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Determining thermal expansion coefficient of stressed thin films at low temperature
Polymer Testing 24 (2005) 839–843 www.elsevier.com/locate/polytest
Test Method
Determining thermal expansion coefficient of stressed thin films at low temperature Zheng-dao Wang*, Xin-xin Zhao, Shao-qing Jiang, Jian-jun Lu School of Civil Engineering, Institute of Mechanics, Beijing JiaoTong University, Beijing 100044, China Received 9 May 2005; accepted 21 June 2005
Abstract The coefficient of thermal expansion (CTE) is one of the critical design parameters for the application of materials in cryogenic engineering and study of available results demonstrates that thin films have different properties from their bulk counterparts. However, few universal techniques have been developed to measure the CTE of thin films at cryogenic temperatures. Furthermore, the possible effect of the applied stress levels on CTE testing results has not received enough attention. In this study, two novel testing methods, one direct and the other indirect, for determining the CTE of thin films under different applied stress levels at cryogenic temperatures are proposed. The test temperature is from room temperature to 77 K and a lower temperature could be reached. Advantages of the two methods are also discussed. q 2005 Elsevier Ltd. All rights reserved. Keywords: Thin film; Coefficient of thermal expansion; Low temperature; Stress
1. Introduction In order to promote the potential applications of microelectronics and micro-electro-mechanical systems (MEMS) in space, superconducting magnet and electronic technologies, the cryogenic mechanical and thermal properties of thin films should receive attention. However, because of the extremely complicated testing system and the high cost, the present studies on thin films are mainly focused on room temperature properties [1–8] and few cryogenic results have been available [9–12]. When thin films are applied at cryogenic temperature, the CTE will become one of the critical parameters. For instance, considering the excellent mechanical and electrical properties, polyimide thin film is thought to be an ideal candidate for electrical insulating and thermal insulating material in a cryogenic environment. However, as an * Corresponding author. Tel.: C86 1051687257; fax: C86 1057682094. E-mail address: [email protected] (Z.-d. Wang).
0142-9418/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2005.06.014
organic material, the CTE of PI is higher than that of the inorganic or metallic substrates, which will cause thermal stress build-up and may result in device failure through peeling and cracking of the PI film and substrate [13,14] with decreasing temperature. In order to reduce the CTE of PI films, attention has been paid to adding low CTE molecular structures in the process of PI synthesis [15–17]. As regards thin film materials, another aspect also deserves being paid attention. That is how to measure the relative properties of thin films accurately. According to the reported study results, it is believed that thin films have different properties from their bulk counterparts. In addition, the film thickness and the fabrication process also have a large effect [18]. Unfortunately, most of the existing testing standards and techniques are developed for bulk specimens. Therefore, it is important to develop new testing methods suitable for thin films and to characterize their properties directly. As regards CTE, although there are several available techniques for measuring the CTE of bulk materials [19–21] and some novel testing methods are also proposed for film materials [7,22], it is still difficult to use the existing
840
Z.-d. Wang et al. / Polymer Testing 24 (2005) 839–843
techniques to measure the CTE of film materials at cryogenic temperatures, especially for organic thin film materials. For instance, the strain gauge technique and quartz-tube dilatometer technique cannot be used for film materials. The X-ray diffraction method is only suitable for materials with crystalline structure. Micro-machined cantilever and other related techniques are not suitable for such specimens, like paper, that have one-dimension of the order of sub millimeter or microns. Moreover, the existing techniques are taken on a constant-stressed-body or a free-body. The question asked here is whether the CTE value is affected by the force level applied on specimens. For the bulk materials, the working stress may be much lower than the failure strength of the materials and can be neglected. However, for thin film materials, it seems to be an interesting topic to investigate the CTE under different stress levels. In this study, two new methods are proposed to measure the CTE of thin films at cryogenic temperature. There are a few features about the two new testing methods. The first is that one is a direct method and the other is an indirect method. The second is that different force levels can be applied on specimens during the experiment. The third is that both methods have the advantages of easy operation, suitable for all film and other materials.
