Chemistry: molecular biology and microbiology – Apparatus – Bioreactor
Reexamination Certificate
1999-02-18
2001-08-07
Redding, David A. (Department: 1744)
Chemistry: molecular biology and microbiology
Apparatus
Bioreactor
C435S286100, C435S287300, C422S105000, C422S109000, C073S865600, C374S057000, C165S254000, C165S258000, C165S264000
Reexamination Certificate
active
06271024
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the field of thermal cycling and thermal cyclers for temperature testing of devices and components such as solar cells and microelectronics.
BACKGROUND OF THE INVENTION
Thermal cyclers have long been used to repetitively heat and cool devices over wide temperature ranges to validate device designs. In the past, the design validation process has been the most prevalent bottleneck encountered in the development of new solar cell designs for critical mission applications. The customer demand to acquire desired thermal cycles before an urgent launch deadline requires accelerated cycling rates for a thermal cycling validation.
Various types of thermal cyclers have been used to perform the thermal cycling validation and testing process. One example is a conductive thermal cycler that has been used to perform life tests on GaAs solar cells. Cycling is achieved by cooling a fairly massive aluminum plate with counter-flowing liquid nitrogen and then heating the plate with symmetrically embedded electric rod-heaters. The devices under test are held in contact with the plate so that cycling occurs primarily by conduction. Typical solar cells mounted on lightweight ¼ inch thick honeycomb panels generally require sixty to ninety minutes to cycle between +80° C. and −80° C. while under 1X10E-7 Torr vacuum. The conductive cycler is well suited for vacuum cycling of cells mounted on heavy ⅛ inch thick solid aluminum panels. However, the conductive thermal cycler has poor cycling rates, because the heat and cool phases work against each other in driving the conductive plate to hot and cold temperatures.
Another type of thermal cycler that has been used is the radiant thermal cycler. In the radiant thermal cycler, quartz-halogen lamp radiation is used in a vacuum with a surrounding cold shroud for heat absorption to cycle the cells, as opposed to the direct conduction method employed by the conductive thermal cycler. A shroud is a cooled copper cylinder surrounding the test device and heating lamps inside the vacuum chamber. Cycle periods of thirty to sixty minutes are attainable on lightweight ¼ inch thick honeycomb panels. Radiant thermal cyclers are well suited for vacuum cycling thin, lightweight specimens with large surface areas. These radiant cyclers have a faster cycling rate than conductive cyclers because only the heat phase works against the cool phase when the heating lamps overcome the cooling shroud effects. The shroud can only recover during the next cool phase even though the shroud is being filled with liquid nitrogen during the heat phase.
Recently, an improved method was used for optimizing the cooling rate for the radiant thermal cycler with the introduction of a small amount of nitrogen gas inside the vacuum chamber so that the conduction of heat from the solar cell coupon under test to the cold shroud is assisted by the nitrogen gas. This nitrogen-assisted cooling is done without a significant degradation of the radiant cooling contribution. The result is a net increase in the cooling rate. It has been experimentally demonstrated that a significant improvement in the cooling late was achieved by maintaining a forty mTorr nitrogen pressure during the cool phase. This pressure yielded shorter cycle periods of twenty-two to forty-five minutes on lightweight ¼ inch thick honeycomb panels. Under nitrogen gas cooling, the disadvantages remain that the heat phase still works against the cool phase, and only panels of very low mass can be cycled rapidly.
Usually, a single chamber has to be repetitively heated and cooled requiring excessive energy and cyclic time depending on the amount of mass being thermally driven. The required time-consuming thermal cyclic tests are needed to qualify solar cells and other components particularly for space applications. In order to validate solar cell panel designs in a more timely manner, faster thermal cyclers are desirable. With the above designs, thermal life testing of devices could last for as long as several years for 50,000 cycles. Also, during heating and cooling phases, undesirable thermal gradients may be created across the device under test. A common solution in the thermal cycling industry is to ignore this problem by using only one control thermocouple positioned in the middle of the test device for customer data logging. These and other disadvantages are solved or reduced using the present invention.
SUMMARY OF THE INVENTION
An object of the invention is to provide fast thermal cycling testing of devices.
Another object of the invention is to provide hot and cold compartments within a thermal cycling chamber to provide fast thermal cycling testing of devices.
Another object of the invention is to provide individually controlled multiple heating elements and temperature sensors for reducing thermal gradients experienced by a device under test.
Still another object of the invention is to provide in-situ electrical testing of the device under test.
The invention is directed to a fast thermal cycling system. A test device, such as a solar cell array coupon, is attached to a panel and placed in a temperature cycling chamber. The temperature cycling chamber has a top hot compartment and bottom cold compartment creating a temperature gradient from a top hot compartment heated by a heater means to a bottom cold compartment cooled by a cooler means. The chamber is filled and pressurized with an ultrapure gas, such as nitrogen, for thermal conduction within the compartments. The panel is repetitively transported between the hot top compartment and the bottom cold compartment for rapid thermal cycling of the device under test. The chamber is constantly pressurized by the gas to slightly above ambient atmospheric pressure, with the gas being vented out the top of the chamber by adjustable vent valves. A motor-pulley system raises and lowers the test device during testing along a vertical track joining the two compartments. The entire chamber is insulated and these two compartments are thermally isolated from one another, except for an opening between the compartments, through which the panel and test device are mechanically cycled.
There are several advantages of the dual compartment thermal cycler invention. The cool phase and heat phase no longer work against each other. Heating lamps keep the top compartment hot, and a cold liquid fluid, such as liquid nitrogen, keeps the bottom compartment cold. The temperature in both compartments is kept stable by gas thermal conduction. One compartment is able to fully recover to its operating temperature while the other compartment is actively heating or cooling the panel and test device. The chamber is large enough to accommodate thick curved fiberglass panels with an aluminum honeycomb filler.
The primary feature of the cycler is the dual compartment chamber for separate heating and cooling phases. Heating lamps, such as quartz-halogen infrared heating lamps, are located in the top hot compartment. These lamps surround the panel during the heat phase and maintain the top hot compartment at an elevated temperature, so that the panel is warmed both by radiation and by gaseous conduction. The panel is lowered into the bottom cold compartment for the cool phase where the surrounding walls are maintained at extremely low temperatures by the cold liquid fluid in an outer container. The panel is cooled both by radiation from the cold walls and by gaseous conduction. The top compartment is maintained at a high temperature during the cool phase, while the bottom compartment is maintained at a low temperature during the heat phase. In this manner, neither compartment expends any time recovering to a respective original operating temperature during use. Exceedingly fast thermal test cycles are practical. These cycles take approximately five to ten minutes. A computer is utilized to control the motor transportation, lamp heating, gas conduction, cold liquid fluid flows, and temperatures.
An in-situ testing feature of the cyc
Francis Robert Walter
Pan Robert Bay
Sve Charles
Valenzuela Pierre Rogelio
VanWormer Steven John
Redding David A.
Reid Derrick Michael
The Aerospace Corporation
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