**2. Magnetic texturing**

## **2.1. Case of Y1Ba2Cu3O7-**<sup>δ</sup>

Rare earth-Ba2Cu3O7-δ crystals are aligned in a magnetic field at room temperature [13] and also when they are imbedded in liquid silver at high temperature [14]. A sufficient paramagnetic anisotropy remains at temperature superior to 1000 °C and the residual anisotropy energy of the crystals is larger than the energy associated with thermal disordering effects [15]. As mentioned previously, the anisotropy energy must also compensate the viscous force in the surrounding liquid where the crystal is free to rotate in a liquid with low resistance. The temperature window inducing a magnetic orientation lies between 1040°C and 1060°C under atmospheric pressure of O2. Below 1040°C, a solid matrix made up of Y1Ba2Cu3O7-δ (Y123) and Y2Ba1Cu1O5 (Y211) remains throughout the annealing treatment while the melting of Y123 above 1040°C leads to a liquid containing precipitates of Y211. Grains containing inclusions of this secondary phase are partially aligned below 1040 °C with their ab-plane perpendicular to the direction of the annealing field, indicating that a partial melting of the precursor takes place during processing. Both X-ray and magnetization reveal that a weak orientation occurs. For samples prepared at temperature above 1040°C, the texturing is very efficient. The sample is cut and oriented to reveal faces perpendicular to the direction of the magnetic field. X-ray spectrum is taken as a first indication of the orientation. A measure of the orientation P00l is given in Figure 1 as a function of annealing temperature [16].

172 Superconductors – Materials, Properties and Applications

moment of forces can be expressed as:

does not destroy the presence of nuclei.

**2. Magnetic texturing** 

**2.1. Case of Y1Ba2Cu3O7-**<sup>δ</sup>

The magnetic field was successfully used to improve texturing in crystals and alloys. The texturing of several alloys under a magnetic field was carefully studied |12]. It depends on the magnetic properties of the crystallizing nuclei and those of the melt. The mechanical force moment allowing the rotation of crystals along the direction of the magnetic field also depends on the degree of homogeneity of the magnetic field. In a homogeneous field, the texture can be induced in a magnetic isotropic crystal due to the moment of forces deriving from the demagnetizing factor and the shape anisotropy of the nucleus. However, at high temperatures and for low susceptibility, this contribution will be neglected. On the contrary, in an anisotropic crystal, the orientation arises from the anisotropic crystal characterized by an difference of magnetic susceptibility Δχ along two mutually perpendicular axes and the

> ܭ ൌ ௱ఞ ଶఓబ

below the liquidus line and to 20% of the crystallization interval.

where α is the angle between B and the axis with maximum χ. Under real conditions, there will be a competition between magnetic orientation forces, viscous forces, convective flows in the melt and interactions between crystals or interaction with the crucible. In some cases, the orienting effect may be limited or even negligible. The window in which the orientation can be induced must be carefully found. For an alloy, Mikelson and al. found that the temperature interval in which the orienting actions of the magnetic field are effective lies

In this article, we will focus on the texturing of high-Tc superconductors under a magnetic field where the main conditions exposed below are taken into account. The main difficulties of superconductors texturing reside in a non congruent melting of the compounds. It is the limiting factor of overheating since the phase diagram is generally complex involving a lot of transformations even above the liquidus temperature. A large overheating above the liquidus will usually lead to the formation of unwanted secondary phases during crystallization and prevent the recombination of the superconducting phase. The latter is usually taken as the critical value for the limit of overheating. The texturing under a magnetic field consist of finding the interval of overheating temperatures preventing the transformation of superconducting-phase nuclei in secondary phases and allowing a sufficient amount of liquid. Usually, the allowable overheating is a dozen of degrees above the melting temperature [7]. As will be shown below, this amount of applied overheating

Rare earth-Ba2Cu3O7-δ crystals are aligned in a magnetic field at room temperature [13] and also when they are imbedded in liquid silver at high temperature [14]. A sufficient paramagnetic anisotropy remains at temperature superior to 1000 °C and the residual anisotropy energy of the crystals is larger than the energy associated with thermal

(1) ߙʹ݅݊ݏܸଶܤ

**Figure 1.** R and P versus annealing temperature in Y1Ba2Cu3O7-<sup>δ</sup> bulk textured sample, where R=∆MH//HA/∆MH┴HA (∆M=M+-M is the hysteresis in the sample magnetization, M+ and M the values of the magnetization measured in an increasing and decreasing field respectively) and P00l =1-Γ where Γ=(Ihkl/I00l)o/( Ihkl/**I**00l)u. Ihkl is the intensity of the strongest forbidden non-(00l)line and I00l the intensity of a (00l) line in a X-ray diffraction spectrum. The superscripts o and u indicate that measurements were performed on oriented and unoriented samples respectively [16].