2. Testing techniques 2.1. Indirect testing method With decreasing temperature, thermal shrinking will occur for most free-body specimens and can be characterized by the CTE. On the contrary, if the specimen is kept a constant length during the cooling process (fixing two sides of the specimen), a thermal stress (tensile stress for most materials) will exist in the specimen. According to the above analysis, the CTE of thin films can be measured by a universal testing machine equipped with a temperature-controlled system. The testing procedure can be explained as following. The first step is to fix the temperature-controlled apparatus and the thin film specimen on a universal testing machine. The process is just as same as a simple tensile experiment. Then a tensile force is loaded on the specimen to a certain stress (s0) level in a displacement control mode at a given temperature, say T0 (normally corresponding to the room temperature). s0 is calculated by s0 Z E0 30
(1)
where 30 and E0 are the pre-set strain and Young’s modulus of the specimen at T0, respectively. By changing 30, different pre-set stress levels can be applied to the specimen. Second, keeping the constant length of the specimen and decreasing the testing temperature from T0 to T1, the corresponding stress will increase from s0 to s1,
then the linear CTE, aT0KT1, between T0 and T1 can be calculated by 3th1 Z s1 =E1 K30
(2)
and aT0KT1 Z 3th1 =ðT1 KT0 Þ
(3)
where 3th1 and E1 are the strain and Young’s modulus of the specimen at T1, respectively. 3th1 can be calculated by Eq. (2) and then aT0KT1will be available by Eq. (3). In Eqs. (2) and (3), T1 and T0 can be measured by a thermometer and s1 will be calculated through the force recorded by the load cell divided by the cross-sectional area of the specimen. However, Young’s modulus, E1, is commonly a function of the testing temperature and is not as a known parameter. In order to determine the Young’s modulus at every testing temperature point, some additional work should be done during the CTE testing process. At every temperature testing point, a tensile experiment is taken simultaneously to determine the Young’s modulus value at this temperature point. It should be ensured that the specimen is only under elastic deformation during this process. The next step is to continue decreasing the temperature from T1 to T2, the calculating equations being similar to Eqs. (2) and (3). The only difference is to use aT1KT2, E1, E2, 3th2, T1 and T2 to replace aT0KT1, E0, E1, 3th1, T0 and T1, respectively. Then, repeating the above step, the linear CTE of the specimen in different temperature ranges can be obtained. Of course, the Young’s modulus at different temperatures can be recorded at the same time. The following details concerning this test method should be considered. The loading apparatus is simply as shown in Fig. 1. In order to avoid the upper (5) and lower jigs (2) shrinking in opposite directions during the cooling process and add testing error, the lower jig is fixed with the flange of the temperature-controlled Dewar (6) by a reverse connecting rod (3). A similar testing method was proposed by Zhu [23], but their test result for the CTE of PI thin film was about three times larger than the reference value. Although they explained that the reference value was also not an accurate value, we think the most possible reason is that their jigs shrunk in two opposite directions and this greatly magnifies the resultant value. Second, in order to realize self-compensation for the deformation of the jigs caused by change of temperature, the same materials should be used for all specimen gripping parts (stainless steel suggested) (4). By taking this measure, only the part of the reverse connecting rod (3) parallel with the specimen cannot be compensated for, but it can be theoretically eliminated since the CTE of the reverse connecting rod (3) is known (for a particular material). Of course, in order to ensure the testing system is really reliable, a specimen made of the same material as the reverse connecting rod (3) should be used to calibrate the testing system before formal experiment.
Z.-d. Wang et al. / Polymer Testing 24 (2005) 839–843 7
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10 11
12 13
6
5
7 6 5 4
4
3 1
3
2 2
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Fig. 1. Schematic depiction of the device for measuring CTE of thin films at cryogenic temperature ((1) universal testing machine, (2) lower jig, (3) reverse connecting rod, (4) specimen, (5) upper jig, (6) temperature-controlled Dewar, (7) load cell, (8) connecting rod).
