- Email: [email protected]
Optical properties of transparent ZnAl2O4 ceramics: A new transparent material prepared by spark plasma sintering
Materials Letters 123 (2014) 142–144
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Optical properties of transparent ZnAl2O4 ceramics: A new transparent material prepared by spark plasma sintering Xu Yong a, Fu Ping a, Zhang Baohua b, Gao Juan a, Zhang Lin c, Wang Xuehua a,n a
School of Materials Science and Engineering, Wuhan Institute of Technology, Wuhan 430073, China Center of Analysis and Measurement, Wuhan Institute of Technology, Wuhan 430073, China c SZY Digital Electronics Co., Ltd., Guangdong 528000, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 17 October 2013 Accepted 2 March 2014 Available online 11 March 2014
Nanosized ZnAl2O4 powders were synthesized by a high-temperature calcinations method at 1100 ºC for 180 min. Transparent ZnAl2O4 ceramics were obtained at 1275 1C for 15 min by spark plasma sintering (SPS) with the home-made nanosized ZnAl2O4 powders. To improve the sinterability, 0.5 wt% tetraethyl orthosilicate (TEOS) was used as a sintering agent, so that a higher transmittance was obtained at a lower sintering temperature of 1260 1C. The transparent ZnAl2O4 ceramics sintered by the improved sintering process had a more uniform microstructure with an average grain size of 500 nm and a high relative density. The in-line transmittance of the transparent ZnAl2O4 ceramics reached 63.5% at 550 nm and 84.7% at 2000 nm. & 2014 Elsevier B.V. All rights reserved.
Keywords: ZnAl2O4 Transparent ceramics Spark plasma sintering (SPS) In-line transmittance
1. Introduction Zinc aluminate spinel (ZnAl2O4) offers many advantages, such as high thermal and chemical stability, hydrophobic behavior, high mechanical resistance, low sintering temperature, and high quantum yields. It has attracted much attention for its wide applications to lasers, lamps, and high temperature windows [1,2]. Recent investigations on ZnAl2O4 compounds explore how these systems can be considered as a new transparent and electroconductive material. Polycrystalline ZnAl2O4, which is used for ultraviolet (UV) photoelectronic devices, has a wide optical band gap (Eg ¼ 3.8 eV), which indicates that the dielectric material is transparent for light possessing wavelengths over 326 nm [3]. Comparing to the opaque ZnAl2O4, the transparent ZnAl2O4 ceramics have a Young modulus of 230–240 GPa, quite high thermal conductivity [20–25 W (mK) 1], a low dielectric constant (ε is about 10) and the Mohs hardness (7.5–8.0). Such properties make the transparent ZnAl2O4 ceramics potentially useful for various applications, such as thermal shock window and high temperature optical window [4]. Goldstein [5] reported a polycrystalline transparent ZnAl2O4 ceramics sintered with nano-ZnAl2O4 powders synthesized by the hydrothermal technique. Such ceramics were sintered by air heating (1500 1C) and then following hot-isostatic pressing (1550 1C), which exhibited an in-line transmission of more than 50% at 550 nm and an average grains size of 1.1 μm.
n
Corresponding author. E-mail address: [email protected] (W. Xuehua).
http://dx.doi.org/10.1016/j.matlet.2014.03.013 0167-577X & 2014 Elsevier B.V. All rights reserved.
In the present work, nanocrystalline ZnAl2O4 powders were firstly synthesized through a high-temperature calcination method. Then the transparent ZnAl2O4 ceramics with singlephase spinel structure, high density and nanosized grain were fabricated by the spark plasma sintering technique. And the microstructures and optical properties of transparent ZnAl2O4 ceramics were studied.
