In electronics, everyone has seen the beneficial impact of miniaturization. Exponential increases in computing power and ever-decreasing package sizes are evident in the evolution of cell phones, MP3 players, laptops and a whole host of other electronic devices. Historical trends have followed Moore's Law, which predicts that the number of transistors on integrated circuits doubles every 18 months. Feature sizes on current computer chips are now measured on the nanometer scale. Can thermoelectrics benefit from this type of scaling? “Smaller” and “more powerful” are generally positive attributes, but what are the implications unique to thermoelectrics when it comes to miniaturization and scaling?
First, let’s consider the dimensions of the thermoelectric elements themselves. The length to area (L/A) ratio dictates the maximum current (Imax) and the heat pumping capacity of the element. In theory, a thermoelectric device consisting of many couples can be scaled up or down in size, and as long as the number of couples and thermoelectric element L/A ratio are maintained, the devices will have equivalent performance. This allows the thermoelectric designer to tailor the thermoelectric device to the specific size and heat pumping needed for a given application.
The degree of miniaturization is a strong function of the type of thermoelectric material used. Production of traditional commercial bulk devices have historically used melt-grown, polycrystalline Bi2Te3 alloys. Polycrystalline Bi2Te3 has some unique characteristics. First, ingots are grown with well-aligned crystals to achieve the best thermoelectric properties. However, these well-aligned crystals are held together with very weak Van der Wal bonds. Production of short elements necessitates thin wafers (less than ~0.75 mm) and those sliced from melt-grown ingots have a tendency to crack and break along their weak crystalline planes. Thicker wafers are less likely to cleave or break along these planes due to the polycrystalline nature of the material.
Newer, fine-grain Bi2Te3 alloys do not suffer from these weak cleavage planes, and much smaller dimensions are possible. Many commercially available fine-grain materials, however, lose performance (lower ZT) relative to their crystalline counterparts. Such is not the case with II-VI Marlow’s Micro Alloyed Material (MAM). MAM provides the highest thermal performance and enables the design engineer to produce virtually any size thermoelectric element needed for a given thermoelectric application.
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