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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.
PbTe-PbS materials are promising for thermoelectric power generation applications. For the composition of (Pb0.95Sn0.05Te)0.92(PbS)0.08 nanostructuring from nucleation and growth and spinodal decomposition has been reported along with thermal conductivity of approximately 1.1 W/m·K at 650 K . Based on temperature-dependent measurements of electrical conductivity, thermopower, and thermal conductivity, the thermoelectric 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 production 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 electric current sintering (PECS) processed (Pb0.95Sn0.05Te)0.92(PbS)0.08 materials and their thermoelectric properties along with the latest advancements in the preparation of these materials.
Thermoelectric modules are of great interest for power generation applications where temperature gradients of approximately 500K exist, and hot side temperatures near 800K. The fabrication of such modules requires optimization of the material compositions, low contact resistivities, and low thermal loss.
AgPbmSbTe2+m (LAST) and Ag(Pb1-xSnx)m SbTe2+m (LASTT) compounds are among the best known materials appropriate for this temperature range. Various measurement systems have been developed and used to characterize bulk samples in the LAST and LASTT systems within this operating temperature range. From the characterized data, modeling of modules based on these materials and segmented legs using LAST(T) with Bi2Te3 have been used to identify the optimal geometry for the individual legs, and the length of the Bi2Te3 segments. We have segmented LAST(T) with Bi2Te3 and achieved contact resistivities of less than 10 μΩ•cm2.
Here we give a detailed presentation on the procedures used in the fabrication of thermoelectric generators based on LAST, LASTT, and segmented with Bi2Te3 materials. We also present the output data on these generators.
Low electrical contact resistance is essential for the fabrication of high efficiency thermoelectric generators in order to convert heat to electricity. These contacts must be stable to high temperatures and through thermal cycling. A ratio of the contact resistance to the leg resistance below 0.1 is the goal for fabrication of a high efficiency thermoelectric power generation device. Here we present the fabrication procedures and characterization of contacts of metal alloys to Pb-Sb-Ag-Te (LAST) and Pb-Sb-Ag-Sn-Te (LASTT) compounds. Contacts were fabricated and measured for both ingot and hot pressed materials. Stainless steel 316 has shown a low resistance contact to these thermoelectric materials when the proper bonding conditions are used. Different time-temperature-pressure conditions for bonding to n-type and to p-type legs are presented. Contact resistances below 10μΩcm2 have been measured. In addition, break tests have shown bond strengths exceeding the semiconductor fracture strength. One of the considerations used in selecting iron alloys for electrical interconnects is the similarity in the coefficient of thermal expansion to the LAST and LASTT materials which is 18 ppm/°C and relatively temperature insensitive. Contacts to the thermoelectric materials were accomplished by diffusion bonding in a furnace developed in our lab at Michigan State University. The furnace is capable of reaching temperatures of up to 1000°C with a controlled atmosphere of a reducing gas. Fabrication procedures and contact data are presented in this paper.
Surface-enhanced Raman scattering (SERS) can enhance the intensity of Raman radiation by many orders of magnitude for molecules bound to metallic nanostructures. SERS is generally thought to have been a result of excitation of surface plasmons in nanostructured metal, which greatly enhances local electric field experienced by the molecule. Common SERS substrates fabricated using noble metal colloidal or electrochemically roughened thin films suffer from lack of homogeneity where only few hot spots yield high enhancement. We explore semiconducting nanowires and metallic nanorods as an economical, stable, and uniform SERS substrate for the detection of trace amount of chemicals and bio-species. We utilize bulk synthesized semiconducting nanowires as nano-scale structures that can be coated with noble metals or their colloidal forms to allow for excitation of surface plasmons over broad frequency range. The use of nanowires as SERS substrates has several advantages. The surface properties of these nanowires are highly reproducible and well defined as compared to other systems like colloidal aggregates, electrochemical roughening etc. The synthesized nanowires offer many unique features (sharp vertices, noncircular cross-sections, inter-wire coupling) that may lead to larger field enhancement factors. High density of nanowires means close interaction between adjacent nanowires, which enables SERS to manifest for a broad selection of excitation sources. We have synthesized germanium dioxide and zinc oxide nanowires using the vapor-liquid-solid growth mechanism utilizing a simple quartz tube furnace set up. The nanowires are grown either using thin gold film (5-15 nm) or colloidal gold (20 nm or 60 nm) as catalyst on substrates of silicon, quartz, and alumina. The resulting nanowires are dense (100-300 nm diameter, 10-40 μm long) and randomly distributed on the substrate. The nanowires are subsequently coated with thin films (10-15 nm) of gold that provide plasmons active surface. We have also investigated silver nanorods on glass formed by grazing angle deposition using e-beam evaporation. These nanorods have a diameter of ∼ 50-70 nm with lengths averaging 300-400 nm. These well aligned high aspect ratio and dense structures are ideal for excitation of surface plasmons. The synthesized structures have been characterized using SEM, TEM, and EDS. The SERS studies were conducted using EzRaman-L system from Enwave Optronics, Inc. Silver nanorods and gold-coated nanowires have been found to exhibit significant Raman enhancement for micro-molar concentrations of Rhodamine 6G and Nile blue, and are promising candidates for SERS applications.
