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Growth and characterization of NiO films on aluminum substrate as thermal interface material for LED application
Surface & Coatings Technology 350 (2018) 462–468
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Growth and characterization of NiO films on aluminum substrate as thermal interface material for LED application
T
⁎
Anas A. Ahmeda,b, , Mutharasu Devarajana, Muna E. Raypaha, Naveed Afzalc a
School of Physics, Universiti Sains Malaysia (USM), 11800, Pulau Pinang, Malaysia Department of Physics, Faculty of Science, Taiz University, Yemen c Centre for Advanced Studies in Physics, GC University, Lahore, Pakistan b
A R T I C LE I N FO
A B S T R A C T
Keywords: NiO film Sol-gel spin coating Thermal interface material Thermal resistance Junction temperature
In this work, NiO films of different layers (5, 10, 15 and 20) were coated on Al substrate by using sol-gel spin coating technique. The crystallinity of the films was improved with increasing the number of layers, as confirmed by the XRD and FESEM results. To investigate the thermal performance of NiO films, high power LED package was fixed on bare Al and NiO coated Al (NiO/Al) substrates. The rise in junction temperature (Tj) and total thermal resistance (Rth-tot) of the LED package were found to be lower on NiO/Al as compared to the bare Al. The Tj and Rth-tot on NiO/Al were increased by increasing the number of NiO layers. The lowest values of Tj and Rth-tot were obtained at 700 mA for the NiO film containing 5 layers. These values were lower than that of the bare Al by the difference of 14.31 °C and 5.73 K/W, respectively. Based on the thermal analysis results, NiO film is suggested to be a suitable thermal interface material for use in high power LED applications.
1. Introduction In recent years, high power light emitting diodes (HP-LEDs) have gained an extensive attention as replacement of conventional lighting owing to their advantages such as low power consumption, light weight, fast response time, long lifetime, etc. [1]. However, it is reported that 70–85% of input electrical power of HP-LEDs can be converted into heat, resulting in an increase in the junction temperature of LEDs [2]. This rise in junction temperature affects the stability, lifetime, quantum efficiency and wavelength shift of the LEDs [3–6]. To avoid overheating, the LEDs can be attached to a cooling system containing a metal substrate (copper or aluminum). However, when LEDs packages are attached to a metal substrate, air gaps are created between the two surfaces. These gaps disrupt the flow of heat from the LED and cause an increase in its junction temperature [7]. Therefore, thermal interface materials (TIMs) are utilized to enhance the heat dissipation by reducing the air gaps and surface irregularities between the LED package and substrate. The TIMs can be categorized into different types such as, thermal grease, thermal fluids, resilient thermal conductors and coatings [8–12]. High thermal conductivity substrates such as alumina, aluminum nitride (AlN) and silicon nitride (Si3N4) are employed to enhance heat dissipation in LED packaging technology. However, disadvantages of using these substrates include high cost of raw materials as well as ⁎
complex techniques that involve high temperature process. Recently, various studies have been conducted on using thin films as TIM for LED applications [13, 14]. It has been observed that thin films introduce lower junction temperature and thermal resistance as compared to bare metal substrates [15–17]. Lim et al. [15] reported that at 300 mA, the junction temperature and thermal resistance of LED package on Cu doped Al2O3/Al films were lower than that on the bare Al by the difference of 4.59 °C and 5.06 K/W, respectively. Shanmugan, et al. [16] also found a decrease in the junction temperature and thermal resistance of LED package on BN/Al films as compared to the bare Al. Similarly, Ong, et al. [17] observed that the thermal resistance and junction temperature of LED package on B-AlN/Cu films were reduced by the difference of 1.43 K/W and 3.37 °C, respectively in comparison with the bare Cu. Apart from TIMs based on alumina, aluminum and boron nitrides, metal oxides such as ZnO has been studied as filler in TIM to improve the heat dissipation [18, 19]. Sim et al. [18] found that the thermal conductivity of silicone rubber was increased by 1.8 when the filling load of ZnO was reached to 10% vol. Fu and co-workers [19] reported that by increasing the filling load of ZnO up to 66.3 wt%, the thermal conductivity of epoxy resin was improved by the factor of 4.70. ZnO thin film sputtered on Al substrate has also been examined as TIM for heat transfer in the LED technology. The results revealed that both junction temperature and thermal resistance were reduced as compared
Corresponding author. E-mail address: [email protected] (A.A. Ahmed).
https://doi.org/10.1016/j.surfcoat.2018.07.052 Received 14 September 2017; Received in revised form 10 July 2018; Accepted 16 July 2018 Available online 17 July 2018 0257-8972/ © 2018 Elsevier B.V. All rights reserved.