In Figure 1, one can note the sharp increase of the orientation for a temperature above 1040°C while the latter gradually decreases above 1050°C. The orientation is deduced from the comparison of R, the hysteresis in sample magnetization ratio between parallel and perpendicular field. R=∆MH//HA/∆MH┴HA and ∆M=M+-M is the hysteresis in the sample magnetization where M+ and M are the values of the magnetization measured in an

increasing and decreasing field respectively. Large samples can be oriented by magnetic texturing. Superconducting critical current properties of such bulk materials were studied [17-20]. The irreversible magnetization data also indicates steady improvement in the superconducting properties of the material with increasing annealing temperature up to 1055°C. The induced orientation within the sample is directly dependent on the magnetic field value as shown in Figure 2. In order to obtain a large degree of induced anisotropy, the magnetic field must be applied continuously in the appropriate temperature regime (from the maximal temperature down to a temperature below the liquidus line), during which crystals are still free to rotate. Steady increase in the induced anisotropy is observed when the magnetic field is increased.

Magnetic Texturing of High-Tc Superconductors 175

: 2CuO→Cu2O +1/2O2 resulting in a rapid decrease of the susceptibility. When the reduction of the magnetic copper is taken into account via the mole fraction loss of oxygen, the susceptibility corrected obeys a classical Curie law as a function of temperature. Consequently above 1070°C, largely above the peritectic temperature, the nuclei composition changes since the melt is transformed and non-superconducting nuclei of secondary phases appear. It is the reason why the magnetic field is no more effective at very high temperature. The value of the supercooling can also be measured while cooling down.

**Figure 3.** Magnetic susceptibility χ of Y1Ba2Cu3O7-δ at high temperature. Usually a change in the slope is

760 800 840 880 920 960 1000 1040

**T(°C)**

Nowadays, top-seed melt texturing is mainly used to texture Y123 pellet where the crystallographic orientation is given by the crystal seed. The largest intrinsic nuclei which are involved in magnetic texturing are melted with an overheating temperature of about 1080 °C in order to be sure that crystallization is induced by the seed and not by intrinsic nuclei. In this technique, the growth of a main Y123 grain starts from a seed which has a higher peritectic decomposition temperature and crystallographic parameters close to Y-123.

In Bi2Sr2CaCu2O8+x (Bi2212), the achievement of the primary phase after melting is not easy. The system Bi2O3-SrO-CaO-CuO is composed of 4 elements. The phase equilibrium is complex and the recombination of the primary phase from the melt is not always reversible when the composition in the liquid is too much changed. Small variations of composition or temperature induce large changes in the equilibrium and distributions of phases. Bi2212 phase can be in equilibrium with most of the system composites Bi2O3-SrO-CaO-CuO. Up to fifteen four-phase equilibriums were mentioned in the literature. The melting give rises to

attributed to a change of phase or to a change in composition.

4 10-7

4,5 10-7

5 10-7

χ**(emu/g)**

5,5 10-7

6 10-7

**2.2. Case of Bi2Sr2CaCu2O8+x** 

*2.2.1. Pellets of Bi2Sr2CaCu2O8+x*

Pellets up to 10 cm of diameter were grown with this technique [21].

**Figure 2.** Variation in the magnitude of the magnetization hysteresis ΔMmax(●) and the ratio R(◊) with the strength of the annealing field at TA=1055°C in Y1Ba2Cu3O7-<sup>δ</sup> bulk textured sample. The magnetic field was applied at all temperatures above 800°C [16].

It was also shown that the magnetic force exerted on a sample (*m*χ*Ha*×d*Ha/*d*z* ) placed in a field gradient (dHa/dz) allows the measurement of the magnetic susceptibility via the weight measurement on an electronic balance. The resulting curves provide information on the fusion, the solidification and the oxygen exchange. A typical curve presenting the magnetic susceptibility variations corrected from the oxygen weight change as a function of temperature is given in Figure 3.

When the sample is heated, the susceptibility first decreases following a Curie law behavior. When Y123 melts, between 1000 °C and 1040°C, the susceptibility increases because the melt is more paramagnetic than the phase. In the liquid state, above 1040°C, the oxygen stoichiometry changes and the mean ratio of copper valence is changed in the melt. The magnetic Cu2+ ions are progressively reduced in non-magnetic ions Cu+ through the reaction : 2CuO→Cu2O +1/2O2 resulting in a rapid decrease of the susceptibility. When the reduction of the magnetic copper is taken into account via the mole fraction loss of oxygen, the susceptibility corrected obeys a classical Curie law as a function of temperature. Consequently above 1070°C, largely above the peritectic temperature, the nuclei composition changes since the melt is transformed and non-superconducting nuclei of secondary phases appear. It is the reason why the magnetic field is no more effective at very high temperature. The value of the supercooling can also be measured while cooling down.

**Figure 3.** Magnetic susceptibility χ of Y1Ba2Cu3O7-δ at high temperature. Usually a change in the slope is attributed to a change of phase or to a change in composition.

Nowadays, top-seed melt texturing is mainly used to texture Y123 pellet where the crystallographic orientation is given by the crystal seed. The largest intrinsic nuclei which are involved in magnetic texturing are melted with an overheating temperature of about 1080 °C in order to be sure that crystallization is induced by the seed and not by intrinsic nuclei. In this technique, the growth of a main Y123 grain starts from a seed which has a higher peritectic decomposition temperature and crystallographic parameters close to Y-123. Pellets up to 10 cm of diameter were grown with this technique [21].