The spectra of the Sloan Digital Sky Survey (SDSS) are being used to construct a catalogue of QSO absorption lines, for use in studies of abundances, relevant radiation fields, number counts as a function of redshift, and other matters, including the evolution of these parameters. The catalogue includes intervening, associated, and BAL absorbers, in order to allow a clearer definition of the relationships between these three classes. We describe the motivation for and the data products of the project to build the SDSS QSO absorption line catalogue.
Low electrical contact resistance is essential for the fabrication of high efficiency thermoelectric generators. These contacts must be stable to high temperatures and through thermal cycling. Here we present the fabrication procedure and characterization of several contacts to Pb-Sb-Ag-Te (LAST) compounds. Contact materials investigated include tungsten, antimony, tin, nickel, and bismuth antimony based solder. The contacts were typically deposited by an electron beam evaporation method after careful preparation of the sample surface. The resistances were measured by using the transmission line model, and ohmic behavior was verified through current vs. voltage measurements. The best contact resistivities of less than 20 µΩ·cm2 have been measured for annealed antimony to n-type LAST samples. We present these procedures for fabricating low resistance contacts and the use of these procedures towards the fabrication of high efficiency thermoelectric generator modules.
Measurements of assembled thermoelectric modules commonly include investigations of the module output power versus load resistance. Such measurements include non-ideal effects such as electrical and thermal contact resistances. Using an AC electrical measurement, two models for a thermoelectric module have been developed utilizing electrical circuits for both the thermal and electrical characteristics of the module.
Measurements were taken over the frequency range of 1mHz to 500Hz using lock-in amplifiers. We present data showing the extraction of ZT from such measurements on commercially available modules utilizing both the magnitude and phase of the measured impedance. Here we extend upon a simple RC equivalent circuit model by utilizing transmission line theory in electrical circuits to explain the thermal activity in a thermoelectric module. This model includes all components of a module such as nickel traces and ceramic end caps, and makes use of their corresponding thermal conductivities, thermal capacitance, and density. This model can then be applied to pn unicouples in either a standard or inline configuration, and to individual p or n legs of the module. Data is presented showing the advantages of both models. Measurements on new thermoelectric materials and modules are also presented.
Lead-Antimony-Silver-Tellurium (L-A-S-T) materials, synthesized at Michigan State University, show promising thermoelectric properties at high temperatures for use in power generation applications. Recent scaled-up quantities of L-A-S-T show a ZT=1.4 at 700 K approaching the figure of merit for samples made in small quantities. These materials are of great interest for power generation applications with hot side temperatures in the range of 600-800 K. Developing these materials into working devices requires minimization of the thermal and electrical parasitic contact resistances, so various fabrication methods are under investigation. To examine each method, a new measurement system has been developed to characterize these devices under various load and temperature gradients. An introduction to the system will be presented, as well as results for devices made of the L-A-S-T materials.
The thermoelectric properties of Pb9.6SbyTe10−xSex were investigated in the intermediate temperature range of 300 – 700 K. The effect of the variation of Sb content (y) on the electronic properties of the materials is remarkable. Samples with compositions Pb9.6Sb0.2Te10−xSex (y = 0.2) show the best combination of low thermal conductivity with moderate electrical conductivity and thermopower. For Pb9.6Sb0.2Te8Se2 (x = 2) a maximum figure of merit of ZT ∼ 1.1 was obtained around 700 K. This value is nearly 1.4 times higher than that of PbTe at 700 K. This enhancement of the figure of merit of Pb9.6Sb0.2Te8Se2 derives from its extremely low thermal conductivity (∼0.7 at W/m.K at 700 K). High resolution transmission electron microscopy of Pb9.6Sb0.2Te10−xSex samples shows broadly distributed Sb-rich nanocrystals, which may be the key feature responsible for the suppression of the thermal conductivity.
High efficiency thermoelectric modules are of great interest for power generation applications where hot side temperatures of approximately 800K exist. The fabrication of such modules requires a multidisciplinary effort for the optimization of the material compositions, the engineering of the module systems, modeling and fabrication of the devices, and constant feedback from characterization. Pb-Sb-Ag-Te (LAST) and Pb-Sb-Ag-Sn-Te (LASTT) compounds are among the best known materials for this temperature range. Modeling of these materials and possible cascaded structures shows efficiencies of 14% can be achieved for low resistance contacts. Using antimony we have achieved contact resistivities less than 20 µΩ·cm2. Here we give a detailed presentation on the procedures used in the fabrication of thermoelectric generators based on these new materials. We also present the characterization systems and measurements on these generators.
A new nanowire growth technique using annealing and pulsed laser deposition is described. The technique demonstrates the control in the precise nanowire growth temperature thereby yielding higher quality nanowires than obtained from simple annealing alone. InSb substrate and gold target were used to demonstrate the new technique. The results from the variation of gold deposits from 2.5 mins to 40 mins at the fixed growth temperature of 510 °C are discussed.
