Timothy P. Hogan

Northwestern University, Evanston, IL, United States

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Publications (86)245.23 Total impact

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    ABSTRACT: Mg2(Si,Sn) compounds are promising candidate low-cost, lightweight, non- toxic thermoelectric materials made from abundant elements and are suited for power generation applications in the intermediate temperature range of 600 K to 800 K. Knowledge on the transport and mechanical properties of Mg2(Si,Sn) compounds is essential to the design of Mg2(Si,Sn)-based ther- moelectric devices. In this work, such materials were synthesized using the molten-salt sealing method and were powder processed, followed by pulsed electric sintering densification. A set of Mg2.08Si0.4?xSn0.6Sbx (0 £ x £ 0.072) compounds were investigated, and a peak ZT of 1.50 was obtained at 716 K in Mg2.08Si0.364Sn0.6Sb0.036. The high ZT is attributed to a high electrical con- ductivity in these samples, possibly caused by a magnesium deficiency in the final product. The mechanical response of the material to stresses is a function of the elastic moduli. The temperature-dependent Young’s modulus, shear modulus, bulk modulus, Poisson’s ratio, acoustic wave speeds, and acoustic Debye temperature of the undoped Mg2(Si,Sn) compounds were measured using resonant ultrasound spectroscopy from 295 K to 603 K. In addition, the hardness and fracture toughness were measured at room temperature.
    Journal of Electronic Materials 12/2014; 43(6):1790-1803. · 1.64 Impact Factor
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    ABSTRACT: This letter reports the thermoelectric properties of Bi-doped Mg2Si0.4Sn0.6 thermoelectric materials. It was found that the ZTs of this material could be greatly enhanced by Bi-doping. Analyses on the transport properties showed that the power factors of the material were enhanced while the lattice thermal conductivities were reduced by Bi-doping. The reduction of the lattice thermal conductivity was likely caused by the interstitial Bi impurities. A peak ZT ≈ 1.55 at 773 K was obtained.
    Applied Physics Letters 11/2014; 105(20):202104. · 3.52 Impact Factor
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    ABSTRACT: Lead chalcogenide thermoelectric systems have been shown to reach record high figure of merit values via modification of the band structure to increase the power factor or via nanostructuring to reduce the thermal conductivity. Recently, (PbTe)1-x(PbSe)x was reported to reach high power factors via a delayed onset of interband crossing. Conversely, the (PbTe)1-x(PbS)x was reported to achieve low thermal conductivities arising from extensive nanostructuring. Here we report the thermoelectric properties of the pseudoternary 2% Na-doped (PbTe)1-2x(PbSe)x(PbS)x system. The (PbTe)1-2x(PbSe)x(PbS)x system is an excellent platform to study phase competition between entropically driven atomic mixing (solid solution behavior) and enthalpy-driven phase separation. We observe that the thermoelectric properties of the PbTe-PbSe-PbS 2% Na doped are superior to those of 2% Na-doped PbTe-PbSe and PbTe-PbS, respectively, achieving a ZT ≈2.0 at 800 K. The material exhibits an increased the power factor by virtue of valence band modification combined with a very reduced lattice thermal conductivity deriving from alloy scattering and point defects. The presence of sulfide ions in the rock-salt structure alters the band structure and creates a plateau in the electrical conductivity and thermopower from 600 to 800 K giving a power factor of 27 μW/cmK(2). The very low total thermal conductivity values of 1.1 W/m·K of the x = 0.07 composition is accounted for essentially by phonon scattering from solid solution defects rather than the assistance of endotaxial nanostructures.