In this case, the testing system should be wholly self-compensating and no force increase is recorded by the load cell (7) with decrease of the temperature. 2.2. Direct testing method Next, we will introduce a new cryogenic CTE testing device to measure CTEs of films under different stress levels directly. The schematic of the testing device is shown in Fig. 2. The specimen (11) is fixed by four magnets and magnet-sheaths (the magnet and magnet-sheath are wholly shown as part 7 in Fig. 2). The left-side magnet and magnetsheath are fixed on the foundation (3) by the supporting rod (6) and cannot move during the experiment. The right side connecting with the weights (13) by a wire can move smoothly along the surface of the temperature-controlled block (10) by the pulley system (12). The pulley system is specially designed for application in low temperatures, and all parts made of the same material (stainless steel) to avoid different CTEs. The force level applied on the specimen can be adjusted by selecting different weights (13) (according to our test, the resistance caused by the pulley system can be neglected). There are two functions for the fixed rod (2). Its upper part is a screw-thread structure, which can ensure that the temperature-controlled block (10) and the foundation (3) firmly contact to each other. Another function is that it can ensure that the testing temperature does not increase too quickly while the height of liquid nitrogen is lower than the lower surface of the foundation (3). In this experiment,
Fig. 2. Schematic depiction of the device for measuring CTE of thin films at cryogenic temperature (1) Dewar, (2) fixed rod, (3) foundation, (4) connecting rod, (5) heater, (6) supporting rod, (7) magnet and its sheath, (8) flange, (9) glass window, (10) temperature-controlled block, (11) specimen, (12) pulley system, (13) weight).
liquid nitrogen will be used as a cooling medium to decrease the testing temperature. Of course, in order to measure the CTE at lower temperature, some other medium, e.g. liquid helium, could be used instead of liquid nitrogen. During the experiment, the specimen (11) will keep close contact with the temperature-controlled block (10). Practically, in order to keep the temperature of the specimen more homogeneous, there is a notch in the middle of the temperature-controlled block (10), and the specimen (11) is put into the notch to directly contact with the surface of temperature-controlled block, which is not shown in Fig. 2. The temperature-controlled block (10) is made of copper, which has a very high thermal conductivity. In our experiment, every temperature point is held for five minutes. Considering the direct contact between the temperaturecontrolled block (10) and the specimen (11), and the micrometer thickness of films, this is enough to assume that there is no temperature difference between the temperaturecontrolled block (10) and the specimen (11). The fixed rod (2) and the foundation (3) are made of stainless steel and have a lower thermal conductivity than that of copper. The testing temperature can be controlled by the heater (5) and recorded by a thermometer, which is fixed in the temperature-controlled block (10) and not shown in Fig 2. The power of the heater (5) is controlled by a standard temperature-controller fixed on the outside of the liquid nitrogen Dewar (1). Two markers that are 80 mm apart were introduced onto the surfaces of the sample before the experiment. During the testing process, with decreasing or increasing temperature, the distance of the two reference points will change due to
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Z.-d. Wang et al. / Polymer Testing 24 (2005) 839–843
increase of the composite CTEs with lower silica content is caused by the contribution of the higher bulk modulus of silica compared to that of polyimide, and the last slow decrease is caused by the difficulty of effectively dispersing nano-sized silica into the PI matrix when the silica content is high. The details of the specimen preparation, experimental results for CTEs of PI/SiO2 composite films, effect caused by the pre-stress level and related analysis are reported in the literature [24].
32
0 wt% 1 wt% 3 wt% 5 wt% 8 wt%
CTE (x10–6K–1)
28 24 20 16 12 8 4 80
120
160
200
240
280
Temperature (K)
4. Conclusions
Fig. 3. Curves of CTE vs. temperature for films with different silica contents.
the thermal deformation of the film specimen, which can be recorded by a microscope through the glass window (9). The microscope is fixed on the outside of the liquid nitrogen Dewar (1) and is not shown in Fig. 2.
3. Experimental results As an application example, the second testing method was used to determine the CTEs of polyimide/silica (PI/SiO2) composite films from 77 to 288 K. The results were obtained with the applied stress level under 2.7 MPa, which is about 3–4.5% of the ultimate tensile strength (UTS) of the specimen (the range is caused by the change of UTS at different temperatures). Plots in Fig. 3 indicate that the CTE of polyimide/silica (PI/SiO2) composite films decreases quickly from about 110 to 288 K (room temperature) and an analogous linear relationship between CTE and temperature exists. It then decreases slowly with further decreasing temperature. The average CTE of composite films with different silica contents from 77 to 288 K is shown in Fig. 4. According to Fig. 4, it shows that the composite CTEs first increases, then decreases quickly. When the silica content is above 10 wt%, the decrease becomes slower. The first 19
Two novel CTE testing techniques for thin films at low temperature are proposed, one is a direct method and another is an indirect method. The common advantages for the two methods are that they are easy to operate and suitable for all thin film materials. Another important characteristic is that different stress levels can be applied and the possible effect of the pre-stress on the CTE testing result can be considered. This may be of interest for thin film materials since even a very low force on the backing material might lead to a stress on the thin film close to its failure strength. In order to demonstrate the effectiveness of the testing methods, the CTEs of PI/SiO2 composite films with different silica content were measured from 77 to 288 K. The results indicated that the CTE of PI/SiO2 composite films first increases, decreases quickly, then decrease slowly with increase of silica content. With decrease of the test temperature, the CTE of PI/SiO2 composite films decreases quickly at first, then become slower from about 110 to 77 K.
