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Plastic shaping of ceramic superconducting discs
Materials
Chemistry
and Physics,
36 (1993)
129
129-133
Plastic shaping of ceramic superconducting L. Risso, Centro
S.J. Stedman
Sviluppo
Materiali
discs
and B. Vicenzi
SPA, A.d.R.
Genova,
Corso
Perrone
24/a,
16152
Genoa
84081
Baronissi
(Italy)
A. Saggese Department
(Received
of Physics,
November
University
of Salerno,
20, 1992; accepted
via Salvatore
Allende,
(Sa)
(Italy)
April 9, 1993)
Abstract Samples of YBa&u,O,_, high-T, superconductor were prepared by simulating ceramic injection moulding technology, which involves mixing organic binders and ceramic powders to permit the fabrication of different shapes. A thermal debinding process was used to remove the organic binder. X-ray diffraction analysis demonstrated that the YBCO powder was not altered by the mixing, pressing and debinding processes. Susceptibility measurements taken after sintering showed that the transition onset is at 92.5 K, with a transition width of 2 K, proving the absence of process-induced deterioration of both superconducting properties and grain couplings.
Introduction
Extrusion [l], tape casting [2] and uniaxial pressing [3] have been used as shaping technologies for the production of ceramic superconductors. The objective of the present work was to simulate the ceramic injection moulding process. The ceramic injection moulding process has the capacity to produce complex shapes, but involves the addition of a large amount of organic binder additives [4]. The systematic development of a suitable binder vehicle for ceramic injection moulding involves consideration of all stages of the ceramic injection moulding process. Consequently, the binder formulation must provide the desired rheological properties for mixing and moulding [5, 61. Moreover, the desired debinding characteristics should allow the removal of the binder vehicle without producing defects in the particle arrangement [7] and grain boundary impurities [8], which reduce the critical current of sintered superconducting ceramics. Hence, in this work the ceramic injection moulding process was simulated by mixing YBCO superconducting powders and organic additives. The effects of the organic additives on the superconducting properties were evaluated during each stage of the process.
Experimental
The YBCO superconducting powders were produced using a pyrolytic technique [9]. The ceramic powders
0254-0584/93/$6.00
produced were characterized by scanning electron microscopy (SEM), X-ray diffraction, specific surface area measurements (BET analysis), helium pycnometry, differential thermal analysis (DTA) and thermogravimetric analysis (TGA). The carbon content of the superconducting powders was measured using a Leco Industries model cs-125 carbon analyzer. The polymeric binder vehicle system consisted of 40 wt.% atactic polypropylene, 15 wt.% hydrocarbon waxes, 15 wt.% ethyl vinyl acetate, 20 wt.% lubricating wax, and 10 wt.% stearic acid [lo]. The polymeric and wax granules were cooled below their glass transition temperatures by immersion in liquid nitrogen and then granulated to produce a finer particle size. The granulated organic binder particle dimensions were less than 100 micrometer. The superconducting YBCO ceramic powder and the granulated organic binder were dry mixed together in a small high-speed mill for 1 min. The mixture consisted of 89.5 wt.% YBCO powder and 10.5 wt.% organic binder, corresponding to approximately 55 vol.% YBCO powder and 45 vol.% organic binder. This feedstock was subsequently used to prepare small pressed discs in a heated mould with a plunger over a temperature range 140-165 “C and pressures in the range 1.94-37.45 MPa. The pressed discs were cooled under pressure and then extracted from the mould. The densities of the pressed discs were calculated from their weights and dimensions. The microhomogeneity of the pressed discs was examined by SEM analysis of a fracture surface and the orientation of
0 1993 - Elsevier Sequoia.
