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Properties of sprayed YSZ buffer layers on alumina substrates for YBCO thick films
Journal of Alloys and Compounds 268 (1998) 226–232
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Properties of sprayed YSZ buffer layers on alumina substrates for YBCO thick films Yoshiharu Matsuoka*, Eriko Ban Department of Physics, Meijo University, Tenpaku-ku, Nagoya 468, Japan Received 11 September 1997; received in revised form 30 October 1997
Abstract Yttria-stabilized-zirconia (YSZ) buffer layers on polycrystalline alumina substrate were fabricated by spraying a suspension consisting of ultrafine YSZ powders. It was found that the spraying conditions such as the flow rate of carrier gas and the aperture of spray nozzle had a great influence on the mechanical properties of the sprayed buffer layers and also on the Jc values of YBCO thick films processed on these buffer layers. The maximum Jc value of screen-printed YBCO film, fired by the solid-phase sintering method, was about 470 A cm 22 at 77 K, 0 T. This value was about four-fifths of those for films on single-crystal YSZ substrates. 1998 Elsevier Science S.A. Keywords: High-T c superconductors; Y-Ba-Cu-O; Thick films; Sprayed buffer layers
1. Introduction To be useful for thick film electronic device applications, the substrate should have a good combination of mechanical and electrical characteristics. Alumina is the most common substrate used in the thick film industry. This material has the required strength, high thermal conductivity and low dielectric losses. There have been, therefore, numerous attempts to produce high T c superconductor thick films on this material [1–5]. Y 1 Ba 2 Cu 3 O 72x (YBCO) thick films on a bare alumina substrate have, however, very poor superconducting properties due to interdiffusion, chemical reactions and the misfit expansion coefficients between film and substrate [6–10]. To overcome the deleterious film-substrate interaction, efforts were made to develop a number of different screen-printed buffer layers such as Ag, Y 2 BaCuO 5 and (Ba,Cu)ZrO 3 [11,12]. Schields and Abell [13] have recently reported that the screen-printed YSZ layers act as a successful barrier to aluminum diffusion and the critical current density Jc of YBCO thick films is improved appreciably to about 100 A cm 22 (77 K, 0 T). However, this Jc value was poorer than that on YSZ substrates. In a previous paper [14], we reported that the YSZ buffer layers produced by spraying a suspension consisting of ultrafine YSZ powders showed a dense and less cracked structure compared with the screen-printed layers and, in *Corresponding author. 0925-8388 / 98 / $19.00 1998 Elsevier Science S.A. All rights reserved. PII S0925-8388( 97 )00561-6
consequence, the Jc value of YBCO films on sprayed layers were much higher than those on screen-printed layers. However, the optimized fabricating conditions and the mechanical properties for sprayed buffer layers have not been elucidated at that experimental stage. The aim of this work is to study the effect of spraying conditions on the mechanical properties of the YSZ buffer layers and the Jc values of YBCO thick films formed on these buffer layers.
2. Experimental procedure Two kinds of commercially available ultrafine YSZ powders (Osaka Cement) with different particle size were ˚ specific used. 8YA; mean diameter of crystallite d5190 A, ˚ S530 m 2 g 21 . surface S510 m 2 g 21 and 8YC; d5370 A, These powders were stabilized with 8 mol % yttria. The schematic diagram of the spraying system used in this study is shown in Fig. 1. The YSZ powders were mixed with ethanol in the weight ratio 1:10 for 30 min in an ultrasonic cleaner. The suspension thus prepared was finely sprayed onto 99.5% polycrystalline alumina substrates through a commercially available atomizer (Olympos Co. Ltd.) using air as the carrier gas. The YSZ powders in the ethanol solution dispersed uniformly and did not deposit at the bottom of the suspension feed during spraying.The size of droplets generated in the spray nozzle was controlled by the following two methods; (i) control
Y. Matsuoka, E. Ban / Journal of Alloys and Compounds 268 (1998) 226 – 232
Fig. 1. Schematic diagram of spraying system.