Acknowledgements This project is funded by Educational Ministry of China and NJTU Paper Foundation of China.
References
CTE, ¦Á (x10–6/K)
18 17 16 15 14 13 0
2
4
6
8
10
12
14
16
Silica content (wt. %)
Fig. 4. Average CTE for films with different silica contents from 77 to 288 K.
[1] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. [2] S.P. Baker, W.W. Gerberich, J. Mater. Res. 9 (1994) 3131. [3] J. Mencik, E. Quandt, D. Munz, Thin Solid Films 287 (1996) 208. [4] W. Fang, J.A. Wickert, J. Micromech. Microeng. 6 (1996) 301. [5] J.J. Vlassak, M.D. Drory, W.D. Nix, J. Mater. Res. 12 (1997) 1900. [6] T.Y. Zhang, L.Q. Chen, R. Fu, Acta. Mater. 47 (1999) 3869. [7] W. Fang, H.C. Tsai, C.Y. Lo, Sens. Actuators A 77 (1999) 21. [8] T. Agag, T. Koga, T. Takeichi, Polymer 42 (2001) 3399. [9] H. Yamaoka, K. Miyata, Adv. Cryo. Eng. (Mater.) 32 (1986) 161. [10] D. Evens, J.T. Morgan, Cryogenics 31 (1991) 220.
Z.-d. Wang et al. / Polymer Testing 24 (2005) 839–843 [11] E. Tschegg, K. Humer, H.W. Weber, Cryogenics 31 (1991) 878. [12] H. Yamaoka, K. Miyata, O. Yano, Cryogenics 35 (1995) 787. [13] G.J. Elsner, Appl. Polym. Sci. 34 (1987) 815. [14] J.A. Thornton, D.W. Hoffman, Thin Solid Film 171 (1989) 5. [15] F.E. Arnold, S.Z.D. Cheng, S.L.C. Hsu, C.J. Lee, F.W. Harris, S.F. Lau, Polymer 33 (1992) 5179. [16] S. Numata, T. Miwa, Polymer 30 (1989) 1170. [17] Z.K. Zhu, J. Yin, F. Cao, X.Y. Shang, Q.H. Lu, Adv. Mater. 12 (2000) 1055. [18] W. Fang, J.A. Wickert, J. Micromech. Microeng. 5 (1995) 276.
[19] [20] [21] [22]
843
R.V. Jones, J. Sci. Instrum. 36 (1959) 90. A.F. Clark, Cryogenics 8 (1968) 282. K.E. Kinzly, Appl. Opt. 6 (1967) 137. H. Rafla-Yuan, B.P. Hichwa, T.H. Allen, J. Vac. Sci. Technol. A 16 (1998) 3119. [23] H. Zhu, W. Li, A.A. Tseng, in: A.T. Nettles, A. Zureick (Eds.), Composite Materials: Testing, Design, and Acceptance Criteria ASTM STP 1416, American Society for Testing and Materials, 2002. [24] Zheng-dao Wang, Jian-Jun Lu, Y. Li, S.Y. Fu, Shao-qing Jiang, Xin-Xin Zhao, Composite Part A, in press.
