Thermal Conductivity Of Superlattices Research Articles

A theoretical insight into phonon heat transport in graphene/biphenylene superlattice nanoribbons: a molecular dynamic study

Manipulating the thermal conductivity of nanomaterials is an efficacious approach to fabricate tailor-made nanodevices for thermoelectric applications. To this end, superlattice nanostructures can be used to achieve minimal thermal conductivity for the employed nanomaterials. Two-dimensional biphenylene is a recently-synthesized sp2-hybridized allotrope of carbon atoms that can be employed in superlattice nanostructures and therefore further investigation in this context is due. In this study, we first determined the thermal conductivity of biphenylene at 142.8 W mK−1 which is significantly lower than that of graphene. As a second step, we studied the effect of the superlattice period () on thermal conductivities of the employed graphene/biphenylene superlattice nanoribbons, using molecular dynamics simulations. We calculated a minimum thermal conductivity of 105.5 W mK−1 at = 5.066 nm which indicates an achieved thermal conductivity reduction of approximately 97% and 26% when compared to pristine graphene and biphenylene, respectively. This superlattice period denotes the phonon coherent length at which the wave-like behavior of phonons starts prevailing over the particle-like behavior. Finally, the effects of temperature and temperature gradient on the thermal conductivity of superlattice were also investigated.

Lattice thermal conductivity in isotope diamond asymmetric superlattices

We study the lattice thermal conductivity of isotope diamond superlattices consisting of 12C and 13C diamond layers at various superlattice periods. It is found that the thermal conductivity of a superlattice is significantly deduced from that of pure diamond because of the reduction of the phonon group velocity near the folded Brillouin zone. The results show that asymmetric superlattices with a different number of layers of 12C and 13C diamonds exhibit higher thermal conductivity than symmetric superlattices even with the same superlattice period, and we find that this can be explained by the trade-off between the effects of phonon specific heat and phonon group velocity. Furthermore, impurities and imperfect superlattice structures are also found to significantly reduce the thermal conductivity, suggesting that these effects can be exploited to control the thermal conductivity over a wide range.

Further Decrease of the Thermal Conductivity of Superlattice Through Embedding Nanoparticle

Further Decrease of the Thermal Conductivity of Superlattice Through Embedding Nanoparticle

Thermal transport in two-dimensional C3N/C2N superlattices: A molecular dynamics approach

Nanostructured superlattices have been the focus of many researchers due to their physical and manipulatable properties. They aim to find promising materials for new electronic and thermoelectric devices. In the present study, we investigate the thermal conductivity of two-dimensional (2D) C3N/ C2N superlattices using non-equilibrium molecular dynamics. We analyze the dependence of thermal conductivity on the total length, temperature, and the temperature difference between thermal baths for the superlattices. The minimum thermal conductivity and the phonon mean free path at a superlattice period of 5.2 nm are 23.2 W/m.K and 24.7 nm, respectively. Our results show that at a specific total length, as the period increases, the number of interfaces decreases, thus the total thermal resistance decreases, and the effective thermal conductivity of the system increases. We found that at long lengths (Lx >80 nm), the high-frequency and low-wavelength phonons are scattered throughout the interfaces, while at short lengths, there is a wave interference that reduces the thermal conductivity. The combination of these two effects, i.e., the wave interference and the interface scattering, is the reason for the existence of a minimum thermal conductivity in superlattices.

Prediction of Bi2Te3-Sb2Te3 Interfacial Conductance and Superlattice Thermal Conductivity Using Molecular Dynamics Simulations.