of the flow rate Q of carrier gas by changing the gas pressure and (ii) control of Dd, where Dd is the difference in diameter between the aperture of spray nozzle and the needle as shown in Fig. 1, by the needle adjuster under the constant carrier gas pressure. After being sprayed, the YSZ layers were fired at T s 51400 and 14508C for a period of time t510 and 60 min with heating and cooling rates of 58C min 21 . The fired buffer layer thickness was kept constant at about 30 mm by controlling the amount of spraying suspension [14]. The mechanical properties of buffer layers thus prepared were studied using the following testers. (i) Vickers hardness tester; Vickers hardness was evaluated as the normal load divided by the pyramidal contact area of an indentation scar, where a pyramidal diamond indenter with diagonal planes at 1368 was used. (ii) Scratch tester; the diamond indenter tip with radius 50 mm was normally loaded across a buffer layer at a continuously increasing rate of 0.16 N s 21 up to the maximum 4.9 N. The preparation and characterization methods of YBCO thick films were similar to those described in our other article [15]. A brief description is presented here. The YBCO powder, the average particle size of which was less than a few micrometers, was mixed thoroughly with an appropriate amount of an organic vehicle to form a paste. This paste was printed through 300-mesh stainless steel screen onto the YSZ buffer layer /Al 2 O 3 substrate. After being dried, the films were sintered at 9408C for 2 min in argon flow of 1 l min 21 . After that, the gas flow was changed from argon to oxygen and then furnace cooled, with a post-annealing at 6008C for 2 h, to room temperature. The fired film thickness were found to be about 20–25 mm, using a surface profile meter. The critical current Ic was measured using a standard d.c. four-probe method at 77 K, 0 T with a 0.5 mV cm 21 criterion. The Jc value was calculated by dividing Ic by the average cross-sectional area of the film. The microstructure of the films was studied using scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy-dispersive X-ray analysis (EDXA).
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Fig. 2. Variation in spray rate (d) and deposition rate (h) of YSZ (8YA) powders with flow rate of carrier gas. Dd (see text) was kept constant at 36 mm.
3. Experimental results and discussion Fig. 2 shows the spray rate of suspension and the deposition rate of YSZ powder (8YA) as a function of flow rate Q of carrier gas, where Dd was kept constant at 36 mm. Here, the spray rate and the deposition rate were, respectively, calculated by dividing the amount of suspension and the fired YSZ layer thickness by the spraying time. It is found from this figure that, as expected, the spray rate increased with increasing flow rate Q. On the other hand, the deposition rate decreased suddenly at Q53.1 l min 21 . This break may be attributed to the lowering of sticking coefficient of YSZ particles due to the increase of the number of following particles: (i) incident particles reflected on the surface, (ii) deposited particles blown off by the carrier gas and (iii) deposited particles knocked out by the incident one. Fig. 3 (a), (b) and (c) show, respectively, typical surface SEM photographs of Q50.37, 1.6 and 3.1 l min 21 YSZ (8YA) films fired at T s 514008C for 10 min. The EDXA over the whole region of SEM image are also presented on the left of these figures. When Q is low (0.37 l min 21 ), a number of large cracks remained over the whole surface region and the distinct peak of Al element was observed from EDXA. This Al peak was inferred to be not the reflection from the alumina produced by the diffusion of Al into the near-surface region, but that mainly from the cracked region such as marked A. In fact, the molar Al:(Zr1Y) ratio in the region A was about 5:1, suggesting that this region A is substrate itself or the Al-rich interface reaction layer. The number and the size of cracks decreased with increasing flow rate Q as seen from Fig. 3. When Q53.1 l min 21 , the peak of Al element vanish completely, indicating that the buffer layer was almost free from large cracks and also that the diffusion of aluminum was suppressed due to the dense structure of the buffer layer. Fig. 4 shows the SEM photographs of a typical scratch channel in the final zones (4.9 N), marked by arrows, of
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Fig. 3. Surface SEM photographs and EDXA for the whole region of the SEM image of three kinds of YSZ (8YA) buffer layers fired at 14008C for 10 min. (a) Q50.37 l min 21 , (b) Q51.6 l min 21 and (c) Q53.1 l min 21 .
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of carrier gas decreases [16]. Hence, when Q is low, the large size droplets reaching the substrate surface would wet it well. In fact, it could be observed even with the naked eyes that the substrate surface was wetted during spray deposition at Q50.3–0.8 l min 21 . This wetting would reduce the apparent density of the deposition layer as a whole and also produce the irregular roughness in this layer. In consequence, the buffer layer after firing become brittle due to the weak interconnection between the YSZ grains. On the contrary, when Q is high, the droplet size are reduced. This reduction would promote the vaporization of solvent due to the increase of specific surface area of droplet. Then, the possibility that the dry solid impinges directly on the substrate surface increases, which would make the deposition layer uniform and dense, resulting in the enhancement of the densification of the fired buffer layer [17]. (ii) When Q is high, the pressure to the deposition layer produced by the droplet impingement and by the carrier gas would increase, leading to the enhancement of the apparent density of the deposition layer. Fig. 5 presents the critical current density Jc (77 K, 0 T) for the YBCO films fired at 9408C for 2 min as a function of flow rate of carrier gas, at which the YSZ (8YA) buffer layers were sprayed and then fired at T s 5 14008C for 10 min. The typical surface SEM photographs, taken at two different magnifications, of YBCO films formed on Q50.37 and 3.1 l min 21 YSZ buffer layers are also shown in Fig. 6. It is found from Fig. 5 that the Jc values tended to increase as the flow rate increases. This result could be explained as follows. (i) The surface state of YBCO film would be affected appreciably by the surface roughness of YSZ buffer layer,
Fig. 4. Surface SEM photographs of a scratch channel in final zones (4.9 N) of three kinds of YSZ (8YA) buffer layers fabricated by the same conditions shown in Fig. 3.