Test Method
Determining thermal expansion coefficient of stressed thin films at low temperature Zheng-dao Wang*, Xin-xin Zhao, Shao-qing Jiang, Jian-jun Lu School of Civil Engineering, Institute of Mechanics, Beijing JiaoTong University, Beijing 100044, China Received 9 May 2005; accepted 21 June 2005
Abstract The coefficient of thermal expansion (CTE) is one of the critical design parameters for the application of materials in cryogenic engineering and study of available results demonstrates that thin films have different properties from their bulk counterparts. However, few universal techniques have been developed to measure the CTE of thin films at cryogenic temperatures. Furthermore, the possible effect of the applied stress levels on CTE testing results has not received enough attention. In this study, two novel testing methods, one direct and the other indirect, for determining the CTE of thin films under different applied stress levels at cryogenic temperatures are proposed. The test temperature is from room temperature to 77 K and a lower temperature could be reached. Advantages of the two methods are also discussed. q 2005 Elsevier Ltd. All rights reserved. Keywords: Thin film; Coefficient of thermal expansion; Low temperature; Stress
1. Introduction In order to promote the potential applications of microelectronics and micro-electro-mechanical systems (MEMS) in space, superconducting magnet and electronic technologies, the cryogenic mechanical and thermal properties of thin films should receive attention. However, because of the extremely complicated testing system and the high cost, the present studies on thin films are mainly focused on room temperature properties [1–8] and few cryogenic results have been available [9–12]. When thin films are applied at cryogenic temperature, the CTE will become one of the critical parameters. For instance, considering the excellent mechanical and electrical properties, polyimide thin film is thought to be an ideal candidate for electrical insulating and thermal insulating material in a cryogenic environment. However, as an * Corresponding author. Tel.: C86 1051687257; fax: C86 1057682094. E-mail address: [email protected] (Z.-d. Wang).
0142-9418/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2005.06.014
organic material, the CTE of PI is higher than that of the inorganic or metallic substrates, which will cause thermal stress build-up and may result in device failure through peeling and cracking of the PI film and substrate [13,14] with decreasing temperature. In order to reduce the CTE of PI films, attention has been paid to adding low CTE molecular structures in the process of PI synthesis [15–17]. As regards thin film materials, another aspect also deserves being paid attention. That is how to measure the relative properties of thin films accurately. According to the reported study results, it is believed that thin films have different properties from their bulk counterparts. In addition, the film thickness and the fabrication process also have a large effect [18]. Unfortunately, most of the existing testing standards and techniques are developed for bulk specimens. Therefore, it is important to develop new testing methods suitable for thin films and to characterize their properties directly. As regards CTE, although there are several available techniques for measuring the CTE of bulk materials [19–21] and some novel testing methods are also proposed for film materials [7,22], it is still difficult to use the existing
840
Z.-d. Wang et al. / Polymer Testing 24 (2005) 839–843
techniques to measure the CTE of film materials at cryogenic temperatures, especially for organic thin film materials. For instance, the strain gauge technique and quartz-tube dilatometer technique cannot be used for film materials. The X-ray diffraction method is only suitable for materials with crystalline structure. Micro-machined cantilever and other related techniques are not suitable for such specimens, like paper, that have one-dimension of the order of sub millimeter or microns. Moreover, the existing techniques are taken on a constant-stressed-body or a free-body. The question asked here is whether the CTE value is affected by the force level applied on specimens. For the bulk materials, the working stress may be much lower than the failure strength of the materials and can be neglected. However, for thin film materials, it seems to be an interesting topic to investigate the CTE under different stress levels. In this study, two new methods are proposed to measure the CTE of thin films at cryogenic temperature. There are a few features about the two new testing methods. The first is that one is a direct method and the other is an indirect method. The second is that different force levels can be applied on specimens during the experiment. The third is that both methods have the advantages of easy operation, suitable for all film and other materials.