Bismuth telluride (Bi2Te3) and its alloys with antimony telluride (Sb2Te3) have long been considered to be the best room-temperature bulk thermoelectric (TE) materials. In recent decades, proof-of-concept demonstrations on Bi2Te3-Sb2Te3 nanostructures have shown high TE performance due to reduction in lattice thermal conductivities. Particularly, ultra-low thermal conductivities have been observed in Bi2Te3-Sb2Te3 1D superlattices, leading to thermoelectric figures of merit (ZT) as high as 2.4. In contrast, very few computational studies have been performed to provide insight into the phonon transport across these nanostructures. In this work, we use non-equilibrium molecular dynamics simulations with previously developed force fields to simulate thermal transport across Bi2Te3-Sb2Te3 interfaces and superlattices. We first calculate the thermal conductance associated with a Bi2Te3-Sb2Te3 interface across a temperature range of 200-400 K. The values are also compared with thermal conductances calculated by a modified Landauer transport formalism using phonon transmission coefficients obtained from the diffuse mismatch model. Our results show that inelastic scattering processes contribute to an increase in interfacial thermal conductance at higher temperatures. Finally, we calculate the thermal conductivities of Bi2Te3-Sb2Te3 superlattices with varying period lengths from 2 to 18 nm. A minimum thermal conductivity of 0.27 W/mK is observed at a period length of 4 nm, which is attributed to the competition between incoherent and coherent phonon transport regimes. In comparison with previous experimental measurements in the literature, our results show good agreement with respect to the range of thermal conductivity values and the period length corresponding to the minimum superlattice thermal conductivity.

Thermal conductivities of different period Si/Ge superlattices

Thermoelectric materials, which can convert wasted heat into electricity, have attracted considerable attention because they provide a solution to energy problems. The Si/Ge superlattices have shown tremendous promise as effective thermoelectric materials. The period lengths of the Si/Ge superlattices can effectively tailor the phonon's transport behaviors and control their thermal conductivities. In this paper, three kinds of Si/Ge superlattices with different period length distributions (uniform, gradient, random) are constructed. The non-equilibrium molecular dynamics (NEMD) method is used to calculate the thermal conductivities of Si/Ge superlattices under the different period length distributions. The effect of the sample’s total length and temperature on the superlattice's thermal conductivity are studied. The simulation result shows that the thermal conductivity of gradient and random periodical Si/Ge superlattices are significantly reduced at room temperature compared with that of the uniform period Si/Ge superlattices. Phonons are transported by wave or particle properties in the different periodical superlattices. The thermal conductivity of uniform period superlattices has an obvious size effect with the increasing of the sample total length. In contrast, the thermal conductivity of gradient, random periodical Si/Ge superlattices are weakly dependent on the sample’s total length. At the same time, temperature is an important factor affecting the heat transport properties. We find that the temperature affects the thermal conductivities of the three kinds of superlattices in different ways. With the increase of the temperature, (i) the thermal conductivity of uniform periodical superlattices shows an obvious temperature effect; (ii) the thermal conductivity of the gradient and random periodical Si/Ge superlattices are nearly unchanged due to the competition between phonon localization weakness and phonon-phonon scattering enhancement. In addition, the phonon densities of states of superlattices with three different periodical length distributions are calculated. We find that in the picture of uniform periodical Si/Ge superlattices, the number of pronounced peaks quickly decreases as the period length increases, particularly at higher frequencies. This indicates that as the period length increases, fewer coherent phonons will be formed over the superlattices. Moreover, the scattering mechanisms of phonons for gradient and random periodical Si/Ge superlattices are basically the same at 100 K and 500 K. These findings provide a developmental way to further reduce the thermal conductivity of superlattices.

Cross-plane thermal conductivity in amorphous Si/SiO2 superlattices

Heat conduction in superlattices demonstrates various atomic-scale effects, one of which is the ultra-low thermal conductivity. Remarkably, theoretical works even promise sub-amorphous thermal conductivity in superlattices made of amorphous materials. Yet, these predictions were not tested experimentally. Here, we experimentally study the cross-plane thermal transport in amorphous Si/SiO2 superlattices at room temperature. Using the micro time-domain thermoreflectance technique, we measured the thermal conductivity of superlattices with periods of 6.6, 11.8, and 25.7 nm. The thermal conductivity values are in the range of 1.1–1.5 W m−1 K−1 and generally agree with the values reported for amorphous Si and SiO2. However, the superlattice with the highest density of interfaces seems to have the thermal conductivity slightly below the amorphous limit. These data suggest that heat conduction below the amorphous limit might be possible in amorphous superlattices with a periodicity shorter than 6.6 nm.