2. Experimental procedure Sample preparation: The nanosized ZnAl2O4 powders were prepared through a high-temperature calcination method at 1100 1C for 3 h via several high purity primary salts (ZnSO4 7H2O and NH4Al(SO4)2 12H2O) with a Zn:Al stoichiometric ratio of 1:2. The nanosized ZnAl2O4 powders were sieved by a 200 mesh sieve. Then, the powders were processed by dry pressing and then by cold isostatic pressing at 250 MPa for 3 min. Finally, the transparent ZnAl2O4 ceramics were sintered by spark plasma sintering (SPS-3.20 MK II, Sumitomo Coal Mining Co., Ltd., Tokyo, Japan). The temperature was increased from 1100 1C to the sintering temperature with a heating rate of 10 1C min 1 and a soaking time of 15 min (named sample A sintered at 1275 1C). As a comparison, sample B was sintered at 1260 1C by using 0.5 wt% TEOS as a sintering agent to improve the sinterability [6]. To remove organic compounds, sample B was pre-calcinated at 800 1C for 3 h before sintering. For optical properties measurement, both surfaces of the circular disks with thickness of 1.0 mm were mirror-polished with diamond polishing paste.
X. Yong et al. / Materials Letters 123 (2014) 142–144
143
Fig.1. Optical transparency of ZnAl2O4 ceramics sintered at different conditions for 15 min: (a) sample A without TEOS; (b) sample B with 0.5 wt% TEOS.
Characterization: The phase analysis of ZnAl2O4 powders and sintered bodies was identified by X-ray diffraction (XRD-7000, Shimadzu Corporation, Kyoto, Japan) using Cu Ka radiation at 40 kV and 30 mA. The relative density of the transparent ZnAl2O4 ceramics was evaluated by using the Archimedes method. The fracture microstructure observation was carried out using a scanning electron microscope (SEM, JSM-5510LV, JOEL Ltd. Tokyo, Japan) after thermal etching at 1200 1C for 60 min. A doublebeam spectrophotometer (Lambda 35, PerkinElmer, Inc., California, USA) and an FT-IR/Raman spectrometer (VERTEX 70, Bruker Corp., Ettlingen, Germany) were used to measure the in-line transmittance in the wavelength range from 190 nm to 800 nm and from 1100 nm to 8000 nm, respectively.
3. Results and discussions Fig. 1 shows the photographs of the ZnAl2O4 ceramics with a diameter of 15 mm and thickness of 1 mm sintered at different conditions. Comparing to the sample A, the transparency of the sample B is good to allow the easy reading of the below-laid writings. All the sintered ZnAl2O4 bodies have a blackish color. It is commonly known that oxide ceramics produced by SPS in a reducing ambient would have a dark color and it could be related to the relative lack of oxygen during SPS [7,8]. Fig. 2 shows the XRD patterns of the calcinated powders and the sintered ZnAl2O4 ceramics. The patterns corresponded precisely with the standard pattern of the cubic spinel structure of ZnAl2O4 (JCPDS card no. 74-1138) and no impurity peak was identified. The XRD result of ZnAl2O4 powders shows that the temperature of 1100 1C was sufficient to synthesize ZnAl2O4 with a single phase. And there was no phase transformation after sintering at 1260 1C. The very strong diffraction intensity of sintered ZnAl2O4 ceramics was a result of well-grown grains of polycrystalline ZnAl2O4 ceramics after sintering. The in-line transmittance of transparent ZnAl2O4 ceramics sintered at different conditions is shown in Fig. 3. Sample B sintered at 1260 1C with the addition of 0.5 wt% TEOS, of which the transmittance was significantly improved at wavelengths of 550 nm and 2000 nm (500 nm and 2000 nm were selected as representative of the samples in visible light and IR ranges to determine the optical properties), the highest transmittance reached as high as 63.5% at 550 nm and 84.7% at 2000 nm, respectively. The high transparency in the present work is better than transparent polycrystalline ZnAl2O4 ceramics sintered by the two stage sintering process [5].
Fig. 2. XRD patterns of ZnAl2O4: (a) sintered ZnAl2O4 ceramics with adding TEOS; (b) calcinated ZnAl2O4 powders; (c) JCPDS card.