All rights reserved
130
the YBCO particles during pressing by X-ray diffraction. The organic binder was removed by slow heating (0.25 “C min-’ up to 500 “C) in air; the discs were supported in an yttria powder bed to avoid contamination. The weight loss during debinding was used to calculate the exact powder volume loading and hence the porosity of the as-pressed discs. The debound discs were inspected visually after debinding and then sintered. The debound samples were heated at 1 “C mine1 up to 1000 “C, held at 1000 “C for 6 h, cooled at 0.5 “C min-’ to 450 “C and then held at this temperature for 8 h with an oxygen flow rate of 1 1 min-‘. After sintering, the linear shrinkage and the densities of the sintered discs were measured. The microstructure of the sintered discs was examined by SEM and optical microscopy. The crystallographic phases were identified by X-ray diffraction (Cu Ka,). The superconducting shielding properties of the sintered discs were measured using a two-coil a.c. susceptometer. The primary coil is in contact with one side of the superconducting disc and generates an a.c. magnetic field. On the other side, the magnetic flux that has partially penetrated through the disc induces an e.m.f. signal on the secondary coil. The temperature variations of the amplitudes of the in-phase and inquadrature components of the induced e.m.f. signal, proportional to the real and imaginary parts, respectively, of the complex susceptibility, can be used to determine the superconducting transition temperature and the transition width [ll]. The apparatus was calibrated with lead discs of various dimensions, at room and liquid helium temperatures, so the values of the components of the e.m.f. signal corresponding to zero and full shielding are known. The YBCO samples were measured using an a.c. magnetic field with a peak value of 1 gauss and a frequency of 17 Hz.
Results
and discussion
The YBCO powders produced by the pyrolytic process are shown in Fig. 1. The powders had an irregular particle morphology and did not contain agglomerates which could disrupt the development of a uniform microstructure during sintering. X-ray diffraction analysis confirmed that the YBCO powders were monophase orthorhombic [12]. The BET surface area was 1.2 m* g -l, and the density of the powders, measured by helium pycnometry, was 6.5 g cme3. The residual carbon content of the powders was 0.11 wt.%. The DTA plot shown in Fig. 2 revealed the peritectic melting point of the YBa,Cu,O,_, phase to the Y2BaCuOS phase and liquid to be 1025 “C in oxygen, in agreement with the results of other researchers [13,
Fig. 1. SEM image (500 X ) of pyrolytic YBa,Cu,O,_, used in this experiment.
p(Jwders
140
Fig. 2. Differential thermal analysis of the powders oxygen; the heating rate was 10 K min-‘.
in flowing
141. A small peak at 949 “C was also noted, which was considered to be due to the eutectic of the YBCO system [15] or to the decomposition of a small residual amount of barium carbonate not detectable by X-ray diffraction. A small weight loss was detected at the same temperature by TGA, and this suggested that the peak was due to BaCO, decomposition. The TGA results also showed that YBCO oxygen loss began at 400 “C and hence indicated that holding the temperature at 450 “C after sintering was an appropriate process for replacing the oxygen lost during sintering and for allowing oxygen diffusion. These powders have been sintered to high densities 1161 and therefore were known to be suitable for this experimental program. The density of the pressed discs depended strongly on the pressing conditions, that is, the applied pressure, pressing time and the temperature of the mould. The results are summarised in Table 1. All the discs were free from visible defects, laminations or cracks after pressing, and hence these discs were suitable for the subsequent steps of debinding and sintering.
131 TABLE Sample no.
1
2 3 4 5 6 7
1. Processing
data and disc densities
Pressure (MPa)
1.9 1.9 18.7 18.7 25.0 25.0 37.5
Pressing time
Die temperature
Density after pressing
(s)
(“C)
60 60 30 60 60 60 60
165 170 170 170 170 170 170
Note: the theoretical density of YBa,Cu,O,_, “Used for microscopy.
of a polished
Density after sintering (g cm-‘)
3.19 2.99 3.34 3.45 3.63 3.38 3.51
2.78 2.57 2.92 3.06 a 3.19 3.25
3.65 3.47 3.81 3.79 a a a
is 6.38 g cmW3.
Optical examination of a polished disc face that was normal to the pressing direction indicated that orientation of the particles could have occurred (Fig. 3); this could have been due to the sample polishing procedure. However, this preferred orientation on the disc surface normal to the pressing direction was confirmed by comparative X-ray analysis of an isotropic sample (YBCO powder) and an as-pressed disc face. The ratio of peaks a and b of Fig. 4 for the isotropic sample was 1.47, whilst the a/b peak ratio for the pressed disc was 4.71. Thus the (010) index of the YBCO crystal was perpendicular to the pressing direction. A preferred orientation of anisotropic particles is a common feature of plastic formation of ceramics [17], but no attempt was made to quantitatively measure the particle orientation. The removal of the binder by thermal degradation did not introduce any visible defects, and the discs were strong enough to be handled. Furthermore, Xray diffraction analysis of powder from the debound disc did not reveal the presence of barium carbonate or other new phases compared with the starting YBCO powder. Barium carbonate could have formed at tem-
Fig. 3. Optical image (500X) polymer-powder blend.
