US3116175A

US3116175A - Method for forming bicrystalline specimens

Info

Publication number: US3116175A
Application number: US711499A
Authority: US
Inventors: Robert B Pond
Original assignee: Marvalaud Inc
Current assignee: Marvalaud Inc
Priority date: 1958-01-27
Filing date: 1958-01-27
Publication date: 1963-12-31
Anticipated expiration: 1980-12-31

Description

Dec. 31, 1963 R. B. POND METHOD FOR FORMING BICRYSTALLINE SPECIMENS 2 Sheets-Sheet 1 Filed Jan. 27, 1958 INVENTOR foafmia pan/0,

BY Y4 ATTORNEYS Dec. 31, 1963 R. B. POND 3, 75

METHOD FOR FORMING BICRYSTALLINE SPECIMENS Filed Jan. 27, 1958 2 Sheets-Sheet 2 BY a w ATTORNEYS United States Patent 3,116,175 METHGD FOR FURMKNG EECRYSTALLIWE SPEClMlENfi Robert B. Pond, Westminster, Md, assignor to Marvalaud, Inc, Westminster, Md, a corporation of Maryland Filed Jan. 27, 1958, Ser. No. 711,499 6 Claims. ((31. EMS-1.6)

This invention relates to a method for forming bicrystalline specimens and more particularly to a method for producing specimens containing two or more crystals wherein the grain boundary between the crystals is disposed substantially transversely to the longitudinal axis of the crystals.

There are various well-known methods which have been utilized heretofore for forming single crystals of metals. One prior art method which is known as the Czochralski method involves lowering a seed crystal into a molten bath of the metal and withdrawing the seed crystal at a specified rate and maintaining the seed and the attached material at a temperature lower than the melting point of the metal. This method produces a single crystal extending longitudinally from the seed crystal along the axis on which the crystal has been moved. Another wellknown prior art method for forming single crystals which is known as the Bridgeman method involves lowering of a tube which is filled with molten metal through a thermal gradient so that at one point in the travel of the tube the temperature of solidification will be passed and all metal below this point will be solidified. If this method is carried out by moving the tube at the proper rate and maintaining the appropriate thermal gradient single crystals will be formed.

Either the Czochralski or Bridgeman techniques may be utilized to produce bicrystalline specimens in which the grain boundary between the crystals is parallel to the specimen axis. This is accomplished by using two seed crystals which are oriented in different manners and carrying out the usual prior art methods. However, it is difiicult, if not impossible, to produce bicrystalline specimens in which the grain boundary is perpendicular to the specimen axis by utilizing the Czochralski or Bridgeman methods. It would be necessary, using such techniques, to seed the molten metal during the growth of one crystal with a second crystal. It is possible to develop bicrystalline specimens by seeding from two ends of a horizontal container with properly oriented seeds disposed in each end of the container nad permitting the crystals which develop to grow together. However, such a method develops unsatisfactory bicrystalline specimens in that a shrinkage cavity is formed at the grain boundary and also excessive impurities are formed in the area adjacent to the grain boundary. This is due to the fact that many of the impurities have a lower melting point than the crystalline element and consequently the impurities will remain in the molten metal and collect adjacent the grain boundary. Hence, prior art methods have been generally unsatisfactory in producing bicrystalline specimens in which the grain boundary is disposed substantially transversely to the longitudinal axis of the crystal.

Bicrystalline specimens which are oriented so that the grain boundary is substantially transverse to the longitudinal axis of the crystals are particularly useful in connection with thermocouples and transistors. It has been found that any impurity will diffuse more readily at a crystal face than through a crystal and consequently bicrystalline specimens of germanium and the like form excellent transistors when indium or boron is diffused across the grain boundary to form the barrier between the donor and the receptor materials. Furthermore, in connection with the study of the physical characteristics of grain boundaries such bicrystalline specimens have well recognized utility. For example, considerable research has been conducted on the effect of grain boundaries upon the formation of slip planes in stress analysis investigations on metals. In this connection reference is made to volume 50, Reprint No. 43 of the Transactions of the American Society for Metals in which the grain boundary movement in bicrystalline aluminum is described.

The presently disclosed method for forming bicrystalline specimens of the type described involves forming a single crystal by any of the known methods, melting a small portion of this single crystal between the ends thereof, rotating one end portion of the crystal with respect to the other end portion and pressing the ends together so that a portion of the molten metal is extruded from the body of the crystal. As the molten metal solidifies a grain boundary is formed between the differently oriented end portions of the specimen.

It could not have been anticipated that such a method would result in the formation of a bicrystalline specimen having the grain boundary disposed substantially transversely with respect to the longitudinal axis of the specimen for the following reasons. It is not possible to apply heat in such a way that the metal is melted along a plane through the specimen. Ordinarily it would be expected that fingers or projections of solid metal would be disposed in the molten portion of the specimen. Upon rotation these fingers or projections break 0% and it would be expected that they would form seeds or randomly disposed crystals within the melted portion as it solidifies. However, by pressing the ends of the specimen together after rotation these nuclei, if they exist, are extruded from the body of the specimen and thus a clearly defined grain boundary may be formed. In addition, as in the case of forming bicrystalline specimens with known prior art methods, it would normally be expected that a shrinkage cavity would be formed as the molten metal solidifies. This is also obviated by reason of the longitudinal translation of the specimen after rotation.

An object of the present invention is to provide a method for forming bicrystalline specimens in which the grain boundary is disposed substantially transversely with respect to the longitudinal axis of the specimen.