Machine learning maximized Anderson localization of phonons in aperiodic superlattices

Nanostructuring materials to achieve ultra-low lattice thermal conductivity has proven to be extremely attractive for numerous applications such as thermoelectric energy conversion. Anderson localization of phonons due to aperiodicity can reduce thermal conductivity in superlattices, but the lower limit of thermal conductivity remains elusive due to the prohibitively large design space. In this work, we demonstrate that an intuition-based manual search for aperiodic superlattice structures (random multilayers or RMLs) with the lowest thermal conductivity yields only a local minimum, while a genetic algorithm (GA) based approach can efficiently identify the globally minimum thermal conductivity by only exploring a small fraction of the design space. Our results show that this minimum value occurs at an average RML period that is, surprisingly, smaller than the period corresponding to the minimum SL thermal conductivity. Above this critical period, scattering of incoherent phonons at interfaces is less, whereas below this period, the room for randomization becomes less, thus putting more coherent phonons out of Anderson localization and causing increased thermal conductivity. Moreover, the lower limit of the thermal conductivity occurs at a moderate rather than maximum randomness of the layer thickness. Our machine learning approach demonstrates a general process of exploring an otherwise prohibitively large design space to gain non-intuitive physical insights.

(Invited) Nanowire-Based Bulk Thermoelectrics

Theoretical and experimental studies over the last two decades have indicated that nanostructuring could be employed to overcome the constraints imposed by Wiedemann-Franz law and thereby allow for independently tuning thermal and electrical transport through materials. Such precise and independent control over electrical and thermal transport through materials allows for tuning their thermoelectric performances, which is typically estimated using the figure of merit, zT. The figure of merit, zT, of thermoelectric materials is given by the following: zT=S 2 σT/(κ e+κ l). Here, S is the Seebeck coefficient of the material, σ is the electrical conductivity of the material, and κ e and κ l are the thermal conductivities of the material from electronic and lattice contributions, respectively. In addition to lowering the κ l of materials through nanostructuring, power factor (S 2 σ) enhancement could also be performed for enhancing their zT values. Strategies such as dopant optimization and resonant scattering aid in enhancing the power factor values of materials. Consequently, multiple nanostructured morphologies of materials could be employed in the fabrication of thermoelectrics, including nanoparticles and quantum dots, superlattices and nanowires. However, the enhanced electrical conductivities expected of single crystal nanowires makes them preferable over nanoparticles. Similarly, incoherent phonon scattering for reducing the thermal conductivity in superlattices is partially masked by coherent phonon scattering. Hence, lowest possible thermal conductivities may not be achieved in superlattices. Furthermore, relative to the mass production of nanowires, mass production of superlattices is difficult. As research in the field of nanowire mass production