    Journal of the American Chemical Society 02/2014; · 10.68 Impact Factor
  • Philosophical Magazine 12/2013; 93(35):4412-4439. · 1.60 Impact Factor
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    ABSTRACT: The coefficient of thermal expansion (CTE) is a key design parameter for thermoelectric (TE) materials, especially in energy harvesting applications since stresses generated by CTE mismatch, thermal gradients, and thermal transients scale with the CTE of the TE material. For the PbTe–PbS-based TE material (Pb 0.95 Sn 0.05 Te) 0.92 (PbS) 0.08 —0.055 % PbI 2 over the temperature ranges of 293–543 and 293–773 K, a CTE, a avg , of 21.4 ± 0.39 10-6 K-1 was measured using (1) dilatometry and (2) high-temperature X-ray diffraction (HT-XRD) for powder and bulk specimens. The CTE values measured via dilatometry and HT-XRD are similar to the literature values for other Pb-based chalcogenides. However, the processing technique was found to impact the thermal expansion such that bloating (which leads to a hysteresis in thermal expansion) occurred for hot pressed billets heated to temperatures [603 K while specimens fabricated by pulsed electric current sintering and as-cast specimens did not show a bloating-modified thermal expansion even for temperatures up to 663 K. The relationship of bloating to the processing techniques is discussed, along with a possible mechanism for inhibiting bloating in powder processed specimens.
    Journal of Materials Science 05/2013; 48:6233-6244. · 2.31 Impact Factor
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    ABSTRACT: Previous efforts to enhance thermoelectric performance have primarily focused on reduction in lattice thermal conductivity caused by broad-based phonon scattering across multiple length scales. Herein, we demonstrate a design strategy which provides for simultaneous improvement of electrical and thermal properties of p-type PbSe and leads to ZT ∼ 1.6 at 923 K, the highest ever reported for a tellurium-free chalcogenide. Our strategy goes beyond the recent ideas of reducing thermal conductivity by adding two key new theory-guided concepts in engineering, both electronic structure and band alignment across nanostructure-matrix interface. Utilizing density functional theory for calculations of valence band energy levels of nanoscale precipitates of CdS, CdSe, ZnS, and ZnSe, we infer favorable valence band alignments between PbSe and compositionally alloyed nanostructures of CdS1-xSex/ZnS1-xSex. Then by alloying Cd on the cation sublattice of PbSe, we tailor the electronic structure of its two valence bands (light hole L and heavy hole Σ) to move closer in energy, thereby enabling the enhancement of the Seebeck coefficients and the power factor.
    Journal of the American Chemical Society 05/2013; · 10.68 Impact Factor
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    ABSTRACT: We present a systematic study of the characterization and thermoelectric properties of nanostructured Na-doped PbSe embedded with 1-4% MSe (M = Ca, Sr, Ba) phases as endotaxial inclusions. The samples were powder-processed by the spark plasma sintering technique, which introduces mesoscale-structured grains. The hierarchical architectures on the atomic scale (Na and M solid solution), nanoscale (MSe nanoprecipitates), and mesoscale (grains) were confirmed by transmission electron microscopy. These structures produce a great reduction in the lattice thermal conductivity relative to pristine PbSe without appreciably affecting the power factor. The lattice thermal conductivity can be reduced by up to ∼29% when the second phase is added. The highest ZT value achieved was ∼1.3 at 923 K for both 2% SrSe-and 3% BaSe-containing samples, while the sample containing 4% CaSe showed a ZT value of ∼1.2 at 923 K. The optimal samples have hole carrier concentration of 1-2 × 10(20) cm(-3). We attribute the high ZT values to the combination of broad-based phonon scattering on multiple length scales and favorable charge transport through coherent interfaces between the PbSe matrix and MSe.
    Journal of the American Chemical Society 03/2013; · 10.68 Impact Factor
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    ABSTRACT: A temperature dependent Hall Effect measurement system with software based data acquisition and control was built and tested. Transport measurements are shown for boron-doped single crystal diamond (SCD) films deposited in a microwave plasma-assisted chemical vapor deposition (MPCVD) reactor. The influence of Ohmic contacts and temperature control accuracy are studied. For a temperature range of 300K-700K IV curves, Hall mobilities and carrier concentrations are presented
    MRS Proceedings. 01/2013; 1511.
