Recently, metal-assisted chemical etching (MaCE) has been proposed as a promising wet-etching method for the fabrication of micro- and nanostructures on silicon with low cost. However, uniform vertical trench etching with high aspect ratio is still of great challenge for traditional MaCE. Here we report an innovated MaCE method, which combined the use of a nanoporous gold thin film as the catalyst and a hydrofluoric acid (HF)-hydrogen peroxide (H2O2) mixture solution with a low HF-to-H2O2 concentration ratio (ρ) as the etchant. The reported method successfully fabricated vertical trenches on silicon with a width down to 2 μm and an aspect ratio of 16. The geometry of the trenches was highly uniform throughout the 3D space. The vertical etching direction was favored on both (100)- and (111)-oriented silicon substrates. The reported method was also capable of producing multiple trenches on the same substrate with individually-tunable lateral geometry. An etching mechanism including a through-catalyst mass-transport process and an electropolishing-favored charge-transport process was identified by a comparative study. The novel method fundamentally solves the problems of distortion and random movement of isolated catalysts in MaCE. The results mark a breakthrough in high-quality silicon trench-etching technology with a cost of more than 2 orders of magnitude lower than that of the currently available methods.

Composites containing Yb-filled CoSb3 and well-distributed Yb2O3 particles are synthesized by in situ reaction method. The structural, chemical, and transport properties of the composites are studied. Some Yb2O3 particles with microsize locate at the grain boundaries of matrix and others distribute within YbyCo4Sb12 grains as nanoscale inclusions. The combination of the “rattling” of Yb ions inside the voids of CoSb3 and the phonon scattering of the oxide defects results in a remarkable reduction in the lattice thermal conductivity. The thermoelectric performance of the composites is significantly improved, and the maximum figures of merit reach 1.3 for the Yb0.25Co4Sb12∕Yb2O3 and 1.2 for the Yb0.21Co4Sb12∕Yb2O3 composites at 850K.

Polycrystalline K-filled CoSb3 are synthesized successfully. The uplimit for K filling is at least 0.45, being higher than those of either alkaline-earth (AE) or rare-earth (RE) metals but being in consistent with our earlier theoretical prediction. The measured transport properties (300–800K) show that K filling does not lower thermal conductivity much in comparison with AE or RE filling due to the relatively low mass of K atom. However, it improves electrical conductivity, retains large Seebeck coefficient, and leads to a reasonably good thermoelectric performance for the filled skutterudites. The maximum figure of merit ZT reaches 1 at 800K for K0.38Co4Sb12.

Strontium-filled skutterudites SryCo4Sb12 have been synthesized by a melting method. The filling fraction of Sr in CoSb3 skutterudite is up to y=0.40, closely consistent with the calculated value by density functional theory methods. The lattice parameters increase linearly with the increase of Sr content, and the relative change in lattice parameters is in agreement with theoretical prediction. Hall effect measurements have been performed by Van de Pauw method at room temperature. All samples filled with Sr atom exhibit n-type conduction. The thermal and electrical transport properties have been measured in the temperature range of 300–850K. The lattice thermal conductivity of SryCo4Sb12 is significantly depressed as compared to that of the unfilled CoSb3. The dimensionless thermoelectric figure of merit, ZT, increases with increasing temperature and reaches a maximum value of 0.9 for Sr0.28Co4Sb12 at 850K.