This Small Business Innovation Research (SBIR) Phase I project will develop a novel and cost-effective processing and manufacturing route to produce high-temperature ceramic oxide thermoelectric materials comprised of nanosized (< 40 nm) grains. Research has shown that a reduction in grain size below 100 nanometers in thermoelectric materials results in an increase in thermal resistance and overall thermoelectric performance by up to 40%. The technical challenge involved is to maintain the correct powder composition and the sintered grain size below 40 nm throughout the required processing steps. The spark plasma sintering (SPS) technique limits grain growth to a minimum, such that grain size of the sintered ceramic remains below 100 nanometers, thus preserving all of the enhanced material properties. This project will evaluate the effects of processing parameters on physical properties of the thermoelectric powder, including particle size, surface area, impurity gain, green and sintered density, grain size and final microstructure. The thermoelectric properties of the resulting powder will be evaluated, including Seebeck coefficient, dc conductivity, thermal conductivity, and ZT. A commercially viable manufacturing process will be demonstrated for scale up to large quantities in the follow-on Phase II project.
The broader impact/commercial potential of this project will be the availability of thermoelectric nanomaterials operating at high temperatures (> 800 C), with enhanced thermoelectric properties. As the conservation of energy resources and associated environmental concerns become more critical, societal interest in utilizing thermoelectric devices to generate electricity from waste heat has grown. Thermoelectrics can function in many specialized applications, but have been hindered by a relatively low efficiency and high material costs. Current materials also have production scalability concerns. A new low-cost, high-temperature, high figure-of-merit thermoelectric material is necessary to satisfy commercial demands. The materials to be developed in this project will enable increased market adoption by allowing waste energy harvesting at high temperatures with a relatively low expected low cost of production (under $3/watt). Potential early adopters of these materials include the glass industry, steelmakers, and the automobile industry. According to recent studies, there is almost $300 million worth of wasted energy per year in the glass industry alone. The estimated market for these materials in vehicles is more than $1 billion. This same material processing technology can be adapted for future applications in high ionic conductors for battery and fuel cell technologies, and other ceramic industrial materials.
Goal of this project is to develop and demonstrate a less than 100 nano meter (1 nano meter = 0.000000001 meter or 0.000001 millimeter) particle size, high temperature oxide thermoelectric powder composition. The powder is to be sintered into a solid ceramic by spark plasma sintering technique. This sintered ceramic is the key building element of a thermoelectric generator (TEG). TEG has been used mostly to convert waste thermal energy into usable electrical energy. The operating principle is based on a physical phenomenon called Seebeck effect where for certain material, a electrical voltage is developed when two sides of the material are subject to a temperature gradient. The material is called n-type if voltage at high temperature side is negative and p-type if the voltage at high temperature side is positive. Many manufacturing industries such as aluminum, glass, metal casting and steel have processes discharging high temperature waste heats. TEG technology offers opportunities to recover and convert those waste heats into usable electricity. Benefits would be to reduce the operating cost for facilities by increasing their energy productivity and energy efficiency. Of particular interests to the general public is the application of TEG technology in vehicle system to convert exhaust heat from engine into usable electricity. The extra electricity thus generated can be used to recharge the battery, to support some added features (car seat warmer, GPS, CD player, etc.), or simply to reduce and save the fuel consumption. It is anticipated in the not so distinct future, TEG technology will replace the need to have an alternator in a passenger car. Materials currently used in TEG are mostly based on metal alloys such as bismuth-antimony-tellurium-selenium (Bi-Sb-Te-Se), lead-telluride (Pb-Te), and silicon-germanium (Si-Ge) for low, mid, and high temperature applications. Oxide ceramic thermoelectric materials have advantages over the metal alloys in 1) they can be operated at much higher temperatures, around 1000 degree Centigrade, 2) they are more reliable, 3) they use less expensive raw materials, 4) they are less expensive to produce in bulk quantities, and 5) they are less environmentally hazardous. The fundamental building block of a TEG device has a Pi shaped configuration as illustrated in the attached image Figure 1. A n-type thermoelectric component is connected in series with a p-type thermoelectric component by a highly conductive metallization material. Each component is also metalized at the other end separately. This building block could be repeated in X, Y, and Z directions to make a device module. When a temperature gradient is established across the thermoelectric legs, a Seebeck voltage is developed and current flows if a load is connected across the other end. The voltage developed per degree Kelvin (Degree Centigrade + 273) of temperature difference across the thermoelectric legs is called Seebeck coefficient (Q). The performance and energy conversion efficiency of a thermoelectric component is also related to its electrical conductivity (S), thermal conductivity (k), and temperature (T) where the TEG material is operated. The overall performance is expressed as a dimensionless parameter called figure of merit , ZT. ZT is defined as; ZT = (Q2 x S x 10-10) x T / k (Dimensionless) (1) Where in equations (1), Q is the Seebeck coefficient in micro-volt/K, S is the electrical conductivity in S/cm or 1/(Ohm-cm), k is the thermal conductivity in Watt/mK of the thermoelectric material. T is the operating temperature in degree Kelvin. One can derive from equation (1) that in order to have thermoelectric material with high ZT, one should maximize Seebeck coefficient Q, electrical conductivity S, and operating temperature T, but minimizing thermal conductivity k. Key outputs of this project are 1) a nano size Niobium Oxide (Nb2O5) doped Strontium Titanate (SrTiO3) high temperature oxide thermoelectric powder composition has been identified and 2) Synthesis and processing steps to manufacture this powder has been identified. The powder was synthesized by low cost mixing of precursor oxides, solid state reaction, and then followed by high energy milling to nano particle size. High energy milling of the powder enhances material's thermoelectric properties. It can be produced in bulk quantities at less than $ 30 per kg. Major impacts of these project outputs are to improve efficiency, operating temperature range, and cost of thermoelectric generator so that its application could be expanded. Thermoelectric properties as a function of temperature of output of this project, a nano size, spark plasma sintered Nb2O5 doped SrTiO3 powder are shown in Fig. 2 to Fig. 5 of the attached images. Powder particle size is less than 50 nano meter. Its thermoelectric properties at 700 degree Centigrade are; Seebeck coefficient: -155 micro-volt/K, Electrical conductivity: 360 S/cm (360/ohm-cm); Thermal conductivity; 3.6 watt/meter-K; and ZT: 0.23. Ceramic made from this powder has high enough ZT for practical application in TEG module for vehicle systems.