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    ABSTRACT: With about two-thirds of all used energy being lost as waste heat, there is a compelling need for high-performance thermoelectric materials that can directly and reversibly convert heat to electrical energy. However, the practical realization of thermoelectric materials is limited by their hitherto low figure of merit, ZT, which governs the Carnot efficiency according to the second law of thermodynamics. The recent successful strategy of nanostructuring to reduce thermal conductivity has achieved record-high ZT values in the range 1.5-1.8 at 750-900 kelvin, but still falls short of the generally desired threshold value of 2. Nanostructures in bulk thermoelectrics allow effective phonon scattering of a significant portion of the phonon spectrum, but phonons with long mean free paths remain largely unaffected. Here we show that heat-carrying phonons with long mean free paths can be scattered by controlling and fine-tuning the mesoscale architecture of nanostructured thermoelectric materials. Thus, by considering sources of scattering on all relevant length scales in a hierarchical fashion--from atomic-scale lattice disorder and nanoscale endotaxial precipitates to mesoscale grain boundaries--we achieve the maximum reduction in lattice thermal conductivity and a large enhancement in the thermoelectric performance of PbTe. By taking such a panoscopic approach to the scattering of heat-carrying phonons across integrated length scales, we go beyond nanostructuring and demonstrate a ZT value of ∼2.2 at 915 kelvin in p-type PbTe endotaxially nanostructured with SrTe at a concentration of 4 mole per cent and mesostructured with powder processing and spark plasma sintering. This increase in ZT beyond the threshold of 2 highlights the role of, and need for, multiscale hierarchical architecture in controlling phonon scattering in bulk thermoelectrics, and offers a realistic prospect of the recovery of a significant portion of waste heat.
    Nature 09/2012; 489(7416):414-8. · 38.60 Impact Factor
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    ABSTRACT: We have investigated in detail the effect of CdS and ZnS as second phases on the thermoelectric properties of p-type PbS. We report a ZT of ∼1.3 at 923 K for 2.5 at.% Na-doped p-type PbS with endotaxially nanostructured 3.0 at.% CdS. We attribute the high ZT to the combination of broad-based phonon scattering on multiple length scales to reduce (lattice) thermal conductivity and favorable charge transport through coherent interfaces between the PbS matrix and metal sulfide nanophase precipitates, which maintains the requisite high carrier conductivity and the associated power factor. Similar to large ionically bonded metal sulfides (ZnS, CaS, and SrS), the covalently bonded CdS can also effectively reduce the lattice thermal conductivity in p-type PbS. The presence of ubiquitous nanostructuring was confirmed by transmission electron microscopy. Valence and conduction band energy levels of the NaCl-type metal sulfides, MS (M = Pb, Cd, Zn, Ca, and Sr) were calculated from density functional theory to gain insight into the band alignment between PbS and the second phases in these materials. The hole transport is controlled by band offset minimization through the alignment of valence bands between the host PbS and the embedded second phases, MS (M = Cd, Zn, Ca, and Sr). The smallest valence band offset of about 0.13 eV at 0 K was found between PbS and CdS which is diminished further by thermal band broadening at elevated temperature. This allows carrier transport between the endotaxially aligned components (i.e., matrix and nanostructure), thus minimizing significant deterioration of the hole mobility and power factor. We conclude the thermoelectric performance of the PbS system and, by extension, other systems can be enhanced by means of a closely coupled phonon-blocking/electron-transmitting approach through embedding endotaxially nanostructured second phases.
    Journal of the American Chemical Society 09/2012; 134(39):16327-36. · 10.68 Impact Factor
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    ABSTRACT: Harvesting of waste heat may lead to macrocrack and/or microcrack damage accumulation in thermoelectrics. No studies in the open literature address the thermal fatigue of any thermoelectric material. This study characterizes the thermal fatigue behavior for two PbTe-based thermoelectric materials, n-type LAST (lead–antimony–silver–tellurium) and p-type LASTT (lead–antimony–silver–tellurium–tin). The mechanical properties (fracture strength, elastic moduli) were evaluated for up to 200 thermal fatigue cycles. In addition, the electrical and thermal transport properties were evaluated for n- and p-type specimens for thermal cycling. The elastic moduli were relatively insensitive to thermal fatigue treatment. The fracture strength, σf, of the thermally fatigued LASTT specimens was in a band of from 25 to 40 MPa while σf of the thermally fatigued LAST ranged from 15 to 38 MPa. The thermopower and electrical conductivity of LASTT samples showed small deviations from the low temperature trend near 600 K and the data repeated well after the first temperature cycle for all samples. For the n-type LAST samples, the electrical conductivity and thermopower showed larger deviations from the low temperature trend near 500 K with some samples requiring several temperature cycles before showing repeatability in the data, suggesting a possible secondary phase in the samples.
    Materials Chemistry and Physics. 06/2012; 134(s 2–3):973–987.