The relative density of samples A and B was 99.45% and 99.68%, respectively. Fig. 4 shows the SEM images of the thermally etched fractures of the transparent ZnAl2O4 ceramics. The fracture morphologies of the two samples are obviously different. Fig. 4 (a) shows that ZnAl2O4 ceramics sintered without TEOS was consisted mainly of non-uniform sized and coarse grains due to the higher sintering temperature, and there were a large number of intergranular pores existing in grain boundaries, which indicated that the structure of the sample appeared to be more unconsolidated and the optical transmittance of the sample was deteriorated. However, with the addition of 0.5 wt% TEOS, the grain size distribution of ZnAl2O4 ceramic was improved evidently (as shown in Fig. 4(b)), the fracture surface is clearly intergranular and the averaged grain size was about 500 nm, and there were not any pores in partial enlargement (as shown in the inset), which is indicative of its high relative density and good optical transparency. Owing to the low sintering temperature and the adding of sintering agent TEOS, grain growth was inhibited, which resulted in a uniform distribution of the grain sizes. Such surface grain morphology was like as transparent yttrium aluminum garnet (YAG) ceramics reported by Dariel et al. [9].
4. Conclusion Transparent ZnAl2O4 ceramics with good transmittance have been fabricated through high-purity nanosized ZnAl2O4 powders by using the spark plasma sintering (SPS) technique at 1260 1C.
144
X. Yong et al. / Materials Letters 123 (2014) 142–144
Fig. 3. In-line transmittance from ultraviolet to infrared ranges of the sintered ZnAl2O4 ceramics: (a) sintered at 1275 1C without TEOS ; (b) sintered at 1260 1C with 0.5 wt % TEOS.
Fig. 4. Fracture morphologies of the ZnAl2O4 ceramics after thermal etching at 1200 1C for 60 min: (a) sintered at 1275 1C without TEOS ; (b) sintered at 1260 1C with 0.5 wt % TEOS.
Optical properties and microstructure of the sintered ZnAl2O4 ceramics were investigated. It is successful to reach a high inline transmittance with 63.5% at 550 nm and 84.7% at 2000 nm. The microstructure, with rare intergranular pores and an average grain size of 500 nm, is indicative of its high relative density. Reference [1] Kumar BR, Rao TS. AC impedance spectroscopy studies on solid-state sintered zinc aluminum oxide (ZnAl2O4) ceramics. AIP Conf Proc 2012;1461:303–6. [2] Gilde G, Patel P, Patterson P, Blodgett D, Duncan D, Hahn D. Evaluation of hot pressing and hot isostastic pressing parameters on the optical properties of spinel. J Am Ceram Soc 2005;88:2747–51. [3] Ciupina V, Carazeanu I, Prodan G. Characteristics of ZnAl2O4 nanocrystals prepared by coprecipitation and microemulsion. J Optoelectron Adv Mater 2004;6:1317–22.
[4] Van der Laag NJ, Snel MD, Magusin PC, de With G. Structural, elastic, thermophysical and dielectric properties of ZnAl2O4. J Eur Ceram Soc 2004;24:2417–24. [5] Goldstein A, Yeshurun Y, Vulfson M, Kravits H. Fabrication of transparent polycrystalline ZnAl2O4 – a new optical bulk ceramic. J Am Ceram Soc 2012;95:879–82. [6] Qin XP, Yang H, Zhou GH, Luo DW, Yang Y, Zhang J, et al. Fabrication and properties of highly transparent Er:YAG ceramics. Opt Mater 2012;34:973–6. [7] Bernard-Granger G, Benameur N, Guizard C, Nygren M. Influence of graphite contamination on the optical properties of transparent spinel obtained by spark plasma sintering. Scr Mater 2009;60:164–7. [8] Fu P, Lu W, Lei W, Xu Y, Wang X, Wu J. Transparent polycrystalline MgAl2O4 ceramic fabricated by spark plasma sintering: microwave dielectric and optical properties. Ceram Int 2013;39:2481–7. [9] Frage N, Kalabukhov S, Sverdlov N, Ezersky V, Dariel MP. Densification of transparent yttrium aluminum garnet (YAG) by SPS processing. J Eur Ceram Soc 2010;30:3331–7.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Optical properties of transparent ZnAl2O4 ceramics: A new transparent material prepared by spark plasma sintering Xu Yong a, Fu Ping a, Zhang Baohua b, Gao Juan a, Zhang Lin c, Wang Xuehua a,n a
