索红莉
论文题目：第二代高温超导线带材Ag基板织构研究及在单晶Ag，多晶Ag和织构的Ag带上外延生长双轴取向REBCO-123薄膜
作者简介：索红莉，女，1968年08月出生，1996年03月师从于北京工业大学周美玲教授和左铁镛教授，于2000年06月获博士学位。
在高温超导成材技术中，具有较强的各向异性，超导转变温度为110K的Bi,Pb(2223)系化合物易于用Ag包套加工制成线带材，应用于强电领域，如变压器，超导磁体，电机，电力传输线等，使其成为第一代走向强电应用的实用超导体，其研制和开发有了长足的进展。然而遗憾的是Bi系化合物的本征特征(例如，温度依赖于不可逆磁场)而限制了它在高温(77K)，高场下的应用，使其目前只能在与液氮相比贵得多的液氦中使用。而以YBCO123系超导材料为主的稀土类(RE=Y，Nb，Gd，Yb，Er等)钡铜氧化物超导体具有较高的不可逆温度线，其jc在外加磁场下没有严重的退降，被称做第二代涂层超导带。近几年在其线带材成材方面有了飞越性的突破，使人们看到了应用的曙光。目前采用双离子束辅助沉积(IBAD)，压延辅助基板(RABiTS)，倾斜基板沉积(ISD)等技术jc值均可超过106A/cm2(77K,0T)。最高的jc值达到5´106A/cm2(77K,0T)。
虽然IBAD和RABiTS技术被用来部分地克服了Y123晶粒间的弱连接问题，使人们看到了Y123系线带材走向应用的曙光，但是由于非常低的沉积率和难于发展成长带，大规模及工业化生产仍遥遥无期。为了降低第二代超导带的制备成本，使其易于工业化，一个更便宜，更易实现工业化的方法是直接在织构的韧性基板上制备Y123厚膜，而无需任何过渡层。为了实现这个目标，需要考虑基板材料的选择。以上研究中所采用的纯Ni金属带尽管通过易于控制的轧制与退火工艺很容易得到千米长度的立方织构Ni带，但由于纯Ni与Y123间严重的相互反应需要有过渡层以及纯Ni中高的居里温度和它的磁性能(623K)，严重限制了其工业应用。
贵金属Ag具有不与超导材料反应的良好性能及对氧化物超导材料的机械性能与电性能有利，分析Ag与Y123之间的相互关系，首先Ag与Y123之间在膜沉积温度下无化学扩散，它本身又有优良的韧性，易于成长带。用Ag做基带，不需使用隔离的过渡层，这样易于大规模生产从而降低线带材制备成本。我们根据这些特性，很早就提出了一个以Ag的带材方式作为直接涂层Y123的思路，基于在轧制的立方织构的Ni带上外延高jcY123膜的报导，我们决心进行难度很大的立方织构Ag基带的制备与研究，至今为止很少见到这方面的研究报导。Ag和Ni同属于易加工的面心立方金属，但其不同点是，Ag有非常低的层错能(SFE24mJ/m2)，其位错非常不易移动，在形变及退火中非常易于形成孪晶。到目前为止，不同的作者报导了在轧制和退火后Ag中的不同的再结晶织构类型，大多数认为难以在Ag中获得立方织构。对我们最终期望在Ag中获得的下面两种稳定织构，{100}<001>立方织构和{110}<uvw>织构，研究的文献报导寥寥无几。
本论文全面系统的研究了做为第二代高温超导线带材的Ag基板的织构形成及转变规律，并可以重复的制备具有{100}<001>立方织构和{110}<112>稳定再结晶织构的Ag带材。通过在(100)，(110)及(111)三种单晶Ag基板，多晶的冷轧Ag基带以及{110}织构的Ag基带上外延生长REBCO-123(RE=Y，Gd，Nd)薄膜，获得了REBCO-123薄膜取向与基板取向之间的关系，并确定了Ag的{110}取向适于制备具有双轴织构的REBCO-123薄膜。现将主要的实验结果总结如下：
我们采用PVD磁控溅射法在多晶的轧制Ag基带上镀制了Y-123薄膜，实验发现Ag基片的织构状态对生长Y-123织构薄膜有重要的影响。分析表明：由于冷轧Ag在镀膜升温过程中织构的一系列变化以及Y-123的Jc值对基板及薄膜晶界错配角的强烈依赖性，很难直接在多晶的冷轧Ag基带上制备具有良好平面取向以及高Jc的Y-123薄膜。因此只有在Ag中获得合适的具有双轴取向的稳定再结晶织构，才可能在其上获得高Jc的具有双轴平面取向的Y-123薄膜。正是在这些研究的基础上，我们提出了系统的研究Ag中织构转变及形成规律。
