wileyonlinelibrary.comAdv. Funct. Mater. 2011, 21, 241-249 the infl uence of nano-particle content, density and size, as well as the infl uence from alloy-scattering and electronic doping effects. In this article, we describe the controlled synthesis and thermoelectric properties of fi ne and uniformly dispersed Ag 2 Te precipitates embedded in PbTe. In contrast to many previously studied PbTe-based systems, [ 5 , 10 , 11 , 13-16 ] the Ag 2 Te precipitates are much larger (50-200 nm), are not isostructural to PbTe and do not introduce considerable electronic doping effect to PbTe. We show that these precipitates scatter the phonons effectively, leading to a low lattice thermal conductivity which approaches the minimum expected of PbTe above 650 K. Moreover, doping with La independently optimizes the carrier concentration and results in a thermoelectric fi gure of merit of 1.6 in La-doped PbTe-Ag 2 Te composites at 775 K. This value is about twice that of the state-of-the-art n-type PbTe [ 1 , 26 ] and arises from the low lattice thermal conductivity at this temperature. (PbTe)1 MicrostructureThe pseudo-binary phase diagram of PbTe-Ag 2 Te (see Figure 1 ) [27][28][29] shows signifi cant and strongly temperature dependent solubility of Ag 2 Te in PbTe. Similar behavior in the PbTe-Sb 2 Te 3 phase diagram has been harnessed to yield Widmanstätten precipitates of Sb 2 Te 3 in a matrix of PbTe. [ 25 ] Here, we utilize the variance in maximum solubility of Ag 2 Te in PbTe, which is about 7-11 mol.% [ 27 , 29 ] at the eutectic temperature of ∼ 970 K and quickly drops to about 1 mol.% at ∼ 770 K. [ 27 ] From these features, one can expect that after melting (step 1 in Figure 1 ) and homogenizing the solid solution at ∼ 970 K (step 2 in Figure 1 ), Ag 2 Te precipitates will be obtained during a lower temperature anneal at ∼ 770 K (step 3 in Figure 1 ).Four compositions of (PbTe) 1 − x (Ag 2 Te) x are considered here ( x = 1.3, 2.7, 4.1, 5.5), all of which have compositions ( Table 1 ) greater than the solubility limit for Ag 2 Te at the annealing temperature (770 K). Following this thermal treatment, Ag-rich precipitates are observed to be homogeneously distributed in the PbTe matrix, as shown by fi eld emission scanning electron microscopy images ( Figure 2 a ). As the Ag 2 Te content increases, the Ag 2 Te is incorporated as a solid solution in the PbTe matrix [27][28][29] ). The open circle at 773 K shows the experimental Ag solubility in PbTe, [ 27 ] consistent with the current study. Starting with a homogeneous melt at a composition of Ag5.5 (point 1), the sample is quenched and then annealed within the single phase region (point 2) for homogenization. Phase separation is then achieved by annealing at 773 K (point 3). up to the solubility limit ( ∼ 1%) at the annealing temperature. [ 27 ] Beyond the solubility limit, the volume fraction of Ag 2 Te particles increases with increasing Ag 2 Te content in the mixture (Figure 2 ), but the Ag content in the PbTe matrix remains constant. The solubility limit is dir...

Cubic boron nitride (cBN) has a number of highly desirable mechanical, thermal, electrical, and optical properties. Because of this, there has been an extensive worldwide effort to synthesize thin films of cBN. Film synthesis is difficult in that without significant levels of ion bombardment during growth, only sp2-bonded BN forms, not sp3-bonded cBN. Recently there has been considerable progress in improving the deposition techniques and cBN film quality. In addition, progress has been made in understanding how energetic deposition conditions can lead to cBN formation. However, unanswered questions remain and process improvements are still needed. In this paper we critically and comprehensively review recent developments in cBN film synthesis and characterization. First, the structures and stability of the BN phases and characterization techniques are described. Next, the key experimental parameters controlling cBN film formation and synthesis techniques are discussed. Following a review of microstructure, the proposed mechanisms of cBN formation and the observed mechanical and electrical properties of cBN films are analyzed. We conclude by highlighting the current impediments to the practical realization of cBN-film technology. 0 1997 Published by Elsevier Science S.A.

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Dynamic control of thermal transport in solid-state systems is a transformative capability with the promise to propel technologies including phononic logic, thermal management, and energy harvesting. A solid-state solution to rapidly manipulate phonons has escaped the scientific community. We demonstrate active and reversible tuning of thermal conductivity by manipulating the nanoscale ferroelastic domain structure of a Pb(Zr0.3Ti0.7)O3 film with applied electric fields. With subsecond response times, the room-temperature thermal conductivity was modulated by 11%.

We experimentally investigate the role of size effects and boundary scattering on the thermal conductivity of silicon-germanium alloys. The thermal conductivities of a series of epitaxially grown Si(1-x)Ge(x) thin films with varying thicknesses and compositions were measured with time-domain thermoreflectance. The resulting conductivities are found to be 3 to 5 times less than bulk values and vary strongly with film thickness. By examining these measured thermal conductivities in the context of a previously established model, it is shown that long wavelength phonons, known to be the dominant heat carriers in alloy films, are strongly scattered by the film boundaries, thereby inducing the observed reductions in heat transport. These results are then generalized to silicon-germanium systems of various thicknesses and compositions; we find that the thermal conductivities of Si(1-x)Ge(x) superlattices are ultimately limited by finite size effects and sample size rather than periodicity or alloying. This demonstrates the strong influence of sample size in alloyed nanosystems. Therefore, if a comparison is to be made between the thermal conductivities of superlattices and alloys, the total sample thicknesses of each must be considered.

We have developed a process for making thick, stress-free, amorphous-tetrahedrally bonded carbon (a-tC) films with hardness and stiffness near that of diamond. Using pulsed-laser deposition, thin a-tC films (0.1–0.2 μm) were deposited at room temperature. The intrinsic stress in these films (6–8 GPa) was relieved by a short (2 min) anneal at 600 °C. Raman and electron energy-loss spectra from single-layer annealed specimens show only subtle changes from as-grown films. Subsequent deposition and annealing steps were used to build up thick layers. Films up to 1.2 μm thick have been grown that are adherent to the substrate and have low residual compressive stress (<0.2 GPa). The values of hardness and modulus determined directly from an Oliver–Pharr analysis of nanoindentation experimental data were 80.2 and 552 GPa, respectively. We used finite-element modeling of the experimental nanoindentation curves to separate the “intrinsic” film response from the measured substrate/film response. We found a hardness of 88 GPa and Young’s modulus of 1100 GPa. From these fits, a lower bound on the compressive yield stress of diamond (∼100 GPa) was determined.

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