progresses rapidly, it is entirely possible to mass produce and deploy terrestrial thermoelectrics for not only scavenging waste heat, but also the generation of electricity from renewable sources (e.g., solar thermal energy). Translating recent successes in the laboratories in enhancing the thermoelectric performances of nanowires to industrial-scale production and terrestrial deployment of nanowire-based devices requires, first and foremost, extending enhanced energy conversion performances achieved in individual nanowires and small-scale nanowire arrays and mats to devices composed of large-scale nanowire assemblies. This in turn requires not only the mass production of nanowires, but also their integration into macroscale devices in a manner that offers the ability to engineer the interfaces between the assembled nanowires. In addition, ensuring reliable thermoelectric performances necessitates that the nanowires and the interfaces comprising the devices retain their respective chemical compositions over extended periods of time. So, nanowires need to be thermally inert and remain unreactive with air and moisture. In other words, convergence of the correct electrical, thermal, mechanical and chemical properties is essential to ensure fabrication of highly efficient bulk thermoelectrics based on nanowires. In this invited talk, the strategies developed by our group for the mass production of nanowires that involve the direct reaction of component elements will be presented (1). Strategies for mass producing nanowires in a by-product free manner will be also be presented and discussed (2). In-situ methods useful for decorating nanowires with inorganic/organic molecules during/immediately after their synthesis for imparting them thermal and chemical stability will also be discussed (1,3-6). Implementation of pressure-assisted and shear-assisted strategies for the interface-engineered assembly of nanowires into either randomly oriented nanowire assemblies or aligned nanowire assemblies will be discussed (4-5,7-9). Illustrations of how nanostructuring, doping optimization and resonant scattering were employed to optimize the thermoelectric performances of bulk nanowires assemblies will be discussed in detail using Si, Zn3P2, ZnO and Mg2Si nanowire systems as illustrative examples (4-5,7-8). REFERENCES Brockway, M. Van Laer, Y. Kang, S. Vaddiraju, Physical Chemistry Chemical Physics, 15, 6260-6267 (2013).Chen, R. Polinnaya, S. Vaddiraju, Materials Research Express, 5, 055042 (2018).Vasiraju, Y. Kang, S. Vaddiraju, Physical Chemistry Chemical Physics, 16, 16150-16157 (2014).Brockway, V. Vasiraju, S. Vaddiraju, Nanotechnology, 25, 125402 (2014).Brockway, V. Vasiraju, H. Asayesh-Ardakani, R. Shahbazian-Yassar, S. Vaddiraju, Nanotechnology, 25, 145401 (2014).Vasiraju, Y. Kang, S. Vaddiraju, Physical Chemistry Chemical Physics, 16, 16150-16157 (2014).Brockway, V. Vasiraju, M. K. Sunkara, S. Vaddiraju, ACS Applied Materials & Interfaces 6, 14923-14930 (2014).Kang, S. Vaddiraju, Chemistry of Materials, 26, 2814-2819 (2014).Vasiraju, L. Brockway, S. Balachandran, A. Srinivasa, S. Vaddiraju, Materials Research Express, 2(1), 015013 (2015).

(Invited) Phonon Heat Transport Processes and Thermal Conductivity of II-Oxide and III-V Superlattices: Wave-like Coherent Transport in CaTiO3/SrTiO3 Superlattices and Ballistic-Diffusive Transport in GaAs/Alas Superlattices

