The Thermopower and Resistivity of Nearly Magnetic Dilute Alloys

<p>In some nearly magnetic dilute alloys, in which the host and impurity are transition metals of similar electronic structure, the thermopower is observed to form a "giant" peak at about the spin fluctuation temperature Tsf deduced from resistivity measurements. Two explanations for these peaks have been postulated: the first is that the peaks are a diffusion thermopower component involving scattering off localized spin fluctuations (LSF) at the impurity sites; the second is that they are an LSF drag effect. We examine the thermopower and resistively of two nearly magnetic alloy systems: Rh(Fe) and Pt(Ni). In the first part of this thesis we describe measurements of the low temperature thermopower and resistivity of several Rh(Fe) alloys to clarify discrepancies in previous measurements and we show, by using a modified Nordheim-Gorter analysis, that the observed thermopower peaks are a diffusion and not a drag effect. In the second part of the thesis we describe measurements of the low temperature thermopower and resistivity of Pt (Ni), for which no previous data had been available. The Pt(Ni) samples are manufactured as thin, evaporated films on glass substrates. However, due to the difficulty encountered in controlling the very high residual resistivity of these samples, we are not able to draw definite conclusions regarding either the thermopower or the resistivity.</p>

The Thermopower and Resistivity of Nearly Magnetic Dilute Alloys

<p>In some nearly magnetic dilute alloys, in which the host and impurity are transition metals of similar electronic structure, the thermopower is observed to form a "giant" peak at about the spin fluctuation temperature Tsf deduced from resistivity measurements. Two explanations for these peaks have been postulated: the first is that the peaks are a diffusion thermopower component involving scattering off localized spin fluctuations (LSF) at the impurity sites; the second is that they are an LSF drag effect. We examine the thermopower and resistively of two nearly magnetic alloy systems: Rh(Fe) and Pt(Ni). In the first part of this thesis we describe measurements of the low temperature thermopower and resistivity of several Rh(Fe) alloys to clarify discrepancies in previous measurements and we show, by using a modified Nordheim-Gorter analysis, that the observed thermopower peaks are a diffusion and not a drag effect. In the second part of the thesis we describe measurements of the low temperature thermopower and resistivity of Pt (Ni), for which no previous data had been available. The Pt(Ni) samples are manufactured as thin, evaporated films on glass substrates. However, due to the difficulty encountered in controlling the very high residual resistivity of these samples, we are not able to draw definite conclusions regarding either the thermopower or the resistivity.</p>

Enhanced Catalytic Activity of PdNi Dilute Nanoalloy for Selective Phenylacetylene Hydrogenation

Coordination of identical metals has significant impact on catalytic activity and selectivity of heterogeneous catalyst. Here, we show that the selectivity for hydrogenation of Pd can be manipulated by altering the coordinate environment. PdNi/SiO2 dilute alloy nanocatalysts have been synthesized at room temperature, which show effectively the unparalleled catalytic performance (about 100% selectivity to styrene) for phenylacetylene hydrogenation at 30 °C with full conversion. Structural and surface analyses show that the improvement in dispersion of the Pd active sites and the changed electronic structure of Pd contribute the catalytic performance significantly. This work is an important step towards developing highly active hydrogenation catalysts by forming dilute alloys.

Specific Heat of Holmium in Gold and Silver at Low Temperatures

The specific heat of dilute alloys of holmium in gold and in silver plays a major role in the optimization of low temperature microcalorimeters with enclosed $$^{163}{{\text {Ho}}}$$ 163 Ho , such as the ones developed for the neutrino mass experiment ECHo. We investigate alloys with atomic concentrations of $$x_{{{\text {Ho}}}}=0.01{-}4\%$$ x Ho = 0.01 - 4 % at temperatures between 10 and $$800\,{{{\hbox {mK}}}}$$ 800 mK . Due to the large total angular momentum $$J=8$$ J = 8 and nuclear spin $$I=7/2$$ I = 7 / 2 of $${{\text {Ho}}}^{3+}$$ Ho 3 + ions, the specific heat of Au:Ho and Ag:Ho depends on the detailed interplay of various interactions, including contributions from the localized 4f electrons and nuclear contributions via hyperfine splitting. This makes it difficult to accurately determine the specific heat of these materials numerically. Instead, we measure their specific heat by using three experimental setups optimized for different concentration and temperature ranges. The results from measurements on six holmium alloys demonstrate that the specific heat of these materials is dominated by a large Schottky anomaly with its maximum at $$T\approx 250\,{{{\hbox {mK}}}}$$ T ≈ 250 mK , which we attribute to hyperfine splitting and crystal field interactions. RKKY and dipole–dipole interactions between the holmium atoms cause additional, concentration-dependent effects. With regard to ECHo, we conclude that for typical operating temperatures of $$T\le 20\,{{{\hbox {mK}}}}$$ T ≤ 20 mK , silver holmium alloys with $$x_{{{\text {Ho}}}}\gtrsim 1\%$$ x Ho ≳ 1 % are suited best.