School of Materials Science and Engineering, Wuhan Institute of Technology, Wuhan 430073, China Center of Analysis and Measurement, Wuhan Institute of Technology, Wuhan 430073, China c SZY Digital Electronics Co., Ltd., Guangdong 528000, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 17 October 2013 Accepted 2 March 2014 Available online 11 March 2014
Nanosized ZnAl2O4 powders were synthesized by a high-temperature calcinations method at 1100 ºC for 180 min. Transparent ZnAl2O4 ceramics were obtained at 1275 1C for 15 min by spark plasma sintering (SPS) with the home-made nanosized ZnAl2O4 powders. To improve the sinterability, 0.5 wt% tetraethyl orthosilicate (TEOS) was used as a sintering agent, so that a higher transmittance was obtained at a lower sintering temperature of 1260 1C. The transparent ZnAl2O4 ceramics sintered by the improved sintering process had a more uniform microstructure with an average grain size of 500 nm and a high relative density. The in-line transmittance of the transparent ZnAl2O4 ceramics reached 63.5% at 550 nm and 84.7% at 2000 nm. & 2014 Elsevier B.V. All rights reserved.
Keywords: ZnAl2O4 Transparent ceramics Spark plasma sintering (SPS) In-line transmittance
1. Introduction Zinc aluminate spinel (ZnAl2O4) offers many advantages, such as high thermal and chemical stability, hydrophobic behavior, high mechanical resistance, low sintering temperature, and high quantum yields. It has attracted much attention for its wide applications to lasers, lamps, and high temperature windows [1,2]. Recent investigations on ZnAl2O4 compounds explore how these systems can be considered as a new transparent and electroconductive material. Polycrystalline ZnAl2O4, which is used for ultraviolet (UV) photoelectronic devices, has a wide optical band gap (Eg ¼ 3.8 eV), which indicates that the dielectric material is transparent for light possessing wavelengths over 326 nm [3]. Comparing to the opaque ZnAl2O4, the transparent ZnAl2O4 ceramics have a Young modulus of 230–240 GPa, quite high thermal conductivity [20–25 W (mK) 1], a low dielectric constant (ε is about 10) and the Mohs hardness (7.5–8.0). Such properties make the transparent ZnAl2O4 ceramics potentially useful for various applications, such as thermal shock window and high temperature optical window [4]. Goldstein [5] reported a polycrystalline transparent ZnAl2O4 ceramics sintered with nano-ZnAl2O4 powders synthesized by the hydrothermal technique. Such ceramics were sintered by air heating (1500 1C) and then following hot-isostatic pressing (1550 1C), which exhibited an in-line transmission of more than 50% at 550 nm and an average grains size of 1.1 μm.
n
Corresponding author. E-mail address: [email protected] (W. Xuehua).
http://dx.doi.org/10.1016/j.matlet.2014.03.013 0167-577X & 2014 Elsevier B.V. All rights reserved.
In the present work, nanocrystalline ZnAl2O4 powders were firstly synthesized through a high-temperature calcination method. Then the transparent ZnAl2O4 ceramics with singlephase spinel structure, high density and nanosized grain were fabricated by the spark plasma sintering technique. And the microstructures and optical properties of transparent ZnAl2O4 ceramics were studied.