### **2.2. Case of Bi2Sr2CaCu2O8+x**

174 Superconductors – Materials, Properties and Applications

the magnetic field is increased.

field was applied at all temperatures above 800°C [16].

0

5

10

Δ

**M**

**(105A/m)**

**max**

15

20

temperature is given in Figure 3.

It was also shown that the magnetic force exerted on a sample (*m*

increasing and decreasing field respectively. Large samples can be oriented by magnetic texturing. Superconducting critical current properties of such bulk materials were studied [17-20]. The irreversible magnetization data also indicates steady improvement in the superconducting properties of the material with increasing annealing temperature up to 1055°C. The induced orientation within the sample is directly dependent on the magnetic field value as shown in Figure 2. In order to obtain a large degree of induced anisotropy, the magnetic field must be applied continuously in the appropriate temperature regime (from the maximal temperature down to a temperature below the liquidus line), during which crystals are still free to rotate. Steady increase in the induced anisotropy is observed when

**Figure 2.** Variation in the magnitude of the magnetization hysteresis ΔMmax(●) and the ratio R(◊) with the strength of the annealing field at TA=1055°C in Y1Ba2Cu3O7-<sup>δ</sup> bulk textured sample. The magnetic

01234567

R

<sup>Δ</sup>Mmax(10<sup>5</sup>

A/m)

**H(T)**

field gradient (dHa/dz) allows the measurement of the magnetic susceptibility via the weight measurement on an electronic balance. The resulting curves provide information on the fusion, the solidification and the oxygen exchange. A typical curve presenting the magnetic susceptibility variations corrected from the oxygen weight change as a function of

When the sample is heated, the susceptibility first decreases following a Curie law behavior. When Y123 melts, between 1000 °C and 1040°C, the susceptibility increases because the melt is more paramagnetic than the phase. In the liquid state, above 1040°C, the oxygen stoichiometry changes and the mean ratio of copper valence is changed in the melt. The magnetic Cu2+ ions are progressively reduced in non-magnetic ions Cu+ through the reaction

χ*Ha*×

0

0,5

1

1,5

2

**R**

2,5

3

3,5

4

d*Ha/*d*z* ) placed in a

### *2.2.1. Pellets of Bi2Sr2CaCu2O8+x*

In Bi2Sr2CaCu2O8+x (Bi2212), the achievement of the primary phase after melting is not easy. The system Bi2O3-SrO-CaO-CuO is composed of 4 elements. The phase equilibrium is complex and the recombination of the primary phase from the melt is not always reversible when the composition in the liquid is too much changed. Small variations of composition or temperature induce large changes in the equilibrium and distributions of phases. Bi2212 phase can be in equilibrium with most of the system composites Bi2O3-SrO-CaO-CuO. Up to fifteen four-phase equilibriums were mentioned in the literature. The melting give rises to secondary non-superconducting phases that are not entirely consumed during solidification. There is a change in the liquid structure about 30 °C above the melting temperature of Bi2212 and Bi2223 in air as shown in Figure 4. [22,23]. The solidification of Bi2212 from the melt requires a precise control of the temperature and composition.

Magnetic Texturing of High-Tc Superconductors 177

to 7 mm. The sample is melt-processed in a furnace inserted in the room temperature bore of a magnet. The melting temperature is determined by measuring the magnetic susceptibility

The study of the magnetic susceptibility represented as a function of temperature in Figure 5 indicates the beginning of the melting process at 877°C and the end of the decomposition of the Bi2212 phases at 905°C [24]. Above 905°C, the liquid composition is stable and no phase is transformed so the magnetic susceptibility follows the classical Curie law and is proportional to 1/T. At 905°C, formation of the phases CuO and (Sr,Ca)O are observed. When the temperature is decreased, the susceptibility first increases before reaching a plateau. The beginning of the solidification can be seen when χ suddenly decreases. During cooling, the phase transformation at 740°C can be noted on the graph and corresponds to the solidification of the eutectic Cu2O/(Sr,Ca)3Bi2O6. In the inset figures, the susceptibility cycle was done for different maximal temperatures. One can observe that the susceptibility plateau is reached only for a maximal temperature of 892°C. Below 871°C, the susceptibility decreases during the cooling because the solidification process immediately takes place. The overheating temperature is not large enough in the interval 877-892°C. When the temperature is superior to 892°C, the susceptibility will reach the plateau before decreasing

**Figure 5.** Magnetic susceptibility of Bi2Sr2CaCu2O8+x at high temperature. Inset: magnetic susceptibility for different maximal temperatures. When the maximal temperature reached is inferior to 892°C, the magnetic susceptibility χ decreases during cooling. When the temperature reached is superior to 892°C,

650 700 750 800 850 900 950 1000

877°C melting

905°C

871°C solidification

**T(°C)**

the susceptibility is first constant during cooling before decreasing [24].