three samples prepared by the same conditions in Fig. 3. For the sample of Q50.37 l min 21 , heavy removal of YSZ film occurred around the indenter tip. This film removal tended to decrease with increasing Q. When Q increased up to 3.1 l min 21 , no film removal occurred and only a slight scar was left on the film surface. The average Vickers hardness values measured on 10 points of the surface of these samples were 6.4, 8.7 and 15 GPa, respectively, i.e. the hardness of the YSZ buffer layers increased with increasing the flow rate Q. Although the spray deposition process is very complex and the detailed mechanisms is not clear at the present stage, we want to discuss this change in hardness of buffer layers. This may be explained by the cumulative effect of the following factors: (i) The droplet size generally increases as the flow rate
Fig. 5. Critical current density Jc (77 K, 0 T) of the YBCO films on YSZ (8YA) buffer layers as a function of flow rate of carrier gas, at which the buffer layers were spray deposited. The YBCO films and YSZ buffer layers were fired at 9408C for 2 min and T s 514008C for 10 min, respectively.
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Fig. 6. Surface SEM photographs of screen-printed YBCO thick films formed on two kinds of YSZ (8YA) buffer layers. (a) Q50.37 l min 21 and (b) Q53.1 l min 21 : left-hand side, low magnification; right-hand side, high magnification. The YBCO films and YSZ buffer layers were fired at 9408 for 2 min and T s 514008 for 10 min, respectively.
i.e. when the surface of buffer layer is smooth and dense, the YBCO film also become dense as being obvious on comparing Figs. 3 and 6. The dense structure of YBCO film leads to high Jc . (ii) For the buffer layer prepared by low flow rate, there remained a number of large cracks. The YBCO films formed on such layer is in contact with the alumina substrate itself or Al-rich region through the cracks. This contact results in the film poisoning due to the severe interface reaction [10], which lead to low Jc . We next studied the effect of the size Dd on the surface state of YSZ (8YA) buffer layers, by keeping the flow rate constant at Q51.6 l min 21 . After being sprayed, the YSZ buffer layers were fired at T s 514008C for 10 min. Fig. 7 shows the critical current density Jc of YBCO films as a function of Dd. The insets to this figure give the surface SEM photographs of Dd523 and 50 mm YSZ (8YA) buffer layers. This result could be also explained as follows: The droplet size and the spray rate of suspension generally increase with increasing Dd [16]. The substrate sprayed with large Dd would therefore be wetted due to the large size droplets reaching the substrate rapidly. This would reduce both the uniformity and the apparent density of the deposition layer, which produce the large cracks, leading to low Jc value of the YBCO films. Next, we fabricated eight kinds of YSZ buffer layers
using both 8YA and 8YC powders on the following conditions; Q53.1 l min 21 , T s 51400 and 14508C, t510 and 60 min. Fig. 8(a) and (b) show, respectively, the surface SEM photographs of 8YA and 8YC samples fired at T s 514508C for 10 and 60 min. For these buffer layers: (i) As expected, the sintering was more promoted in the smaller size powder, i.e. 8YA, than 8YC samples, when compared at the same firing conditions. (ii) A fairly large number of micropores remained in the 10 min fired samples of both 8YA and 8YC. (iii) The densification and the grain growth of the YSZ layer were promoted as the sintering time increased from 10 to 60 min. The Jc value of YBCO films formed on these eight kinds of sample are given in Table 1. Several features are noticeable in this table: (i) The maximum Jc of about 470 A cm 22 , whose value is about four-fifths of those for films on single-crystal YSZ substrate (|600 A cm 22 ) [15], was obtained for the 8YA sample fired at T s 514508C for 10 min. The reason for the lower Jc in YBCO / YSZ samples than that in single-crystal samples could be due to the difference in the surface roughness of the substrates, but not be due to the diffusion of Al into the YBCO film across the YSZ buffer layer [14]. The rough surface could prevent the interconnection between YBCO grains and grain growth of the ab plane along the substrate surface, resulting in low Jc value. In fact, the ratio of XRD
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Fig. 7. Critical current density Jc (77 K, 0 T) of the YBCO thick films as a function of Dd (see text). The flow rate of carrier gas was kept constant at Q51.6 l min 21 . The insets show the surface SEM photographs of two kinds of YSZ (8YA) buffer layers. (a) Dd523 mm and (b) Dd550 mm.
