Single-crystal – oriented-crystal – and epitaxy growth processes; – Processes of growth from liquid or supercritical state
Reexamination Certificate
1999-03-19
2001-01-09
Utech, Benjamin L. (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Processes of growth from liquid or supercritical state
Reexamination Certificate
active
06171390
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for preparing large oxide single crystalline materials useful as a “RE123 oxide superconductor” or “LiNdO
3
oxide laser transmitting element”, respectively.
2. Description of the Related Art
Recently, there has been remarkably developed a new technology utilizing specific properties of oxide crystalline materials and, for example, crystalline materials of RE123 oxides (wherein RE represents one or more rare earth elements) and LiNdO
3
crystalline materials play an important part as a high temperature superconductive material and a laser transmitting element, respectively.
The “Solidification process” has been conventionally known as a general procedure to yield oxide crystalline materials used for the above mentioned purpose, in which a molten material, i.e., crystalline precursor material, is cooled at a slow speed from a temperature around the melting point to promote solidification and crystal growth.
In such a conventional process as described above, however, it takes a long period of time to yield the desired crystals, which is practically disadvantageous.
In order to improve the conventional disadvantage by reducing the time period of crystal growth, the “supercooling (undercooling) solidification process” has been proposed as a novel crystal growing method.
The supercooling solidification process is a single crystal growing method utilizing such characteristics that “when a molten or semi-molten liquid of an oxide crystal precursor is supercooled, i.e., kept in a supercooled condition at a temperature below the melting point, the crystal growth rate is remarkably accelerated”. Several methods for preparing Y-oxide superconductive crystals based on the above mentioned supercooling method have been described in, for example, JP-A No. 6-211,588; Journal of Materials Research, Vol. 11, No. 4, pp. 795-803, April 1996; and ibid., Vol. 11, No. 5, pp. 1114-1119, May 1996.
According to the above mentioned references including JP-A No. 6-211,588, the most characteristic feature is that “A Y-oxide superconductor precursor material, which has been added with a seed crystal as nuclei for crystal growth followed by raising the temperature above the peritectic temperature or raising the temperature above the peritectic temperature followed by addition of the seed crystal, is subjected to supercooling (undercooling) by cooling it under the peritectic temperature and further cooling it continuously but slowly, for example at a cooling rate of 1° C./ hr. or isothermally keeping it at the supercooled temperature to grow Y-oxide superconductor crystals, the oxides never solidify at once completely when they are supercooled, which is distinct from pure metals”. On the other hand, the “supercooling solidification process” is a method for growing crystals in which a molten precursor material is supercooled to a temperature region below the melting point in a molten or semi-molten state and then slowly cooled, usually at a cooling rate of 1 to 10° C./hr., or kept at the supercooled temperature, and the existence of the above mentioned “seed crystal” is not necessarily essential to the process. That is, such a process is employed for a purpose to increase the crystal growth rate.
FIG. 1
shows a relationship between the crystal growth rate of Sm123 and the degree of supercooling. It is apparent from
FIG. 1
that the crystal grow rate increases as an increase in the degree of supercooling.
The inventors have investigated the preparation of larger oxide single crystals from various viewpoints and come to a conclusion that the supercooling solidification process has a limit in the preparation of large single crystals and is not sufficient to yield homogeneous large single crystals with less defects.
As evident from
FIG. 1
, the crystal growth rate increases as an increase in the degree of supercooling, resulting in it being possible to prepare large crystals in a short time.
However, as evident from the relationship between supercooling degree and frequency of nucleation in
FIG. 2
, the nucleation increases rapidly relative to an increase in the degree of supercooling.
The increase in the frequency of nucleation inhibits the preparation of large single crystals (for example, crystals grown from seed crystals), resulting in a limitation of the size of single crystals.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a method for easily preparing large and perfect oxide single crystals without consuming a longer time.
To accomplish this and further objects of the present invention, the inventors have keenly investigated and finally obtained novel information in which, when a highly heated oxide crystal precursor material is supercooled below the melting point and then subjected to continuous slow heating within the supercooling region to grow crystals, nucleation which hinders single crystal particles from growing is controlled, while keeping a high crystal growth rate, so that large single crystals can be grown without consuming a longer period of time.
The present invention has been achieved on the basis of the above mentioned information and provides a method for preparing a large oxide single crystalline material as in the following.
1. A method for preparing a large oxide single crystalline material, characterized in that a crystal precursor material is supercooled prior to the solidification thereof in the course of the crystal growth of an oxide by a supercooling solidification process, followed by subjecting said precursor material to continuous slow heating while keeping the supercooled condition to promote crystal growth.
2. A method for preparing a large RE123 oxide superconductive single crystalline material, wherein RE is one or more rare earth elements in which Y is not excluded, characterized in that a RE123 oxide superconductive crystalline precursor material added with seed crystals is supercooled below its peritectic temperature prior to the solidification thereof in the course of crystal growth of the RE123 oxide superconductive crystal, followed by subjecting said precursor material to continuous slow heating while keeping the supercooled condition to promote crystal growth.
Oxide crystals prepared by the present invention are not limited to a specific oxide but may cover any material including the above mentioned LiNdO
3
oxide crystal used as a laser transmitting element. Especially, an excellent practical effect is exhibited when the present invention is applied to prepare RE123 oxide superconductive single crystalline materials in which the growth of large crystals has been difficult.
REFERENCES:
patent: 5611854 (1997-03-01), Veal et al.
patent: 5776864 (1998-07-01), Todt et al.
patent: 5869431 (1999-02-01), Veal et al.
Kambara Makoto
Miyake Kouichi
Satoh Hidetaka
Shiohara Yuh
Anderson Matthew
Flynn ,Thiel, Boutell & Tanis, P.C.
Superconductivity Research Laboratory
Utech Benjamin L.
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