0.5

1

1.5

2

χ**(10-7emu/g)**

2.5

3 3.2 3.4 3.6 3.8 4

3

χ**(10-7emu/g)**

3.5

at the solidification. The liquidus temperature is equal to 892 °C [6].

T(°C)

850 860 870 880 890 900

T(°C)

T(°C)

as a function of temperature [6, 24,28].

**Figure 4.** Part of the phase diagram of Bi1.6Pb0.4Sr2Can-1CunOx as described in [23] for n varying between 1.5 and 3 and temperatures between 800 and 950°C.Phase {1} Bi2Sr2Ca0Cu1Ox (n=1) ; phase {2} Bi2Sr2Ca1Cu2Ox (n=2) ; phase {3} Bi2Sr2Ca2Cu3Ox (n=3); phase {2:1} (Sr,Ca)2Cu1 ;phase {14:24} (Sr,Ca)14Cu24 ; phase {1:2} (Sr,Ca)1Cu2.

In BSSCO bulk compounds, several methods can be used and eventually mixed to induce alignment in the c-axis and overcome weak links. The magnetic field was successfully applied in this compound and important critical current densities were obtained [24-27]. A technique combining magnetic melt processing and hot forging was developed to improve the critical current densities in Bi2212 [28]. The magnetic field acts as an orienting tool while the pressing reinforces the orientation and the density. The addition of MgO in Bi2212 allows the material to keep high viscosity in the melt state at high temperature, making it compatible with a magnetic field texturing followed by hot forging. Moreover the MgO or Ag addition induces an improvement of superconducting properties.

Bi2212 powder added with 10 wt.% MgO is used as a starting powder. This mixture is cold pressed under an uniaxial pressure of 1 GPa in 20-mm diameter pellet with a thickness of 5 to 7 mm. The sample is melt-processed in a furnace inserted in the room temperature bore of a magnet. The melting temperature is determined by measuring the magnetic susceptibility as a function of temperature [6, 24,28].

176 Superconductors – Materials, Properties and Applications

(Sr,Ca)14Cu24 ; phase {1:2} (Sr,Ca)1Cu2.

secondary non-superconducting phases that are not entirely consumed during solidification. There is a change in the liquid structure about 30 °C above the melting temperature of Bi2212 and Bi2223 in air as shown in Figure 4. [22,23]. The solidification of Bi2212 from the

**Figure 4.** Part of the phase diagram of Bi1.6Pb0.4Sr2Can-1CunOx as described in [23] for n varying between

In BSSCO bulk compounds, several methods can be used and eventually mixed to induce alignment in the c-axis and overcome weak links. The magnetic field was successfully applied in this compound and important critical current densities were obtained [24-27]. A technique combining magnetic melt processing and hot forging was developed to improve the critical current densities in Bi2212 [28]. The magnetic field acts as an orienting tool while the pressing reinforces the orientation and the density. The addition of MgO in Bi2212 allows the material to keep high viscosity in the melt state at high temperature, making it compatible with a magnetic field texturing followed by hot forging. Moreover the MgO or

Bi2212 powder added with 10 wt.% MgO is used as a starting powder. This mixture is cold pressed under an uniaxial pressure of 1 GPa in 20-mm diameter pellet with a thickness of 5

1.5 and 3 and temperatures between 800 and 950°C.Phase {1} Bi2Sr2Ca0Cu1Ox (n=1) ; phase {2} Bi2Sr2Ca1Cu2Ox (n=2) ; phase {3} Bi2Sr2Ca2Cu3Ox (n=3); phase {2:1} (Sr,Ca)2Cu1 ;phase {14:24}

Ag addition induces an improvement of superconducting properties.

melt requires a precise control of the temperature and composition.

The study of the magnetic susceptibility represented as a function of temperature in Figure 5 indicates the beginning of the melting process at 877°C and the end of the decomposition of the Bi2212 phases at 905°C [24]. Above 905°C, the liquid composition is stable and no phase is transformed so the magnetic susceptibility follows the classical Curie law and is proportional to 1/T. At 905°C, formation of the phases CuO and (Sr,Ca)O are observed. When the temperature is decreased, the susceptibility first increases before reaching a plateau. The beginning of the solidification can be seen when χ suddenly decreases. During cooling, the phase transformation at 740°C can be noted on the graph and corresponds to the solidification of the eutectic Cu2O/(Sr,Ca)3Bi2O6. In the inset figures, the susceptibility cycle was done for different maximal temperatures. One can observe that the susceptibility plateau is reached only for a maximal temperature of 892°C. Below 871°C, the susceptibility decreases during the cooling because the solidification process immediately takes place. The overheating temperature is not large enough in the interval 877-892°C. When the temperature is superior to 892°C, the susceptibility will reach the plateau before decreasing at the solidification. The liquidus temperature is equal to 892 °C [6].

**Figure 5.** Magnetic susceptibility of Bi2Sr2CaCu2O8+x at high temperature. Inset: magnetic susceptibility for different maximal temperatures. When the maximal temperature reached is inferior to 892°C, the magnetic susceptibility χ decreases during cooling. When the temperature reached is superior to 892°C, the susceptibility is first constant during cooling before decreasing [24].

