U.S. patent number 3,607,137 [Application Number 04/683,896] was granted by the patent office on 1971-09-21 for method of avoiding strain in phase transitions of single crystals.
This patent grant is currently assigned to Hayakawa Denki Kogyo Kabushiki Kaisha. Invention is credited to Toshio Inoguchi, Katsuro Nakazawa, Ichihiko Niwa.
United States Patent |
3,607,137 |
Inoguchi , et al. |
September 21, 1971 |
METHOD OF AVOIDING STRAIN IN PHASE TRANSITIONS OF SINGLE
CRYSTALS
Abstract
Method for growing crystals of inorganic metal salts from a melt
which comprises cooling a single crystal of an inorganic metal salt
having at least one solid-solid transition point in a stable
structure of a higher temperature in such a manner that the crystal
passes through the transition point from an end of the crystal to
another end of the same to obtain the corresponding single crystal
in a stable structure of a lower temperature, the transition
interface being at such a distance from the solid-liquid interface
that the strain caused by the solid-solid transition does not exert
any influence on the solid-liquid interface.
Inventors: |
Inoguchi; Toshio (Osaka,
JA), Niwa; Ichihiko (Osaka, JA), Nakazawa;
Katsuro (Osaka, JA) |
Assignee: |
Hayakawa Denki Kogyo Kabushiki
Kaisha (Osaka, JA)
|
Family
ID: |
13588963 |
Appl.
No.: |
04/683,896 |
Filed: |
November 17, 1967 |
Foreign Application Priority Data
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|
|
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Nov 18, 1966 [JA] |
|
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41/75878 |
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Current U.S.
Class: |
117/10; 23/305R;
117/900; 117/940; 117/37 |
Current CPC
Class: |
C30B
11/00 (20130101); C30B 29/12 (20130101); Y10S
117/90 (20130101) |
Current International
Class: |
C30B
11/00 (20060101); B01j 017/04 (); C01g
003/04 () |
Field of
Search: |
;23/31SP,97,296 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Yudkoff; Norman
Assistant Examiner: Foster; R. T.
Claims
What is claimed is:
1. A method for growing a single crystal of an inorganic metal salt
having a solid-solid transition point contiguous with its melting
point which comprises cooling a single crystal of the inorganic
metal salt from a melt, the single crystal forming at a
solid-liquid interface, the cooling being in such a manner that the
crystal passes through the transition point from one end of the
crystal to the other whereby the single crystal is converted from
the form stable above the transition point to the form stable below
the transition point, the temperature gradient on cooling being so
gradual and the solid-liquid interface being at such a distance
from the solid-solid transition interface that the strain caused by
the solid-solid transition does not exert any influence on the
liquid-solid interface.
2. The method according to claim 1, wherein the single crystal
passes through the transition point after its crystallization from
the corresponding melt is completed.
3. The method according to claim 1, wherein one portion of the
single crystal passes through the transition point while another
portion is being crystallized from a melt.
4. The method according to claim 1, wherein the cooling is effected
in a quartz glass tube provided with a carbon film on the inner
wall.
5. The method according to claim 1, wherein the inorganic metal
salt is cuprous chloride.
6. The method according to claim 1 wherein, the distance between
the solid-liquid interface and the solid-solid transition interface
is more than 1 cm.
7. A method according to claim 1, wherein the final single crystal
is obtained by first passing the inorganic metal salt through one
heat zone at a temperature above the melting point of the inorganic
metal salt and gradually passing the melt through a second heat
zone at a temperature lower than the melting temperature of the
inorganic salt but above the transition point of the inorganic
metal salt to form a single crystal of a stable structure at a
higher temperature and then gradually passing the crystal through a
heat zone which has a temperature lower than the solid to solid
transition temperature of the inorganic metal salt whereby a single
crystal having a structure stable at a lower temperature is
obtained, the distance between the solid-liquid interface and the
solid-solid transition interface being more than 1 cm.
Description
The present invention relates to a method for growing single
crystals. More particularly, it relates to a method for growing
single crystals of inorganic metal salts having at least one
solid-solid transition point.