In the field of thermoelectrics, the figure of merit of new materials is based on the electrical conductivity, thermoelectric power, and thermal conductivity of the sample, however additional insight is gained through knowledge of the carrier concentrations and mobility in the materials. The figure of merit is commonly related to the material properties through the B factor which is directly dependent on the mobility of the carriers as well as the effective mass.
To gain additional insight on the new materials of interest for thermoelectric applications, a Hall Effect system has been developed for measuring the temperature dependent carrier concentrations and mobilities. In this paper, the measurement system will be described, and recent results for several new materials will be presented.
New thermoelectric bulk materials such as CsBi4Te6 have shown superior properties to traditional materials, however, optimal performance requires continuing investigations of doping and alloying trends. A recently modified high throughput measurement system is presented for doping and alloying investigations in several new thermoelectric materials. The modification includes a four-probe configuration for more accurate measurements while maintaining a relatively short sample preparation time. The system is fully computer controlled and provides flexible contacts to accommodate various sample dimensions. Optimal compositions are then identified for further investigations in thermoelectric prototype modules. The most promising materials will be further characterized for electrical conductivity, thermoelectric power, thermal conductivity, and Hall effect measurements as a function of temperature.
The possibility of a prototype thermoelectric cooling device for operation near liquid nitrogen temperatures has been explored. In these devices, the figure of merit involves a combination of the properties of the two branches of the module. Here, we investigate the fabrication of a module with a new low temperature material, CsBi4Te6 (p-type), and the best known low temperature n-type materials Bi85Sb15. Transport measurements for each of these materials show high performance at low temperatures. Known values for the figure of merit Zmax of CsBi4Te6 is 3.5 × 10−3 K−1 at 225K and for Bi85Sb15 is 6.5 × 10−3 K−1 at 77K. At 100K these values drop to 2.0×10−3 K−1 for CsBi4Te6 and 6.0×10−3 K−1 for Bi85Sb15. Theoretical simulations based on these data show a cooling of δT = 12K at 100K, which is almost three times the efficiency of a Bi2Te3 module at that temperature. We present transport measurements of elements used in the fabrication of a low temperature thermoelectric module and properties of the resulting module.
Several new materials in the CsBi4Te6, A2Bi8Se13, (A = K, Rb, Cs), HoNiSb, Ba/Ge/B (B = In, Sn), and AgPbBiQ3 (Q = S, Se, Te) systems have shown promising characteristics for thermoelectric applications. New synthesis techniques are able to produce samples at much higher rates than previously possible. This has led to a persistent challenge in thermoelectric materials research of rapid and comprehensive characterization of samples. This paper presents a description of a new 4-sample transport measurement system and the related measurement techniques. Special features of the system include fully computer-controlled operation (implemented in LabView™) for simultaneous measurement of electrical conductivity, thermo-electric power, and thermal conductivity. This system has been successfully used to characterize several new thermoelectric materials (including some of the above-mentioned compounds) and reference materials exhibiting a wide range of thermal conductivities.
Epitaxial Tl2Ba2CaCu2O8 thin films with excellent electrical transport characteristics are grown in a two-step process involving metal-organic chemical vapor deposition (MOCVD) of a BaCaCuO(F) thin film followed by a postanneal in the presence of Tl2O vapor. Vapor pressure characteristics of the recently developed liquid metal-organic precursors Ba(hfa)2 • mep (hfa = hexafluoroacetylacetonate, mep = methylethylpentaglyme), Ca(hfa)2 • tet (tet = tetraglyme), and the solid precursor Cu(dpm)2 (dpm = dipivaloylmethanate) are characterized by low pressure thermogravimetric analysis. Under typical film growth conditions, transport is shown to be diffusion limited. The transport rate of Ba(hfa)2 • mep is demonstrated to be stable for over 85 h at typical MOCVD temperatures (120 °C). In contrast, the vapor pressure stability of the commonly used Ba precursor, Ba(dpm)2, deteriorates rapidly at typical growth temperatures, and the decrease in vapor pressure is approximately exponential with a half-life of ∼9.4 h. These precursors are employed in a low pressure (5 Torr) horizontal, hot-wall, film growth reactor for growth of BaCaCuO(F) thin films on (110) LaAlO3 substrates. From the dependence of film deposition rate on substrate temperature and precursor partial pressure, the kinetics of deposition are shown to be mass-transport limited over the temperature range 350–650 °C at a 20 nm/min deposition rate. A ligand exchange process which yields volatile Cu(hfa)2 and Cu(hfa) (dpm) is also observed under film growth conditions. The MOCVD-derived BaCaCuO(F) films are postannealed in the presence of bulk Tl2Ba2CaCu2O8 at temperatures of 720–890 °C in flowing atmospheres ranging from 0–100% O2. The resulting Tl2Ba2CaCu2O8 films are shown to be epitaxial by x-ray diffraction and transmission electron microscopic (TEM) analysis with the c-axis normal to the substrate surface, with in-plane alignment, and with abrupt film-substrate interfaces. The best films exhibit a Tc = 105 K, transport-measured Jc= 1.2 × 105 A/cm2 at 77 K, and surface resistances as low as 0.4 mΩ (40 K, 10 GHz).
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