An economical room-temperature mechanical alloying process has been shown to be an effective means of making a homogeneous powder that can be hot-pressed to synthesize a thermoelectric material having reproducible chemical composition. The thermoelectric materials to which the technique has thus far been applied with success include rare-earth chalcogenides [La3–xTe4 (0 < x < 0.33) and La3–xYbyTe4 (0 < x < 1, 0 < y < 1)] and Zintl compounds (including Yb14MnSb11 and Yb14BiSb11). The synthesis of a given material consists of the room-temperature thermomechanical- alloying process followed by a hot-pressing process. Relative to synthesis of nominally the same material by a traditional process that includes hot melting, this synthesis is simpler and yields a material having superior thermoelectric properties.
The room-temperature mechanical alloying process is, more specifically, a ball-milling process. It begins inside an argon-filled glove box, wherein elemental constituents in amounts corresponding to their desired proportions in the thermoelectric material to be synthesized are loaded into a vial that contains milling balls. The vial and milling balls are made of a material compatible with the material to be synthesized. (For synthesizing Yb14MnSb11, one uses a vial and balls made of tungsten carbide; for synthesizing La3–xTe4 or La3–xYbyTe4, one uses a vial and balls made of stainless steel.) Next, the filled vial is removed from the glove box and clamped onto a commercially available mixer/mill machine, which is used to shake the vial for as long as 40 hours to effect ball milling.
After ball milling, the vial is returned to the glove box, wherein the powder produced by the ball milling is loaded into a graphite die for hot pressing.
- In the case of Yb14MnSb11, it is necessary to sandwich the powder between two graphite foil layers at each end. In ascending order, the resulting assembly inside the die consists of one or more spacer(s), two graphite foil layers, the powder, two more graphite foil layers, and a plunger that presses down on the aforementioned components.
- In the case of La3–xTe4 or La3–xYbyTe4, the plunger is made of graphite, the inside of the die is lined with graphite foil, and the powder touches the top and bottom spacers, which are coated with boron nitride to prevent adhesion.
The die and its contents are then placed in a hot press, wherein the powder is subjected to a temperature-vs.-time and a pressure- (or force)-vs.-time profile, specified for the material to be synthesized (for example, see figure), to consolidate the powder into a solid mass of requisite density. After this hot pressing, the mass is removed from the die. In the case of La3–xTe4 or La3–xYbyTe4, the mass is sanded to remove graphite foil and boron nitride from its surface.
This work was done by Chen-Kuo Huang, Jean-Pierre Fleurial, G. Jeffrey Snyder, Richard Blair, and Andrew May of Caltech for NASA's Jet Propulsion Laboratory.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:
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Refer to NPO-44356, volume and number of this NASA Tech Briefs issue, and the page number.