(g cm-3)
Density after debinding (g cm-‘)
disc of the pressed
A
/ \
t
i’ \
Fig. 4. Comparison between two lines in the X-ray diffraction spectra of powders (---) and of as-pressed discs (-). Peak a is the (060) line, and peak b is the (200) line. The a/b ratio is 1.47 for powders and 4.71 for the discs, showing surface grain orientation in the latter.
peratures below 500 “C, but it decomposes at much higher temperatures, and as barium carbonate was not detected after debinding it is clear that barium carbonate was not formed as a transient phase [18]. Hence, it was possible to remove the organic binder formulation without altering the YBCO powder characteristics. There was a slight increase in the carbon content, from 0.11 wt.% to 0.37 wt.%, during the debinding process, but even after sintering barium carbonate was not detected, and the final carbon content of the sintered disc was the same as that of the starting powder, indicating that the excess carbon burned off during the sintering cycle. The sintered discs did not achieve very high sintered densities (Table 1). This was probably due to the low green density, which would require a linear sintering shrinkage of 21.16% in order for the theoretical density to be obtained. In fact, uniaxially pressed discs of this YBCO powder with green densities of 57.1% (referred to the theoretical density) have been sintered up to 83.5% [16]. It was not possible to sinter at higher
132
temperatures to improve the density because of the irreversible phase transformations to nonsuperconducting phases that occur above 1025 “C, as mentioned earlier [15]. A higher green density could be obtained if the binder/powder ratio could be reduced and higher moulding pressures used. The sintered discs still contained macropores left by some relatively large binder particles removed by thermal degradation, but in the homogeneous regions between the pores the sintered YBCO microstructure consisted of grains of uniform dimensions (Fig. 5). Xray analysis of the sintered discs confirmed the monophase orthorhombic crystal structure, but also revealed that the preferred orientation of the YBCO grains of
the green discs had disappeared during sintering. Preferred orientation of the YBCO grains in the same crystallographic direction would allow a higher critical current density [19] and therefore more advantageous properties. The superconducting transition temperature was measured using a two-coil a.c. susceptometer [20], as previously mentioned. The amplitudes of the in-phase and in-quadrature components of the e.m.f. signal for disc no. 2 (see Table l), with a diameter of 9 mm and a height of 4 mm, are shown in Fig. 6(a) and (b). For this sample the transition temperature onset is 92.5 K and the transition width is about 2 K. All of the other samples show the same behaviour, with an almost complete shielding at temperatures as high as 88 K. In spite of the absence of quantitative measurements of transport critical currents, the presence of a complete shielding state a few degrees below the transition temperature shows a rather good grain quality and strong superconducting grain coupling.
Conclusions
Fig. 5. Optical image (1000x, etching in acetic acid) of a sintered sample showing a uniform microstructure containing macropores.
The simulated injection moulding process, as can be seen from the values of the transition temperature onset and the transition width, has not affected either the superconducting properties or the superconducting coupling between grains. Therefore the injection moulding process can be considered a viable method for the fabrication of superconducting components.
60
Acknowledgements We would like to thank Dr M. Tului of C.S.M. and Professor S. Pace of Salerno University for useful discussions on the preparation of the YBCO powders and the magnetic measurements. a0
a0
a5
90
Temperature
01 a0
a5
95
90
Temperature
100
(K)
References
95
I
100
(K)
Fig. 6. In-phase (a) and in-quadrature (b) components e.m.f. signal of sample no. 2. The transition temperature is 92.5 K and the transition width is 2 K.
of the onset
J.K. Wright, R.M. Thomson and J.R.G. Evans, J. Muter. Sci., 25 (1990) 149. C. His, N. Chardon, R. Kuentzler and S. Vilminot, J. Mater. Sci., 26 (1991) 4829. N. McN. Alford, J.D. Birchall, W.J. Clegg, M.A. Harmer, K. Kendall and D.H. Jones, J. Muter. Sci., 23 (1988) 761. R.M. German, Powder Injection Molding, Metal Powder Industries Federation, Princeton, NJ, 1990, p. 125. M.J. Edirisinghe and J.R.G. Evans, Int. J. High Technol. Cerum., 2 (1986) 1. M.J. Edirisinghe and J.R.G. Evans, Znt. J. High Technol. Ceram., 2 (1986) 249. S.J. Stedman, J.R.G. Evans and J. Woodthorpe, Ceram. IN., 16 (1990) 107.