˚ and (b) 8YC (d5370 A): ˚ left-hand side, t510 min; Fig. 8. Surface SEM photographs of YSZ buffer layers fired at T s 514508C: (a) 8YA (d5190 A) right-hand side, t560 min.
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Table 1 Critical current density Jc (77 K, 0 T) of the YBCO thick films formed on eight kinds of sprayed YSZ buffer layers where all YBCO films were fired at 9408C for 2 min. Buffer layer
T s (8C)
Jc (A cm 22 ) t510 min
8YA (d5190 8YC (d5370
1400 1450 1400 1450
430 470 370 400
˚ A) ˚ A)
t560 min 300 380 370 340
intensity of (006) peak to the intensity of the (103) peak, which give a tentative criteria for the preferred orientation of the c axis, were 1.2 and 2.0 for the YBCO films formed on YSZ buffer layer (8YA, T s 514508C, t510 min) and a single-crystal YSZ, respectively. (ii) For the samples fired for 10 min, the Jc values of the YBCO films on 14508C buffer layer were higher than those on 14008C buffer layer. This may be due to the higher densification and lesser micropores of 14508C layer, leading to higher Jc of YBCO films as described before. (iii) For the YBCO films on 60 min buffer layers, the Jc values tended to decrease as compared with samples fired for 10 min, in spite of the promotion of densification of buffer layers. This could be attributed to the following two reasons. First, as confirmed by a surface profile meter, the YSZ buffer layer on Al 2 O 3 substrate tended to bend slightly when fired for long times over 60 min. This bending of substrate introduced the cracks on the YBCO films, resulting in the degradation of Jc . Second, although considered not to be a major factor, the YBCO film may be poisoned due to the diffusion of Al ion, whose concentration near the surface of the buffer layer was increased due to the long period (|60 min) heat-treatment.
4. Conclusions The main conclusions of this paper are summarized below. (i) When spray droplets were reduced in size by increasing the flow rate of carrier gas or by reducing the needle aperture, the mechanical properties of YSZ buffer layers were enhanced and the Jc value of YBCO as also improved. (ii) The densification of the YSZ buffer layers was more ˚ samples compared with 8YC promoted in 8YA (d5190 A) ˚ when fired on the same conditions. The Jc (d5370 A) values of YBCO films tended to increase with increasing
degree of densification of these buffer layers. However, this tendency broke when the YSZ layer /Al 2 O 3 substrates were fired for periods longer than 60 min, probably due to the cracks, introduced by the slight bending of the substrates, on YBCO films. (iii) The maximum Jc of about 470 A cm 22 (77 K, 0 T) was obtained when the YSZ(8YA) buffer layer was fired at 14508C for 10 min. This value was about four-fifths of those for films on single-crystal YSZ substrates.
Acknowledgements The authors would like to express their sincere thanks to Mr. T. Yoshimura of Meijo University for his technical assistance and to Dr. H. Ogawa of Meijo University and Dr. K. Kurosawa of Aichi Prefectural Government for their fruitful discussions during this work.
