Recent studies focusing on enhancing the thermoelectric performance of metal oxides were primarily motivated by their low cost, large availability of the component elements in the earth's crust, and their high stability. So far, these studies indicate that n-type materials, such as ZnO, have much lower thermoelectric performance than their p-type counterparts. Overcoming this limitation requires precisely tuning the thermal and electrical transport through n-type metal oxides. One way to accomplish this is through the use of optimally doped bulk assemblies of ZnO nanowires. In this study, the thermoelectric properties of n-type aluminum and gallium dually doped bulk assembles of ZnO nanowires were determined. The results indicated that a high zT of 0.6 at 1000 °C, the highest experimentally observed for any n-type oxide, is possible. The high performance is attributed to the tailoring of the ZnO phase composition, nanostructuring of the material, and Zn-III band hybridization-based resonant scattering.

A simple method for the large-scale synthesis of gram quantities of compound semiconductor nanowires without the need for any external catalysts or templates is presented. This method is demonstrated using zinc phosphide (Zn3P2) and zinc antimonide (β-Zn4Sb3) nanowires as example systems. Large-scale synthesis of Zn3P2 and Zn4Sb3 nanowire powders was accomplished using a hot-walled chemical vapor deposition chamber by transporting phosphorus and antimony, respectively, via the vapor phase onto heated zinc foils. The zinc foils were rolled concentrically into coils to maximize the substrate surface area, and consequently, the nanowire yield. Using this method, 250 mg of Zn3P2 nanowires were obtained on 480 cm(2) of zinc foil in a span of 45 minutes. Furthermore, a process of exposing the synthesized nanowires to a vapor of organic functional molecules immediately after their synthesis and before their removal from the vacuum chamber was developed to obtain large quantities of surface functionalized nanowire powders. This in situ vapor-phase functionalization procedure passivated the nanowire surfaces without adversely affecting their morphology or dimensions. Our studies revealed that both 4-aminothiophenol and 3-propanedithiol functionalized Zn3P2 nanowires were stable over a 120 day duration without any agglomeration or degradation. This method of mass producing nanowires can also be extended to other binary semiconductors.

Gram quantities of both unfunctionalized and 1,4-benzenedithiol (BDT) functionalized zinc phosphide (Zn3P2) nanowire powders, synthesized using direct reaction of zinc and phosphorus, were hot-pressed into highly dense pellets (≥98% of the theoretical density) for the determination of their thermoelectric performance. It was deduced that mechanical flexibility of the nanowires is essential for consolidating them in randomly oriented fashion into dense pellets, without making any major changes to their morphologies. Electrical and thermal transport measurements indicated that the enhanced thermoelectric performance expected of individual Zn3P2 nanowires is still retained within large-scale nanowire assemblies. A maximum reduction of 28% in the thermal conductivity of Zn3P2 resulted from nanostructuring. Use of nanowire morphology also led to enhanced electrical conductivity in Zn3P2. Interface engineering of the nanowires in the pellets, accomplished by hot-pressing BDT functionalized nanowires, resulted in an increase on both the Seebeck coefficient and the electrical conductivity of the nanowire pellets. It is believed that filtering of low energy carriers resulting from the variation of the chemical compositions at the nanowire interfaces is responsible for this phenomenon. Overall, this study indicated that mechanical properties of the nanowires along with the chemical compositions of their surfaces play a hitherto unknown, but vital, role in realizing highly efficient bulk thermoelectric modules based on nanowires.

