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.

These Temperature and Force Schedules are followed in hot pressing of La3–xTe4or La3–xYbyTe4 powder to consolidate it into a solid mass having superior thermoelectricproperties. The force indicated here is calculated to yield a specifiedpressure when exerted over an area slightly more than 12 mm in diameter.
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:

Innovative Technology Assets Management

JPL

Mail Stop 202-233

4800 Oak Grove Drive

Pasadena, CA 91109-8099

(818) 354-2240

E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.

Refer to NPO-44356, volume and number of this NASA Tech Briefs issue, and the page number.

Topics:
Alloys Assembling Chemicals Ferrous metals Forming Graphite Machining processes Manufacturing & Prototyping Manufacturing processes Metals Milling Stamping Steel
This Brief includes a Technical Support Package (TSP).
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Mechanical Alloying for Making Thermoelectric Compounds

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NASA Tech Briefs Magazine

This article first appeared in the September, 2007 issue of NASA Tech Briefs Magazine (Vol. 31 No. 9).

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Overview

The document from NASA's Jet Propulsion Laboratory (JPL) discusses advancements in the synthesis of thermoelectric materials through a novel room temperature mechanical alloying technique. Thermoelectric generators are devices that convert heat energy directly into electrical energy without moving parts, making them reliable and suitable for use in hostile environments. However, traditional thermoelectric materials face challenges such as low efficiency and high manufacturing costs, which have spurred a demand for improved materials and methods.

The innovation detailed in the document focuses on the synthesis of advanced thermoelectric materials, specifically rare earth chalcogenides (like LaTe-type and LaYbTe) and Zintl compounds (such as Yb14MnSb11 and Yb14BiSb11). These materials were synthesized at room temperature using mechanical alloying, a process that is not only simpler than traditional high-temperature melting techniques but also yields homogeneous materials with superior thermal and electrical properties.

The document emphasizes the reproducibility and economic advantages of the new synthesis process, which allows for the creation of various compositions and the investigation of their thermoelectric properties. Initial results indicate that the synthesized materials exhibit enhanced performance, demonstrating their potential for various thermoelectric applications.

In summary, the document outlines a significant technological advancement in the field of thermoelectric materials, highlighting the benefits of room temperature mechanical alloying in producing high-quality compounds. This innovation could lead to more efficient and cost-effective thermoelectric generators, addressing the limitations of existing technologies and expanding their applicability in diverse fields. The work is part of NASA's broader efforts to promote aerospace-related developments with potential commercial and scientific applications, and further information is available through the NASA Innovative Partnerships Program.