Zhixi Bian

Nanoscale design to enable the revolution in renewable energy

The creation of a sustainable energy generation, storage, and distribution infrastructure represents a global grand challenge that requires massive transnational investments in the research and development of energy technologies that will provide the amount of energy needed on a sufficient scale and timeframe with minimal impact on the environment and have limited economic and societal disruption during implementation. In this opinion paper, we focus on an important set of solar, thermal, and electrochemical energy conversion, storage, and conservation technologies specifically related to recent and prospective advances in nanoscale science and technology that offer high potential in addressing the energy challenge. We approach this task from a two-fold perspective: analyzing the fundamental physicochemical principles and engineering aspects of these energy technologies and identifying unique opportunities enabled by nanoscale design of materials, processes, and systems in order to improve performance and reduce costs. Our principal goal is to establish a roadmap for research and development activities in nanoscale science and technology that would significantly advance and accelerate the implementation of renewable energy technologies. In all cases we make specific recommendations for research needs in the near-term (2–5 years), mid-term (5–10 years) and long-term (>10 years), as well as projecting a timeline for maturation of each technological solution. We also identify a number of priority themes in basic energy science that cut across the entire spectrum of energy conversion, storage, and conservation technologies. We anticipate that the conclusions and recommendations herein will be of use not only to the technical community, but also to policy makers and the broader public, occasionally with an admitted emphasis on the US perspective.

Effect of nanoparticle scattering on thermoelectric power factor

The effect of nanoparticles on the thermoelectric power factor is investigated using the relaxation time approximation. The partial-wave technique is used for calculating the nanoparticle scattering cross section exactly. We validate our model by comparing its results to the experimental data obtained for ErAs:InGaAlAs samples. We use the theory to maximize the power factor with respect to nanoparticle and electron concentrations as well as the barrier height. We found that at the optimum of the power factor, the electron concentration is usually higher in the sample with nanoparticles, implying that Seebeck is usually unchanged and conductivity is increased.

Cross-plane lattice and electronic thermal conductivities of ErAs:InGaAs/InGaAlAs superlattices

We studied the cross-plane lattice and electronic thermal conductivities of superlattices made of InGaAlAs and InGaAs films, with the latter containing embedded ErAs nanoparticles (denoted as ErAs:InGaAs). Measurements of total thermal conductivity at four doping levels and a theoretical analysis were used to estimate the cross-plane electronic thermal conductivity of the superlattices. The results show that the lattice and electronic thermal conductivities have marginal dependence on doping levels. This suggests that there is lateral conservation of electronic momentum during thermionic emission in the superlattices, which limits the fraction of available electrons for thermionic emission, thereby affecting the performance of thermoelectric devices.

Cross-plane Seebeck coefficient of ErAs:InGaAs/InGaAlAs superlattices

We characterize cross-plane and in-plane Seebeck coefficients for ErAs:InGaAs∕InGaAlAs superlattices with different carrier concentrations using test patterns integrated with microheaters. The microheater creates a local temperature difference, and the cross-plane Seebeck coefficients of the superlattices are determined by a combination of experimental measurements and finite element simulations. The cross-plane Seebeck coefficients are compared to the in-plane Seebeck coefficients and a significant increase in the cross-plane Seebeck coefficient over the in-plane Seebeck coefficient is observed. Differences between cross-plane and in-plane Seebeck coefficients decrease as the carrier concentration increases, which is indicative of heterostructure thermionic emission in the cross-plane direction.

Low-temperature thermoelectric power factor enhancement by controlling nanoparticle size distribution.

Coherent potential approximation is used to study the effect of adding doped spherical nanoparticles inside a host matrix on the thermoelectric properties. This takes into account electron multiple scatterings that are important in samples with relatively high volume fraction of nanoparticles (>1%). We show that with large fraction of uniform small size nanoparticles (∼1 nm), the power factor can be enhanced significantly. The improvement could be large (up to 450% for GaAs) especially at low temperatures when the mobility is limited by impurity or nanoparticle scattering. The advantage of doping via embedded nanoparticles compared to the conventional shallow impurities is quantified. At the optimum thermoelectric power factor, the electrical conductivity of the nanoparticle-doped material is larger than that of impurity-doped one at the studied temperature range (50-500 K) whereas the Seebeck coefficient of the nanoparticle doped material is enhanced only at low temperatures (∼50 K).