关于Ag织构，最初的研究表明，在低层错能的Ag中，一般退火后可得到黄铜型再结晶织构，如{236}<385>,{023}<032>以及{012}<021>等。分析认为Ag中形变织构向退火织构的转变可分为孪晶和快速增长两个阶段，第一阶段的取向形成是由孪晶引起和控制，由于Ag的低层错能，在非常低的温度下，由于形变应力的回复，孪晶非常易于产生在形变基体基础上。而第二个阶段主要由扩散控制，在这个阶段，取向由{110}型向黄铜型{236}<385>，{023}<032>以及{012}<021>明显地转变。在Ag中，低温下，形变与再结晶织构的取向关系可以被描述为：再结晶织构晶体的一个(111)面总是接近垂直于形变晶体的最活跃的滑移面和滑移方向，其产生原因是由于诸如位错堆积等缺陷分布在滑移面上所引起的原子的各向异性扩散。 在Ag中主要的再结晶织构如{236}<385>，{023}<032>以及{012}<021>的形成可被理解为这样一个过程，即一次孪晶后在形变基体中的原子各向异性扩散及在再结晶晶体中的各向异性增长过程。
为了获得所需要的稳定的再结晶{100}<001>立方织构和{110}型织构，我们精确的控制了所有的形变与退火工艺参数，最终可重复制备上述两种织构。对于立方织构的形成，实验发现100°C预热轧制，150°C和250°C的退火分级加热以及快速升温到700°C并保温30分钟是优化后的工艺参数。实验的重复性非常好。其技术核心是采用适当温度的预热轧制以及采用分级加热退火方式以使在低温下尽可能少的形成黄铜型再结晶织构。理论分析表明取向形核理论及选择增长理论在立方织构的形成过程中起了共同的作用。实验结果指出了形变计谋的重要性，即合适的预热温度的选择不仅要使在形变后获得一些铜型织构(C和S含量)的形变含量，而且还要避免在形变中发生动态再结晶。热轧不是一种好的形变方式，因为无论是高温或低温热轧都在形变过程中发生了动再结晶，因而不能在退火中形成立方织构。当预热温度增加时，主要的形变织构从黄铜型{110}<112>向高氏型{110}<001>含量转变。当预热轧制温度为100°C时，较高含量的铜型轧制织构的存在在某种程度上相当于轻轻的增长了其层错能，从而有利于退火后形成立方织构。而当预热轧制温度升到130°C时，在形变过程中发生了动态再结晶。
为了得到{110}<uvw>织构的Ag带， 实验采用99.95%的商业纯Ag块，使用标准的冷轧及退火工艺。结果发现真空退火是获得具有{110}<112>织构的大晶粒的必要条件，其优化的工艺条件为采用分级加热。首先为了形成大量的精细的小晶粒和少量的大晶粒，使用快速升温到500°C保温几分钟，然后利用极慢的升温速度加热到800°C保温4小时，这个过程使具有{110}取向的大晶粒逐步吞并小晶粒而逐渐优先长大。总结技术的核心是在初次再结晶后拥有大量数目的精细晶粒及少量的大晶粒，并在整个过程中在真空状态下退火。实验发现10%的道次压下量有利于{110}<uvw>织构的形成，X-ray极图，ODF分析以及EBSD测量结果展示了高质量的{110}<112>织构，其{110}<112>峰半高宽小于6°。从EBSD分析可知，{110}面的微取向角甚至小于4°，这表明了强的织构度。实验还通过在高温900°C下再次退火已具有{110}<112>织构的Ag带研究了Ag带织构的稳定性。结果表明了此带可被用来在高温下外延生长高温超导涂层。在Ag中具有大晶粒的{110}<112>织构的形成可被解释为选择晶粒增长及孪晶转变机制共同作用的结果。实验及分析均表明了两组{110}<112>织构的峰共同存在，其中一组是另一组的孪晶织构，它们共同拥有{110}面和<112>方向，其原始织构与它的孪晶织构之间的夹角为70.3°。
关于超导材料的研究，一些实验表明用元素替代(用Yb,Er等替代Y)，在适当的条件下可大大降低其包晶反应温度(低于Ag的熔点)。因此为了发展高温超导长线带材及将来在Ag基带上制备REBCO-123涂层厚膜，我们用柠檬酸盐高温热分解方法试制并获得了非常精细的几乎完全单相的无碳RE-123(RE=Y,Er,Yb)粉末。