133
8 S.B. Newcomb, Proc.
B.A. Glowacki, J.E. Ewetts and W.M. Stubbs, Sot., 40 (1988) 161. M. Tului and L. Risso, Ital. Pat. Appl. RM
Br. Ceram.
9 S. Fortunati, 9IAOOO555
(1991).
10 L. Risso, S.J. Stedman 924000794
and B. Vicenzi, Ital. Pat. Appl.
RM
(1992).
11 F. Celani, S. Pace, N. Sparvieri, A. Saggese, C. Giovannella and R. Messi, Mater. Chem. Phys., 25 (1990) 43. 12 Powder Diffraction File Search Manual, Vol. 2, No. 3, International Center for Diffraction Data, Swarthmore, PA, 1987, p. 192. 13 T. Aselage and K. Keefer, J. Mater. Rex, 3 (1988) 1279.
14 G.A. Costa, M. Ferretti 91 (1988)
and G.L. Olcese, J. Ctysr. Growth,
392.
15 B.J. Lee and D.N. Lee, J. Am. Ceram. Sot., 72 (1989) 314. 16 B. Lagomarsino, Thesis, University of Genoa (Italy), Chemical Engineering Department, 1991. 17 J.V. Milewski, Adv. Ceram. Mater., 1 (1986) 36. 18 D.R. Clarke, T.M. Shaw and D. Dimos, J. Am. Ceram. Sot., 72 (1989) 1103. 19 N. Klein, G. Miiller, H. Piel, B. Roas, L. Shultz, U. Klein and M. Peiniger, Appl. Phys. Lett., 54 (1989) 757. 20 F. Celani, A. Saggese, V. Calzona, M. Putti, L. Liberatori, S. Pace, B. Polichetti and R. Scafuro, Physica C, 162 (1989) 903.
Chemistry
and Physics,
36 (1993)
129
129-133
Plastic shaping of ceramic superconducting L. Risso, Centro
S.J. Stedman
Sviluppo
Materiali
discs
and B. Vicenzi
SPA, A.d.R.
Genova,
Corso
Perrone
24/a,
16152
Genoa
84081
Baronissi
(Italy)
A. Saggese Department
(Received
of Physics,
November
University
of Salerno,
20, 1992; accepted
via Salvatore
Allende,
(Sa)
(Italy)
April 9, 1993)
Abstract Samples of YBa&u,O,_, high-T, superconductor were prepared by simulating ceramic injection moulding technology, which involves mixing organic binders and ceramic powders to permit the fabrication of different shapes. A thermal debinding process was used to remove the organic binder. X-ray diffraction analysis demonstrated that the YBCO powder was not altered by the mixing, pressing and debinding processes. Susceptibility measurements taken after sintering showed that the transition onset is at 92.5 K, with a transition width of 2 K, proving the absence of process-induced deterioration of both superconducting properties and grain couplings.
Introduction
Extrusion [l], tape casting [2] and uniaxial pressing [3] have been used as shaping technologies for the production of ceramic superconductors. The objective of the present work was to simulate the ceramic injection moulding process. The ceramic injection moulding process has the capacity to produce complex shapes, but involves the addition of a large amount of organic binder additives [4]. The systematic development of a suitable binder vehicle for ceramic injection moulding involves consideration of all stages of the ceramic injection moulding process. Consequently, the binder formulation must provide the desired rheological properties for mixing and moulding [5, 61. Moreover, the desired debinding characteristics should allow the removal of the binder vehicle without producing defects in the particle arrangement [7] and grain boundary impurities [8], which reduce the critical current of sintered superconducting ceramics. Hence, in this work the ceramic injection moulding process was simulated by mixing YBCO superconducting powders and organic additives. The effects of the organic additives on the superconducting properties were evaluated during each stage of the process.
