Research

 

Nanostructured Thermoelectrics

 

Background

 

In a typical thermoelectric device, a junction is formed between p-type and n-type semiconductor materials. The device acts as a Peltier heat pump when electrical current is passed through the junction or as a Seebeck electrical generator when placed along a temperature gradient. In either case the device performance is limited by the non-dimensional thermoelectric figure of merit (ZT) of the material, where T is the absolute temperature and Z is the thermoelectric figure of merit, Z=sa2/k (a-thermoelectric power, s-electrical conductivity, k-thermal conductivity).

Increasing the thermoelectric figure of merit in conventional solids faces many challenges. For instance, increasing the thermoelectric power usually decreases the electrical conductivity. Moreover, an increase in the electrical conductivity also increases the electronic contribution to thermal conductivity. Therefore, a limit is rapidly approached where a modification to any of the three parameters s, a, k, adversely affects the other transport parameters, and Z does not change significantly.

 

Motivation

 

Harvesting medium-grade heat from industrial stacks alone could generate at least 50 GW of electrical power, which is half the energy produced by the US nuclear fleet. Current technologies for waste heat recovery systems employ thermodynamics cycles that work with low-boiling temperature fluids. At present there are no adequate technologies for efficiently utilizing abundant sources of waste heat such as low pressure exhaust gas streams, or non-gas heat sources (e.g., hot surfaces). On another hand efficient solid-state refrigeration will lead to a revolution in air conditioning applications from cars to buildings. Advances in solid-state thermoelectric energy conversion will translate into large scale implementation of these systems in real world applications. Immediate societal benefits include a decreased dependence on oil reserves, decreased threat from CO2 environmental damage and creation of a new global market for waste heat technologies.

 

Nanostructures and nanostructured materials are ideal candidates for highly efficient thermoelectric energy conversion. The reasons are rooted in quantum and size effects of the charge and heat carriers respectively. In suitable nanostructures, quantum confinement of the charge carriers may enhance the Seebeck coefficient and electrical conductivity of existing materials due to an increased density of states at the Fermi level. On the other hand, enhanced boundary scattering of heat carriers lead to a large reduction in thermal conductivity of nanostructures. The simultaneous increase of the power factor and the reduction of thermal conductivity lead to large figures of merit Z.

 

Goals

 

·         Investigate nanoscience based strategies to achieve high figures of merit in novel materials for solid state refrigeration and power generation.

·         Develop metrology techniques for thermoelectric properties characterization of nanostructured thermoelectric materials and their interfaces with electrodes.

·         Develop fabrication and testing strategies for nanostructured thermoelectric energy conversion devices.

 

Recent contributions and relevant publications

 

·   Discovery (through a long-term collaboration with Prof. G. Ramanath’s group from MSE Dept. at RPI) of a new class of highly scalable, high figure of merit, nanostructured bulk thermoelectric materials (patent pending and licensed by ThermoAura Inc.). The high ZT results reported were enabled by reductions in thermal conductivity due to nanopores and nanograins. There is also a promise for ZT>2 by sulfur doping control facilitated by the new bottom-up synthesis method.

 

R. J. Mehta, Y. Zhang, H. Zhu, D. S. Parker, M. Belley, D. J. Singh, R. Ramprasad, T. Borca-Tasciuc, and G. Ramanath

Seebeck and Figure of Merit Enhancement in Nanostructured Antimony Telluride by Antisite Defect Suppression through Sulfur Doping,”

Nano Letters, Vol. 12, 4523–4529, 2012 (PDF).

 

Yanliang Zhang, Rutvik J. Mehta, Matthew Belley, Liang Han, Ganpati Ramanath, and Theodorian Borca-Tasciuc

“Lattice Thermal Conductivity Diminution and High Thermoelectric Power Factor Retention in Nanoporous Macroassemblies of Sulfur-Doped Bismuth Telluride Nanocrystals

Applied Physics Letters, Vol. 100, 1193113 1-4, 2012 (PDF).

