Study on the Influence of Product Thermal Effect on Thermal Vacuum Test

Due to different applications, the size, shape and surface state of the power supply on spacecraft vary greatly, but they all need to experience a harsh environment, such as extreme vacuum, cold and hot alternation. Therefore, it is necessary to undergo the thermal vacuum test on the ground. During the thermal vacuum test, the aerospace power supply products with different surface states have different internal temperature rise effects due to their own emissivity and absorption during testing. The higher the temperature rise effect, the lower the product reliability. Therefore, in order to reduce the temperature rise effect of products in the process of thermal vacuum test, the temperature rise effect of products with different surface states or the same surface state can be reduced by using different shading measures in the thermal vacuum test of products is analysed and verified by experiments. The test results show that the product with bright surface has lower temperature rise effect than the product with blackened surface, which can reduce the maximum temperature difference, enhance the uniformity of temperature field and enhance the reliability of the product. In addition, using measures such as covering the product with aluminized film or overhanging it can reduce the surface temperature difference, with the overhanging shade being more effective.


Introduction
Before the final assembly and sub assembly of spacecraft components, vacuum thermal test shall be carried out according to the specified requirements, and the components shall undergo multiple cycles of cold and heat transformation load under vacuum environment to verify if it meets the requirements for use in its working environment [1][2][3].With the increasing complexity of the size, thermal control, and testing status of individual product structures, the requirements for vacuum thermal testing are also increasing, and the technical challenges encountered during vacuum thermal testing are becoming more numerous.For example, the back of the Chang'e-3 lander's battery panel is pasted with a fluorine-46 conductive film.The surface of the battery panel's current plug and the surface of the sun sensor are smooth aluminum surfaces [4,5].The absorptivity and emissivity of the smooth surface state are low, which is unfavorable for heating and cooling.For the thermal vacuum test with strict heating or cooling rate requirements, the test results often fail to meet the requirements of the test outline.Therefore, this paper conducts experimental verification, analysis and research on the temperature rise effect caused by the surface state during the thermal vacuum test of products, verifies

Analysis of Product Thermal Effect
With the vigorous development of aerospace technology, light metals are used more and more widely.Due to the excellent characteristics of aluminum alloy, such as lightweight, corrosion resistance, high hardness, high strength, high temperature resistance and impact resistance, most power supply products currently use aluminum alloy shells, but aluminum alloy have a smooth surface [6,7].When conducting vacuum thermal test, its surface has low heat absorption and emission rate.Therefore, it is difficult to achieve the heating or cooling rate simulated by the test requirements.In recent years, some enterprises have proposed to spray a thermal conductive coating on the surface of products, that is, to spray a layer of high thermal conductivity, long-term service high thermal conductive coating on the external surface of devices and equipment.During the on-orbit operation of space power, the heat received is first conducted and dissipated to the surface of the coating, relying on the joint action of the coating's thermal conductivity and radiation to quickly dissipate heat and reduce the surface and internal temperature of the object, ultimately achieving the goal of cooling and heat dissipation.In addition, the power supply is composed of numerous electronic components.According to foreign statistics, the reliability of electronic components decreases by 10% every time the temperature rises by 2 °C when they are working, and the service life is only 1/6 of 25 °C when the temperature rises by 50 ° C. Therefore, technical measures should be taken to limit the temperature rise of components [8][9][10].
(1) For product perspective The power supply product will generate a large amount of heat during the process of powering on.Generally, the power supply product will undergo thermal design to reduce the heat consumption effect of the product itself or through heat dissipation design, such as using good thermal conductivity materials like plates and casings, to quickly dissipate the heat generated by the product.In addition, there is a large amount of thermal radiation in the on orbit working environment of aerospace power supply products, so the traditional smooth aluminum alloy surface will accumulate a large amount of heat on the surface area after being exposed to solar radiation for a long time, and the surface heat transferred to the interior will ablate the circuit board, reducing product reliability.Therefore, at present, the surface of radiator or power supply is generally treated by oxidation blackening process to improve the emissivity, diffuse reflection and light absorption effect of power supply products, improve the surface state of products, and thereby improve the temperature rise effect.
(2) For test status The thermal vacuum test mainly simulates the solar irradiation and cold black environment.Under the controllable operating conditions, the solar irradiation intensity can be reduced through the intervention of external conditions to control the temperature rise effect of the product.Therefore, during the thermal vacuum test, the highest part of the product temperature can be shielded to reduce the radiant heat absorbed by the product.