2. Testing techniques 2.1. Indirect testing method With decreasing temperature, thermal shrinking will occur for most free-body specimens and can be characterized by the CTE. On the contrary, if the specimen is kept a constant length during the cooling process (fixing two sides of the specimen), a thermal stress (tensile stress for most materials) will exist in the specimen. According to the above analysis, the CTE of thin films can be measured by a universal testing machine equipped with a temperature-controlled system. The testing procedure can be explained as following. The first step is to fix the temperature-controlled apparatus and the thin film specimen on a universal testing machine. The process is just as same as a simple tensile experiment. Then a tensile force is loaded on the specimen to a certain stress (s0) level in a displacement control mode at a given temperature, say T0 (normally corresponding to the room temperature). s0 is calculated by s0 Z E0 30
(1)
where 30 and E0 are the pre-set strain and Young’s modulus of the specimen at T0, respectively. By changing 30, different pre-set stress levels can be applied to the specimen. Second, keeping the constant length of the specimen and decreasing the testing temperature from T0 to T1, the corresponding stress will increase from s0 to s1,
then the linear CTE, aT0KT1, between T0 and T1 can be calculated by 3th1 Z s1 =E1 K30
(2)
and aT0KT1 Z 3th1 =ðT1 KT0 Þ
(3)
where 3th1 and E1 are the strain and Young’s modulus of the specimen at T1, respectively. 3th1 can be calculated by Eq. (2) and then aT0KT1will be available by Eq. (3). In Eqs. (2) and (3), T1 and T0 can be measured by a thermometer and s1 will be calculated through the force recorded by the load cell divided by the cross-sectional area of the specimen. However, Young’s modulus, E1, is commonly a function of the testing temperature and is not as a known parameter. In order to determine the Young’s modulus at every testing temperature point, some additional work should be done during the CTE testing process. At every temperature testing point, a tensile experiment is taken simultaneously to determine the Young’s modulus value at this temperature point. It should be ensured that the specimen is only under elastic deformation during this process. The next step is to continue decreasing the temperature from T1 to T2, the calculating equations being similar to Eqs. (2) and (3). The only difference is to use aT1KT2, E1, E2, 3th2, T1 and T2 to replace aT0KT1, E0, E1, 3th1, T0 and T1, respectively. Then, repeating the above step, the linear CTE of the specimen in different temperature ranges can be obtained. Of course, the Young’s modulus at different temperatures can be recorded at the same time. The following details concerning this test method should be considered. The loading apparatus is simply as shown in Fig. 1. In order to avoid the upper (5) and lower jigs (2) shrinking in opposite directions during the cooling process and add testing error, the lower jig is fixed with the flange of the temperature-controlled Dewar (6) by a reverse connecting rod (3). A similar testing method was proposed by Zhu [23], but their test result for the CTE of PI thin film was about three times larger than the reference value. Although they explained that the reference value was also not an accurate value, we think the most possible reason is that their jigs shrunk in two opposite directions and this greatly magnifies the resultant value. Second, in order to realize self-compensation for the deformation of the jigs caused by change of temperature, the same materials should be used for all specimen gripping parts (stainless steel suggested) (4). By taking this measure, only the part of the reverse connecting rod (3) parallel with the specimen cannot be compensated for, but it can be theoretically eliminated since the CTE of the reverse connecting rod (3) is known (for a particular material). Of course, in order to ensure the testing system is really reliable, a specimen made of the same material as the reverse connecting rod (3) should be used to calibrate the testing system before formal experiment.
Z.-d. Wang et al. / Polymer Testing 24 (2005) 839–843 7
8
8
9
841
10 11
12 13
6
5
7 6 5 4
4
3 1
3
2 2
1
Fig. 1. Schematic depiction of the device for measuring CTE of thin films at cryogenic temperature ((1) universal testing machine, (2) lower jig, (3) reverse connecting rod, (4) specimen, (5) upper jig, (6) temperature-controlled Dewar, (7) load cell, (8) connecting rod).
In this case, the testing system should be wholly self-compensating and no force increase is recorded by the load cell (7) with decrease of the temperature. 2.2. Direct testing method Next, we will introduce a new cryogenic CTE testing device to measure CTEs of films under different stress levels directly. The schematic of the testing device is shown in Fig. 2. The specimen (11) is fixed by four magnets and magnet-sheaths (the magnet and magnet-sheath are wholly shown as part 7 in Fig. 2). The left-side magnet and magnetsheath are fixed on the foundation (3) by the supporting rod (6) and cannot move during the experiment. The right side connecting with the weights (13) by a wire can move smoothly along the surface of the temperature-controlled block (10) by the pulley system (12). The pulley system is specially designed for application in low temperatures, and all parts made of the same material (stainless steel) to avoid different CTEs. The force level applied on the specimen can be adjusted by selecting different weights (13) (according to our test, the resistance caused by the pulley system can be neglected). There are two functions for the fixed rod (2). Its upper part is a screw-thread structure, which can ensure that the temperature-controlled block (10) and the foundation (3) firmly contact to each other. Another function is that it can ensure that the testing temperature does not increase too quickly while the height of liquid nitrogen is lower than the lower surface of the foundation (3). In this experiment,
Fig. 2. Schematic depiction of the device for measuring CTE of thin films at cryogenic temperature (1) Dewar, (2) fixed rod, (3) foundation, (4) connecting rod, (5) heater, (6) supporting rod, (7) magnet and its sheath, (8) flange, (9) glass window, (10) temperature-controlled block, (11) specimen, (12) pulley system, (13) weight).