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We describe a new approach to modeling the wetting behavior of micro-and nano-textured surfaces with varying degrees of geometrical heterogeneity. Surfaces are modeled as pore arrays with a Gaussian distribution of sidewall reentrant angles and a characteristic wall roughness. Unlike conventional wettability models, our model considers the fraction of a surface's pores that are filled at any time, allowing us to capture more subtle dependences of a liquid's apparent contact angle on its surface tension. The model has four fitting parameters and is calibrated for a particular surface by measuring the apparent contact angles between the surface and at least four probe liquids. We have calibrated the model for three heterogeneous nanoporous surfaces that we have fabricated: a hydrothermally grown zinc oxide, a film of polyvinylidene fluoride (PVDF) microspheres formed by spinodal decomposition, and a polytetrafluoroethylene (PTFE) film with pores defined by sacrificial polystyrene microspheres. These three surfaces show markedly different dependences of a liquid's apparent contact angle on the liquid's surface tension, and the results can be explained by considering geometric variability. The highly variable PTFE pores yield the most gradual variation of apparent contact angle with probe liquid surface tension. The PVDF microspheres are more regular in diameter and, although connected in an irregular manner, result in a much sharper transition from non-wetting to wetting behavior as surface tension reduces. We also demonstrate, by terminating porous zinc oxide with three alternative hydrophobic molecules, that a single geometrical model can capture a structure's wetting behavior for multiple surface chemistries and liquids. Finally, we contrast our results with those from a highly regular, lithographically-produced structure which shows an extremely sharp dependence of wettability on surface tension. This new model could be valuable in designing and evaluating processes for manufacturing liquid-repellent surfaces on an industrial scale.

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Recent studies indicated that nanowire format of materials is ideal for enhancing the thermoelectric performance of materials. Most of these studies were performed using individual nanowires as the test elements. It is not currently clear whether bulk assemblies of nanowires replicate this enhanced thermoelectric performance of individual nanowires. Therefore, it is imperative to understand whether enhanced thermoelectric performance exhibited by individual nanowires can be extended to bulk assemblies of nanowires. It is also imperative to know whether the addition of metal nanoparticle to semiconductor nanowires can be employed for enhancing their thermoelectric performance further. Specifically, it is important to understand the effect of microstructure and composition on the thermoelectric performance on bulk compound semiconductor nanowire-metal nanoparticle composites. In this study, bulk composites composed of mixtures of copper nanoparticles with either unfunctionalized or 1,4-benzenedithiol (BDT) functionalized Zn₃P₂ nanowires were fabricated and analyzed for their thermoelectric performance. The results indicated that use of BDT functionalized nanowires for the fabrication of composites leads to interface-engineered composites that have uniform composition all across their cross-section. The interface engineering allows for increasing their Seebeck coefficients and electrical conductivities, relative to the Zn₃P₂ nanowire pellets. In contrast, the use of unfunctionalized Zn₃P₂ nanowires for the fabrication of composite leads to the formation of composites that are non-uniform in composition across their cross-section. Ultimately, the composites were found to have Zn₃P₂ nanowires interspersed with metal alloy nanoparticles. Such non-uniform composites exhibited very high electrical conductivities, but slightly lower Seebeck coefficients, relative to Zn₃P₂ nanowire pellets. These composites were found to show a very high zT of 0.23 at 770 K, orders of magnitude higher than either interface-engineered composites or Zn₃P₂ nanowire pellets. The results indicate that microstructural composition of semiconductor nanowire-metal nanoparticle composites plays a major role in determining their thermoelectric performance, and such composites exhibit enhanced thermoelectric performance.

Many studies have shown that dropwise condensation can enhance air-side heat transfer coefficients by at least an order of magnitude relative to filmwise condensation. However, among the hundreds of superhydrophobic surface-modification processes previously reported, there remains a lack of coating methods that enable stable dropwise condensation and can be applied to aluminum -by far the most common material for the air side of heat exchangers, e.g. in air conditioning. Here we present a bottom-up synthesis technique to grow zinc oxide-based films on to aluminum with tunable nanoporosity and strongly re-entrant surface features. These surfaces exhibit exceptional static water contact angles of up to 178° with a hysteresis less than 3° and a slide angle of 1°. We have further characterized the surfaces in the presence of six different liquids, and show that our optimal surface can repel even dipropylene glycol with a contact angle of 124°, even though its surface tension is less than half that of water. Crucially, we have also tested our films under water-condensing conditions in flowing air, characterizing the dropletshedding behavior, and we have understood how to tune the growth process to deliver stable dropletshedding instead of flooding. The process uses inexpensive reagents, can operate below 100 °C via immersion in an aqueous bath, and takes 1-3 hours to complete, making it readily scalable to areas of many square meters and complex geometries.2