The thermal conductivity of superlattices is strongly reduced as compared to the thermal conductivity of the individual parent materials comprising these multilayered structures due to phonon-scattering and thermal boundary resistances at the superlattice period interfaces. However, given the relatively length scale of the superlattice periods compared to typical phonon wavelengths and mean free paths, the vibrational heat transport in superlattices can give rise to unique phonon scattering processes. For example, partially ballistic and partially diffusive phonon transport in superlattices in which long wavelength phonons effectively do not “see” the interfaces on the order of the superlattice period can occur if superlattice periodicity is short enough. As another example, under certain conditions in which phonon coherence is not destroyed at superlattice interfaces, the emergence of a “minimum” in the superlattice thermal conductivity as a function of interface density can be realized. In this work, we present a series of works on III-V (e.g., GaAs/AlAs) and II-oxide (CaTiO3/SrTiO3) superlattices in which we observe unique phonon thermal transport properties based on measurements of thermal conductivity of these superlattice thin films. First, we will discuss our work on CaTiO3/SrTiO3 superlattices in which we observe coherent phonon transport based on the observation of a minimum in thermal conductivity of superlattice thin films as a function of interface density. When the period thickness decreases to below the phonon coherence length of the phonons in CaTiO3 and SrTiO3, an increase in interface density leads to an increase in thermal conductivity, a phenomenon that indicates a “particle-wave-like” crossover and coherent phonon transport influencing the in phonon thermal conductivity in superlattices. This observation is enabled by the chemically and structurally abrupt nature of the CaTiO3/SrTiO3 interfaces in these superlattices. Next, we will discuss our work on GaAs/AlAs superlattices in which we observe a ballistic-diffusive cross over based on the total thickness of the superlattice. We report on the room temperature thermal conductivity of AlAs-GaAs superlattices (SLs), in which we systematically vary the period thickness and total thickness between 2–24 nm and 20.1–2,160 nm, respectively. The thermal conductivity increases with the SL thickness and plateaus at a thickness around 200 nm, showing a clear transition from a quasiballistic to a diffusive phonon transport regime. These results demonstrate the existence of classical size effects in SLs, even at the highest interface density samples. Our results reveal that the change in thermal conductivity with period thickness is dominated by incoherent (particlelike) phonons in these III-V superlattice films, whose properties are not dictated by changes in the AlAs or GaAs phonon dispersion relations. This work demonstrates the importance of studying both period and sample thickness dependencies of thermal conductivity to understand the relative contributions of coherent and incoherent phonon transport in the thermal conductivity in SLs. “Crossover from incoherent to coherent phonon scattering in epitaxial oxide superlattices,” Nature Materials, 13, 168–172 (2014).“Interplay between total thickness and period thickness in the phonon thermal conductivity of superlattices from the nanoscale to the microscale: Coherent versus incoherent phonon transport,” Physical Review B, 97, 085306 (2018).

Interplay between total thickness and period thickness in the phonon thermal conductivity of superlattices from the nanoscale to the microscale: Coherent versus incoherent phonon transport

We report on the room temperature thermal conductivity of AlAs-GaAs superlattices (SLs), in which we systematically vary the period thickness and total thickness between $2--24\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ and $20.1--2,160\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$, respectively. The thermal conductivity increases with the SL thickness and plateaus at a thickness around 200 nm, showing a clear transition from a quasiballistic to a diffusive phonon transport regime. These results demonstrate the existence of classical size effects in SLs, even at the highest interface density samples. We use harmonic atomistic Green's function calculations to capture incoherence in phonon transport by averaging the calculated transmission over several purely coherent simulations of independent SL with different random mixing at the AlAs-GaAs interfaces. These simulations demonstrate the significant contribution of incoherent phonon transport through the decrease in the transmission and conductance in the SLs as the number of interfaces increases. In spite of this conductance decrease, our simulations show a quasilinear increase in thermal conductivity with the superlattice thickness. This suggests that the observation of a quasilinear increase in thermal conductivity can have important contributions from incoherent phonon transport. Furthermore, this seemingly linear slope in thermal conductivity versus SL thickness data may actually be nonlinear when extended to a larger number of periods, which is a signature of incoherent effects. Indeed, this trend for superlattices with interatomic mixing at the interfaces could easily be interpreted as linear when the number of periods is small. Our results reveal that the change in thermal conductivity with period thickness is dominated by incoherent (particlelike) phonons, whose properties are not dictated by changes in the AlAs or GaAs phonon dispersion relations. This work demonstrates the importance of studying both period and sample thickness dependencies of thermal conductivity to understand the relative contributions of coherent and incoherent phonon transport in the thermal conductivity in SLs.

Effect of crystalline/amorphous interfaces on thermal transport across confined thin films and superlattices