本论文详细地对比和分析了在不同取向Ag单晶上REBCO-123(RE=Gd,Nd)薄膜取向与Ag基板取向之间的关系。在三种不同取向的单晶Ag基板，SrTiO3单晶基板以及(110)织构的Ag基带上，采用RF磁控溅射技术成功的制备了Gd-123和Nd-123膜。在Ag(100)取向基板上，Gd-123膜和Nd-123膜有两个呈45°分开的外延取向生长。在Ag(110)基板上，只发现了一个单一的平面外延取向。同时发现使用Nd替代，薄膜质量大有改进，这主要表现为在(100)Ag基板上，Nd-123膜的二次取向沿着Ag[011]方向的密度10倍的低于主要的沿着Ag[001]方向的外延取向的密度。而在Ag(110)基板上，获得了非常强的单一取向的的沿着Ag[110]方向的Nd-123外延生长，且(103)峰的f扫描半高宽等于2°，这几乎非常接近于在SrTiO3单晶基板上的Nd-123薄膜的半高宽值。在(111)单晶Ag基板上，我们没有发现择优取向生长的Gd-123峰，而在它上面的Nd-123薄膜却存在24个峰，6个方向的取向生长。这一切都表明了使用Nd替代大大改进了薄膜质量。除此之外，在我们的大晶粒Ag{110}织构的基带上同样获得了单一取向的Nd-123膜，所有的晶粒都朝一个方向排列并且取向生长，其f扫描半高宽等于4°表明了非常强的平面取向。这个结果确信并证明了{110}织构的Ag带适于REBCO-123薄膜的制备。
总结以上研究成果，在Ag织构研究方面，我们在国际上首次报导了在Ag中可得到重复制备的立方织构及1米长{110}<112>织构，并将织构的Ag带材用于外延薄膜的实验研究。另外实验在三种取向的Ag单晶上外延生长了REBCO-123薄膜(RE=Nd，Gd)并详细比较和分析了各种单晶Ag基板取向与外延的REBCO-123薄膜的取向关系，特别是采用Nd的替代发现大大的改进了薄膜的外延质量，这将具有重要的意义。
关键词：基板；超导带；第二代涂层超导带；双轴取向的REBCO-123（RE=Y，Gd，Nd）膜；涂层带；超导厚膜；n级孪晶；立方织构；预热轧制；热轧；黄铜型织构；铜型织构；取向增长理论；取向形核理论；{110}＜uvw＞再结晶Ag织构；二次再结晶；选择晶粒增长；织构的Ag带；Ag单晶；RF磁控溅射；外延增长；Nd替代。
Study on texture in Ag for the second generation of
coated superconducting tapes and epitaxial growth of biacially oriented
REBCO=123(RE=Y,Gd,Nd) thin films on Ag single crystal, polycrystalline Ag and
textured Ag ribbons
Abstract (English, 2950 words)
The only high temperature superconductor being used for
long conductors at 77K is actually Bi,Pb(2223). This very anisotropic compound
has a high Tc value, 110K, but exhibits only a quite low reversible
field, which strongly limits its applications, to magnetic fields below 0.5 T
at 77K. This is sufficient for cables and transformers, but not for motors or
energy storage. The so-called “Coated conductors”, based on Y(123) or R.E.(123)
offer a very promising alternative. Very high critical current densities have
so far been reached by means of various techniques: IBAD, RABITS, ISD. The
critical current values at 77K obtained using these techniques are well above 1
x 106 A/cm2, and maximum values of 5 x 106
A/cm2 have already been reported.