Other objects and many of the attendant advantages of the present invention will become more fully apparent upon consideration of the following detailed specification in connection with the accompanying drawing wherein:

FIG. 1 is a longitudinal cross section through a single crystal showing a portion of the crystal in the molten form,

FIG. 2 is a similar longitudinal section showing the specimen in the bicrystalline form, and

FIGS. 3 to 7 inclusive are diagrammatic illustrations of various embodiments of the present invention.

Referring now more specifically to the drawing wherein like numerals indicate like parts throughout the several views there is shown at 1 in FIG. 1 a single metallic crystalline specimen. This crystalline specimen may be formed by any of the well known prior art methods which are described more fully hereinbfeore. Heat is applied on one side of the specimen between the ends thereof so that a portion 2 is melted. In the FIG. 1 embodiment this melted portion contacts the solid metallic end portions along two angular faces. It may readily be appreciated that under ordinary circumstances plane or angular faces would not be formed but they are shown in this manner for convenience of illustration.

In carrying out the method for forming bicrystalline specimens one end portion of the specimen is held stationary while the other end portion is rotated about the longitudinal axis of the specimen until the end portions 3 and 4 are in the relative position shown in FIG. 2. The end portions are then pressed together so that a portion of the molten material is extruded from the body of the specimen around the periphery thereof, as shown at 5. By extruding the material many of the impurities in the specimen are removed and also randomly disposed seed crystals which would tend to form a polycrystalline specimen are extruded. A clearly defined grain boundary which is disposed transversely to the longitudinal axis or" the specimen is formed between the end portions 3 and 4.

Bicrystalline specimens have been made in which the end portions of the single crystal are turned or twisted about axes other than the longitudinal axis of the specimen. In FIG. 3 a specimen is shown in which a grain boundary 6 is formed between the difierently oriented crystals 7 and 8 and the grain boundary 9 is formed between the crystals 8 and 10.

By applying heat substantially uniformly around the periphery of the crystal a grain boundary which is substantially perpendicular to the longitudinal axis of the specimen may be formed. In FIG. 4 there is shown a specimen in which the end portions 11 and 11.2 are rotated through 180 about the longitudinal axis of the specimen and the grain boundary is perpendicular to the axis of the specimen. The dotted line showings represent similar planes in the end portions of the specimen so as to illustrate the degree of rotation.

FIG. 5 shows a specimen having crystals 13 and 14 in which a 180 rotation has been eifected and in which the grain barrier is angularly disposed with respect to the specimen axis in a manner similar to that disclosed in FIGS. 1 and 2.

In FIG. 6 a 90 rotation has been effected and FIG. 7 illustrates a 45 specimen rotation. In both FIG. 6 and PEG. 7 a grain boundary is disposed substantially perpendicular to the specimen annexes.

As a further modification of the present invention it is possible to combine the Czochralski or Bridgeman methods for producing crystals with the presently disclosed method to produce a novel crystalline structure. By growing a plurality of crystals from adjacently disposed seeds with the Czochralski or Bridgeman techniques a multicrystalline specimen is obtained in which the grain boundaries lie in planes parallel to the specimen axis. Then by carrying out the presently disclosed process on this specimen through melting a portion of all of the crystals through the grain boundaries thereof and rotating one end of the specimen with respect to the other end a completely novel crystalline specimen is obtained. Each of the single crystals of the original specimen will now be bicrystalline and each of the crystals in the specimen will have a grain boundary and another crystal disposed at one end thereof and on at least one side thereof.

Obviously many modifications and variations of the present invention are possible in light of the above teachings. What is claimed as new and desired to be secured by Letters Patent is:

1. A method for forming bicrystalline metallic specimens comprising forming a single crystal from molten metal, heating to the melting point a portion of the single crystal across the entire cross-sectional area thereof, rotating one end of the crystal about the longitudinal axis thereof and solidifying the melted portion to form a grain boundary.

2. A method according to claim 1 wherein said one end of the crystal is rotated through 180.

3. In a process of the class described comprising the steps of preparing a single metallic crystal, melting a portion of the crystal across the entire cross-sectional area thereof at a point between the ends thereof, rotating one end portion of the crystal with respect to the other, pressing the ends of the crystal towards each other so as to extrude the molten metal from the body of the crystal and solidifying the molten metal to form a grain boundary at the point of rotation of the crystal.

4. In a process according to claim 3 wherein the crystal ends are rotated through 180 about the longitudinal axis of the crystal.

5. In a process according to claim 3 wherein the crystal ends are rotated through about an axis transverse to the longitudinal axis.

6. A method for forming bicrystalline metallic specimens comprising the steps of forming a single metallic crystal, melting a portion of the crystal across the entire cross-sectional area thereof, holding one end of the crystal stationary while rotating the other end about the longitudinal axis of the crystal, pressing one end of the crystal towards the other end to extrude the molten metal and solidifying the molten metal to form a grain boundary.

References Cited in the file of this patent UNITED STATES PATENTS 2,743,200 Hannay Apr. 24, 1956 2,743,201 Johnson et al. Apr. 24, 1956 2,964,396 Rummel et al Dec. 13, 1960 FOREIGN PATENTS 1,125,277 France July 9, 1956 742,237 Great Britain Dec. 21, 1955 OTHER REFERENCES Review of Scientific Instruments, vol. 25, No. 4, pages 331-334, April 1954.