We report on the thermal boundary resistances across crystalline and amorphous confined thin films and the thermal conductivities of amorphous/crystalline superlattices for Si/Ge systems as determined via non-equilibrium molecular dynamics simulations. Thermal resistances across disordered Si or Ge thin films increase with increasing length of the interfacial thin films and in general demonstrate higher thermal boundary resistances in comparison to ordered films. However, for films ≲3 nm, the resistances are highly dependent on the spectral overlap of the density of states between the film and leads. Furthermore, the resistances at a single amorphous/crystalline interface in these structures are much lower than those at interfaces between the corresponding crystalline materials, suggesting that diffusive scattering at an interface could result in higher energy transmissions in these systems. We use these findings, together with the fact that high mass ratios between amorphous and crystalline materials can lead to higher thermal resistances across thin films, to design amorphous/crystalline superlattices with very low thermal conductivities. In this regard, we study the thermal conductivities of amorphous/crystalline superlattices and show that the thermal conductivities decrease monotonically with increasing interface densities above 0.1 nm−1. These thermal conductivities are lower than that of the homogeneous amorphous counterparts, which alludes to the fact that interfaces non-negligibly contribute to thermal resistance in these superlattices. Our results suggest that the thermal conductivity of superlattices can be reduced below the amorphous limit of its material constituent even when one of the materials remains crystalline.

Phonon heat transport in superlattices: Case of Si/SiGe and SiGe/SiGe superlattices

We present a predictive Boltzmann model for the cross-plane thermal conductivity in superlattices. The developed model considers particle-like phonons exhibiting wave characteristics at the interfaces and makes the assumption that the phonon heat transport in a superlattice has a mixed character. Exact Boltzmann equation comprising spatial dependence of phonon distribution function is solved to yield a general expression for the lattice thermal conductivity. The intrinsic phonon scattering rates are calculated from Fermi’s golden rule, and the model vibrational parameters are derived as functions of temperature and crystallographic directions by using elasticity theory-based lattice dynamics approach. The developed theory is then adapted to calculate the cross-plane thermal conductivity of superlattices. It is assumed that the phonons of wavelengths comparable or smaller than the superlattice period or the root mean square irregularity at the superlattice interfaces may be subject to a resistive scattering mechanism at the interfaces, whereas the phonons of wavelengths much greater than the superlattice period undergo ballistic transmission through the interfaces and obey dispersion relations determined by the Brillouin zone folding effects of the superlattice. The accuracy of the concept of mixed phonon transport regime in superlattices is demonstrated clearly with reference to experimental measurements regarding the effects of period thickness and temperature on the cross-plane thermal conductivity of Si/Si0.7Ge0.3 and Si0.84Ge0.16/Si0.76Ge0.3 superlattices.

Contributions of strain relaxation and interface modes to thermal transport in superlattices

Superlattice structures are widely used in electronic and optoelectronic devices, many of which depend heavily on thermal management for performance and reliability. It has been observed that silicon/germanium superlattices exhibit an enhancement in thermal conductivity at very short period lengths, which has been attributed to the contribution of coherent phonons. Here we investigate additional potential contributions to enhanced thermal conductivity in superlattices as period length is reduced, finding that a reduction in strain relaxation as well as increased contributions of interface modes that have vibrational character intermediate between those of the two constituent materials offer additional mechanisms for increased thermal conductivity.

Phonon Transport in SiGe-Based Nanocomposites and Nanowires for Thermoelectric Applications

ABSTRACTSilicon-germanium (SiGe) superlattices (SLs) have been proposed for application as efficient thermoelectrics because of their low thermal conductivity, below that of bulk SiGe alloys. However, the cost of growing SLs is prohibitive, so nanocomposites, made by a ball-milling and sintering, have been proposed as a cost-effective replacement with similar properties. Lattice thermal conductivity in SiGe SLs is reduced by scattering from the rough interfaces between layers. Therefore, it is expected that interface properties, such as roughness, orientation, and composition, will play a significant role in thermal transport in nanocomposites and offer many additional degrees of freedom to control the thermal conductivity in nanocomposites by tailoring grain size, shape, and crystal angle distributions. We previously demonstrated the sensitivity of the lattice thermal conductivity in SLs to the interface properties, based on solving the phonon Boltzmann transport equation under the relaxation time approximation. Here we adapt the model to a broad range of SiGe nanocomposites. We model nanocomposite structures using a Voronoi tessellation to mimic the grains and their distribution in the nanocomposite and show excellent agreement with experimentally observed structures, while for nanowires we use the Monte Carlo method to solve the phonon Boltzmann equation. In order to accurately treat phonon scattering from a series of atomically rough interfaces between the grains in the nanocomposite and at the boundaries of nanowires, we employ a momentum-dependent specularity parameter. Our results show thermal transport in SiGe nanocomposites and nanowires is reduced significantly below their bulk alloy counterparts.