Experimental
The YBCO superconducting powders were produced using a pyrolytic technique [9]. The ceramic powders
0254-0584/93/$6.00
produced were characterized by scanning electron microscopy (SEM), X-ray diffraction, specific surface area measurements (BET analysis), helium pycnometry, differential thermal analysis (DTA) and thermogravimetric analysis (TGA). The carbon content of the superconducting powders was measured using a Leco Industries model cs-125 carbon analyzer. The polymeric binder vehicle system consisted of 40 wt.% atactic polypropylene, 15 wt.% hydrocarbon waxes, 15 wt.% ethyl vinyl acetate, 20 wt.% lubricating wax, and 10 wt.% stearic acid [lo]. The polymeric and wax granules were cooled below their glass transition temperatures by immersion in liquid nitrogen and then granulated to produce a finer particle size. The granulated organic binder particle dimensions were less than 100 micrometer. The superconducting YBCO ceramic powder and the granulated organic binder were dry mixed together in a small high-speed mill for 1 min. The mixture consisted of 89.5 wt.% YBCO powder and 10.5 wt.% organic binder, corresponding to approximately 55 vol.% YBCO powder and 45 vol.% organic binder. This feedstock was subsequently used to prepare small pressed discs in a heated mould with a plunger over a temperature range 140-165 “C and pressures in the range 1.94-37.45 MPa. The pressed discs were cooled under pressure and then extracted from the mould. The densities of the pressed discs were calculated from their weights and dimensions. The microhomogeneity of the pressed discs was examined by SEM analysis of a fracture surface and the orientation of
0 1993 - Elsevier Sequoia.
All rights reserved
130
the YBCO particles during pressing by X-ray diffraction. The organic binder was removed by slow heating (0.25 “C min-’ up to 500 “C) in air; the discs were supported in an yttria powder bed to avoid contamination. The weight loss during debinding was used to calculate the exact powder volume loading and hence the porosity of the as-pressed discs. The debound discs were inspected visually after debinding and then sintered. The debound samples were heated at 1 “C mine1 up to 1000 “C, held at 1000 “C for 6 h, cooled at 0.5 “C min-’ to 450 “C and then held at this temperature for 8 h with an oxygen flow rate of 1 1 min-‘. After sintering, the linear shrinkage and the densities of the sintered discs were measured. The microstructure of the sintered discs was examined by SEM and optical microscopy. The crystallographic phases were identified by X-ray diffraction (Cu Ka,). The superconducting shielding properties of the sintered discs were measured using a two-coil a.c. susceptometer. The primary coil is in contact with one side of the superconducting disc and generates an a.c. magnetic field. On the other side, the magnetic flux that has partially penetrated through the disc induces an e.m.f. signal on the secondary coil. The temperature variations of the amplitudes of the in-phase and inquadrature components of the induced e.m.f. signal, proportional to the real and imaginary parts, respectively, of the complex susceptibility, can be used to determine the superconducting transition temperature and the transition width [ll]. The apparatus was calibrated with lead discs of various dimensions, at room and liquid helium temperatures, so the values of the components of the e.m.f. signal corresponding to zero and full shielding are known. The YBCO samples were measured using an a.c. magnetic field with a peak value of 1 gauss and a frequency of 17 Hz.
Results
and discussion
The YBCO powders produced by the pyrolytic process are shown in Fig. 1. The powders had an irregular particle morphology and did not contain agglomerates which could disrupt the development of a uniform microstructure during sintering. X-ray diffraction analysis confirmed that the YBCO powders were monophase orthorhombic [12]. The BET surface area was 1.2 m* g -l, and the density of the powders, measured by helium pycnometry, was 6.5 g cme3. The residual carbon content of the powders was 0.11 wt.%. The DTA plot shown in Fig. 2 revealed the peritectic melting point of the YBa,Cu,O,_, phase to the Y2BaCuOS phase and liquid to be 1025 “C in oxygen, in agreement with the results of other researchers [13,
Fig. 1. SEM image (500 X ) of pyrolytic YBa,Cu,O,_, used in this experiment.
p(Jwders
140
Fig. 2. Differential thermal analysis of the powders oxygen; the heating rate was 10 K min-‘.