References [1] R.C. Budhani, Sing-Mo H. Tzeng, H.J. Doerr, R.F. Bunshan, Appl. Phys. Lett. 51 (1987) 1277. [2] N.P. Bansal, R.N. Simons, D.E. Farrell, Appl. Phys. Lett. 53 (1988) 603. [3] L.H. Perng, T.S. Chin, K.C. Chen, C.H. Lin, W.Y. Lin, S.E. Hsu, Supercon. Sci. Technol. 3 (1990) 238. [4] J. McKittrick, R. Contreras, Thin Solid Films 206 (1991) 146. [5] Y. Ohasi, K. Kawabata, M. Niwa, M. Fukuchi, Jpn. J. Appl. Phys. 30 (1991) L32. [6] H. Koinuma, K. Fukuda, T. Hasimoto, K. Fueki, Jpn. J. Appl. Phys. 27 (1988) L1216. [7] L. Madhavrao, R. Rajagopalan, J. Mater. Sci. 25 (1990) 2349. ˇˇ ˇ V. Skacel, ´ [8] P. Stastny, R. Kuzel, J. Less-Common Met. 164–165 (1990) 464. [9] X.M. Li, Y.T. Chou, Y.H. Hu, C.L. Booth, J. Mater. Sci. Lett. 9 (1990) 669. [10] X.M. Li, Y.T. Chou, Y.H. Hu, C.L. Booth, J. Mater. Sci. 26 (1991) 3057. [11] M.V.S. Lakshmi, K. Ramkumar, M. Satyam, J. Mater. Sci. 26 (1991) 4092. [12] A. Bailey, K. Sealey, T. Puzzer, G.J. Russell, K.N.R. Taylor, Mater. Sci. Eng. B12 (1992) 237. [13] T.C. Shields, J.S. Abell, Supercon. Sci. Technol. 5 (1992) 627. [14] Y. Matsuoka, E. Ban, H. Ogawa, K. Kurosawa, J. Alloys Comp. 239 (1996) 55. [15] Y. Matsuoka, E. Ban, H. Ogawa, Supercon. Sci. Technol. 2 (1989) 300. [16] N. Nagai, in: T. Kurabayashi (Ed.), Ekitai no Biryuuka Gijyutsu, IPC Pubulisher, Tokyo, 1995, p.57 (in Japanese). [17] N. Morita, in: T. Kurabayashi (Ed.), Ekitai no Biryuuka Gijyutsu, IPC Pubulisher, Tokyo, 1995, p. 193 (in Japanese).
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Properties of sprayed YSZ buffer layers on alumina substrates for YBCO thick films Yoshiharu Matsuoka*, Eriko Ban Department of Physics, Meijo University, Tenpaku-ku, Nagoya 468, Japan Received 11 September 1997; received in revised form 30 October 1997
Abstract Yttria-stabilized-zirconia (YSZ) buffer layers on polycrystalline alumina substrate were fabricated by spraying a suspension consisting of ultrafine YSZ powders. It was found that the spraying conditions such as the flow rate of carrier gas and the aperture of spray nozzle had a great influence on the mechanical properties of the sprayed buffer layers and also on the Jc values of YBCO thick films processed on these buffer layers. The maximum Jc value of screen-printed YBCO film, fired by the solid-phase sintering method, was about 470 A cm 22 at 77 K, 0 T. This value was about four-fifths of those for films on single-crystal YSZ substrates. 1998 Elsevier Science S.A. Keywords: High-T c superconductors; Y-Ba-Cu-O; Thick films; Sprayed buffer layers
1. Introduction To be useful for thick film electronic device applications, the substrate should have a good combination of mechanical and electrical characteristics. Alumina is the most common substrate used in the thick film industry. This material has the required strength, high thermal conductivity and low dielectric losses. There have been, therefore, numerous attempts to produce high T c superconductor thick films on this material [1–5]. Y 1 Ba 2 Cu 3 O 72x (YBCO) thick films on a bare alumina substrate have, however, very poor superconducting properties due to interdiffusion, chemical reactions and the misfit expansion coefficients between film and substrate [6–10]. To overcome the deleterious film-substrate interaction, efforts were made to develop a number of different screen-printed buffer layers such as Ag, Y 2 BaCuO 5 and (Ba,Cu)ZrO 3 [11,12]. Schields and Abell [13] have recently reported that the screen-printed YSZ layers act as a successful barrier to aluminum diffusion and the critical current density Jc of YBCO thick films is improved appreciably to about 100 A cm 22 (77 K, 0 T). However, this Jc value was poorer than that on YSZ substrates. In a previous paper [14], we reported that the YSZ buffer layers produced by spraying a suspension consisting of ultrafine YSZ powders showed a dense and less cracked structure compared with the screen-printed layers and, in *Corresponding author. 0925-8388 / 98 / $19.00 1998 Elsevier Science S.A. All rights reserved. PII S0925-8388( 97 )00561-6
consequence, the Jc value of YBCO films on sprayed layers were much higher than those on screen-printed layers. However, the optimized fabricating conditions and the mechanical properties for sprayed buffer layers have not been elucidated at that experimental stage. The aim of this work is to study the effect of spraying conditions on the mechanical properties of the YSZ buffer layers and the Jc values of YBCO thick films formed on these buffer layers.