liquid nitrogen will be used as a cooling medium to decrease the testing temperature. Of course, in order to measure the CTE at lower temperature, some other medium, e.g. liquid helium, could be used instead of liquid nitrogen. During the experiment, the specimen (11) will keep close contact with the temperature-controlled block (10). Practically, in order to keep the temperature of the specimen more homogeneous, there is a notch in the middle of the temperature-controlled block (10), and the specimen (11) is put into the notch to directly contact with the surface of temperature-controlled block, which is not shown in Fig. 2. The temperature-controlled block (10) is made of copper, which has a very high thermal conductivity. In our experiment, every temperature point is held for five minutes. Considering the direct contact between the temperaturecontrolled block (10) and the specimen (11), and the micrometer thickness of films, this is enough to assume that there is no temperature difference between the temperaturecontrolled block (10) and the specimen (11). The fixed rod (2) and the foundation (3) are made of stainless steel and have a lower thermal conductivity than that of copper. The testing temperature can be controlled by the heater (5) and recorded by a thermometer, which is fixed in the temperature-controlled block (10) and not shown in Fig 2. The power of the heater (5) is controlled by a standard temperature-controller fixed on the outside of the liquid nitrogen Dewar (1). Two markers that are 80 mm apart were introduced onto the surfaces of the sample before the experiment. During the testing process, with decreasing or increasing temperature, the distance of the two reference points will change due to
842
Z.-d. Wang et al. / Polymer Testing 24 (2005) 839–843
increase of the composite CTEs with lower silica content is caused by the contribution of the higher bulk modulus of silica compared to that of polyimide, and the last slow decrease is caused by the difficulty of effectively dispersing nano-sized silica into the PI matrix when the silica content is high. The details of the specimen preparation, experimental results for CTEs of PI/SiO2 composite films, effect caused by the pre-stress level and related analysis are reported in the literature [24].
32
0 wt% 1 wt% 3 wt% 5 wt% 8 wt%
CTE (x10–6K–1)
28 24 20 16 12 8 4 80
120
160
200
240
280
Temperature (K)
4. Conclusions
Fig. 3. Curves of CTE vs. temperature for films with different silica contents.
the thermal deformation of the film specimen, which can be recorded by a microscope through the glass window (9). The microscope is fixed on the outside of the liquid nitrogen Dewar (1) and is not shown in Fig. 2.
3. Experimental results As an application example, the second testing method was used to determine the CTEs of polyimide/silica (PI/SiO2) composite films from 77 to 288 K. The results were obtained with the applied stress level under 2.7 MPa, which is about 3–4.5% of the ultimate tensile strength (UTS) of the specimen (the range is caused by the change of UTS at different temperatures). Plots in Fig. 3 indicate that the CTE of polyimide/silica (PI/SiO2) composite films decreases quickly from about 110 to 288 K (room temperature) and an analogous linear relationship between CTE and temperature exists. It then decreases slowly with further decreasing temperature. The average CTE of composite films with different silica contents from 77 to 288 K is shown in Fig. 4. According to Fig. 4, it shows that the composite CTEs first increases, then decreases quickly. When the silica content is above 10 wt%, the decrease becomes slower. The first 19
Two novel CTE testing techniques for thin films at low temperature are proposed, one is a direct method and another is an indirect method. The common advantages for the two methods are that they are easy to operate and suitable for all thin film materials. Another important characteristic is that different stress levels can be applied and the possible effect of the pre-stress on the CTE testing result can be considered. This may be of interest for thin film materials since even a very low force on the backing material might lead to a stress on the thin film close to its failure strength. In order to demonstrate the effectiveness of the testing methods, the CTEs of PI/SiO2 composite films with different silica content were measured from 77 to 288 K. The results indicated that the CTE of PI/SiO2 composite films first increases, decreases quickly, then decrease slowly with increase of silica content. With decrease of the test temperature, the CTE of PI/SiO2 composite films decreases quickly at first, then become slower from about 110 to 77 K.
Acknowledgements This project is funded by Educational Ministry of China and NJTU Paper Foundation of China.
References
CTE, ¦Á (x10–6/K)
18 17 16 15 14 13 0
2
4
6
8
10
12
14
16
Silica content (wt. %)
Fig. 4. Average CTE for films with different silica contents from 77 to 288 K.
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