Although there have been known various methods for growing single
crystals, none of them can be applied to an inorganic metal salt
that the transition point is comparatively contiguous to the
melting point. In the case, they afford fine and polycrystalline
bodies, which are not suited to any practical use, e.g., as an
electro-optical element.
As the result of various investigations, the present inventors
previously provided a method for growing single crystals of
inorganic metal salts which comprises cooling a melt of an
inorganic metal salt having a transition point admixed with a
melting point depressing agent in an amount that the melting point
of the inorganic metal salt is lowered below the transition point.
The melting point depressing agent for the said method should have
such a small segregation constant that it is sufficiently
segregated from the mother melt on crystallization. In this
respect, for instance, potassium chloride and strontium chloride
are effective melting point depressing agents for cuprous chloride.
However, it is inevitable that the afforded single crystal is
contaminated with the melting point depressing agent in such a very
small amount that the contaminant can not be detected by a usual
analytical method, even though the most suitable melting point
depressing agent is employed. As well known, trace contaminant
affords more or less an influence of the electric and optical
characteristics a single crystal, and such influence may be
regarded as significant depending on the use of the crystal. Thus,
the said method can afford single crystals being large enough to be
practically used but can not give extremely pure ones.
Considering the reason for the difficulty in growing a single
crystal of an inorganic metal salt having the transition point
contiguous to the melting point, the temperature gradient at the
crystal-growing portion in the growing direction is considerably
steep in a conventional crystallization method such as the
so-called ascending method (e.g. Czochralski Method Kyropoulos
method) or the so-called temperature gradient method (e.g. Bridgman
method, Stockbarger method), and the solid-liquid transition
interface (i.e., an isothermal interface corresponding to the
melting point) where the inorganic metal salt is crystallized from
the melt in a stable structure of a higher temperature is close to
the solid-solid transition interface (i.e., an isothermal interface
corresponding to the transition point) where the transition of the
crystal from the stable structure of a higher temperature to that
of a lower temperature takes place. For instance, in the case of
cuprous chloride, the interval between the said two interfaces is
less than 1 cm., i.e., several mm. Therefore, the strain caused by
the solid-solid transition is propagated to the solid-liquid
transition interface where crystallization is taking place, and the
propagated strain results in the occurrence of undesirable crystal
nuclei which prevent the growth of a single crystal. The said
method using a melting point depressing agent overcomes this
difficulty caused by the crystalline transition. As above
mentioned, however, it can not avoid the contamination of the
obtained single crystal with a trace amount of the used melting
point depressing agent.
Through the advanced investigation, it came to the notice of the
present inventors that a single crystal of an inorganic metal salt
having a transition point can be readily grown at the temperature
range from the melting point to the transition point in the same
manner as the one lacking a transition point and also that
difficulty is present in how to cool the thus-afforded crystal in a
stable structure of a higher temperature to a lower temperature,
e.g., room temperature, through the transition point. Then, the
polycrystallization occuring when a single crystal in a stable
structure of a higher temperature is cooled through the transition
point while keeping a unified temperature gradient in a whole
crystal was interpreted to be caused by initiation of the
solid-solid transition at innumerable points in the crystal and
growth of the crystals in a stable structure of a lower temperature
from the said innumerable points. If this was correctly
interpreted, the gradual cooling of a single crystal in a stable
structure of a higher temperature through the transition point from
an end of the crystal to another end of the same should afford the
corresponding single crystal in a stable structure of a lower
temperature. The correctness of the interpretation has been
evidenced by the experiments, and there have been successfully
produced single crystals of inorganic metal salts being
satisfactory in size and purity and suitable for electric and
optional use.
Accordingly, a basic object of the present invention is to embody a
single crystal of an inorganic metal salt having at least one
transition point. Another object of this invention is to embody a
method for growing a single crystal of an inorganic metal salt of
which the transition point is contiguous to the melting point. A
further object of the invention is to embody a method for growing a
single crystal of an inorganic metal salt possessing a practically
utilizable size in high purity. These and other objects will be
apparent to those conversant with the art to which the present
invention pertains from the foregoing and subsequent
descriptions.