The melting temperature being defined, the process can be optimized. In the process, the temperature can reach 1100°C and the vertical magnetic field used was 5.7 Tesla. The maximal processing temperature of 892°C was consequently defined as the optimum annealing temperature. Hot forging can be applied to bulk Bi2212/MgO magnetic melt processed. Hot Forging at 25 MPa during 2 hours at 880°C induces an increase in the orientation degree and of the density [28].

Magnetic Texturing of High-Tc Superconductors 179

Tm −40°C

840°C 6°C/h

820°C

**Bi 2.0 short samples Bi2.3 short samples Bi 2.0 long samples Bi 2.3 long samples**

f

measurement at 4.2 K, self field

/

**Figure 6.** Generalized temperature profile for MMP Bi2Sr2CaCu2O8+x tapes in magnetic field up to 15T [26]. Tmax is the maximal annealing temperature. The magnetic field is applied at Tmax and shut down at

Time

Tmax×12mn

18°C/h 48°C/h

Tm −10°C

H

**Figure 7.** Critical current density in Bi2Sr2CaCu2O8+x Ag-sheathed tapes as a function of maximal temperatures [34] for two different compositions. Pre-reacted Bi2Sr2CaCu2O8+x powders are used.

882 884 886 888 890 892 894 896 898 900 902 904 906 **T max (°C)**

Tmax - 40°C. The atmosphere is flowing O2.

0

100

200

300

400 500

600

**Je (A/mm2)** 

700

800

900

1000

500°C

120°C/h

Temperature

The magnetic melt processing performed 15°C above the onset of melting leads to the best orientation degree and thus to the best Jc. At this temperature, as was seen previously with YBaCuO the remaining magnetic susceptibility is still large enough to overcome the thermal disordering effect and there is enough liquid to allow free rotation of the crystallites. The anisotropy of susceptibility at 800 °C is equal to 4×10-8 emu/g [6]. For a magnetic field of 5.7T, and a temperature of 800°C, the crystals which are aligned in magnetic fields below 892 °C are larger than 10-21m3 as determined by (1). Above this temperature, the texture is gradually lost because of the transformation of Bi2212 nuclei in secondary phases leading to a difficulty in recombining the 2212 phase.

The results are consistent with the one of H. Maeda et al. where Bi2212 bulks and tapes were processed by the MMP in magnetic fields up to 15 T following the heat treatment shown in figure 6 [26]. A texture is also developed due to the anisotropy of magnetic susceptibility. The degree of texture and the anisotropy factor in magnetization increase almost linearly with the magnetic field strength during MMP. The anisotropy factor in magnetization reaches 3.2 and 6.5 at 13 T in Bi(Pb)2212 bulks and Ag-doped Bi2212 bulks respectively. For bulk materials, the doping of some fine particles, which induce melting and crystal nucleation, is required to achieve highly textured structures by MMP. The transport critical current density Jc and the transport critical current Ic of Bi2212 tapes increase with increasing Ha due to the texture development. These results indicate that MMP is effective to enhance the texture development and Jc values for Bi2212 bulks and tapes with thick cores, making it possible to fabricate tapes with high Ic.

These observations are also consistent with the fact that above the melting point, remaining nuclei subsist that tend to be aligned in the direction of the magnetic field when they are cooled in the window of solidification. However, leaving aside the difficulty to form different phases and to lose the initial composition when the overheating is too large, exceeding too much the melting temperature reduces the presence of the intrinsic nuclei which cannot act as growth crystals during solidification and the orientation and texture are progressively lost when the temperature is 30 °C above 892 °C.

## *2.2.2. Ag-sheated tapes of Bi2Sr2CaCu2O8+x*

Magnetic texturing of Ag-sheathed tapes was studied in very high fields and lead to high critical current densities [27,29-32]. E. Flahaut demonstrated in his PhD the possibility to texture highly homogeneous Bi2212 tapes by moving the tape in the furnace and using a continuous melting process with large critical currents (Jc = 230kA/cm2 at 4.2K, self field) [33].

orientation degree and of the density [28].

a difficulty in recombining the 2212 phase.

possible to fabricate tapes with high Ic.

*2.2.2. Ag-sheated tapes of Bi2Sr2CaCu2O8+x*

[33].

progressively lost when the temperature is 30 °C above 892 °C.