Surface & Coatings Technology 350 (2018) 462–468
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and 700 mA) for 900 s to reach the thermal steady state. Afterwards, it was switched to the cooling mode, in which the transient cooling curve was recorded for another 900 s at 1 mA to prevent self-heating effect at the junction. The rise in junction temperature of the LED and thermal resistance were obtained by analyzing the transient cooling curve through T3ster Master Software.
to bare Al substrate by the difference of 3.33 °C and 2.11 K/W, respectively [20]. Nickel oxide (NiO) film has received a significant interest in recent years because of its potential applications in different fields. Various studies have reported the synthesis of NiO films by using sol-gel spin coating technique [21–24]. Roro, et al. [21] prepared multiwall carbon nanotube (MWCNT)/NiO nanocomposite for solar thermal application. They studied the effect of MWCNT concentration on the photothermal conversion efficiency. The optimum efficiency was found to be 71% when the concentration of MWCNT to NiO reached 50%. Soleimanpour, et al. [22] fabricated hydrogen gas sensor based on NiO thin film. The sensor showed fast response and recovery time at low operating temperature (below 200 °C). It has been found that the NiO film shows good thermal conductivity even at high temperature. Lewis, et al. [25] investigated the thermal conductivity of bulk NiO films. The thermal conductivity was measured to be ~49 W/mK at room temperature and dropped to ~22 W/mK as the temperature was increased to 500 K. Similarly, Pranati, et al. [26] showed that the thermal conductivity of nanostructured NiO was ~21 W/mK at room temperature which was decreased to ~10 W/mK at 500 K. Therefore, it is interesting to investigate NiO films as TIM for enhancing heat transfer in LED packaging. To the best of the author's knowledge, no previous study has reported the use of NiO films as TIM for heat dissipation in HP-LEDs packages. This work reports the growth of NiO films by using sol-gel spin coating technique. The structural, morphological and thermal (the junction temperature and the thermal resistance) properties of NiO films attached to LED package have been investigated.
3. Results and discussion 3.1. Structural analysis The structural properties of NiO films coated on Al substrates were investigated by using X-ray diffraction (XRD) analysis. The XRD patterns of NiO films are shown in Fig. 2. The films show face centered cubic structure (fcc) with diffraction peaks corresponding to (111), (200) and (220) planes. No diffraction peak related to other phases was detected, indicating that the prepared films have pure NiO phase. In addition, it is clearly observed that by increasing the number of layers, the intensity of all diffraction peaks gradually increases which is accompanied with a decrease in their full width at half maximum (FWHM) values. The structural parameters such as crystallite size and the lattice strain were estimated from the XRD results. The FWHM, crystallite size and lattice strain are correlated to each other via Willamson-Hall formula [27, 28];
β cosθ =
2. Experimental section
0.94λ + 4ε sinθ D
(1)
where, λ is the wavelength of X-ray (1.54056 Å), β is the FWHM of the diffraction peak, θ is the diffraction angle, D is the crystallite size and Ɛ is the lattice strain. The crystallite size and lattice strain as a function of the number of layers were calculated using the aforementioned formula and these are plotted in Fig. 3. With increasing the number of layers from 5 to 20, the crystallite size increased from 25.35 to 33.66 nm, whereas the lattice strain decreased from 2.57 × 10−3 to 1.45 × 10−3. This suggests improvement in the crystallinity of NiO films with increasing the number of layers [29, 30]. The improvement in the crystalline structure reduces the number of grain boundaries which in turn enhances the thermal conductivity of the film. It is reported that the thermal conductivity of thin films is affected by the grain size of thin films. The thinner films have smaller grain size (more grain boundaries) as compared to the thicker films. As a result, the thermal conductivity of thinner films decreases due to scattering of the lattice vibration waves (phonons) at the multiple grain boundaries [31, 32].
2.1. Synthesis of NiO thin films NiO thin films were coated on Al substrate by using sol-gel spin coating technique. Nickel acetate tetrahydrate, 2-methoxyethanol and diethanolamine (DEA) were used as source of Ni, solvent and stabilizer, respectively. All the chemicals were purchased from Sigma Aldrich and used without further purification. Nickel acetate was dissolved in 2methoxyethanol and the solution was stirred on a hot plate at 60 °C for 2 h to get a clear and homogenous solution. The DEA was added dropwise to the solution while stirring. The concentration of nickel acetate was 0.5 M and the molar ratio of nickel acetate to DEA was maintained at 1.0. The obtained sol-gel solution was aged for 24 h at room temperature. Prior to the coating, Al substrates having dimensions of l × w × t = 25 × 25 × 1.5 mm3 were cleaned with isopropanol and acetone successively in an ultrasonic bath. After that, the substrates were washed out with deionized water and then dried by using high purity N2 gas. The coating solution was dropped on Al substrate and rotated at acceleration of 100 rpm/s for 30 s and then dried on the hot plate at 300 °C for 10 min to get rid of organic residuals. The coating and drying process was repeated for 5, 10, 15 and 20 times to achieve different thicknesses of NiO film. Thereafter, the samples were annealed in atmospheric air at 500 °C for 4 h to enhance the crystalline quality of NiO films. The structural and morphological properties of the prepared samples were characterized by means of X-ray diffractometer (Bruker D8 Advance, equipped with Cu Kα radiation tube (λ = 1.540 Å)) and field emission scanning electron microscopy (FESEM), respectively.