The method of this invention comprises cooling a single crystal of
an inorganic metal salt having at least one solid-solid transition
point in a stable structure of a higher temperature in such a
manner that the crystal passes through the transition point from an
end of the crystal to another end of the same to obtain the
corresponding single crystal in a stable structure of a lower
temperature. The present method can be applied equally to any
inorganic metal salts so far as they have one or more transition
point(s). Examples of such inorganic metal salts are cuprous
bromide, cuprous iodide, zinc sulfide, cadmium sulfide, etc. For
the convenience of illustration, however, cuprous chloride is taken
as an example in the following disclosure.
Cuprous chloride has a melting point at 422.degree. C. and a
transition point 407.degree. C. That is, the crystal is formed in
the wurtzite structure at a temperature from 407.degree. C. to
422.degree. C. and in the zinchblende structure at a temperature
lower than 407.degree. C. Accordingly, when a melt of cuprous
chloride is cooled to a room temperature in a conventional manner,
there are formed polycrystals in the zincblende structure, of which
size is at the largest about 3 mm.
To grow a single crystal of cuprous chloride in the zincblende
structure according to the method of this invention, it is
necessary first to form a single crystal in the wurtzite structure,
i.e., a stable structure of a higher temperature. For this purpose,
a melt of cuprous chloride may be cooled from an end to another end
to a temperature ranging from the melting point to the transition
point. Then, the crystal in the wurtzite structure is gradually
cooled so that is passes through the transition point from an end
to another end, whereby there is formed a single crystal in the
zincblende structure, i.e., a stable structure of a lower
temperature. It is not necessary to initiate the transition from
the wurtzite structure to the zincblende structure after completion
of the crystallization of the whole melt of cuprous chloride into
the wurtzite structure. That is, the solid-solid transition can be
performed simultaneously with the liquid-solid transition unless
the strain caused by the solid-solid transition exerts influence on
the liquid-solid transition interface. For instance, an interval of
more than 1 cm. between the two transition interfaces is sufficient
for this purpose.
In view of the object of the present invention, the starting
cuprous chloride should be sufficiently pure. For instance, it is
suited to the present method, if neither turbidity nor coloration
is observed on melting. The purification of a commercially
available cuprous chloride may be effected in conventional
procedures, e.g., by washing with a solvent such as glacial acetic
acid, ethanol, acetone or the like, recrystallization from
concentrated hydrochloric acid, sublimation under reduced pressure
and zone melting under reduced pressure.
The crystallization may be performed in such a tube usually
employed for crystallizing a single crystal as made of quartz glass
or hard glass. The glass tube that a carbon film is formed on the
inner wall is advantageous for prevention of wetting with the melt
of cuprous chloride, the occasionally occasionally resulting in the
occurrence of strain on cooling which may sometimes lead to the
production of cracks and polycrystalline bodies. The formation of
the carbon film may be preferably effected by decomposing an
organic compound such as methane, ethane, ether, benzene or acetone
on the inner wall of the glass tube while heating. Although the
carbon film can be also formed by any other conventional procedure
(e.g., application of colloidal black lead), such film is apt to be
eliminated. It is also advantageous to use the glass tube having a
specifically formed bottom part for selecting a crystal nucleus so
that the production of polycrystalline bodies can be inhibited.
Such glass tube consists of an upper part for growing a crystal and
a lower part (i.e., a bottom part) for selecting a crystal nucleus,
these parts satisfying the following requirements: (a) the former
and the latter being formed as a body intervening a neck of which
the opening has a diameter suitable for introduction of a melt and
selection of a crystal nucleus; (b) the latter having at least one
curve so that the former and the latter are not coaxial; and (c)
the said opening being not within the solid angle viewed from the
end of the latter along the wall of the glass tube. Some typical
examples of the glass tube provided with the above requirements are
disclosed in our copending application, Ser. No. 544,605, filed
Apr. 22, 1966 now U.S. Pat. No. 3,433,602.
The cooling manner per se may be effected according to a known
method (e.g., Bridgman method, Stockbarber method).