However, in contrast to the Bi based conductors, the
particular conditions at the grain boundaries of Y(123) lead to the necessity
to form biaxially textured layers. This problem is responsible for the strong
difficulties encountered when preparing long conductors: The challenge consists
now in developing cheap and scaleable techniques suitable for industrial
applications. The superconducting layers in coated conductors have been
epitaxially deposited on various substrates materials, among which the most
promising seem to be Ni, Ni-V or Ni-W. Due to reactions between substrate and
superconductor, various buffer layers have been introduced, based essentially
on MgO and CeO2. Another buffer layer material, NiO, simply formed
by oxidizing the Ni surface, did not yet give satisfactory results. It was
found that the maximum jc in coated Y(123) layers is obtained on {100} textured (or cube
textured) Ni grains, the lowest
misalignement angles being actually of the order of 4°. Because of its high irreversibility field,
the compounds of the RE-Ba2Cu3O7 (RE: Y, rare
earth) family are of high interest for the next generation of superconductor
applications. However, the scalabilities and cost-effectivenesses of the
processes are not yet proven, the main obstacles being slow and costly several
buffer layer production. There are also questions about the poisoning of the
YBCO layer by the underlying Ni.
Silver is the ideal substrate from the point of view of
chemical compatability, it is thus a promising substrate for the second
generation of coated superconductors. However, it is difficult obtain a high
level of biaxial texture in Ag. There are not many investigations about Y(123)
deposition on biaxially textured Ag substrates: this is due to the difficulties
encountered when preparing biaxially textured Ag, as a consequence of its
considerably lower stacking fault energy with respect to Ni.
In this thesis, the texture formation and texturing
mechanisms in rolled and recrystallised silver were systematically studied. The
goal is to obtain cube textured {100}<001> and {110}<uvw> textured
Ag ribbons as well as to explore the commensurability between the REBCO-123
films and the Ag substrates. In order to achieve this goal, careful attention
was paid to preparation of the Ag ingot to be rolled, deformation percentage,
initial and final foil thickness, and recrystallisation heat treatment
conditions. A clear understanding of the epitaxial growth mechanism of RE-123
on different oriented Ag substrate is the another key issue of this thesis. The
epitaxial growth of RE-123 on the different orientated Ag single crystals and
textured Ag ribbons was presented. The influence of the rare earth element on
the superconducting phase growth mechanism was discussed.
Initial studies of Y-123 films on polycrystalline Ag substrates show that in-plane orientated Y-123 is difficult to obtain directly on cold rolled Ag ribbons. This is due to texture changes and grain growth in the rolled Ag ribbons that occur when the substrate temperature is increased for deposition. According to those results, it is not possible to get high quality Y-123 films directly on cold rolled Ag ribbons. It is a necessary to get a stable recrystallization texture in Ag before the deposition stage. Thus, it becomes important to study the texturing mechanisms in Ag in order to get a stable recrystallization texture. What we expected is to get the {100}<001> cube texture or the {110}<uvw> one. Initial studies on Ag texture show that recrystallization Brass-type texture components such as {236}<385>, {023}<032> and {012}<021> can generally be obtained after annealing in deformed Ag. The orientation changes during annealing of deformed Ag are assumed to be determined by n-generation twinning and by fast growing orientation. Those obtained recrystallization Brass-type texture components in Ag can be explained by the above mentioned model in which the boundary migration direction is determined by the surface energy difference between neighboring crystal planes. A careful control of all the processing parameters allows us to form high quality of {100}<001> cube textured and {110}<112> textured Ag ribbons after annealing.