Thermoelectric power generator module of 16×16 Bi2Te3 and 0.6% ErAs:(InGaAs)1−x(InAlAs)x segmented elements

We report the fabrication and characterization of thermoelectric power generator modules of 16×16 segmented elements consisting of 0.8 mm thick Bi2Te3 and 50 μm thick ErAs:(InGaAs)1−x(InAlAs)x with 0.6% ErAs by volume. An output power up to 6.3 W was measured when the heat source temperature was at 610 K. The thermoelectric properties of (InGaAs)1−x(InAlAs)x were characterized from 300 up to 830 K. The finite element modeling shows that the performance of the generator modules can further be enhanced by improving the thermoelectric properties of the element materials, and reducing the electrical and thermal parasitic losses.

A comparison of thin film microrefrigerators based on Si/SiGe superlattice and bulk SiGe

Most of the conventional thermal management techniques can be used to cool the whole chip. Since thermal design requirements are mostly driven by the peak temperatures, reducing or eliminating hot spots could alleviate the design requirements for the whole package. Monolithic solid-state microcoolers offer an attractive way to eliminate hot spots. In this paper, we review theoretical and experimental cooling performance of silicon-based microrefrigerators on a chip. Both Si/SiGe superlattice and also bulk SiGe thin film devices have been fabricated and characterized. Direct measurement of the cooling along with material characterization allows us to extract the key factors limiting the performance of these microrefrigerators. Although Si/SiGe superlattice has larger thermoelectric power factor, the maximum cooling of thin film refrigerators based on SiGe alloys are comparable to that of superlattices. This is due to the fact that the superlattice thermal conductivity is larger than bulk SiGe alloy by about 30%.

Pulsed cooling of inhomogeneous thermoelectric materials

It has been proven that the maximum cooling temperature of a thermoelectric material can be increased by using either pulsed operation or graded Seebeck profiles. In this paper, we show that the maximum cooling temperature can be further increased by the pulsed operation of optimal inhomogeneous thermoelectric materials. A random sampling method is used to obtain the optimal electrical conductivity profile of inhomogeneous materials, which can achieve a much higher cooling temperature than the best uniform materials under the steady-state condition. Numerical simulations of pulsed operation are then carried out in the time domain. In the limit of low thermoelectric figure-of-merit ZT, the finite-difference time-domain simulations are verified by an analytical solution for homogeneous material. This numerical method is applied to high ZT BiTe materials and simulations show that the effective figure-of-merit can be improved by 153% when both optimal graded electrical conductivity profiles and pulsed operation are used.

ErAs : InGaAs/InGaAlAs superlattice thin-film power generator array

We report a wafer scale approach for the fabrication of thin-film power generators composed of arrays of 400 p and n type ErAs:InGaAs/InGaAlAs superlattice thermoelectric elements. The elements incorporate ErAs metallic nanoparticles into the semiconductor superlattice structure to provide charge carriers and create scattering centers for phonons. p- and n-type ErAs:InGaAs/InGaAlAs superlattices with a total thickness of 5 mu m were grown on InP substrate using molecular beam epitaxy. The cross-plane Seebeck coefficients and cross-plane thermal conductivity of the superlattice were measured using test pattern devices and the 3 omega method, respectively. Four hundred element power generators were fabricated from these 5 mu m thick, 200 mu mx200 mu m in area superlattice elements. The output power was over 0.7 mW for an external resistor of 100 Omega with a 30 K temperature difference drop across the generator. We discuss the limitations to the generator performance and provide suggestions for improvements. (c) 2006 American Institute of Physics.

InP-based passive ring-resonator-coupled lasers

The design of passive ring-coupled lasers based on InGaAsP waveguides is investigated using a beam propagation method. Mode coupling, propagation loss due to bending, and scattering loss from waveguide sidewall roughness are taken into account. By compromising threshold gain, linewidth and side-mode suppression ratio (SMSR), suitable waveguide width and coupling strength are determined for different ring sizes. Using a ring with radius ranging from 20 to 200 /spl mu/m, it is possible to design passive ring-coupled lasers with threshold gain less than 60/cm and 80/cm for waveguide sidewall roughness 5 and 10 nm, respectively, SMSR larger than 50 dB, and linewidth in the range of /spl sim/3-500 kHz.