The melting temperature being defined, the process can be optimized. In the process, the temperature can reach 1100°C and the vertical magnetic field used was 5.7 Tesla. The maximal processing temperature of 892°C was consequently defined as the optimum annealing temperature. Hot forging can be applied to bulk Bi2212/MgO magnetic melt processed. Hot Forging at 25 MPa during 2 hours at 880°C induces an increase in the

The magnetic melt processing performed 15°C above the onset of melting leads to the best orientation degree and thus to the best Jc. At this temperature, as was seen previously with YBaCuO the remaining magnetic susceptibility is still large enough to overcome the thermal disordering effect and there is enough liquid to allow free rotation of the crystallites. The anisotropy of susceptibility at 800 °C is equal to 4×10-8 emu/g [6]. For a magnetic field of 5.7T, and a temperature of 800°C, the crystals which are aligned in magnetic fields below 892 °C are larger than 10-21m3 as determined by (1). Above this temperature, the texture is gradually lost because of the transformation of Bi2212 nuclei in secondary phases leading to

The results are consistent with the one of H. Maeda et al. where Bi2212 bulks and tapes were processed by the MMP in magnetic fields up to 15 T following the heat treatment shown in figure 6 [26]. A texture is also developed due to the anisotropy of magnetic susceptibility. The degree of texture and the anisotropy factor in magnetization increase almost linearly with the magnetic field strength during MMP. The anisotropy factor in magnetization reaches 3.2 and 6.5 at 13 T in Bi(Pb)2212 bulks and Ag-doped Bi2212 bulks respectively. For bulk materials, the doping of some fine particles, which induce melting and crystal nucleation, is required to achieve highly textured structures by MMP. The transport critical current density Jc and the transport critical current Ic of Bi2212 tapes increase with increasing Ha due to the texture development. These results indicate that MMP is effective to enhance the texture development and Jc values for Bi2212 bulks and tapes with thick cores, making it

These observations are also consistent with the fact that above the melting point, remaining nuclei subsist that tend to be aligned in the direction of the magnetic field when they are cooled in the window of solidification. However, leaving aside the difficulty to form different phases and to lose the initial composition when the overheating is too large, exceeding too much the melting temperature reduces the presence of the intrinsic nuclei which cannot act as growth crystals during solidification and the orientation and texture are

Magnetic texturing of Ag-sheathed tapes was studied in very high fields and lead to high critical current densities [27,29-32]. E. Flahaut demonstrated in his PhD the possibility to texture highly homogeneous Bi2212 tapes by moving the tape in the furnace and using a continuous melting process with large critical currents (Jc = 230kA/cm2 at 4.2K, self field)

**Figure 6.** Generalized temperature profile for MMP Bi2Sr2CaCu2O8+x tapes in magnetic field up to 15T [26]. Tmax is the maximal annealing temperature. The magnetic field is applied at Tmax and shut down at Tmax - 40°C. The atmosphere is flowing O2.

**Figure 7.** Critical current density in Bi2Sr2CaCu2O8+x Ag-sheathed tapes as a function of maximal temperatures [34] for two different compositions. Pre-reacted Bi2Sr2CaCu2O8+x powders are used.

It was shown that the critical current density can be improved with this method compared to a standard static heat treatment. The continuous process is applied to the fusion stage, the most sensitive treatment where the temperature must be carefully controlled, in order to improve the homogeneity of the sample. There are two reasons why a continuous process improves the critical current. Firstly, it allows the whole tape to be subjected to the same maximal temperature which is not the case in a furnace where the temperature must be perfectly homogeneous along the sample volume. Secondly, the texture can be improved by the thermal gradient that induces an orientation of the crystallites along the silver sheath of each filament. The engineering critical current density of Bi2212 tapes at 4.2K is plotted as a function of maximum annealing temperature used in the melting process in figure 7 [34]. The critical current is measured by a classical four point method. It is a good representation of the superconducting and crystallographic properties of the samples. Usually, a poor critical current is coming from a poor connectivity between the grains resulting from a lack of orientation and accumulation of secondary phases at the grain boundaries. The critical current density is maximal at the melting temperature and in a very narrow window of temperatures above the melting temperature. The Bi2212 nuclei acting in the magnetic texturing are also present in the narrow sheaths and induce the crystal growth. When the overheating temperature is too large, the critical current density progressively decreases for all the reasons mentioned above.

Magnetic Texturing of High-Tc Superconductors 181

Multizone furnace

**Figure 8.** Dynamic heat process under a magnetic field. A multizone furnace is inserted in a superconducting coil reaching 5T. Flowing O2, maximal temperature and processing speed are

Pay off

O

Ic (0T) Ic(5T)

B

5T Coil horizontal field

Take up

**Figure 9.** Critical current in Ag sheathed tapes of Bi2Sr2CaCu2O8+x as a function of maximal annealing

890 891 892 893 894 895 896 897 898

**T(K)**

temperature measured at 4.2K with field and without field [35].

300

350

400

450

500

**Critical current Ic(A)**

550

600

650

important parameters that can be controlled [35].

### *2.2.3. Magnetic field texturing of 2212 tapes using a continuous displacement in the furnace*

Bi2212 multilament tapes were processed by the well-established powder-in-tube (PIT) technique. The tape is composed of 78 laments embedded in a Ag–Mg (0.2% Mg) sheath [35]. The tapes are different from the ones before. A dynamic heating process was inserted into a superconducting coil to enable a dynamic heat treatment under a magnetic eld (5 T) in order to obtain homogeneous and high critical current densities (figure 8).