2. Experimental procedure Two kinds of commercially available ultrafine YSZ powders (Osaka Cement) with different particle size were ˚ specific used. 8YA; mean diameter of crystallite d5190 A, ˚ S530 m 2 g 21 . surface S510 m 2 g 21 and 8YC; d5370 A, These powders were stabilized with 8 mol % yttria. The schematic diagram of the spraying system used in this study is shown in Fig. 1. The YSZ powders were mixed with ethanol in the weight ratio 1:10 for 30 min in an ultrasonic cleaner. The suspension thus prepared was finely sprayed onto 99.5% polycrystalline alumina substrates through a commercially available atomizer (Olympos Co. Ltd.) using air as the carrier gas. The YSZ powders in the ethanol solution dispersed uniformly and did not deposit at the bottom of the suspension feed during spraying.The size of droplets generated in the spray nozzle was controlled by the following two methods; (i) control
Y. Matsuoka, E. Ban / Journal of Alloys and Compounds 268 (1998) 226 – 232
Fig. 1. Schematic diagram of spraying system.
of the flow rate Q of carrier gas by changing the gas pressure and (ii) control of Dd, where Dd is the difference in diameter between the aperture of spray nozzle and the needle as shown in Fig. 1, by the needle adjuster under the constant carrier gas pressure. After being sprayed, the YSZ layers were fired at T s 51400 and 14508C for a period of time t510 and 60 min with heating and cooling rates of 58C min 21 . The fired buffer layer thickness was kept constant at about 30 mm by controlling the amount of spraying suspension [14]. The mechanical properties of buffer layers thus prepared were studied using the following testers. (i) Vickers hardness tester; Vickers hardness was evaluated as the normal load divided by the pyramidal contact area of an indentation scar, where a pyramidal diamond indenter with diagonal planes at 1368 was used. (ii) Scratch tester; the diamond indenter tip with radius 50 mm was normally loaded across a buffer layer at a continuously increasing rate of 0.16 N s 21 up to the maximum 4.9 N. The preparation and characterization methods of YBCO thick films were similar to those described in our other article [15]. A brief description is presented here. The YBCO powder, the average particle size of which was less than a few micrometers, was mixed thoroughly with an appropriate amount of an organic vehicle to form a paste. This paste was printed through 300-mesh stainless steel screen onto the YSZ buffer layer /Al 2 O 3 substrate. After being dried, the films were sintered at 9408C for 2 min in argon flow of 1 l min 21 . After that, the gas flow was changed from argon to oxygen and then furnace cooled, with a post-annealing at 6008C for 2 h, to room temperature. The fired film thickness were found to be about 20–25 mm, using a surface profile meter. The critical current Ic was measured using a standard d.c. four-probe method at 77 K, 0 T with a 0.5 mV cm 21 criterion. The Jc value was calculated by dividing Ic by the average cross-sectional area of the film. The microstructure of the films was studied using scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy-dispersive X-ray analysis (EDXA).
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Fig. 2. Variation in spray rate (d) and deposition rate (h) of YSZ (8YA) powders with flow rate of carrier gas. Dd (see text) was kept constant at 36 mm.
3. Experimental results and discussion Fig. 2 shows the spray rate of suspension and the deposition rate of YSZ powder (8YA) as a function of flow rate Q of carrier gas, where Dd was kept constant at 36 mm. Here, the spray rate and the deposition rate were, respectively, calculated by dividing the amount of suspension and the fired YSZ layer thickness by the spraying time. It is found from this figure that, as expected, the spray rate increased with increasing flow rate Q. On the other hand, the deposition rate decreased suddenly at Q53.1 l min 21 . This break may be attributed to the lowering of sticking coefficient of YSZ particles due to the increase of the number of following particles: (i) incident particles reflected on the surface, (ii) deposited particles blown off by the carrier gas and (iii) deposited particles knocked out by the incident one. Fig. 3 (a), (b) and (c) show, respectively, typical surface SEM photographs of Q50.37, 1.6 and 3.1 l min 21 YSZ (8YA) films fired at T s 514008C for 10 min. The EDXA over the whole region of SEM image are also presented on the left of these figures. When Q is low (0.37 l min 21 ), a number of large cracks remained over the whole surface region and the distinct peak of Al element was observed from EDXA. This Al peak was inferred to be not the reflection from the alumina produced by the diffusion of Al into the near-surface region, but that mainly from the cracked region such as marked A. In fact, the molar Al:(Zr1Y) ratio in the region A was about 5:1, suggesting that this region A is substrate itself or the Al-rich interface reaction layer. The number and the size of cracks decreased with increasing flow rate Q as seen from Fig. 3. When Q53.1 l min 21 , the peak of Al element vanish completely, indicating that the buffer layer was almost free from large cracks and also that the diffusion of aluminum was suppressed due to the dense structure of the buffer layer. Fig. 4 shows the SEM photographs of a typical scratch channel in the final zones (4.9 N), marked by arrows, of
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Fig. 3. Surface SEM photographs and EDXA for the whole region of the SEM image of three kinds of YSZ (8YA) buffer layers fired at 14008C for 10 min. (a) Q50.37 l min 21 , (b) Q51.6 l min 21 and (c) Q53.1 l min 21 .
