The decomposition of PdSi by adding a small amount of Ag
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The decomposition of the intermetallic compound PdSi by adding a small amount of Ag has been investigated by thermal analysis, metallographic measurement, and x-ray photoemission spectroscopy (XPS). Within the range of Ag concentration from 0 to 6 at. %, PdSi is decomposed in linear proportion to the concentration of Ag into Si and the ternary compound Pd2Si(Ag), which has the ratio of 16 PdSi molecules per single Ag atom. The PdSi is fully decomposed at Ag 10 at. %. In the ternary alloy included in the Pd-Si-Ag alloy involving Ag below 12 at. %, the atomic ratio of Si to metal atoms, which are Pd plus Ag, is nearly the same as the ratio in Pd-rich Pd2Si. Metallographic and XPS measurements indicate that the Ag atom included in the PdSi behaves as an electron donor with a large radius of the wave function; the Ag electron transfers to the anti-bonding state of PdSi and acts to decompose the compound. The Ag atoms seem to occupy the sites of Pd atoms in Pd2Si(Ag) which has been decomposed from PdSi.
I. INTRODUCTION
II. EXPERIMENTAL
The phase properties and crystallographic parameters of Pd-silicides have been investigated by several authors.1 6 The intermetallic compound Pd2Si is a stable constitution of Pd-silicides, and there exists an allowable range of Si concentration in the compound 33.0 to 34.2 at. %, corresponding to the so-called "Pd-rich" and "Si-rich" Pd2Si.6'9 The lattice constants of Pd2Si and Si crystals fit well within 2%,4 so that if the alloy consists of only dual constitutions of Si crystals and Pd2Si, the alloy would be stable for thermal cycles. Such a stability may be required for the interface between Si crystal and Pd metal in applications in semiconductor devices. The same requirement is also met in an attempt to produce the negative hydrogen ions8; the requirements to the complex material are that the complex material consists of Si crystal and material permeable to hydrogen as Pd metal, and that the element is airtight and stable against thermal stress. However, the Pd/Si boundary inherently includes the unstable intermetallic compound PdSi. In the metallurgical investigation, the compound PdSi is formed through peritectic reaction at 901 °C, and decomposes gradually into Si-rich Pd2Si and Si below the eutectoidal temperature at 824 °C.6 Addition of a small amount of Ag into the Pd-Si alloy decomposes the compound PdSi into Si and a compound similar to Pd2Si.7 However, the experiment has been insufficient for a detailed discussion about the nature of decomposition of the constitution. In this paper the decomposing ratio of PdSi with respect to Ag concentration is investigated, and its mechanism is discussed with the use of x-ray photoemission spectroscopy, metallographic measurement, and thermal analysis.
The Pd-Si binary alloys were made of the mixtures of PdSi powder, 99.95 purity Pd metal, and 7'9 purity Si lumps. The PdSi powder, which was made by CERAC Inc., was used as a buffer to avoid an explosive reaction of Pd and Si; the composition of the powder was Pd:Si = 50.57:49.43 at.%.
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