Design of Validation Scheme for the Influence of Product Thermal Effect on Thermal Vacuum Test
According to the above analysis, there are two methods to reduce the temperature rise effect during the product test, one is the oxidation and blackening treatment of the product surface, and the other is the covering the product surface.Therefore, the test plan will be designed for these two situations.The test conditions are as follows: the test temperature range is set to -30℃~+70℃, the test vacuum degree is better than 6.65×10 -3 Pa, the temperature rise and fall rate is 1.5 ℃/min, the product is not powered on during the test, the test fixture is fully contacted with the cold plate and compacted, and the specific test status is shown in table 1.Two kinds of products are selected: one with a polished surface and the other with an oxidized black surface.Except that the product surface state is different, other states are all the same.The two products were tested under the same conditions, and the influence of their surface thermal characteristics on the thermal vacuum test was analyzed according to the test results.
Test 2: To verify the shielding of the highest temperature part, a single product will be used for this test.Considering different shielding areas and different effects in different areas, the same product is used here, and two working conditions are respectively adopted, namely, overhanging shade and covering the upper part of the product with aluminum film.The two test results are compared with the working condition without shielding, and the impact of the highest temperature part of shielding on the thermal vacuum test is analyzed according to the test results.

Analysis of the Effect of Product Surface State on Temperature Rise
Two products with the same structure dimensions will be selected for testing, one is blackening treatment, and the other is smooth surface.Both will be tested using the same type of vacuum simulation equipment in succession.In the figure, 3 # and 4 # are temperature control points, and the rest are temperature monitoring points.The test temperature curve of the test is shown in the figure below.It can be seen from the above figure that under the same heating rate, the heating rate of the polished surface has a heating rate of 0.69 ℃/min, while the heating rate of the surface blackening treatment products can reach 1.2 ℃/min, and when the temperature control point temperature of the polished surface is 70℃, the maximum temperature is 84 ℃, the temperature difference is 14 ℃, the maximum temperature difference of the oxidation blackened product is 48.9 ℃, and the oxidation blackened surface has a large temperature overshoot phenomenon.The test results show that: in the thermal vacuum test, the product with polished surface has weaker heat absorption capacity than the product with blackened surface, slower heating rate, and can reduce the maximum temperature difference, improving the surface thermal effect of the product.

Effect of Covering the Highest Part of the Product Temperature
Use blackened product in vacuum tank (size: 322mm × 275mm × 260mm), during the hot vacuum test, the product is installed on the tooling plate with screws, and the tooling plate is pressed on the cold plate of the vacuum tank by the pressure rod without any other treatment.According to the above test design, the shielding methods are divided into the following two types: Method 1: Cover the upper part of the product with aluminum film Method 2: A "hat" made of iron wire and aluminum film is used to suspend the upper part of the product (the aluminum film does not contact the product surface).
The on-site state of the test is shown in figure 3. Temperature sensors were attached at different locations on the product.T12 and T14 are temperature control points.T9 temperature monitoring points are located in the center of the upper surface of the product.T11 and T17 are located in the front of the product.T10 and T16 temperature monitoring points are located at the same height on the left and right sides of the product.T13 and T18 temperature monitoring points are located on the rear heating surface of the product.The test temperature curve of the experiment is shown in figure 4. It can be seen from the figure that the maximum temperature of product surface is 161.1℃ in the test without aluminum film; Method 1: In the test of directly covered with aluminum film, the maximum temperature of the product surface was 154.3℃, and the maximum temperature was reduced by 6.8℃; Method 2: In the test of using aluminum film "hat", the maximum temperature of the product surface is 136.1℃, and the maximum temperature is reduced by 25℃.The test results show that the use of covered with aluminum film and overhanging shade can reduce the surface temperature difference of the product, and the overhanging shade effect is more obvious.Compared with the working condition without covering measures, suspension reduce the temperature difference by 25℃, improve the uniformity of the temperature field of the entire vacuum tank, and reduce the ablation phenomenon caused by local high temperature during the product test.

Conclusion
In view of the influence of product surface state on temperature rise effect, through the experimental verification and analysis of different product surface states in vacuum thermal test, partially shielded products in the test process and different shielding methods, the experimental research shows that products with bright surfaces have weaker heat absorption capacity than those with blackened surfaces, and the heating rate is slow, which can reduce the maximum temperature difference.The vacuum thermal test with shielding showed that both methods of covering and hanging shielding with aluminum-coated film can reduce the surface temperature of the product.The effect of hanging shielding is more significant.Compared with the working conditions without shielding measures, hanging shielding can reduce the temperature difference by 25℃.

Figure 1 .
Figure 1.Time temperature curve of hot vacuum test on smooth surface.

Figure 2 .
Figure 2. Time temperature curve of thermal vacuum test on blackened surface.

Figure 3 .
Figure 3. Thermal vacuum test of products covered with aluminum film (left) and "hat" (right).

Figure 4 .
Figure 4. Time temperature curve of test condition comparison.

Table 1 .
Test Status of Comparison Products.