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    ABSTRACT: Mg2Si is of interest as a thermoelectric (TE) material in part due to its low materials cost, lack of toxic components, and low mass density. However, harvesting of waste heat subjects TE materials to a range of mechanical and thermal stresses. To understand and model the material’s response to such stresses, the mechanical properties of the TE material must be known. The Mg2Si specimens included in this study were powder processed and then sintered via pulsed electrical current sintering. The elastic moduli (Young’s modulus, shear modulus, and Poisson’s ratio) were measured using resonant ultrasound spectroscopy, while the hardness and fracture toughness were examined using Vickers indentation. Also, the Vickers indentation crack lengths were measured as a function of time in room air to determine the susceptibility of Mg2Si to slow crack growth.
    Journal of Electronic Materials 06/2012; 41(6). · 1.64 Impact Factor
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    ABSTRACT: We report high thermoelectric performance in nanostructured p-type PbS, a material consisting of highly earth abundant and inexpensive elements. The high level of Na doping switched intrinsic n-type PbS to p-type and substantially raised the power factor maximum for pure PbS to ~9.0 μW cm(-1) K(-2) at >723 K using 2.5 at. % Na as the hole dopant. Contrary to that of PbTe, no enhancement in the Hall coefficient occurs at high temperature for heavily doped p-type PbS, indicating a single band model and no heavy hole band. We also report that the lattice thermal conductivity of PbS can be greatly reduced by adding SrS or CaS, which form a combination of a nanostructured/solid solution material as determined by transmission electron microscopy. We find that both nanoscale precipitates and point defects play an important role in reducing the lattice thermal conductivity, but the contribution from nanoscale precipitates of SrS is greater than that of CaS, whereas the contribution from point defects in the case of CaS is greater than that of SrS. Theoretical calculations of the lattice thermal conductivity based on the modified Callaway model reveal that both nanostructures and point defects (solid solution) effectively scatter phonons in this system. The lattice thermal conductivity at 723 K can be reduced by ~50% by introducing up to 4.0 at. % of either SrS or CaS. As a consequence, ZT values as high as 1.22 and 1.12 at 923 K can be achieved for nominal Pb(0.975)Na(0.025)S with 3.0 at. % SrS and CaS, respectively. No deterioration was observed after a 15 d annealing treatment of the samples, indicating the excellent thermal stability for these high performance thermoelectrics. The promising thermoelectric properties of nanostructured PbS point to a robust low cost alternative to other high performance thermoelectric materials.
    Journal of the American Chemical Society 04/2012; 134(18):7902-12. · 10.68 Impact Factor
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    ABSTRACT: High quality single crystal boron-doped diamond films are deposited in a microwave plasma-assisted CVD reactor with feedgas mixtures including hydrogen, methane, diborane, and carbon dioxide at reactor pressures of 160 Torr. The effect of diborane levels and other growth parameters on the incorporated boron levels are investigated, and the doping efficiency is calculated over a wide range of boron concentrations. The boron level is investigated using infrared absorption, and compared to SIMS measurements, and defects are shown to affect the doping uniformity.
    MRS Online Proceeding Library 01/2012; 1395.
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    ABSTRACT: Lead chalcogenide materials have drawn attention in recent years because of their outstanding thermoelectric properties. Bulk n-type materials of AgPbm SbTe2+m have been reported to exhibit high figure of merit, ZT, as high as 1.7 at 700 K. Recent reports have shown p-type lead selenide-based compounds with comparable ZT. The analogous material AgPbm SbSe17 shares a similar cubic rock-salt structure with PbTe-based compounds; however, it exhibits a higher melting point, and selenium is more abundant than tellurium. Using solid solution chemistry, we have fabricated cast AgPb15SbSe17 samples that show a peak power factor of approximately 17 μW/cm K2 at 450 K. Increasing the strength of such materials is commonly achieved through powder processing, which also helps to homogenize the source materials. Pulsed electric current sintering (PECS) is a hot-pressing technique that utilizes electric current through the die and sample for direct Joule heating during pressing. The mechanisms present during PECS processing have captured significant research interest and have led to some notable improvements in sample properties compared with other densification techniques. We report the thermoelectric properties of PECS samples of AgPbm SbSe17 along with sample fabrication and processing details.