3.2. Surface morphology The surface morphology of NiO films was investigated through field emission scanning electron microscope (FESEM). Fig. 4(a–e) represents the surface morphology of bare Al and NiO films coated on the Al substrate. It is clearly seen that the surface of bare Al (Fig. 4a) is rough and it is not uniform as compared to the NiO film surface (Fig. 4(b–e)). This indicates that the surface of bare Al was noticeably modified by the NiO film coating that enhances heat dissipation by decreasing the contact resistance. In addition, all images of NiO films show spherically shaped grains. The average grain size of NiO film containing 5 layers was calculated using ImageJ software and it was found to be ~24.52 nm (Fig. 2b). When the number of layers were increased to 10, the average grain size was increased to ~26.74 nm (Fig. 4c). With further increase in the number of NiO layers to 15 and 20, the average grain size became ~30.10 nm and ~33.52 nm, respectively (Fig. 4d and e). The increasing trend of grain size with increasing layers agrees well with the XRD data as shown in Fig. 3.
2.2. Thermal transient measurement The thermal performance of NiO films coated on Al substrates was evaluated by using T3Ster measurement system. For this purpose, Golden Dragon LED with maximum power of 3.6 W was fixed on NiO film coated substrates. Fig. 1 shows the schematic diagram of the testing configuration. The thermal transient measurement was conducted in still-air box at room temperature (25 °C ± 1 °C). The LED package was forward biased at different driving current (150, 350, 500 463
Surface & Coatings Technology 350 (2018) 462–468
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Fig. 1. Schematic diagram of the LED package fixed on NiO film coated Al substrate.
3.3. Heat transfer characteristic of LED fixed on NiO film coated Al substrates 3.3.1. K-factor calibration To obtain the junction temperature of LED, the K factor should be estimated. It is known that the forward voltage applied to LED is a temperature dependent property, where the relationship between the forward voltage and the junction temperature is given by the following formula [33];
K=
∆Tj ∆Vf
(2)
here, K is a calibration factor, ΔTj is a variation in the junction temperature and ΔVf is a variation in the forward voltage. To perform the calibration, the LED was placed inside dry thermostat chamber and operated at low driving current (1 mA). The thermostat temperature was changed from 25 to 75 °C with an interval of 10 °C. The voltage drop across the junction due to the change in the junction temperature was measured once the LED reaches the thermal equilibrium with the thermostat chamber. Fig. 5 shows a change in forward voltage as function of the junction temperature of the LED. The slope was estimated to be 1.54 mV/°C.
Fig. 2. X-ray diffraction patterns of different layers of NiO film coated on Al substrate.
3.3.2. Junction temperature characteristic Thermal management in LED packages is of a great interest in enhancing the reliability and performance of the lighting systems. The Tj and Rth are critical parameters that should be minimized to improve the thermal management. One of the solutions of minimizing Tj and Rth is using TIMs. The influence of different number of NiO layers as TIM on heat dissipation from the LED package to the ambient was conducted through thermal transient measurement. Fig. 6(a–d) shows the cooling curve of the LED package attached to bare Al substrate and NiO film coated substrates at different values of the driving current (If). It is clearly seen that ΔTj of NiO film coated substrates are lower than that of the bare Al at all values of If. In addition, at a certain If value, the rise in junction temperature shows an increasing trend as the number of layers increases. The measured values of ΔTj for bare and films coated Al substrates are summarized in Table 1. The table shows that at 700 mA, the highest value of ΔTj (117.17 °C) was recorded for the LED package attached to bare Al, whereas the lowest value was observed to be 102.86 °C for the LED fixed on NiO film containing 5 layers. The difference between the rise in junction temperature on bare Al and NiO/Al was found to be 14.31 °C which is much better than the results reported in previous studies using thin films as TIM [20, 34–36].
Fig. 3. The average crystallite size and lattice strain of NiO films as a function of the number of layers.
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(a) bare Al substrate
(b)
(c) NiO film (10 layers)
(e)
(d)
NiO film (5 layers)
NiO film (15 layers)
NiO film (20 layers) 465
(caption on next page)
Surface & Coatings Technology 350 (2018) 462–468
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Fig. 4. (a–e) FESEM micrographs of (a) bare Al and (b–e) NiO films coated substrates.
Table 1 Rise in junction temperature (Tj) of LED fixed on bare and NiO film coated Al substrates at various driving current. Driving current
No. of layers
150 mA 350 mA 500 mA 700 mA
Zero layer (bare Al) TJ (°C)
5 layers of NiO film TJ (°C)
10 layers of NiO film TJ (°C)
15 layers of NiO film TJ (°C)
20 layers of NiO film TJ (°C)
21.82 53.70 79.91 117.17
19.85 48.57 71.87 102.86
20.78 51.08 76.22 111.93
20.92 52.26 78.49 115.29
21.17 53.22 79.15 115.13
between the junction temperature (Tj) of LED chip and the ambient temperature (Ta) at heat sink divided by the electrical power (Pel) applied to that LED [32];
Rth = Fig. 5. K-factor calibration curve of high power LED package.