2. Experimental procedure Sample preparation: The nanosized ZnAl2O4 powders were prepared through a high-temperature calcination method at 1100 1C for 3 h via several high purity primary salts (ZnSO4 7H2O and NH4Al(SO4)2 12H2O) with a Zn:Al stoichiometric ratio of 1:2. The nanosized ZnAl2O4 powders were sieved by a 200 mesh sieve. Then, the powders were processed by dry pressing and then by cold isostatic pressing at 250 MPa for 3 min. Finally, the transparent ZnAl2O4 ceramics were sintered by spark plasma sintering (SPS-3.20 MK II, Sumitomo Coal Mining Co., Ltd., Tokyo, Japan). The temperature was increased from 1100 1C to the sintering temperature with a heating rate of 10 1C min 1 and a soaking time of 15 min (named sample A sintered at 1275 1C). As a comparison, sample B was sintered at 1260 1C by using 0.5 wt% TEOS as a sintering agent to improve the sinterability [6]. To remove organic compounds, sample B was pre-calcinated at 800 1C for 3 h before sintering. For optical properties measurement, both surfaces of the circular disks with thickness of 1.0 mm were mirror-polished with diamond polishing paste.
X. Yong et al. / Materials Letters 123 (2014) 142–144
143
Fig.1. Optical transparency of ZnAl2O4 ceramics sintered at different conditions for 15 min: (a) sample A without TEOS; (b) sample B with 0.5 wt% TEOS.
Characterization: The phase analysis of ZnAl2O4 powders and sintered bodies was identified by X-ray diffraction (XRD-7000, Shimadzu Corporation, Kyoto, Japan) using Cu Ka radiation at 40 kV and 30 mA. The relative density of the transparent ZnAl2O4 ceramics was evaluated by using the Archimedes method. The fracture microstructure observation was carried out using a scanning electron microscope (SEM, JSM-5510LV, JOEL Ltd. Tokyo, Japan) after thermal etching at 1200 1C for 60 min. A doublebeam spectrophotometer (Lambda 35, PerkinElmer, Inc., California, USA) and an FT-IR/Raman spectrometer (VERTEX 70, Bruker Corp., Ettlingen, Germany) were used to measure the in-line transmittance in the wavelength range from 190 nm to 800 nm and from 1100 nm to 8000 nm, respectively.
3. Results and discussions Fig. 1 shows the photographs of the ZnAl2O4 ceramics with a diameter of 15 mm and thickness of 1 mm sintered at different conditions. Comparing to the sample A, the transparency of the sample B is good to allow the easy reading of the below-laid writings. All the sintered ZnAl2O4 bodies have a blackish color. It is commonly known that oxide ceramics produced by SPS in a reducing ambient would have a dark color and it could be related to the relative lack of oxygen during SPS [7,8]. Fig. 2 shows the XRD patterns of the calcinated powders and the sintered ZnAl2O4 ceramics. The patterns corresponded precisely with the standard pattern of the cubic spinel structure of ZnAl2O4 (JCPDS card no. 74-1138) and no impurity peak was identified. The XRD result of ZnAl2O4 powders shows that the temperature of 1100 1C was sufficient to synthesize ZnAl2O4 with a single phase. And there was no phase transformation after sintering at 1260 1C. The very strong diffraction intensity of sintered ZnAl2O4 ceramics was a result of well-grown grains of polycrystalline ZnAl2O4 ceramics after sintering. The in-line transmittance of transparent ZnAl2O4 ceramics sintered at different conditions is shown in Fig. 3. Sample B sintered at 1260 1C with the addition of 0.5 wt% TEOS, of which the transmittance was significantly improved at wavelengths of 550 nm and 2000 nm (500 nm and 2000 nm were selected as representative of the samples in visible light and IR ranges to determine the optical properties), the highest transmittance reached as high as 63.5% at 550 nm and 84.7% at 2000 nm, respectively. The high transparency in the present work is better than transparent polycrystalline ZnAl2O4 ceramics sintered by the two stage sintering process [5].
Fig. 2. XRD patterns of ZnAl2O4: (a) sintered ZnAl2O4 ceramics with adding TEOS; (b) calcinated ZnAl2O4 powders; (c) JCPDS card.