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of carrier gas decreases [16]. Hence, when Q is low, the large size droplets reaching the substrate surface would wet it well. In fact, it could be observed even with the naked eyes that the substrate surface was wetted during spray deposition at Q50.3–0.8 l min 21 . This wetting would reduce the apparent density of the deposition layer as a whole and also produce the irregular roughness in this layer. In consequence, the buffer layer after firing become brittle due to the weak interconnection between the YSZ grains. On the contrary, when Q is high, the droplet size are reduced. This reduction would promote the vaporization of solvent due to the increase of specific surface area of droplet. Then, the possibility that the dry solid impinges directly on the substrate surface increases, which would make the deposition layer uniform and dense, resulting in the enhancement of the densification of the fired buffer layer [17]. (ii) When Q is high, the pressure to the deposition layer produced by the droplet impingement and by the carrier gas would increase, leading to the enhancement of the apparent density of the deposition layer. Fig. 5 presents the critical current density Jc (77 K, 0 T) for the YBCO films fired at 9408C for 2 min as a function of flow rate of carrier gas, at which the YSZ (8YA) buffer layers were sprayed and then fired at T s 5 14008C for 10 min. The typical surface SEM photographs, taken at two different magnifications, of YBCO films formed on Q50.37 and 3.1 l min 21 YSZ buffer layers are also shown in Fig. 6. It is found from Fig. 5 that the Jc values tended to increase as the flow rate increases. This result could be explained as follows. (i) The surface state of YBCO film would be affected appreciably by the surface roughness of YSZ buffer layer,
Fig. 4. Surface SEM photographs of a scratch channel in final zones (4.9 N) of three kinds of YSZ (8YA) buffer layers fabricated by the same conditions shown in Fig. 3.
three samples prepared by the same conditions in Fig. 3. For the sample of Q50.37 l min 21 , heavy removal of YSZ film occurred around the indenter tip. This film removal tended to decrease with increasing Q. When Q increased up to 3.1 l min 21 , no film removal occurred and only a slight scar was left on the film surface. The average Vickers hardness values measured on 10 points of the surface of these samples were 6.4, 8.7 and 15 GPa, respectively, i.e. the hardness of the YSZ buffer layers increased with increasing the flow rate Q. Although the spray deposition process is very complex and the detailed mechanisms is not clear at the present stage, we want to discuss this change in hardness of buffer layers. This may be explained by the cumulative effect of the following factors: (i) The droplet size generally increases as the flow rate
Fig. 5. Critical current density Jc (77 K, 0 T) of the YBCO films on YSZ (8YA) buffer layers as a function of flow rate of carrier gas, at which the buffer layers were spray deposited. The YBCO films and YSZ buffer layers were fired at 9408C for 2 min and T s 514008C for 10 min, respectively.
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Fig. 6. Surface SEM photographs of screen-printed YBCO thick films formed on two kinds of YSZ (8YA) buffer layers. (a) Q50.37 l min 21 and (b) Q53.1 l min 21 : left-hand side, low magnification; right-hand side, high magnification. The YBCO films and YSZ buffer layers were fired at 9408 for 2 min and T s 514008 for 10 min, respectively.
i.e. when the surface of buffer layer is smooth and dense, the YBCO film also become dense as being obvious on comparing Figs. 3 and 6. The dense structure of YBCO film leads to high Jc . (ii) For the buffer layer prepared by low flow rate, there remained a number of large cracks. The YBCO films formed on such layer is in contact with the alumina substrate itself or Al-rich region through the cracks. This contact results in the film poisoning due to the severe interface reaction [10], which lead to low Jc . We next studied the effect of the size Dd on the surface state of YSZ (8YA) buffer layers, by keeping the flow rate constant at Q51.6 l min 21 . After being sprayed, the YSZ buffer layers were fired at T s 514008C for 10 min. Fig. 7 shows the critical current density Jc of YBCO films as a function of Dd. The insets to this figure give the surface SEM photographs of Dd523 and 50 mm YSZ (8YA) buffer layers. This result could be also explained as follows: The droplet size and the spray rate of suspension generally increase with increasing Dd [16]. The substrate sprayed with large Dd would therefore be wetted due to the large size droplets reaching the substrate rapidly. This would reduce both the uniformity and the apparent density of the deposition layer, which produce the large cracks, leading to low Jc value of the YBCO films. Next, we fabricated eight kinds of YSZ buffer layers