To obtain {100}<001> cube texture, commercial 99.99% pure Ag powder was used with a maximum grain size of 50 mm. This powder was first cold-pressed under 4 kbar and then sintered at 750°C in air. The sintering temperature was chosen to ensure a sufficiently high density (>90%) without important grain growth. Finally, starting materials with the original geometric shape of 20´15´5.5mm3 bars were produced with grain sizes of about 15mm. Pre-heated rolling and hot rolling are used to deform the ingots. The pre-heated temperature is the temperature at which the ingot is heated before standard cold rolling, it is controlled in the following range: 77K, room temperature, 70°C, 90°C, 95°C, 100°C, 110°C, 130°C. Hot rolling is carried out by preheating the rolls to the temperatures of 50°C, 130°C and 400°C. All the starting materials are rolled down to a 80 mm thickness for a total reduction larger than 95%. A 20% deformation is controlled at each pass. Typically, a 20 m long ingot or 6 kg would be used to produce a 1 km long rolled tape. After rolling, the samples are annealed in a primary vacuum at different temperatures between 100°C to 900°c for different periods of time from 1 min to 4 hours, followed by slow cooling. For each sample, both after rolling and annealing, three incomplete (111), (200) and (220) pole figures (from 0 to 70°) are measured on a Siemens X-ray diffractometer (Cu Ka radiation). For texture analysis, the three pole figures data are used to compute the Orientation Distribution Function (ODF) according to the series expansion method (rank of harmonic expansion: Lmax =22). The ODF gives a complete representation in the Euler space of all the different orientations presented in the tape. To obtain a local information on the texture, diffraction of the backscattered electrons (electron backscattered EBSD) is performed during SEM. A diffraction pattern is recorded each 10µm on a 2×2mm2 map. The pseudo-Kikuchi bands are automatically indexed to get the crystallographic orientation for each point.
A sharp cube texture was obtained in rolled and annealed Ag tapes. A pre-heating at 100°C before cold rolling and an annealing at 700°C for 30 min in a primary vacuum are the optimal conditions to get the cube texture. It is found that Oriented-Nucleation and Oriented-Growth play a common role in the development of cube texture in Ag tapes. The main deformation texture transition from the Brass component {110}<112> to Goss component {110}<001> in Ag takes place gradually as the pre-heated rolling temperature is increased. The existence of a significant Copper-type component with relatively high ODF values after 100°C pre-heated deformation may indicate that pre-heated rolling slightly increases the Stacking Fault Energy of Ag which has a favorable effect on the recrystallization cube texture. The pre-heating temperature must be chosen not only to obtain some amount of Copper-type texture component but also to avoid the occurrence of dynamic recrystallization during deformation. A precise control of this deformation stage is a key factor to get the final cube texture after recrystallization. With the optimal parameters, the recrystallization texture is a sharp cube texture {100}<001>. The FWHMs of the three pole figures are as low as 10°. The ODF value of the cube texture is three times higher than the one of the main secondary orientation, the rotated cube texture. The ODF values of the other secondary orientations are ten times smaller. The presence of this weak amount of {012}<122> type components, of rotated cube texture {100}<011> and {212}<122> twins are apparently due to the fact that Brass component still exists in the tapes. The stability of the cube texture has been checked at high temperature up to 900°C which is a practical temperature for superconducting tape processing. In spite of the very low stacking fault energy of pure Ag, the present investigation shows that the texture of Ag tapes can be controlled and that {100}<001> tapes of good quality can be produced.
As it will
be discussed below in the epitaxial growth of REBCO film on textured
substrates. The drawback of the cube texture, however, is that it generally
gives two biaxial orientations of the superconducting film. We believe that to
obtain a low-cost product, a buffer layer should be avoided. The {110} texture
could open the possibility to deposit RE123 material directly onto and it is
probably the most appropriate Ag texture to use to obtain a biaxially-aligned,
high-temperature superconductor material, RE123. It has been shown that, in the
present thesis, that it is possible to obtain pure and stable {110}<112>
textured Ag ribbons on 1 meter long-length.
