Si-Cu contact mass

More intricate treatment of silicon and copper was reported by Mui52. The Si/Cu contact mass was prepared by (1) treating Si particles with Cu catalyst vapor in a mechanical mixer, (2) mixing the Si particles with the Cu catalyst vapor, (3) contacting the vapor with the particles and (4) forming active Cu-Si alloys on the Si particles. The CuCl vapor was formed at 1200 °C and fed to silicon at 320 °C. This author claims that treating... 

Summary The question of the nature of the catalytically active copper species in Rochow contact masses has been investigated using the SEM/EDX technique. The results do not support the hypothesis of active ri-CusSi, but they provide more direct evidence for the existence and the catalytic action of X-ray amorphous Cu-Si surface species, i.e., extremely dispersed particles or even two-dimensional species like Cu Si surface compounds, which we proposed recently. The investigation of zinc-promoted and non-promoted contact masses on basis of the pure and technical-grade silicon showed that the mode of operation of the famous Rochow promoter zinc can be understood rather as a moderation than as a real acceleration. By moderating the initial reaction rate, the promoter enables a sufficiently high stationarity of the reaction. 

The catalytic activity of the Cu-Si surface species explained above was a logical, but only an indirect derivation from our former experiments. For some time, we were not able to present more direct evidence. The reason was the complicated surface topography of a working contact mass. According to all experiences, the Rochow reaction exclusively takes place in sharply demarcated pits on the silicon surface, the shape of which usually corresponds to the respective crystallographic planes. The inner surface of these pits mostly has an own structure, i.e, it is partly covered by particles of copper species, often doubtless by ri-CuaSi particles [15]. But, the characteristic dimensions of these structures are generally below the resolving power of appropriate surface methods like the combination SEM/EDX. 

Fig. 2. Typical surface image of a contact mass sample CuCb/Siteci/Zn after 20 min reaction time, and EDX analysis 1) non-attacked Si surface, < 0.5 wt.% Cu, 96 wt.% Si 2) copper-containing particle, 52 wt.% Cu, 41 wt.% Si 3) surface within a pit beside a copper-containing particle, 2 wt.% Cu, 90 wt.% Si.

With our view of the active copper state, we do not exclude that ri-CujSi can play a role in the reacting system. -q-CusSi is probably not the main carrier of the catalytic activity, but possibly it plays a role as a copper reservoir. In the period after 20 min reaction time, i.e., after the state in Fig. 2, redispersion of q-CujSi can be observed. Already after 60 min, the simple structure of the pits is destroyed and a lot of small particles are to be observed within the pit. (The surface then looks like that in Fig. 6, which is discussed only in the second section.) This redispersion process, probably comparable to our observations [12, 15], demonstrates a certain involvement of q-CusSi in the reaction, which is perhaps linked with a minor catalytic activity. It can be imagined that the q-Cu Si particles act as a copper pool for the formation of new Cu-Si surface species. From such point of view, q-CusSi could be regarded as a precursor of the active species. If so, however, there should exist also other ways to form these species there are also active contact masses without any detectable q-phase. 

As can be concluded from Fig. 4, the initial reactivity of the surface of Sited, is very high The relevant copper species present, possibly copper chlorides, which can easily form copper-silicidic phases by reaction with silicon, can easily attack the whole of the surface and react with surface silicon, resulting in Cu-Si species and finally metallic copper, e g, [26]. Due to this very fast formation of catalytically active Cu-Si species and of precursors thereof, the reaction becomes very fast already after a short time. But, on the other hand, this overall attack on the silicon surface gives copper species the possibility to be deposited practically over the whole of the silicon grain This means in terms of our model of catalytically active Cu-Si surface species, explained in the first section of this paper, that there is a lack of still free silicon surface area, which is needed in order to form the active "two-dimensional" Cu-Si species. The surface is simply blocked by thick copper-containing layers. As consequence, the reaction goes down after a short time and the contact mass reaches only low stationary activity. 

It is possible that the improvements described above are due to pre-formation of silicon copper alloys, as claimed by Elattar. Improved activity of MCS contact mass was obtained by forming a contact mass of particulate Si and Cu (16.9% metallic Cu, 45.5% CU2O, 36.9% CuO) by heating the mass in the presence of MeCl gas at 305 °C for a time sufficient to form active spots of Cu-Si alloy on the surface. The nature of these copper-silicon alloys may include those described by some former E. German workers who showed the importance of forming CusSi and CuSia among others. ... 

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