Minimum thermal conductivity in superlattices: A first-principles formalism

The thermal conductivity of silicon-germanium superlattices is computed from density-functional perturbation theory using relaxation times that include both anharmonic and interface roughness effects. A decrease in the group velocity of low-frequency phonons in addition to the interface-disorder-induced scattering of high-frequency phonons drives the superlattice thermal conductivity to below the alloy limit. At short periods, interplay between decrease in group velocity and increase in phonon lifetimes with increase in superlattice period leads to a minimum in the cross-plane thermal conductivity. Increasing the mass mismatch between the constituent materials in the superlattice further lowers the thermal conductivity below the alloy limit, pointing to avenues for higher efficiency thermoelectric materials.

Thermal conductivity of GaAs/AlAs superlattices and the puzzle of interfaces

We present a molecular dynamics investigation of the cross-plane thermal conductivity ofsuperlattices using the non-equilibrium molecular dynamics method. The purpose is toinvestigate the influence of the interfaces, which is expected to be important in thosenanostructures where the superlattice period is smaller than the phonon mean free path. Incontrast to previous studies, more realistic interfaces are considered: interfacial roughness ismodeled using atomic rectangular islands and interdiffusion is taken into account. It isshown that thermal conductivity is very sensitive to the detailed interfacial shape andto the presence of interdiffusion. This may be relevant to recent experiments.

Reduction in coherent phonon lifetime in Bi2Te3/Sb2Te3 superlattices

Femtosecond pulses are used to excite A1g optical phonons in Bi2Te3, Sb2Te3, and Bi2Te3/Sb2Te3 superlattice. Time-resolved reflectivity measurements show both the low-frequency and high-frequency components of A1g phonon modes. By comparing the phonon lifetime, it is found that the scattering rate (inverse of lifetime) in superlattice is significantly higher than those in Bi2Te3 and Sb2Te3. This represents the direct measurement of coherent phonon lifetime reduction in superlattice structures, consistent with the observed reduction in thermal conductivity in superlattices.

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%.

Enhanced Phonon Scattering in Oxide Superlattices to Improve Laser Induced Thermoelectric Voltages

AbstractOptimizing the figure of merit for thermoelectric applications, ZT = S2σ2T/κ is currently at the core of materials oriented research in thermoelectricity. Here, one promising approach is to reduce ther thermal conductivity without sacrificing the electrical conductivity. Constructing superlattices of structurally compatible materials is one way to accomplish this goal. We report an enhanced laser induced thermoelectric voltage (LITV) effect observed in (YBa2Cu3O7/La1-xPbxMnO3)n multilayer thin films for the first time. Two groups of multilayer thin films grown on vicinal cut LaAlO3 substrates were prepared by pulsed laser deposition technique. The first group were grown on different substrates vicinal cut at different angles, and were used for checking the mechanism of the induced voltages. The second group samples were made at different period number n and for studying the number dependence of the peak values of LITV. The substrate angle dependence proved that this is a thermoelectric effect [1]. It was found that the LITV signals were enhanced significantly for these multilayer thin films comparing with the single layer ones. It is natural that the conductivity is going to be anisotropic due to the layered structure, and the same holds for the Seebeck coefficients. The enlarged Seebeck anisotropy will lead to higher induced voltages. Another possible reason is the reduced thermal conductivity in the layered structure. The maximum enhancement of LITV signals takes place at period number of 7, which seems in agreement with the prediction of minimum thermal conductivity in superlattices by Simkin and Mahan.