By application of the present invention, there can be readily
obtained the substantially unstrained and highly pure single
crystal of cuprous chloride in such a size as practically
utilizable (e.g., cylindrical single crystal of 20 mm. in diameter
and 50 mm. in length).
The present invention has been hereinabove illustrated on the
production of the single crystal of cuprous chloride. However, it
is clear that this invention can be generally applied for the
production of single crystals of inorganic metal salts having a
transition point in the substantially same manner as in the
production of the single crystal of cuprous chloride .
Practical embodiments of the present invention are illustratively
shown in the following examples with reference to the attached
drawings.
Example 1
A. Purification of cuprous chloride:
Commercially available cuprous chloride (reagent grade) is washed
with glacial acetic acid, ethanol and ether in order in nitrogen
atmosphere and dried at 75.degree. to 100.degree. C. in nitrogen
stream. The resultant cuprous chloride is charged in a transparent
quartz glass tube. After evacuation by heating at 300.degree. C.
under reduced pressure for 5 to 8 hours, the quartz glass tube is
sealed and subjected to zone melting purification with a zone
temperature 550.degree. to 600.degree. C. and a rate of movement of
8 cm. per hour. The color of the melt is dark green to green until
passing about 10 zones and then becomes blackish brown to yellowish
brown while passing further zones. Finally, the solid part is made
colorless and transparent. The purification is accomplished by
passing about 20 zones.
B. Formation of carbon film in crystallizing tube:
An end of a transparent quartz glass pipe is sealed to form a
bottom part as shown in FIG. 1, the ultimate end shaping a cone of
about 60.degree. in vertical angle. The resultant tube is heated at
600.degree. to 800.degree. C. under reduced pressure and vaporized
acetone is introduced therein. The acetone is decomposed to form a
carbon film on the inner wall of the tube.
C. Growth of single crystal:
The quartz glass tube prepared above is heated under reduced
pressure to eliminate the air therein, charged with cuprous
chloride, heated at about 300.degree. C. under reduced pressure for
5 hours and then sealed. The quartz glass tube is suspended in a
vertical furnace as shown in FIG. 2 (a) having a temperature
distribution as shown in FIG. 2 (b) and gradually descended. In the
zone I (the temperature (T.sub.1 ) being higher than the melting
point of cuprous chloride (MP), e.g., 450.degree. to 500.degree.
C.), the melt of cuprous chloride is formed. Subsequently, in the
zone II (the temperature (T.sub.2) being lower than the melting
point of the cuprous chloride (mp) and higher than the transition
point (TP), e.g., 410.degree. to 415.degree. C.), a single crystal
in a stable structure of a higher temperature (the wurtzite
structure) grows. After the whole melt has been crystallized into a
stable structure of a higher temperature, the end of the tube
reached the zone III (the temperature (T.sub.being lower than the
transition point (TP), e.g., 350.degree. to 400.degree. C.), where
the solid-solid transition is initiated. Finally, the whole crystal
is changed to a single crystal in a stable structure of a lower
temperature (the zincblende structure). The speed of descent should
be decided by the temperature gradients of the zones II and III. In
this example, a good result is achieved with a speed of about 0.5
mm. per hour, when the temperature gradient of the zone II is
10.degree. C. per cm. and that of the zone III 4.degree. C. per
cm.
Example 2
The purification of cuprous chloride, the formation of carbon film
in crystallizing tube and the growth of single crystal are executed
as in example 1 A, B and C but using a vertical furnace as shown in
FIG. 3 a having a temperature distribution as shown in FIG. 3 b or
b'. Contrary to the foregoing example 1 wherein the distance of the
zone II D is longer than the length of the sample L so that the
solid-solid transition takes place after completion of the
liquid-solid transition, the length of the sample L is longer than
the distance of the zone II D in this example. Therefore, the
solid-solid transition takes place before completion of the
liquid-solid transition. In the case, the distance of the zone II D
is adjusted to such a distance that the strain caused by the
solid-solid transition does not prevent the growth of the single
crystal at the liquid-solid interface, i.e., more than 1 cm.
* * * * *