S. Andrew Ning and Dr. David Allred, Physics and Astronomy
As the technology was readily available here at BYU, we decided to make our own electrochromic material to meet our specifications. After a literature review we found a study done by Wang et al.  which suggests a preparation method using a thin film of vanadium dioxide to coat glass. Vanadium dioxide is a fascinating material for its property of undergoing a phase transition from a semiconductor to a metal at a temperature of around 68°C . This change in state is accompanied with a change in its optical properties. It can reversibly change from a state of reflectance to a state of transmittance. For this material, the change of state is controlled by temperature (thermochromic) rather than by voltage (electrochromic).
This ability to undergo a phase transition according to temperature is advantageous for our space suit material application. This allows our material to act as a self-regulating thermostat. Our objective in making thermochromic material is to prepare it so that it demonstrates switchable properties in the microwave region. Secondly, we would like to prepare our material so that its critical temperature is at an appropriate level for heating astronauts by conduction. To achieve these objectives requires careful setup of the preparation conditions.
We planned to deposit our glass samples with vanadium dioxide using a RF magnetron sputterer located in U234 of the Eyring Science Center (Fig. 1). To product a single phase of vanadium dioxide, Wang suggests the need to reach temperatures greater than 600° C . Higher temperature will not only allow us to access different structures, but will also increase atom mobility allowing the atoms to diffuse. This will allow us to create bigger crystals with fewer defects. The challenge was that our RF magnetron sputterer did not have the capability to heat our samples to the desired temperature.
In order to achieve temperatures greater than 600° C we decided to design a substrate heater. To connect a substrate heater to our RF magnetron sputterer required that we purchase a feedthrough to act as a connector. It was necessary that this feedthrough be made of conductive material so that it could carry high currents. It is important to drop as much of the power as possible into our heater substrate not only to improve efficiency but also to prevent damage of other parts. Copper was chosen as a suitable material to meet these needs. Copper has a high conductivity that should allow us to reach temperatures well over 600° C. We found a suitable copper feedthrough made by CeramTec capable of conducting 185 Amps of electricity. The part we purchased was a ¼ inch high current feedthrough for a 2 ¾ inch ConFlat flange.
After the feedthrough was purchased we began the design of a bracket. To ensure a proper fit we measured the dimensions of our feedthrough cross section, our quartz substrate area, and the diameter of our RF Magnetron Sputter opening. These dimensions served as the constraints to create a first draft on paper. This design was further modified and later modeled on the computer using SolidWorks. This bracket was designed be assembled with the feedthrough and the quartz slide as seen in Figure 2. To cut the copper we decided to use a water jet which is available in the Precision Maching Laboratory in the basement of the Crabtree Building. We created a drawing based on out computer model and then used its dimensions to program the water jet. The water jet was chosen over a saw because of the small size of the piece and the curved surface on the bottom of the piece. Water jet cutting is not as precise as other methods such as the Wire EDM, but tight tolerances were not necessary for this design. After the part was cut out the holes were drilled by use of an end mill. Finally, to attach this bracket to the feedthroughs we tapped a hold on the sides for a set screw which could be tightened by use of an Allen wrench.
Now that the bracket was designed and attached to the feedthrough, we needed to modify our quartz slide to clamp it to the brackets. The clamps were designed to hold down the quartz slide, but the slide needed to be modified to fit between the clamps. This requires the slide to be machined as seen in Figure 3.
Cutting quartz however, was a much greater challenge than cutting copper. Our quartz substrates our highly cross-linked to allow them to heat up to high temperatures. Though this is good for our heating application, this cross linking makes the quarts break in unpredictable ways. Because the quartz is brittle rather than ductile, and its size is small, cutting our substrate was a challenge. In 1995 Dr. Allred and Dr. Todd published a paper on techniques for cutting glass using a water jet . This paper suggests that by using Styrofoam for support and weights to hold down the material, glass can be cut by use of the waterjet. This method is currently being pursued in the machining lab .
- X. Wang, et al., “Preparation of Thermochromic VO2 Thin Films on Fused Silica and Soda-lime Glass by RF Magnetron Sputtering,” Jpn. J. Appl. Phys., 41, 2002, pp. 312-313
- H. T. Yuan, et al., “Research on Optical Property of Phase Transition PcNi/VO2 Films,” Applied Surface Science, 243, 2005, pp. 36-39.
- F. Yuan, J. A. Johnson, D. D. Allred, R. H. Todd, “Waterjet cutting of cross-linked glass,” J. Vac. Sci. Technol., A 13(1), 1995, pp. 136-139.
- This research was supported by the Office of Research and Creative Activities and in part by the Rocky Mountain NASA Space Grant Consortium.