ALBERT

Notes: Solid state reactions between thin films of titanium and amorphous silicon (a-Si) were studied using differential scanning calorimetry. Multilayered diffusion couples were heated from just above room temperature to 900 K while monitoring the heat flow from the sample during the reaction. From analysis of our data we have calculated the enthalpy of formation to be ΔHf=−62±5 kJ/mol for the reaction, a-Si+Ti→C49−TiSi2 and have subsequently estimated the heat evolved during the reaction c-Si+Ti→C54−TiSi2 as ΔHf=−56±5 kJ/mol. Based upon this estimate, we find the enthalpy as determined from thin film samples for this system agrees with previous studies which made use of bulk sampling techniques. © 1997 American Institute of Physics.

Notes: The heat capacity of TiSi2 has been measured in the temperature range 105–500 K. The heat capacity of TiSi2 varies monotonically between temperatures of 100 and 500 K with a reference value of 22.0±0.2 J/g atom K at 298.15 K. Based upon our heat capacity data, the standard molar entropy of TiSi2 at 298.15 K is estimated to be 22.2±0.8 J/g atom K. Our data support estimates of the higher temperature heat capacity of TiSi2 based upon previously measured heat capacities of different, but similar, substances. A number of TiSi2 samples were prepared by rapidly quenching ((approximately-greater-than)105 K/s) from the melt. The structure and the measured heat capacity of these samples were similar to those of well annealed samples, underscoring the thermal stability of this material.

Notes: We have investigated solid state amorphization reactions in mechanically deformed composites in both the Ni-Ti system and the Ni-Zr system. The growth of amorphous material in our Ni/Ti composites is apparently facilitated by the relatively large degree of disorder induced in the metal layers by the mechanical deformation process. The growth of amorphous material is slower in Ni/Ti composites than in Ni/Zr composites, while we found similar kinetic constraints on the formation of equilibrium compounds in both systems. Thus the maximum thickness of amorphous Ni-Ti layers was an order of magnitude less than the 1000-A(ring) layers grown in the Ni-Zr system.

Notes: The heat capacities of V3P and V3Si were measured in the temperature range from 150–350 K. At 298.15 K, we find Cp(V3P)=22.3±.2 J/g atom K and Cp(V3Si)=22.6±0.2 J/g atom K. At 150 K the heat capacity of V3P is 6% smaller than that of V3Si. Our data provide support for a value of 25.6 J/g atom for the standard entropy of V3Si at 298.15 K, and result in an estimated value of 23.5 J/g atom for the standard entropy of V3P at 298.15 K.

Notes: It has been shown that significant changes in the course of solid state reactions can be realized by decreasing length scale, temperature, or by varying parent microstructures. In the case of the formation of Cu3Si by interdiffusion of Cu and Si, previous research has shown that over a large temperature range reaction rates are determined by the rate of grain boundary diffusion of Cu through the growing Cu3Si phase. We have examined the effect of replacing crystalline Si with amorphous Si (a-Si) on these solid state reactions, as well as the effect of decreasing the temperatures and length scales of the reactions. Multilayered thin film diffusion couples of Cu and a-Si were prepared by sputter deposition, with most average composite stoichiometries close to that of the equilibrium phase Cu3Si. Layer thicknesses of the two materials were changed such that the modulation (sum of the thickness of one layer of Cu and a-Si), λ, varied between 5 and 160 nm. X-ray diffraction analysis and transmission electron microscopy analysis were used to identify phases present in as prepared and reacted diffusion couples. Complete reactions to form a single phase or mixtures of the three low temperature equilibrium silicides (Cu3Si, Cu15Si4, and Cu5Si) were observed. Upon initial heating of samples from room temperature, heat flow signals were observed with differential scanning calorimetry corresponding to the growth of Cu3Si. At higher temperatures (〉525 K) and in the presence of excess Cu, the more Cu-rich silicides, Cu15Si, and Cu5Si formed. Based on differential scanning calorimetry results for samples with average stoichiometry of the phases Cu3Si and Cu5Si, enthalpies of formation of these compounds were measured. Considering the reaction of these phases forming from Cu and a-Si, the enthalpies were found to be −13.6±0.3 kJ/mol for Cu3Si and −10.5±0.6 kJ/mol for Cu5Si. The growth of Cu3Si was found to obey a parabolic growth law: x2=k2t, where x is the thickness of the growing silicide, k2 is the temperature dependent reaction constant, and t is the reaction time. Also, the form of the reaction constant, k2, was Arrhenius: k2=k0 exp(−Ea/kbT) with kb being Boltzmann's constant and the prefactor, k0=1.5×10−3 cm2/s, and activation energy, Ea=0.98 eV. These results indicate a much slower reaction to form Cu3Si in thin film Cu/a-Si diffusion couples than indicated by previous researchers using mostly bulk samples of Cu and crystalline Si (x-Si). © 1999 American Institute of Physics.

Notes: Differential scanning calorimetry was used to characterize the energetics and kinetics of interdiffusion in solder/metal diffusion couples. The heat of formation of Cu3Sn from Cu6Sn5 and Cu thin films was found to be ΔHr=−4.3±0.3 kJ/mol, similar to the results of previous measurements on bulk samples. We have seen that the nucleation of Cu3Sn begins at temperatures near 360 K, but that the nucleation and initial growth of Cu3Sn is not a well-defined Arrhenius process in these diffusion couples. Later portions of our differential scanning calorimetry scans were identified with diffusion-limited growth of Cu3Sn. From these calorimetry data we have estimated the averaged interdiffusion coefficient, D˜(cm2/s)=D0 exp (−E/kbT), where kb is Boltzmann's constant and D0=3.2×10−2 cm2/s and E=0.87 eV/atom. © 1995 American Institute of Physics.

Notes: The specific heat of high-purity copper has been measured over the temperature range of 0.03–0.2 K. It is shown that an anomaly appears in the specific heats of some samples. The temperature dependence is ~T \s-2 at T \〉 0.03 K, and at 0.03 K the magnitude is equal to that of the contribution from conduction electrons. We have not been able to explain the origin of this anomaly.