∆Tj Pel
=
Tj − Ta Pel
(3)
In accordance with the JEDEC JESD-51 standard, the Rth of LED package attached to bare and NiO/Al substrates was measured by using T3ster unit. Fig. 7(a–d) shows a comparison of cumulative structure function of LED package fixed on bare Al and NiO films coated Al substrates. The cumulative structure function was extracted from
3.3.3. Thermal resistance characteristic The thermal resistance (Rth) is a measure of device capability to transfer heat from one point to another (the higher the Rth, the slower the heat transfer). The Rth of LED package is defined as the difference
Fig. 6. (a–b) Cooling transient curve of LED fixed on bare Al and NiO film coated substrates at driving current of (a) 150 mA (b) 350 mA (c) 500 mA and (d) 700 mA. 466
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Fig. 7. (a–b) Cumulative structure functions of LED fixed on bare Al and NiO film coated substrates at driving current of (a) 150 mA (b) 350 mA (c) 500 mA and (d) 700 mA.
transient cooling curve at different values of If. As shown in Fig. 7(a), the cumulative structure function is composed of three regions (region I, region II and region III). Region I represents the thermal resistance between the junction and the board of the LED package (Rthj-b), while region II demonstrates the thermal resistance from the board to the ambient (Rthb-a). Region III is the total thermal resistance (Rth-tot), which is a thermal resistance from the junction to the ambient. It is clearly seen in the insets of Fig. 7 that Al substrates coated with different number of NiO film layers introduce lower Rth-tot as compared to the bare Al at all values of If. A noticeable reduction in Rth-tot was observed for the NiO film coated Al substrate with 5 layers. To get a clear picture, the Rth-tot values of bare Al and NiO/Al were extracted from the cumulative structure function and are summarized in Table 2. These values support our discussion above and indicate that with 5 layers coated NiO film, the lowest value of Rth-tot is obtained. The difference ΔRth-tot between the bare and NiO film containing 5 layers was measured to be 4.56 K/W at 150 mA and it was increased to its maximum value (5.73 K/W) with increasing If to 700 mA. The maximum value of ΔRth-tot reported in this study is noticed to be higher than the results reported by other studies [15, 16, 35, 36]. Moreover, it is observed that Rth-tot increases with increasing the number of NiO film layers. The better performance of NiO film coated substrates than the bare
Table 2 Total thermal resistance (Rth-tot) of LED fixed on NiO film coated Al substrates at various driving currents. Driving current
150 mA 350 mA 500 mA 700 mA
No. of layers Zero layer (bare Al) Rth-total(K/ W)
5 layers of NiO film Rth-total(K/ W)
10 layers of NiO film Rth-total(K/ W)
15 layers of NiO film Rth-total(K/ W)
20 layers of NiO film Rth-total(K/ W)
52.67 52.69 53.69 54.75
48.11 48.03 48.92 49.02
50.28 50.60 51.94 53.56
50.63 51.77 53.17 53.74
50.89 52.27 52.63 53.90
Al can be attributed to the high thermal conductivity of the NiO films. The increase in crystallite size, which is conjugated with decrease in the lattice strain as the number of layers increases, indicates improvement in the crystallinity of NiO films. This results in enhancing the thermal conductivity of the films by increasing the mean free path of phonons due to reduction of grain boundaries and microstructural defects that act as scattering centers of the phonons [31, 37]. The increase in Rth-tot with increasing number of layers is due to an increase in the thickness of NiO films. Similar findings were reported by the other researchers 467
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[38, 39]. The thickness of NiO films plays a significant role in modifying the heat dissipation from the LED package to the ambient. The increase in film thickness leads to improvement in the crystallinity of the film which enhances its thermal conductivity. However, the increase in the film thickness leads to an increase in the thermal resistance of the film which in turn reduces the heat dissipation. Overall, (as shown in Table 1 and Table 2) the NiO film with lowest thickness (5 layers) shows the optimum performance during the heat transfer. Consequently, an increase in the thickness of NiO films adversely affects the heat flow from the LED package to the environment. The increase in Rth-tot with increase of If can be correlated to the current crowding effect, in which the heat dissipation area decreases at higher value of the driving current causing an increase in the thermal resistance [40, 41]. Moreover, at higher values of If, the temperature of NiO films increases which affects the heat transfer due to a decrease in the thermal conductivity of the NiO film. It was reported by Lewis et al. [25] and Pranati et al. [26] that the thermal conductivity of NiO decreases with increase of the temperature. This is because of phonons scattering at grain boundaries that becomes more pronounced at higher temperatures. Overall, the low values of Tj and Rth-tot, when NiO films were coated as TIM between LED package and bare Al substrate, indicate that the heat transfer from LED chip to ambient was improved. As a result, NiO films are recommended to be coated as thermally conductive layers on metal substrates for heat dissipation.
[12] [13] [14] [15]
[16]
[17]
[18]
[19] [20]
[21]
[22]
[23]
4. Conclusion [24]
Polycrystalline NiO films containing different number of layers (5, 10, 15 and 20) were successfully coated on Al substrate by using sol-gel spin coating technique. XRD and FESEM analyses revealed that the crystallinity of the films was increased with increasing the number of layers. The rise in junction temperature (Tj) and total thermal resistance (Rth-tot) of the LED package attached to bare Al substrate and NiO/Al were measured and compared with each other. The Tj and Rth-tot of the LED on NiO/Al were found to be lower than the Tj and Rth-tot of LED on the bare Al. The Tj and Rth-tot values on NiO/Al were increased by increasing the number of layers. The NiO film coated on Al with 5 layers showed a remarkable decrease in the values of Tj and Rth-tot. These were estimated to be lower than that of the bare Al by the difference of 14.31 °C and 5.73 K/W, respectively at 700 mA.
