A high-performance design, and fabrication and growth processes to implement the design, have been devised for encapsulating a hydrated salt in a container that both protects the salt and provides thermal conductance between the salt and the environment surrounding the container. The unitary salt/container structure is known in the art as a salt pill. In the original application of the present design and processes, the salt is, more specifically, a hydrated paramagnetic salt, for use as a refrigerant in a very-low-temperature adiabatic-demagnetization refrigerator (ADR). The design and process can also be applied, with modifications, to other hydrated salts.
Hydrated paramagnetic salts have long been used in ADRs because they have the desired magnetic properties at low temperatures. They also have some properties, disadvantageous for ADRs, that dictate the kind of enclosures in which they must be housed:
• Being hydrated, they lose water if exposed to less than 100-percent relative humidity. Because any dehydration compromises their magnetic properties, salts used in ADRs must be sealed in hermetic containers.
• Because they have relatively poor thermal conductivities in the temperature range of interest (<0.1 K), integral thermal buses are needed as means of efficiently transferring heat to and from the salts during refrigeration cycles. A thermal bus is typically made from a high-thermal-conductivity metal (such as copper or gold), and the salt is configured to make intimate thermal contact with the metal. Commonly in current practice (and in the present design), the thermal bus includes a matrix of wires or rods, and the salt is grown onto this matrix. The density and spacing of the conductors depend on the heat fluxes that must be accommodated during operation.
Because the salt is hydrated, it must be grown from solution onto the matrix, in a container that, immediately after growth, must be hermetically sealed to complete the salt pill. In the present design and fabrication process, the thermal bus is initially fabricated in two pieces: (1) a unitary piece comprising a square array of parallel copper fingers protruding from a copper disk, and (2) a copper cap that can be bolted into thermal contact with an external object. The disk-and-fingers piece is made from a single copper rod by using automated electrical-discharge machining (EDM) to create the gaps between the rods. Prior to EDM, the bolt holes (for subsequent connection to other parts of the ADR) and two access holes (for use in growing the magnetic salt) are machined into the copper rod.
In a single brazing operation, the two copper pieces constituting the thermal bus are joined together, two stainlesssteel weldment rings are joined to the copper (one at each end), and two stainless- steel collars surrounding the access holes are joined to the copper. After brazing, an outer stainless-steel containment tube is welded to the weldment rings. At this point, the salt pill is hermetically sealed except for the collared openings, which are to be welded shut after the salt is grown.
The size and spacing of the copper fingers are set to provide very high thermal conductance to the salt while minimizing complications caused by surfacetension forces on the salt solution during growth of the salt. The salt is grown by use of a continuous-counterflow technique in which saturated solution is pumped into, and depleted solution is withdrawn from, the salt pill in such a way that crystallites are first nucleated at the bottom, and then salt crystals grow from the bottom upward in a controlled manner until the entire container is filled with salt. The salt solution is circulated by a dual peristaltic pump, using tubes of different sizes for supply and return so that the flow capability for return exceeds that for supply: This is key to ensuring that the saturated solution occupies only a thin layer above the growing salt, ensuring that salt grows only by extending itself rather than by nucleation at random locations throughout the salt pill. Growing the salt in this way ensures that regardless of the configuration and thermal conductance of the thermal bus, there is no premature formation of salt in the upper volume of the salt pill. If allowed to occur, such premature formation could trap pockets of solution.
This salt-growth process can yield a high fill fraction (>98 percent). The process can be automated at a high growth rate. The fabrication and saltgrowth processes are suitable for mass production of salt pills for ADRs.
This work was done by Peter J. Shirron, Michael J. DiPirro, and Edgar R. Canavan of Goddard Space Flight Center. GSC-14873-1