using both 8YA and 8YC powders on the following conditions; Q53.1 l min 21 , T s 51400 and 14508C, t510 and 60 min. Fig. 8(a) and (b) show, respectively, the surface SEM photographs of 8YA and 8YC samples fired at T s 514508C for 10 and 60 min. For these buffer layers: (i) As expected, the sintering was more promoted in the smaller size powder, i.e. 8YA, than 8YC samples, when compared at the same firing conditions. (ii) A fairly large number of micropores remained in the 10 min fired samples of both 8YA and 8YC. (iii) The densification and the grain growth of the YSZ layer were promoted as the sintering time increased from 10 to 60 min. The Jc value of YBCO films formed on these eight kinds of sample are given in Table 1. Several features are noticeable in this table: (i) The maximum Jc of about 470 A cm 22 , whose value is about four-fifths of those for films on single-crystal YSZ substrate (|600 A cm 22 ) [15], was obtained for the 8YA sample fired at T s 514508C for 10 min. The reason for the lower Jc in YBCO / YSZ samples than that in single-crystal samples could be due to the difference in the surface roughness of the substrates, but not be due to the diffusion of Al into the YBCO film across the YSZ buffer layer [14]. The rough surface could prevent the interconnection between YBCO grains and grain growth of the ab plane along the substrate surface, resulting in low Jc value. In fact, the ratio of XRD
Y. Matsuoka, E. Ban / Journal of Alloys and Compounds 268 (1998) 226 – 232
231
Fig. 7. Critical current density Jc (77 K, 0 T) of the YBCO thick films as a function of Dd (see text). The flow rate of carrier gas was kept constant at Q51.6 l min 21 . The insets show the surface SEM photographs of two kinds of YSZ (8YA) buffer layers. (a) Dd523 mm and (b) Dd550 mm.
˚ and (b) 8YC (d5370 A): ˚ left-hand side, t510 min; Fig. 8. Surface SEM photographs of YSZ buffer layers fired at T s 514508C: (a) 8YA (d5190 A) right-hand side, t560 min.
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Table 1 Critical current density Jc (77 K, 0 T) of the YBCO thick films formed on eight kinds of sprayed YSZ buffer layers where all YBCO films were fired at 9408C for 2 min. Buffer layer
T s (8C)
Jc (A cm 22 ) t510 min
8YA (d5190 8YC (d5370
1400 1450 1400 1450
430 470 370 400
˚ A) ˚ A)
t560 min 300 380 370 340
intensity of (006) peak to the intensity of the (103) peak, which give a tentative criteria for the preferred orientation of the c axis, were 1.2 and 2.0 for the YBCO films formed on YSZ buffer layer (8YA, T s 514508C, t510 min) and a single-crystal YSZ, respectively. (ii) For the samples fired for 10 min, the Jc values of the YBCO films on 14508C buffer layer were higher than those on 14008C buffer layer. This may be due to the higher densification and lesser micropores of 14508C layer, leading to higher Jc of YBCO films as described before. (iii) For the YBCO films on 60 min buffer layers, the Jc values tended to decrease as compared with samples fired for 10 min, in spite of the promotion of densification of buffer layers. This could be attributed to the following two reasons. First, as confirmed by a surface profile meter, the YSZ buffer layer on Al 2 O 3 substrate tended to bend slightly when fired for long times over 60 min. This bending of substrate introduced the cracks on the YBCO films, resulting in the degradation of Jc . Second, although considered not to be a major factor, the YBCO film may be poisoned due to the diffusion of Al ion, whose concentration near the surface of the buffer layer was increased due to the long period (|60 min) heat-treatment.
4. Conclusions The main conclusions of this paper are summarized below. (i) When spray droplets were reduced in size by increasing the flow rate of carrier gas or by reducing the needle aperture, the mechanical properties of YSZ buffer layers were enhanced and the Jc value of YBCO as also improved. (ii) The densification of the YSZ buffer layers was more ˚ samples compared with 8YC promoted in 8YA (d5190 A) ˚ when fired on the same conditions. The Jc (d5370 A) values of YBCO films tended to increase with increasing
degree of densification of these buffer layers. However, this tendency broke when the YSZ layer /Al 2 O 3 substrates were fired for periods longer than 60 min, probably due to the cracks, introduced by the slight bending of the substrates, on YBCO films. (iii) The maximum Jc of about 470 A cm 22 (77 K, 0 T) was obtained when the YSZ(8YA) buffer layer was fired at 14508C for 10 min. This value was about four-fifths of those for films on single-crystal YSZ substrates.
Acknowledgements The authors would like to express their sincere thanks to Mr. T. Yoshimura of Meijo University for his technical assistance and to Dr. H. Ogawa of Meijo University and Dr. K. Kurosawa of Aichi Prefectural Government for their fruitful discussions during this work.
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