[25] [26] [27] [28]
[29]
[30] [31] [32]
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Growth and characterization of NiO films on aluminum substrate as thermal interface material for LED application
T
⁎
Anas A. Ahmeda,b, , Mutharasu Devarajana, Muna E. Raypaha, Naveed Afzalc a
School of Physics, Universiti Sains Malaysia (USM), 11800, Pulau Pinang, Malaysia Department of Physics, Faculty of Science, Taiz University, Yemen c Centre for Advanced Studies in Physics, GC University, Lahore, Pakistan b
A R T I C LE I N FO
A B S T R A C T
Keywords: NiO film Sol-gel spin coating Thermal interface material Thermal resistance Junction temperature
In this work, NiO films of different layers (5, 10, 15 and 20) were coated on Al substrate by using sol-gel spin coating technique. The crystallinity of the films was improved with increasing the number of layers, as confirmed by the XRD and FESEM results. To investigate the thermal performance of NiO films, high power LED package was fixed on bare Al and NiO coated Al (NiO/Al) substrates. The rise in junction temperature (Tj) and total thermal resistance (Rth-tot) of the LED package were found to be lower on NiO/Al as compared to the bare Al. The Tj and Rth-tot on NiO/Al were increased by increasing the number of NiO layers. The lowest values of Tj and Rth-tot were obtained at 700 mA for the NiO film containing 5 layers. These values were lower than that of the bare Al by the difference of 14.31 °C and 5.73 K/W, respectively. Based on the thermal analysis results, NiO film is suggested to be a suitable thermal interface material for use in high power LED applications.
1. Introduction In recent years, high power light emitting diodes (HP-LEDs) have gained an extensive attention as replacement of conventional lighting owing to their advantages such as low power consumption, light weight, fast response time, long lifetime, etc. [1]. However, it is reported that 70–85% of input electrical power of HP-LEDs can be converted into heat, resulting in an increase in the junction temperature of LEDs [2]. This rise in junction temperature affects the stability, lifetime, quantum efficiency and wavelength shift of the LEDs [3–6]. To avoid overheating, the LEDs can be attached to a cooling system containing a metal substrate (copper or aluminum). However, when LEDs packages are attached to a metal substrate, air gaps are created between the two surfaces. These gaps disrupt the flow of heat from the LED and cause an increase in its junction temperature [7]. Therefore, thermal interface materials (TIMs) are utilized to enhance the heat dissipation by reducing the air gaps and surface irregularities between the LED package and substrate. The TIMs can be categorized into different types such as, thermal grease, thermal fluids, resilient thermal conductors and coatings [8–12]. High thermal conductivity substrates such as alumina, aluminum nitride (AlN) and silicon nitride (Si3N4) are employed to enhance heat dissipation in LED packaging technology. However, disadvantages of using these substrates include high cost of raw materials as well as ⁎
complex techniques that involve high temperature process. Recently, various studies have been conducted on using thin films as TIM for LED applications [13, 14]. It has been observed that thin films introduce lower junction temperature and thermal resistance as compared to bare metal substrates [15–17]. Lim et al. [15] reported that at 300 mA, the junction temperature and thermal resistance of LED package on Cu doped Al2O3/Al films were lower than that on the bare Al by the difference of 4.59 °C and 5.06 K/W, respectively. Shanmugan, et al. [16] also found a decrease in the junction temperature and thermal resistance of LED package on BN/Al films as compared to the bare Al. Similarly, Ong, et al. [17] observed that the thermal resistance and junction temperature of LED package on B-AlN/Cu films were reduced by the difference of 1.43 K/W and 3.37 °C, respectively in comparison with the bare Cu. Apart from TIMs based on alumina, aluminum and boron nitrides, metal oxides such as ZnO has been studied as filler in TIM to improve the heat dissipation [18, 19]. Sim et al. [18] found that the thermal conductivity of silicone rubber was increased by 1.8 when the filling load of ZnO was reached to 10% vol. Fu and co-workers [19] reported that by increasing the filling load of ZnO up to 66.3 wt%, the thermal conductivity of epoxy resin was improved by the factor of 4.70. ZnO thin film sputtered on Al substrate has also been examined as TIM for heat transfer in the LED technology. The results revealed that both junction temperature and thermal resistance were reduced as compared
Corresponding author. E-mail address: [email protected] (A.A. Ahmed).
https://doi.org/10.1016/j.surfcoat.2018.07.052 Received 14 September 2017; Received in revised form 10 July 2018; Accepted 16 July 2018 Available online 17 July 2018 0257-8972/ © 2018 Elsevier B.V. All rights reserved.