in flowing
141. A small peak at 949 “C was also noted, which was considered to be due to the eutectic of the YBCO system [15] or to the decomposition of a small residual amount of barium carbonate not detectable by X-ray diffraction. A small weight loss was detected at the same temperature by TGA, and this suggested that the peak was due to BaCO, decomposition. The TGA results also showed that YBCO oxygen loss began at 400 “C and hence indicated that holding the temperature at 450 “C after sintering was an appropriate process for replacing the oxygen lost during sintering and for allowing oxygen diffusion. These powders have been sintered to high densities 1161 and therefore were known to be suitable for this experimental program. The density of the pressed discs depended strongly on the pressing conditions, that is, the applied pressure, pressing time and the temperature of the mould. The results are summarised in Table 1. All the discs were free from visible defects, laminations or cracks after pressing, and hence these discs were suitable for the subsequent steps of debinding and sintering.
131 TABLE Sample no.
1
2 3 4 5 6 7
1. Processing
data and disc densities
Pressure (MPa)
1.9 1.9 18.7 18.7 25.0 25.0 37.5
Pressing time
Die temperature
Density after pressing
(s)
(“C)
60 60 30 60 60 60 60
165 170 170 170 170 170 170
Note: the theoretical density of YBa,Cu,O,_, “Used for microscopy.
of a polished
Density after sintering (g cm-‘)
3.19 2.99 3.34 3.45 3.63 3.38 3.51
2.78 2.57 2.92 3.06 a 3.19 3.25
3.65 3.47 3.81 3.79 a a a
is 6.38 g cmW3.
Optical examination of a polished disc face that was normal to the pressing direction indicated that orientation of the particles could have occurred (Fig. 3); this could have been due to the sample polishing procedure. However, this preferred orientation on the disc surface normal to the pressing direction was confirmed by comparative X-ray analysis of an isotropic sample (YBCO powder) and an as-pressed disc face. The ratio of peaks a and b of Fig. 4 for the isotropic sample was 1.47, whilst the a/b peak ratio for the pressed disc was 4.71. Thus the (010) index of the YBCO crystal was perpendicular to the pressing direction. A preferred orientation of anisotropic particles is a common feature of plastic formation of ceramics [17], but no attempt was made to quantitatively measure the particle orientation. The removal of the binder by thermal degradation did not introduce any visible defects, and the discs were strong enough to be handled. Furthermore, Xray diffraction analysis of powder from the debound disc did not reveal the presence of barium carbonate or other new phases compared with the starting YBCO powder. Barium carbonate could have formed at tem-
Fig. 3. Optical image (500X) polymer-powder blend.
(g cm-3)
Density after debinding (g cm-‘)
disc of the pressed
A
/ \
t
i’ \
Fig. 4. Comparison between two lines in the X-ray diffraction spectra of powders (---) and of as-pressed discs (-). Peak a is the (060) line, and peak b is the (200) line. The a/b ratio is 1.47 for powders and 4.71 for the discs, showing surface grain orientation in the latter.
peratures below 500 “C, but it decomposes at much higher temperatures, and as barium carbonate was not detected after debinding it is clear that barium carbonate was not formed as a transient phase [18]. Hence, it was possible to remove the organic binder formulation without altering the YBCO powder characteristics. There was a slight increase in the carbon content, from 0.11 wt.% to 0.37 wt.%, during the debinding process, but even after sintering barium carbonate was not detected, and the final carbon content of the sintered disc was the same as that of the starting powder, indicating that the excess carbon burned off during the sintering cycle. The sintered discs did not achieve very high sintered densities (Table 1). This was probably due to the low green density, which would require a linear sintering shrinkage of 21.16% in order for the theoretical density to be obtained. In fact, uniaxially pressed discs of this YBCO powder with green densities of 57.1% (referred to the theoretical density) have been sintered up to 83.5% [16]. It was not possible to sinter at higher
132
temperatures to improve the density because of the irreversible phase transformations to nonsuperconducting phases that occur above 1025 “C, as mentioned earlier [15]. A higher green density could be obtained if the binder/powder ratio could be reduced and higher moulding pressures used. The sintered discs still contained macropores left by some relatively large binder particles removed by thermal degradation, but in the homogeneous regions between the pores the sintered YBCO microstructure consisted of grains of uniform dimensions (Fig. 5). Xray analysis of the sintered discs confirmed the monophase orthorhombic crystal structure, but also revealed that the preferred orientation of the YBCO grains of
the green discs had disappeared during sintering. Preferred orientation of the YBCO grains in the same crystallographic direction would allow a higher critical current density [19] and therefore more advantageous properties. The superconducting transition temperature was measured using a two-coil a.c. susceptometer [20], as previously mentioned. The amplitudes of the in-phase and in-quadrature components of the e.m.f. signal for disc no. 2 (see Table l), with a diameter of 9 mm and a height of 4 mm, are shown in Fig. 6(a) and (b). For this sample the transition temperature onset is 92.5 K and the transition width is about 2 K. All of the other samples show the same behaviour, with an almost complete shielding at temperatures as high as 88 K. In spite of the absence of quantitative measurements of transport critical currents, the presence of a complete shielding state a few degrees below the transition temperature shows a rather good grain quality and strong superconducting grain coupling.