The thermal treatment consists in a melting step for 250 s and a slow temperature decrease (0.1 °C s− 1). The atmosphere is 40% owing O2. The maximum applied eld is 5T. Critical current densities in Bi2212 tapes were measured in self field at 4.2K to compare the benefit of a magnetic field during the dynamic heat treatment. An increase of the critical current density is obtained under the application of a magnetic eld at T= 894 °C as can be seen in figure 9. The processing speed is around 1cm/min. The results are reproducible and this gain in current densities is observed for temperature around 894°C. The results are consistent with the ones previously presented for Y123 and Bi2212 bulk samples. The rotation of the crystallites under a magnetic field is possible when the liquid at the interface of the grains is sufficient and its viscosity is high which implies a relatively high temperature of the melt. Above 896°C, the refractory and non-superconductive phases tend to grow rapidly, limiting the critical current density. The dynamic heat treatment under a magnetic field offers several benefits. Firstly, the temperature is highly homogeneous. Secondly, no long treatments are necessary and the texture can be obtained homogenously and rapidly. This is also due to the facts that the nuclei causing the growth of the right phase subsist in the melt and don't need the recombination in the solid state that requires long time.

all the reasons mentioned above.

It was shown that the critical current density can be improved with this method compared to a standard static heat treatment. The continuous process is applied to the fusion stage, the most sensitive treatment where the temperature must be carefully controlled, in order to improve the homogeneity of the sample. There are two reasons why a continuous process improves the critical current. Firstly, it allows the whole tape to be subjected to the same maximal temperature which is not the case in a furnace where the temperature must be perfectly homogeneous along the sample volume. Secondly, the texture can be improved by the thermal gradient that induces an orientation of the crystallites along the silver sheath of each filament. The engineering critical current density of Bi2212 tapes at 4.2K is plotted as a function of maximum annealing temperature used in the melting process in figure 7 [34]. The critical current is measured by a classical four point method. It is a good representation of the superconducting and crystallographic properties of the samples. Usually, a poor critical current is coming from a poor connectivity between the grains resulting from a lack of orientation and accumulation of secondary phases at the grain boundaries. The critical current density is maximal at the melting temperature and in a very narrow window of temperatures above the melting temperature. The Bi2212 nuclei acting in the magnetic texturing are also present in the narrow sheaths and induce the crystal growth. When the overheating temperature is too large, the critical current density progressively decreases for

*2.2.3. Magnetic field texturing of 2212 tapes using a continuous displacement in the furnace* 

Bi2212 multilament tapes were processed by the well-established powder-in-tube (PIT) technique. The tape is composed of 78 laments embedded in a Ag–Mg (0.2% Mg) sheath [35]. The tapes are different from the ones before. A dynamic heating process was inserted into a superconducting coil to enable a dynamic heat treatment under a magnetic eld (5 T)

The thermal treatment consists in a melting step for 250 s and a slow temperature decrease (0.1 °C s− 1). The atmosphere is 40% owing O2. The maximum applied eld is 5T. Critical current densities in Bi2212 tapes were measured in self field at 4.2K to compare the benefit of a magnetic field during the dynamic heat treatment. An increase of the critical current density is obtained under the application of a magnetic eld at T= 894 °C as can be seen in figure 9. The processing speed is around 1cm/min. The results are reproducible and this gain in current densities is observed for temperature around 894°C. The results are consistent with the ones previously presented for Y123 and Bi2212 bulk samples. The rotation of the crystallites under a magnetic field is possible when the liquid at the interface of the grains is sufficient and its viscosity is high which implies a relatively high temperature of the melt. Above 896°C, the refractory and non-superconductive phases tend to grow rapidly, limiting the critical current density. The dynamic heat treatment under a magnetic field offers several benefits. Firstly, the temperature is highly homogeneous. Secondly, no long treatments are necessary and the texture can be obtained homogenously and rapidly. This is also due to the facts that the nuclei causing the growth of the right phase subsist in the melt

in order to obtain homogeneous and high critical current densities (figure 8).

and don't need the recombination in the solid state that requires long time.

**Figure 8.** Dynamic heat process under a magnetic field. A multizone furnace is inserted in a superconducting coil reaching 5T. Flowing O2, maximal temperature and processing speed are important parameters that can be controlled [35].

**Figure 9.** Critical current in Ag sheathed tapes of Bi2Sr2CaCu2O8+x as a function of maximal annealing temperature measured at 4.2K with field and without field [35].

## **2.3. Case of Bi2Sr2Ca2Cu3O8+x**

### *2.3.1. Pellets of Bi2Sr2Ca2Cu3O8+x*

J. Noudem demonstrates the possibility to form the Bi2Sr2Ca2Cu3O8+x **(**Bi2223) phase after a liquid transformation above the melting temperature [36 - 38]. As seen previously in the Bi2212 compounds, the phase diagram is complex which requires a precise control of the temperature and composition [23].

