purities from the mortar is often a reason for notgrinding the substance; and if it is necessary to do sothe mortar must be chosen with very great care forthe reason just mentioned.

The analytical chemist normally has a choice be-tween mortars of porcelain, agate and various metalcarbides for the grinding of solid substances [1]. Ingeneral, mortars of these kinds are not sufficientlyhard and resistant to abrasion for the detection oftrace elements. The recently available mortars ofaluminium oxide and boron carbide - which cannow be sintered to almost the theoretical density -are a great improvement in this respect, and havebecome practically indispensible for trace analysis [2].

However, for the grinding of very hard materials,even these mortars are not really good enough: toomuch material is still removed from the mortar toallow certain trace analyses to be carried out withsuccess.It the course of investigations in this laboratory on

silicon carbide, long known as a hard material (car-borundum), we have succeeded in making a mortarof this material. This mortar was originally mainlyof use for our own investigations [3]: it is an idealsituation to be able to grind a substance in a mortarmade of the same (pure) material - a principle onewould like to apply more often, were it not that formost substances it remains impossible owing to theassociated technical difficulties.It was later found that this SiC mortar also has very

favourable properties for general analytical purposes,apart from the investigation on SiC. Silicon carbideis not only hard and resistant to abrasion, but alsochemically resistant, and we have moreover succeededin obtaining it in a very pure form. For wet analyses,the chemical resistance is the most important, since itmeans that the little SiC which is worn off will not

Dr. W. F. Knippenberg, G. Verspui and J. Visser are researchworkers at Philips Research Laboratories, Eindhoven.

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interfere with the analysis. For the spectral analyses,it is rather the high purity of the SiC material whichmakes the mortar useful. Only the silicon (and inexceptional cases also the carbon) then occurs as animpurity in these analyses. (Since the substance un-der investigation in spectral analyses is normallyevaporated between two carbon electrodes, the carbonof the mortar is seldom a factor of importance; itonly makes its presence felt when other materials are

Fig. 1. Making a silicon carbide mortar in a carbon cylinder.Silicon carbide crystallizes out from silicon and carbon-con-taining vapour on the lid, which has the form of the negativeof the desired mortar.

used for the electrodes.) It is also worthy of mentionthat the high chemical resistance of silicon carbideallows a mortar made of this material to be verythoroughly cleaned. Contamination from substancesleft over from a previous analysis can thus generallybe avoided.It will be clear, then, that this mortar forms a very

welcome addition to the usual assortment of mortarsin an analytical laboratory; together with the mortarof pure boron carbide, which will still be needed for

1965, No. 11/12 SILICON CARBIDE MORTAR

the determination of silicon, and thealuminium oxide mortar in case one alsowishes to determine carbon, it makestrace analysis of all elements possible.

The mortar is made of polycrystallinesilicon carbide of high purity (the impur-ities amounting at the most to a fewhundred-thousandths of a percent). Thismaterial is obtained by the thermal de-cornposiuon of methyl dichlorosilane inthe presence of hydrogen [41. The siliconcarbide thus obtained is heated in a car-bon crucible at 2600°C in an inert gas at-mosphere. At this temperature, the sili-con carbide decomposes into carbon(solid and vapour) and silicon (vapour).

The crucible is covered with a carbonlid in the form of a negative of the desiredmortar. The temperature of this lid is main-tained between 2400 and 2500°C. At thistemperature silicon carbide is depositedon the lid, while recrystallization processesensure a high density of the depositedlayer. Relatively large single crystals occurin this layer, about 0.1 cm thick andwith an area of about 1 cm''. Fig. 1shows a section through the carbon lidwith the silicon carbide deposited on it.

When the layer has reached the desired thickness,the carbon lid is burnt and the mortar finished with

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Fig. 2. A mortar of silicon carbide in a plastic base. The relatively large singlecrystals of which the mortar is built up can be seen at the left-hand edge.The pestle is also of silicon carbide (with a plastic handle).

diamond powder. Fig. 2 shows a SiC mortar cementedinto a plastic base. mond powder.

The pestle for use with the mortar can be made ina similar way. A certain amount of silicon carbide is The pure "sublimed" silicon carbide is also used as a

material for cutting tools. Such tools are needed forthe turning or milling of materials which have to beanalyzed in shaving form, and which must not be con-taminated with the usual materials of which chiselsare made. The SiC tools, like the pestles, are groundto the desired shape. The starting material can be apolycrystalline fragment, or if so desired one of thecomponent single crystals, the single crystals beinglarge enough for this purpose. Fig. 3 shows a toolwhere the SiC head is welded on to a molybdenumrod with the aid of a gold-tantalum alloy.

!!/!!!!/I!IIIIIII 1111 1111\1111

~ ~ 4Fig. 3. A silicon carbide cutting tool ground from a single crystal(the triangle at the tip). The chisel is welded on to a molybde-num rod with the aid of a gold-tantalum alloy at a temperatureof 1300 -c.

"sublimed" on to a flat carbon plate as describedabove, and the pestle is ground out of a rough lumpof the material obtained, again with the aid of dia-

[1] See e.g. O. G. Koch and G. A. Koch-Dedic, Ha ndbuch derSpurenanalyse, Springer, Berlin 1964.

[2] See e.g. N. W. H. Addink, Chem. Weekblad 56, 622, 1960and A. Claassen, Chem. Weekblad 58, 33, 1962.

[3] W. F. Knippenberg, Philips Res. Repts. 18,251, 1963.[4] W. F. Knippenberg, Philips Res. Repts. 18, 205, 1963.