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and 700 mA) for 900 s to reach the thermal steady state. Afterwards, it was switched to the cooling mode, in which the transient cooling curve was recorded for another 900 s at 1 mA to prevent self-heating effect at the junction. The rise in junction temperature of the LED and thermal resistance were obtained by analyzing the transient cooling curve through T3ster Master Software.
to bare Al substrate by the difference of 3.33 °C and 2.11 K/W, respectively [20]. Nickel oxide (NiO) film has received a significant interest in recent years because of its potential applications in different fields. Various studies have reported the synthesis of NiO films by using sol-gel spin coating technique [21–24]. Roro, et al. [21] prepared multiwall carbon nanotube (MWCNT)/NiO nanocomposite for solar thermal application. They studied the effect of MWCNT concentration on the photothermal conversion efficiency. The optimum efficiency was found to be 71% when the concentration of MWCNT to NiO reached 50%. Soleimanpour, et al. [22] fabricated hydrogen gas sensor based on NiO thin film. The sensor showed fast response and recovery time at low operating temperature (below 200 °C). It has been found that the NiO film shows good thermal conductivity even at high temperature. Lewis, et al. [25] investigated the thermal conductivity of bulk NiO films. The thermal conductivity was measured to be ~49 W/mK at room temperature and dropped to ~22 W/mK as the temperature was increased to 500 K. Similarly, Pranati, et al. [26] showed that the thermal conductivity of nanostructured NiO was ~21 W/mK at room temperature which was decreased to ~10 W/mK at 500 K. Therefore, it is interesting to investigate NiO films as TIM for enhancing heat transfer in LED packaging. To the best of the author's knowledge, no previous study has reported the use of NiO films as TIM for heat dissipation in HP-LEDs packages. This work reports the growth of NiO films by using sol-gel spin coating technique. The structural, morphological and thermal (the junction temperature and the thermal resistance) properties of NiO films attached to LED package have been investigated.
3. Results and discussion 3.1. Structural analysis The structural properties of NiO films coated on Al substrates were investigated by using X-ray diffraction (XRD) analysis. The XRD patterns of NiO films are shown in Fig. 2. The films show face centered cubic structure (fcc) with diffraction peaks corresponding to (111), (200) and (220) planes. No diffraction peak related to other phases was detected, indicating that the prepared films have pure NiO phase. In addition, it is clearly observed that by increasing the number of layers, the intensity of all diffraction peaks gradually increases which is accompanied with a decrease in their full width at half maximum (FWHM) values. The structural parameters such as crystallite size and the lattice strain were estimated from the XRD results. The FWHM, crystallite size and lattice strain are correlated to each other via Willamson-Hall formula [27, 28];
β cosθ =
2. Experimental section
0.94λ + 4ε sinθ D
(1)
where, λ is the wavelength of X-ray (1.54056 Å), β is the FWHM of the diffraction peak, θ is the diffraction angle, D is the crystallite size and Ɛ is the lattice strain. The crystallite size and lattice strain as a function of the number of layers were calculated using the aforementioned formula and these are plotted in Fig. 3. With increasing the number of layers from 5 to 20, the crystallite size increased from 25.35 to 33.66 nm, whereas the lattice strain decreased from 2.57 × 10−3 to 1.45 × 10−3. This suggests improvement in the crystallinity of NiO films with increasing the number of layers [29, 30]. The improvement in the crystalline structure reduces the number of grain boundaries which in turn enhances the thermal conductivity of the film. It is reported that the thermal conductivity of thin films is affected by the grain size of thin films. The thinner films have smaller grain size (more grain boundaries) as compared to the thicker films. As a result, the thermal conductivity of thinner films decreases due to scattering of the lattice vibration waves (phonons) at the multiple grain boundaries [31, 32].
2.1. Synthesis of NiO thin films NiO thin films were coated on Al substrate by using sol-gel spin coating technique. Nickel acetate tetrahydrate, 2-methoxyethanol and diethanolamine (DEA) were used as source of Ni, solvent and stabilizer, respectively. All the chemicals were purchased from Sigma Aldrich and used without further purification. Nickel acetate was dissolved in 2methoxyethanol and the solution was stirred on a hot plate at 60 °C for 2 h to get a clear and homogenous solution. The DEA was added dropwise to the solution while stirring. The concentration of nickel acetate was 0.5 M and the molar ratio of nickel acetate to DEA was maintained at 1.0. The obtained sol-gel solution was aged for 24 h at room temperature. Prior to the coating, Al substrates having dimensions of l × w × t = 25 × 25 × 1.5 mm3 were cleaned with isopropanol and acetone successively in an ultrasonic bath. After that, the substrates were washed out with deionized water and then dried by using high purity N2 gas. The coating solution was dropped on Al substrate and rotated at acceleration of 100 rpm/s for 30 s and then dried on the hot plate at 300 °C for 10 min to get rid of organic residuals. The coating and drying process was repeated for 5, 10, 15 and 20 times to achieve different thicknesses of NiO film. Thereafter, the samples were annealed in atmospheric air at 500 °C for 4 h to enhance the crystalline quality of NiO films. The structural and morphological properties of the prepared samples were characterized by means of X-ray diffractometer (Bruker D8 Advance, equipped with Cu Kα radiation tube (λ = 1.540 Å)) and field emission scanning electron microscopy (FESEM), respectively.