It is
known, the {110} texture is one of the deformation textures. It is formed in Ag
and other metals with low stacking fault energy (SFE) after cold rolling, but
is usually not stable upon further annealing. It was reported that the {110}
texture appears after recrystallization in bcc metals such as Fe–Si alloys
where it plays an important role for industrial applications. It was also found
in pure Fe and low carbon steels as well as in high-SFE metals such as Ni, Al,
etc. The formation mechanism of this kind of stable {110} recrystallization
texture appears as a selective grain growth, also called secondary
recrystallization. The principle of the secondary recrystallization is that the
nuclei of {110} grains emerge from the microstructure of normal primary
recrystallization and grows preferentially among the other orientations because
of the anisotropic grain boundary mobility. This leads to large grains with the
{110} orientation. Nevertheless, a stable {110} orientation is difficult to
obtain in Ag. Indeed, low SFE fcc metals such as Ag commonly form a brass-type
texture after recrystallization.
A reproducible way to form
{110}<112> textured Ag ribbons with a misorientation angle smaller than
4° was found in this work. The {110} texture appears
after recrystallization into large grains, which may result from secondary
recrystallization (selective grain growth). In our experiment, annealing in the
vacuum is a condition necessary to get the {110} orientation. Finally, the
optimized annealing process to get large grain is annealing at 800°C in the
vacuum for 4 hours. Prior to going to 800°C, one 500°C step was used to form
fine grains and big grains. The optimized result indicates that the
significance of technical is to have a few big grain with orientation of {110}
after primary recrystallization. A 20% deformation reduction at each step is
found to be the appreciate rolling condition to get normal oriented deformation
texture. The FWHM values of {110}<112> orientation with no more than 6°
measured by three X-ray pole figures, and the misorientation angles of less
than 4° from other orientation measured by EBSD indicate high quality of
{110}<112> texture. The stability of the {110}<112> texture was
studied by again annealing the {110}<112> textured Ag ribbon at 900°C and
the result indicates that the tapes are suitable for epitaxial growth of
superconducting oxide coatings. The dimensions of the samples are also found to have some effects on the
formation of {110}<112> texture. Because of the
complication of influence factors on texture development of Ag, the
significance of the energetic state and the history of the starting material
was also emphasized. The formation of the {110}<112> texture is
interpreted in terms of common function of secondary recrystallization and
twinning transition mechanism. Two orientations belonging to the same {110}<112> family can be
found, one being the twin of the other.
To study the epitaxial growth of RE-123 on the different Ag
orientations, single crystals were grown from a 99.9% pure silver by a Bridgman
technique. The 5 cm long crystals were oriented by X-ray Laue method and cut
into 1 mm slices with surfaces oriented according to the (100), (110) or (111)
directions. The slices were finally polished and chemically etched to get clean
oriented surfaces. The superconducting films were grown directly on pure Ag
using a RF planar magnetron sputtering system. GdBa2Cu3O7-d (Gd-123) and Nd1.12Ba1.88Cu3O7-d (Nd-123) targets with a diameter of 55 mm were used, the input RF
power was 100 W for Gd123 and 50 W for Nd123. The substrates were mounted on a
high temperature heater using silver paste. The deposition temperature ranged
from 710° to 760°C. A total pressure of 140 mTorr consisting of a 95% Ar, 5% O2
gas mixture was used for sputtering. Deposition rate was 500 Å/h, and the
thickness 2500 Å, those values were calibrated from finite-size effect on a
thinner film. After deposition, the films were hold at 500°C for 1-2h in 500
Torr of O2 for oxygenation.