 

Rutvik J. Mehta, Yanliang Zhang, Chinnathambi Karthik, Binay Singh, Richard W. Siegel, Theodorian Borca-Tasciuc & Ganpati Ramanath

“A New Class of Doped Nanobulk High-Figure-of-Merit Thermoelectrics by Scalable Bottom-up Assembly”

Nature Materials, Vol. 11, 233-240, 2012 (PDF).

 

 

·   Development of a scanning thermal microprobe for quantitative characterization of the thermal conductivity and Seebeck coefficient with microscale resolution.

 

 T. Borca-Tasciuc

“Scanning Probe Methods for Thermal and Thermoelectric Property Measurements,”

for the volume Experimental Techniques for Micro/Nanoscale Thermal and Thermoelectric Measurements

Annual Reviews of Heat Transfer, Invited paper (accepted).

 

Y. Zhang, E. Castillo, R. Mehta, G. Ramanath, and T. Borca-Tasciuc

“A non-contact thermal microprobe for local thermal conductivity measurement”

Review of Scientific Instruments, Vol. 82, 024902, 2011. (PDF)

 

Y. Zhang, C. L. Hapenciuc, E. E. Castillo, T. Borca-Tasciuc, R. J. Mehta, C. Karthik, and G. Ramanath

“A microprobe technique for simultaneously measuring thermal conductivity and Seebeck coefficient of thin films”

 Applied Physics Letters, Vol. 96, 062107, 2010. (PDF)

 

·   Development of a transient method for measurement of all thermoelectric properties as well as electrical and thermal contact resistances in films.

 

E. E. Castillo, C. L. Hapenciuc, and T. Borca-Tasciuc

“Thermoelectric characterization by transient Harman method under non-ideal contact and boundary conditions”

 Review of Scientific instruments, Vol. 81, 044902, 2010. (PDF)

 

 

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Thermal Transport in Nanostructures and Nanocomposites

 

Background

 

As nanostructures approach fundamental length scales of the thermal energy carriers, size and quantum effects may strongly influence their effective thermophysical properties. A good example is the well-documented reduction of the thermal conductivity in thin films and superlattices, which is attributed to the intense surface/interface scattering of heat carriers. Similar effects are partially responsible for large reductions in the thermal conductivity of nanowires and quantum dots. This affects the failure rate of devices, which depends heavily on the operating temperature. Similarly, overheating problems affect the operation and performance of solid state lasers and LEDs. However, the reduced thermal conductivity effect may be used to advantage in thermoelectric applications, in which reduced phonon thermal conductivity and enhanced electrical conductivity and Seebeck coefficient are required. On the other hand, novel nanoscale arrangements of atoms could have very high thermal conductivities, as predicted for single wall carbon nanotubes. This raises new challenges and also brings new opportunities in heat transfer, energy conversion, and thermal energy storage using nanomaterials.

 

Motivation

 

Structures such as thin films, quantum wells, superlattices, have already numerous applications in microelectronics, optoelectronics, micro-electro-mechanical-systems, solid-state lasers, and most recently in solid-state energy conversion. Moreover, with the emergence of a new era of nanotechnology one-dimensional structures such as nanowires and carbon nanotubes or zero-dimensional structures such as quantum dots may emerge in the next generations of nanoelectronics and optoelectronis devices or nanoelectromechanical systems. New materials and structures bring new opportunities for engineering thermal transport in a variety of applications from thermal management, to thermal energy storage, to thermoelectric energy conversion.

 

Goals

 

 

Recent contributions and relevant publications

 

·         Development of a photothermoelectric method to determine the anisotropic thermal conductivity and the interface thermal resistance in thin film-on-substrate systems.

 

Borca-Tasciuc, T., Borca-Tasciuc, D.-A., and Chen G.