3.2. Surface morphology The surface morphology of NiO films was investigated through field emission scanning electron microscope (FESEM). Fig. 4(a–e) represents the surface morphology of bare Al and NiO films coated on the Al substrate. It is clearly seen that the surface of bare Al (Fig. 4a) is rough and it is not uniform as compared to the NiO film surface (Fig. 4(b–e)). This indicates that the surface of bare Al was noticeably modified by the NiO film coating that enhances heat dissipation by decreasing the contact resistance. In addition, all images of NiO films show spherically shaped grains. The average grain size of NiO film containing 5 layers was calculated using ImageJ software and it was found to be ~24.52 nm (Fig. 2b). When the number of layers were increased to 10, the average grain size was increased to ~26.74 nm (Fig. 4c). With further increase in the number of NiO layers to 15 and 20, the average grain size became ~30.10 nm and ~33.52 nm, respectively (Fig. 4d and e). The increasing trend of grain size with increasing layers agrees well with the XRD data as shown in Fig. 3.
2.2. Thermal transient measurement The thermal performance of NiO films coated on Al substrates was evaluated by using T3Ster measurement system. For this purpose, Golden Dragon LED with maximum power of 3.6 W was fixed on NiO film coated substrates. Fig. 1 shows the schematic diagram of the testing configuration. The thermal transient measurement was conducted in still-air box at room temperature (25 °C ± 1 °C). The LED package was forward biased at different driving current (150, 350, 500 463
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Fig. 1. Schematic diagram of the LED package fixed on NiO film coated Al substrate.
3.3. Heat transfer characteristic of LED fixed on NiO film coated Al substrates 3.3.1. K-factor calibration To obtain the junction temperature of LED, the K factor should be estimated. It is known that the forward voltage applied to LED is a temperature dependent property, where the relationship between the forward voltage and the junction temperature is given by the following formula [33];
K=
∆Tj ∆Vf
(2)
here, K is a calibration factor, ΔTj is a variation in the junction temperature and ΔVf is a variation in the forward voltage. To perform the calibration, the LED was placed inside dry thermostat chamber and operated at low driving current (1 mA). The thermostat temperature was changed from 25 to 75 °C with an interval of 10 °C. The voltage drop across the junction due to the change in the junction temperature was measured once the LED reaches the thermal equilibrium with the thermostat chamber. Fig. 5 shows a change in forward voltage as function of the junction temperature of the LED. The slope was estimated to be 1.54 mV/°C.
Fig. 2. X-ray diffraction patterns of different layers of NiO film coated on Al substrate.
3.3.2. Junction temperature characteristic Thermal management in LED packages is of a great interest in enhancing the reliability and performance of the lighting systems. The Tj and Rth are critical parameters that should be minimized to improve the thermal management. One of the solutions of minimizing Tj and Rth is using TIMs. The influence of different number of NiO layers as TIM on heat dissipation from the LED package to the ambient was conducted through thermal transient measurement. Fig. 6(a–d) shows the cooling curve of the LED package attached to bare Al substrate and NiO film coated substrates at different values of the driving current (If). It is clearly seen that ΔTj of NiO film coated substrates are lower than that of the bare Al at all values of If. In addition, at a certain If value, the rise in junction temperature shows an increasing trend as the number of layers increases. The measured values of ΔTj for bare and films coated Al substrates are summarized in Table 1. The table shows that at 700 mA, the highest value of ΔTj (117.17 °C) was recorded for the LED package attached to bare Al, whereas the lowest value was observed to be 102.86 °C for the LED fixed on NiO film containing 5 layers. The difference between the rise in junction temperature on bare Al and NiO/Al was found to be 14.31 °C which is much better than the results reported in previous studies using thin films as TIM [20, 34–36].
Fig. 3. The average crystallite size and lattice strain of NiO films as a function of the number of layers.
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(a) bare Al substrate
(b)
(c) NiO film (10 layers)
(e)
(d)
NiO film (5 layers)
NiO film (15 layers)
NiO film (20 layers) 465
(caption on next page)
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Fig. 4. (a–e) FESEM micrographs of (a) bare Al and (b–e) NiO films coated substrates.
Table 1 Rise in junction temperature (Tj) of LED fixed on bare and NiO film coated Al substrates at various driving current. Driving current
No. of layers
150 mA 350 mA 500 mA 700 mA
Zero layer (bare Al) TJ (°C)
5 layers of NiO film TJ (°C)
10 layers of NiO film TJ (°C)
15 layers of NiO film TJ (°C)
20 layers of NiO film TJ (°C)
21.82 53.70 79.91 117.17
19.85 48.57 71.87 102.86
20.78 51.08 76.22 111.93
20.92 52.26 78.49 115.29
21.17 53.22 79.15 115.13
between the junction temperature (Tj) of LED chip and the ambient temperature (Ta) at heat sink divided by the electrical power (Pel) applied to that LED [32];
Rth = Fig. 5. K-factor calibration curve of high power LED package.