Firstly, from the normal q to 2q scan of a Nd-123 on a Ag (110) sample, the presence of the (00L) peaks indicates that the film is highly c-axis oriented. A little amount of green phase (Nd-211) can be detected. The proportion of green phase increases with the increasing of temperature. Its presence can be related to the use Ag (110) substrates since no green phase is found on Nd-123 deposited on SrTiO3 (100) substrates. In the Gd-123 case, no green phase was detected. According to the SEM micrographs of a Nd123 film. There is a clear c-axis orientation and an apparent in-plane orientation that is also confirmed by the j-scans. The film surface is not as flat as the one obtained on SrTiO3 single crystal substrates because of the non-perfect flatness of the silver. Apart from a small porosity large flat areas with connected grains are seen. The green phase appears as small bright grains. By the (103) j-scans of the Gd-123 and Nd-123 films on the different silver single crystals. It is shown that on an Ag (100) substrate, the Gd-123 phase grows in two orientations, 45° apart. FWHM is about 12°. For the two orientations, the peaks intensity is similar although four of them appear higher because their direction coincides with the (110) direction of silver. A unique orientation is obtained when the Gd-123 phase is sputtered on a Ag (110) substrate, but the peaks are rather broad with a FWMH of 30°. On a Ag (111) substrate, no preferential orientation was found. A possible orientation may be masked by the peaks broadness and their low intensity. A high improvement of the film quality was obtained with Nd1.12Ba1.88Cu3O7-d. On a Ag (100) substrate, there are still two growth directions, but the peaks intensity of the diagonal orientation is reduced by of factor of 10 (in that case the intensity of the (110) Ag direction is small compared to the Nd peaks so that the relative intensity of the two Nd directions really comes from the Nd). FWHM is much sharper, of about 3.5°. For the sample grown on Ag (110) single crystal, we see a very good improvement of the FWHM, comparing to the Gd-123 case. It is as sharp as 2°. This value is close to the FWHM of a Nd-123 film sputtered on a SrTiO3 substrate (about 0.5°) with the same sputtering conditions. For the film grown on {110} textured Ag ribbons, the result also show a unique biaxial orientation with a FWHM of 4° in the X-ray pole figures, which nicely indicates that those {110} ribbons are suitable for RE-123 epitaxial growth. This result strongly proved that the high interest of silver substrate and shows that the use of Nd greatly improves the film quality with respect to Gd-123. Finally, on a Ag (111) substrate, the phase can take six different orientations (24 peaks on 360°). The improvement obtained with Nd-123 can be explained by a better lattice matching, the cell parameters of Nd-123 (a=3.86 Å, b=3.91 Å) being bigger than the one of Gd-123 (a=3.82Å, b=3.89 Å), and then closer to the silver cell parameter (a=4.09 Å) but a more complex growth mechanism due to the particular Nd-Ba sites substitution could be an other explanation. It is related to the excess Nd that has a benefit effect on the film growth. A ZFC-SQUID data of a Nd-123 film on Ag (110) performed under a 5 Oe magnetic field shown that the onset Tc is 80 K and the transition is wide. It can be explained by the morphology of the film and by non- stoichiometry of the Nd-123 phase. This off-stoichiometry has a negative effect on the critical temperature but can also be of high interest to improve the pinning properties of the films. An optimization of the deposition parameters is expected to overcome this problem. High critical temperature Nd-123 (Tc=90K) can be grown on SrTiO3, the same should be possible on silver too. Gd-123 films on SrTiO3 show high Tc of 90 K, no substitution occurring in that case.
In a word, silver is a promising substrate for epitaxial growth of RE-123 thin films. The cube texture of {100}<001> and 1 meter length of {110}<112> textured Ag ribbons have been successfully prepared. The film grown on {110} textured Ag ribbons show a unique biaxial orientations, which nicely indicates that those {110} ribbons are suitable for RE-123 epitaxial growth. The use of Nd greatly improves the film quality with respect to Gd-123.
Keywords: Substrates; Superconducting tapes; The second generation of superconducting tapes; Biaxially aligened REBCO(RE=Y,Gd, Nd) films ;Coated tapes; Superconducting thick films; n-generation twinning; Cube texture; Pre-heated rolling; Hot-rolling; Brass-type texture ;Copper-type texture ; Oriented growth theory; Oriented nuclear theory; Secondary recrystallization; Selective grains growth ; Textured Ag ribbons; Ag ribbons; Ag single crystal; RF Sputtering ;Epitaxial growth; Nd substitution.