The relative density of samples A and B was 99.45% and 99.68%, respectively. Fig. 4 shows the SEM images of the thermally etched fractures of the transparent ZnAl2O4 ceramics. The fracture morphologies of the two samples are obviously different. Fig. 4 (a) shows that ZnAl2O4 ceramics sintered without TEOS was consisted mainly of non-uniform sized and coarse grains due to the higher sintering temperature, and there were a large number of intergranular pores existing in grain boundaries, which indicated that the structure of the sample appeared to be more unconsolidated and the optical transmittance of the sample was deteriorated. However, with the addition of 0.5 wt% TEOS, the grain size distribution of ZnAl2O4 ceramic was improved evidently (as shown in Fig. 4(b)), the fracture surface is clearly intergranular and the averaged grain size was about 500 nm, and there were not any pores in partial enlargement (as shown in the inset), which is indicative of its high relative density and good optical transparency. Owing to the low sintering temperature and the adding of sintering agent TEOS, grain growth was inhibited, which resulted in a uniform distribution of the grain sizes. Such surface grain morphology was like as transparent yttrium aluminum garnet (YAG) ceramics reported by Dariel et al. [9].
4. Conclusion Transparent ZnAl2O4 ceramics with good transmittance have been fabricated through high-purity nanosized ZnAl2O4 powders by using the spark plasma sintering (SPS) technique at 1260 1C.
144
X. Yong et al. / Materials Letters 123 (2014) 142–144
Fig. 3. In-line transmittance from ultraviolet to infrared ranges of the sintered ZnAl2O4 ceramics: (a) sintered at 1275 1C without TEOS ; (b) sintered at 1260 1C with 0.5 wt % TEOS.
Fig. 4. Fracture morphologies of the ZnAl2O4 ceramics after thermal etching at 1200 1C for 60 min: (a) sintered at 1275 1C without TEOS ; (b) sintered at 1260 1C with 0.5 wt % TEOS.
Optical properties and microstructure of the sintered ZnAl2O4 ceramics were investigated. It is successful to reach a high inline transmittance with 63.5% at 550 nm and 84.7% at 2000 nm. The microstructure, with rare intergranular pores and an average grain size of 500 nm, is indicative of its high relative density. Reference [1] Kumar BR, Rao TS. AC impedance spectroscopy studies on solid-state sintered zinc aluminum oxide (ZnAl2O4) ceramics. AIP Conf Proc 2012;1461:303–6. [2] Gilde G, Patel P, Patterson P, Blodgett D, Duncan D, Hahn D. Evaluation of hot pressing and hot isostastic pressing parameters on the optical properties of spinel. J Am Ceram Soc 2005;88:2747–51. [3] Ciupina V, Carazeanu I, Prodan G. Characteristics of ZnAl2O4 nanocrystals prepared by coprecipitation and microemulsion. J Optoelectron Adv Mater 2004;6:1317–22.
[4] Van der Laag NJ, Snel MD, Magusin PC, de With G. Structural, elastic, thermophysical and dielectric properties of ZnAl2O4. J Eur Ceram Soc 2004;24:2417–24. [5] Goldstein A, Yeshurun Y, Vulfson M, Kravits H. Fabrication of transparent polycrystalline ZnAl2O4 – a new optical bulk ceramic. J Am Ceram Soc 2012;95:879–82. [6] Qin XP, Yang H, Zhou GH, Luo DW, Yang Y, Zhang J, et al. Fabrication and properties of highly transparent Er:YAG ceramics. Opt Mater 2012;34:973–6. [7] Bernard-Granger G, Benameur N, Guizard C, Nygren M. Influence of graphite contamination on the optical properties of transparent spinel obtained by spark plasma sintering. Scr Mater 2009;60:164–7. [8] Fu P, Lu W, Lei W, Xu Y, Wang X, Wu J. Transparent polycrystalline MgAl2O4 ceramic fabricated by spark plasma sintering: microwave dielectric and optical properties. Ceram Int 2013;39:2481–7. [9] Frage N, Kalabukhov S, Sverdlov N, Ezersky V, Dariel MP. Densification of transparent yttrium aluminum garnet (YAG) by SPS processing. J Eur Ceram Soc 2010;30:3331–7.