 “A Photothermoelectric Technique for Anisotropic Thermal Diffusivity Characterization of Nanowire/Nanotube Composites,”

IEEE Transactions on Components and Packaging Technologies, Vol. 30, 609-617, 2007. (PDF)

 

·         Discovery of novel mechanics for formation of high thermal conductivity networks in polymer composites filled with nanoparticles (patent pending). Self assembly of nanoparticles in high aspect ratio structures, followed by low temperature sintering enabled by small particles size, leads to high thermal conductivity pathways which impart high thermal conductivity to the nanocomposite.

 

Kamyar Pashayi, Hafez Raeisi Fard, Fengyuan Lai, Sushumna Iruvanti, Joel Plawsky, and Theodorian Borca-Tasciuc

“High Thermal Conductivity Epoxy-Silver Composites Based on Self-Constructed Nanostructured Metallic Networks”

Journal of Applied Physics, Vol. 111, 104310 1-6, 2012 (PDF).

 

 

·         Characterization of anisotropic thermal properties in aligned carbon nanotube arrays and aligned carbon-nanotube polymer composites. The findings reported include the anisotropy of aligned carbon nanotube arrays and the large thermal conductivity degradation due to defects.

 

Sunil K. Pal, Youngsuk Son, Theodorian Borca-Tasciuc, Diana-Andra Borca-Tasciuc, Swastik Kar, Robert Vajtai, Pulickel M. Ajayan

”Thermal and electrical transport along MWCNT arrays grown on Inconel substrates,”

Journal of Materials Research, Vol. 23, 2099, 2008. (PDF)

 

Borca-Tasciuc, T.,  Mazumder, M., Pal, S. K., Son, Y., L., Schadler, L. S., and Ajayan, P.

“Anisotropic thermal diffusivity characterization of aligned carbon nanotube-polymer composites”

(Invited) Journal of Nanoscience and Nanotechnology Vol. 7, 1581-1588, 2007. (PDF)

 

Borca-Tasciuc, T., Vafaei, S., Borca-Tasciuc, D.-A., Wei, B. Q, Vajtai, R., and Ajayan, P.

“Anisotropic Thermal Diffusivity of aligned multiwall carbon nanotube arrays

 Journal of Applied Physics, Vol. 98, 054309, 2005. (PDF)

 

 

·         Characterization of the interface thermal resistance at the native interface between carbon nanotube arrays and the silicon substrate. The main finding is that the native interface after Chemical Vapor Deposition (CVD) has a high thermal resistivity due to imperfect substrate-CNT contacts and the low volume fraction of CNTs in the studied arrays.

 

Figure 1Youngsuk Son, Sunil K. Pal, and Theodorian Borca-Tasciuc, Pulickel M. Ajayan, and Richard W. Siegel

”Thermal resistance of the native interface between vertically aligned multiwalled carbon nanotube arrays and their SiO2/Si substrate,”

Journal of Applied Physics, Vol. 103, 024911, 2008. (PDF).

 

 

·         Characterization of thermal transport in Si/Ge and Si/SiC multilayers. The large thermal conductivity reductions measured in the cross-plane direction, with proper doping control, can make these materials good candidates for high temperature thermoelectric energy conversion applications.

 

M. Mazumder, T. Borca-Tasciuc, S. Teehan, H. Efstathiadis, E. Stinzianni, and V. Solovyov

 “Temperature dependent thermal conductivity of Si/SiC amorphous multilayer films”

 Applied Physics Letters, Vol. 96, 093103, 2010. (PDF)

 

 

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Micro and Nano-Electro-Mechanical-Systems

 

The synergy between microfabrication and nanotechnology may bring tremendous opportunities for the development of novel sensing and actuation concepts. We explore using MEMS fabrication technologies and nanostructured materials with novel properties for the following applications. The microfabrication efforts are facilitated by the class 100 Microfabrication Clean Room (MCR) at Rensselaer Polytechnic Institute. 

 

Research interests in MEMS/NEMS:

 

·         Energy conversion

·         Chemical & biological detection

·         Fast optical switching

·         Flow control

·         Inertial sensors

·         Test-structures for thermo-fluid and thermoelectric characterization.

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