∆Tj Pel
=
Tj − Ta Pel
(3)
In accordance with the JEDEC JESD-51 standard, the Rth of LED package attached to bare and NiO/Al substrates was measured by using T3ster unit. Fig. 7(a–d) shows a comparison of cumulative structure function of LED package fixed on bare Al and NiO films coated Al substrates. The cumulative structure function was extracted from
3.3.3. Thermal resistance characteristic The thermal resistance (Rth) is a measure of device capability to transfer heat from one point to another (the higher the Rth, the slower the heat transfer). The Rth of LED package is defined as the difference
Fig. 6. (a–b) Cooling transient curve of LED fixed on bare Al and NiO film coated substrates at driving current of (a) 150 mA (b) 350 mA (c) 500 mA and (d) 700 mA. 466
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Fig. 7. (a–b) Cumulative structure functions of LED fixed on bare Al and NiO film coated substrates at driving current of (a) 150 mA (b) 350 mA (c) 500 mA and (d) 700 mA.
transient cooling curve at different values of If. As shown in Fig. 7(a), the cumulative structure function is composed of three regions (region I, region II and region III). Region I represents the thermal resistance between the junction and the board of the LED package (Rthj-b), while region II demonstrates the thermal resistance from the board to the ambient (Rthb-a). Region III is the total thermal resistance (Rth-tot), which is a thermal resistance from the junction to the ambient. It is clearly seen in the insets of Fig. 7 that Al substrates coated with different number of NiO film layers introduce lower Rth-tot as compared to the bare Al at all values of If. A noticeable reduction in Rth-tot was observed for the NiO film coated Al substrate with 5 layers. To get a clear picture, the Rth-tot values of bare Al and NiO/Al were extracted from the cumulative structure function and are summarized in Table 2. These values support our discussion above and indicate that with 5 layers coated NiO film, the lowest value of Rth-tot is obtained. The difference ΔRth-tot between the bare and NiO film containing 5 layers was measured to be 4.56 K/W at 150 mA and it was increased to its maximum value (5.73 K/W) with increasing If to 700 mA. The maximum value of ΔRth-tot reported in this study is noticed to be higher than the results reported by other studies [15, 16, 35, 36]. Moreover, it is observed that Rth-tot increases with increasing the number of NiO film layers. The better performance of NiO film coated substrates than the bare
Table 2 Total thermal resistance (Rth-tot) of LED fixed on NiO film coated Al substrates at various driving currents. Driving current
150 mA 350 mA 500 mA 700 mA
No. of layers Zero layer (bare Al) Rth-total(K/ W)
5 layers of NiO film Rth-total(K/ W)
10 layers of NiO film Rth-total(K/ W)
15 layers of NiO film Rth-total(K/ W)
20 layers of NiO film Rth-total(K/ W)
52.67 52.69 53.69 54.75
48.11 48.03 48.92 49.02
50.28 50.60 51.94 53.56
50.63 51.77 53.17 53.74
50.89 52.27 52.63 53.90
Al can be attributed to the high thermal conductivity of the NiO films. The increase in crystallite size, which is conjugated with decrease in the lattice strain as the number of layers increases, indicates improvement in the crystallinity of NiO films. This results in enhancing the thermal conductivity of the films by increasing the mean free path of phonons due to reduction of grain boundaries and microstructural defects that act as scattering centers of the phonons [31, 37]. The increase in Rth-tot with increasing number of layers is due to an increase in the thickness of NiO films. Similar findings were reported by the other researchers 467
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[38, 39]. The thickness of NiO films plays a significant role in modifying the heat dissipation from the LED package to the ambient. The increase in film thickness leads to improvement in the crystallinity of the film which enhances its thermal conductivity. However, the increase in the film thickness leads to an increase in the thermal resistance of the film which in turn reduces the heat dissipation. Overall, (as shown in Table 1 and Table 2) the NiO film with lowest thickness (5 layers) shows the optimum performance during the heat transfer. Consequently, an increase in the thickness of NiO films adversely affects the heat flow from the LED package to the environment. The increase in Rth-tot with increase of If can be correlated to the current crowding effect, in which the heat dissipation area decreases at higher value of the driving current causing an increase in the thermal resistance [40, 41]. Moreover, at higher values of If, the temperature of NiO films increases which affects the heat transfer due to a decrease in the thermal conductivity of the NiO film. It was reported by Lewis et al. [25] and Pranati et al. [26] that the thermal conductivity of NiO decreases with increase of the temperature. This is because of phonons scattering at grain boundaries that becomes more pronounced at higher temperatures. Overall, the low values of Tj and Rth-tot, when NiO films were coated as TIM between LED package and bare Al substrate, indicate that the heat transfer from LED chip to ambient was improved. As a result, NiO films are recommended to be coated as thermally conductive layers on metal substrates for heat dissipation.
[12] [13] [14] [15]
[16]
[17]
[18]
[19] [20]
[21]
[22]
[23]
4. Conclusion [24]
Polycrystalline NiO films containing different number of layers (5, 10, 15 and 20) were successfully coated on Al substrate by using sol-gel spin coating technique. XRD and FESEM analyses revealed that the crystallinity of the films was increased with increasing the number of layers. The rise in junction temperature (Tj) and total thermal resistance (Rth-tot) of the LED package attached to bare Al substrate and NiO/Al were measured and compared with each other. The Tj and Rth-tot of the LED on NiO/Al were found to be lower than the Tj and Rth-tot of LED on the bare Al. The Tj and Rth-tot values on NiO/Al were increased by increasing the number of layers. The NiO film coated on Al with 5 layers showed a remarkable decrease in the values of Tj and Rth-tot. These were estimated to be lower than that of the bare Al by the difference of 14.31 °C and 5.73 K/W, respectively at 700 mA.
[25] [26] [27] [28]
[29]
[30] [31] [32]
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