Conclusions
Fig. 5. Optical image (1000x, etching in acetic acid) of a sintered sample showing a uniform microstructure containing macropores.
The simulated injection moulding process, as can be seen from the values of the transition temperature onset and the transition width, has not affected either the superconducting properties or the superconducting coupling between grains. Therefore the injection moulding process can be considered a viable method for the fabrication of superconducting components.
60
Acknowledgements We would like to thank Dr M. Tului of C.S.M. and Professor S. Pace of Salerno University for useful discussions on the preparation of the YBCO powders and the magnetic measurements. a0
a0
a5
90
Temperature
01 a0
a5
95
90
Temperature
100
(K)
References
95
I
100
(K)
Fig. 6. In-phase (a) and in-quadrature (b) components e.m.f. signal of sample no. 2. The transition temperature is 92.5 K and the transition width is 2 K.
of the onset
J.K. Wright, R.M. Thomson and J.R.G. Evans, J. Muter. Sci., 25 (1990) 149. C. His, N. Chardon, R. Kuentzler and S. Vilminot, J. Mater. Sci., 26 (1991) 4829. N. McN. Alford, J.D. Birchall, W.J. Clegg, M.A. Harmer, K. Kendall and D.H. Jones, J. Muter. Sci., 23 (1988) 761. R.M. German, Powder Injection Molding, Metal Powder Industries Federation, Princeton, NJ, 1990, p. 125. M.J. Edirisinghe and J.R.G. Evans, Int. J. High Technol. Cerum., 2 (1986) 1. M.J. Edirisinghe and J.R.G. Evans, Znt. J. High Technol. Ceram., 2 (1986) 249. S.J. Stedman, J.R.G. Evans and J. Woodthorpe, Ceram. IN., 16 (1990) 107.
133
8 S.B. Newcomb, Proc.
B.A. Glowacki, J.E. Ewetts and W.M. Stubbs, Sot., 40 (1988) 161. M. Tului and L. Risso, Ital. Pat. Appl. RM
Br. Ceram.
9 S. Fortunati, 9IAOOO555
(1991).
10 L. Risso, S.J. Stedman 924000794
and B. Vicenzi, Ital. Pat. Appl.
RM
(1992).
11 F. Celani, S. Pace, N. Sparvieri, A. Saggese, C. Giovannella and R. Messi, Mater. Chem. Phys., 25 (1990) 43. 12 Powder Diffraction File Search Manual, Vol. 2, No. 3, International Center for Diffraction Data, Swarthmore, PA, 1987, p. 192. 13 T. Aselage and K. Keefer, J. Mater. Rex, 3 (1988) 1279.
14 G.A. Costa, M. Ferretti 91 (1988)
and G.L. Olcese, J. Ctysr. Growth,
392.
15 B.J. Lee and D.N. Lee, J. Am. Ceram. Sot., 72 (1989) 314. 16 B. Lagomarsino, Thesis, University of Genoa (Italy), Chemical Engineering Department, 1991. 17 J.V. Milewski, Adv. Ceram. Mater., 1 (1986) 36. 18 D.R. Clarke, T.M. Shaw and D. Dimos, J. Am. Ceram. Sot., 72 (1989) 1103. 19 N. Klein, G. Miiller, H. Piel, B. Roas, L. Shultz, U. Klein and M. Peiniger, Appl. Phys. Lett., 54 (1989) 757. 20 F. Celani, A. Saggese, V. Calzona, M. Putti, L. Liberatori, S. Pace, B. Polichetti and R. Scafuro, Physica C, 162 (1989) 903.