Magnetic Texturing of High-Tc Superconductors 183

the Bi,Pb(2223) phase diagram and the presence of numerous secondary phases coexisting with the Bi,Pb(2223) phase as shown in Figure 4 has led to a standard route of tapes preparation where precursor powders are calcined and pre-reacted before the cold deformation inside the Ag tubes. This route implies the use of Bi2212 phase as the predominant phase and a series of cold deformations (swaging, drawing and rolling) before heat-treated the Ag-sheathed tapes to achieve Bi,Pb(2223) phase. However, intrinsic limits to this route seem to come from the formation mechanisms where the transient liquid assisting the Bi,Pb(2223) formation is not stable and results in a poor structural homogeneity. E. Giannini et al showed that it is possible to form the Bi,Pb(2223) phase from a liquid close to the equilibrium conditions following a reversible melting of the Bi,Pb(2223) phase exactly in the same way as J. Noudem et al. did in bulk samples [39]. Then, Bi2223 intrinsic nuclei exist in the melt. This new route is extremely sensitive to the temperature profile and is dependent of the local Pb content [40]. This result confirms the possibility to texture Ag-sheathed Bi,Pb(2223)

tapes by means of a scrolling continuous heat treatment under a magnetic field.

**3.1. Intrinsic nuclei surviving above the melting temperature** 

**the melting temperature** 

equation [5,6].

εls = 0:

**3. Magnetic texturing induced by intrinsic growth nuclei surviving above** 

Undercooling temperatures of liquid alloys depend on the overheating rate above the melting temperature Tm of liquid alloys and elements [41-43]. The experiments presented in section 2 lead to the conclusion that intrinsic nuclei exist above Tm and are aligned in magnetic field during the growth time evolved in the solidification window between liquidus and solidus temperatures [7]. These results are in contradiction with the idea [8-10] that all crystals have to disappear at the melting temperature Tm by surface melting. The classical equation of Gibbs free energy change associated with crystal formation also predicts that the critical radius for crystal growth becomes infinite at Tm and all crystals are expected to melt [8-10]. It has been recently proposed to add an energy saving εv produced by the equalization of Fermi energies of out-of-equilibrium crystals and their melt in this

Transformation of liquid-solid always induces changes of the conduction electron number per volume unit, and sometimes per atom. The equalization of Fermi energies of a spherical particle having a radius R smaller than a critical value R\*2ls (θ) produces an unknown energy saving −εv per volume unit. The εv value is equal to a fraction εls of the fusion heat ΔHm per molar volume Vm. This energy was included in the classical Gibbs free energy change Δ θ G ( ,R) 1ls associated with crystal formation in metallic melts [5,6,10]. The modified free energy change is ΔG2ls(R,θ) given by (2) and ΔG1ls(θ,R) is obtained with

3

H H <sup>R</sup> 12k V lnK G (R, ) ( )4 4 R (1 )( ) <sup>V</sup> 3 V 432 S

2ls ls ls

Δ Δ Δ θ = θ−ε π + π +ε

m m 2 B m ls 1/3

π×Δ (2)

m mm

**Figure 10.** Critical current density in Bi2223 textured bulk compounds measured at 77K as a function of the maximal annealing temperature [38].

The texture along the c-axis is induced by means of a magnetic field. Pellets of Bi2223 were first uniaxialy pressed before annealing. The heating cycle took place in a vertical furnace placed in a superconducting magnet reaching 8T. The optimum maximum temperature lies within the range 855° and 900°C and is followed by a slow decrease in temperature of 2°C/h. A maximal value of critical current density of 1450 A/cm2 was obtained for a temperature reaching 860°C during 1 hour. Below this value, the proportion of liquid phase is too small to allow rotation of the platelets under the influence of the magnetic field and the critical current consequently sharply decreases. Above this optimum processing temperature, the transformation of Bi2223 nuclei in secondary phases (Ca/Sr)14Cu24Oy, CuO and (Ca/Sr)2CuO3 prevent the recombination of Bi2223. Thus, the critical current density gradually decreased until zero around 900°C as shown in Figure 10 in agreement with the phase diagram of Figure 4.

### *2.3.2. Ag-sheated tapes of Bi2Sr2Ca2Cu3O8+x*

The possibility to obtain high critical current and a good homogeneity in Ag-sheathed Bi2223 tapes could follow accordingly to the previous results However, the complexity of the Bi,Pb(2223) phase diagram and the presence of numerous secondary phases coexisting with the Bi,Pb(2223) phase as shown in Figure 4 has led to a standard route of tapes preparation where precursor powders are calcined and pre-reacted before the cold deformation inside the Ag tubes. This route implies the use of Bi2212 phase as the predominant phase and a series of cold deformations (swaging, drawing and rolling) before heat-treated the Ag-sheathed tapes to achieve Bi,Pb(2223) phase. However, intrinsic limits to this route seem to come from the formation mechanisms where the transient liquid assisting the Bi,Pb(2223) formation is not stable and results in a poor structural homogeneity. E. Giannini et al showed that it is possible to form the Bi,Pb(2223) phase from a liquid close to the equilibrium conditions following a reversible melting of the Bi,Pb(2223) phase exactly in the same way as J. Noudem et al. did in bulk samples [39]. Then, Bi2223 intrinsic nuclei exist in the melt. This new route is extremely sensitive to the temperature profile and is dependent of the local Pb content [40]. This result confirms the possibility to texture Ag-sheathed Bi,Pb(2223) tapes by means of a scrolling continuous heat treatment under a magnetic field.