    Journal of Electronic Materials 01/2012; 41. · 1.64 Impact Factor
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    ABSTRACT: This investigation reports the output parameters of thermoelectric modules on based on metal/LAST(T) hot side contacts.
    Applied Solar Energy 01/2012; 48(1).
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    ABSTRACT: Lead sulfide, a compound consisting of elements with high natural abundance, can be converted into an excellent thermoelectric material. We report extensive doping studies, which show that the power factor maximum for pure n-type PbS can be raised substantially to ~12 μW cm(-1) K(-2) at >723 K using 1.0 mol % PbCl(2) as the electron donor dopant. We also report that the lattice thermal conductivity of PbS can be greatly reduced by adding selected metal sulfide phases. The thermal conductivity at 723 K can be reduced by ~50%, 52%, 30%, and 42% through introduction of up to 5.0 mol % Bi(2)S(3), Sb(2)S(3), SrS, and CaS, respectively. These phases form as nanoscale precipitates in the PbS matrix, as confirmed by transmission electron microscopy (TEM), and the experimental results show that they cause huge phonon scattering. As a consequence of this nanostructuring, ZT values as high as 0.8 and 0.78 at 723 K can be obtained for nominal bulk PbS material. When processed with spark plasma sintering, PbS samples with 1.0 mol % Bi(2)S(3) dispersion phase and doped with 1.0 mol % PbCl(2) show even lower levels of lattice thermal conductivity and further enhanced ZT values of 1.1 at 923 K. The promising thermoelectric properties promote PbS as a robust alternative to PbTe and other thermoelectric materials.
    Journal of the American Chemical Society 11/2011; 133(50):20476-87. · 10.68 Impact Factor
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    ABSTRACT: PbTe-PbS materials are promising for thermoelec. power generation applications. For the compn. of (Pb0.95Sn0.05Te)0.92(PbS)0.08 nanostructuring from nucleation and growth and spinodal decompn. has been reported along with thermal cond. of approx. 1.1 W/m·K at 650 K. Based on temp.-dependent measurements of elec. cond., thermopower, and thermal cond., the thermoelec. figure of merit, ZT, are ∼1.5 at 650 K for cast ingots. To develop larger quantities of material for device fabrication, advancement in the synthesis, processing and prodn. of (Pb0.95Sn0.05Te)0.92(PbS)0.08 is necessary. Powder processing of samples is a well-known technique for increasing sample strength, and uniformity. In this presentation, we show sample fabrication and processing details of pulsed elec. current sintering (PECS) processed (Pb0.95Sn0.05Te)0.92(PbS)0.08 materials and their thermoelec. properties along with the latest advancements in the prepn. of these materials. [on SciFinder(R)]
    MRS Online Proc. Libr. 01/2011; 1314(Copyright (C) 2013 American Chemical Society (ACS). All Rights Reserved.):No pp. given.
  • MRS Online Proceeding Library 01/2011; 453.
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    ABSTRACT: The electrical characteristics of high quality single crystal boron-doped diamond are studied. Samples are synthesized in a high power-density microwave plasma-assisted chemical vapor deposition (CVD) reactor at a pressure of 160 Torr. The boron-doped diamond films are grown using diborane in the feedgas at concentrations of 0-0.25 ppm, and are compared to those grown previously with 1-10 ppm. The boron acceptor concentration is investigated using infrared absorption, and compared to the boron concentration obtained by SIMS. A four point probe is used to study the conductivity. The temperature dependent conductivity is analyzed to determine the boron dopant activation energy
    MRS Proceedings. 01/2011; 1282.

Publication Stats

425 Citations
245.23 Total Impact Points

Institutions

  • 1992–2013
    • Northwestern University
      • • Department of Chemistry
      • • Department of Electrical Engineering and Computer Science
      Evanston, IL, United States
  • 1997–2012
    • Michigan State University
      • • Department of Electrical and Computer Engineering
      • • Department of Chemistry
      East Lansing, Michigan, United States
  • 2010
    • Concordia University–Ann Arbor
      Ann Arbor, Michigan, United States
  • 2009–2010
    • Northwest University
      Evanston, Illinois, United States
  • 1993–2010
    • CSU Mentor
      • Department of Chemistry
      Long Beach, California, United States