Note: Descriptions are shown in the official language in which they were submitted.
1034~68
This invention relates to a process for growing single crystals
from a melt.
The processes contemplated in this invention are based on the con-
ventional procedure of cooling a body of molten material in a crucible con-
tained in a furnace. Such processes are known to have several disadvantages,
all of which detract from the quality of the crystal produced. Typical ones
are turbulence in the melt, the presence of gas bubbles, the presence of
adventitious impurities, localised super cooling, and other problems associated
with high temperature gradients within the melt.
It has now been discovered that many of these problems are overcome
if the crucible is cooled from the bottom in such a way that the crucible
,.
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walls are maintained above the melting point of the material being crystal-
lized until crystal growth is substantially complete. In all processes accord-
, - ~
ing to the present invention, the material within the crucible is grown into
a single crystal by independently controlling the temperatures of the crucible
' $
-~' side walls and bottom to provide the desired and necessary temperature grad-
ients in the solid and liquid portions of the material within the crucible.
:~ .
' Thus in one embodiment this invention provides a proceiss for growing; single crystals including the steps of placing material in a crucible, heating
the crucible to above the melting point of the material to melt it, and there-
after solidifying the melted material by extracting heat from a bottom portion
of the crucible wherein the temperature of at least those portions of the
-~ side walls of the crucible that are in contact with the material within the' 2, crucible are maintained above the melting point of the material and simultane-
s ously the temperature of said bottom portion is reduced below the melting
point of the material, until~substantially all the material within the cruc-
~ ible has solidified.
i The crucible is made from a material chemically inert with respect
-~ to the material being crystallized : typical materials are refractory metals
.:!
~ 30 (molybdenum, tungsten, iridium, rhenium, etc), high purity graphite, or
'7 quartz. Conveniently the crucible is substantially cylindrical, with a height
. equal to or greater than its radius. A key feature of this process ~ the
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1~:)38268
independent control of crucible wall and bottom temperatures. It is there-
fore desirable that heat flow between the walls and bottom be obstructed as
much as possible. Conveniently this is achieved by incorporating a thin wall
portion, for example by means of an external groove, in the crucible adjacent
to its base.
Preferably heat is removed from the crucible by means of a circular
heat exchanger engaged substantially centrally with the base of the crucible.
Preferably the diameter of the crucible is from about two to about ten times
the diameter of the heat exchanger.
` 10 The rates at which the crucible walls and bottom are cooled during
crystal growth are critical. Therefore the size of the heat exchanger
ideally is determined by the particular material being crystallized. For
~,
ceramic materials, such as sapphire, the bottom temperature should not
decrease at more than 50C per hour, whilst for a metal such as germanium a
. ;~
rate of 25C per hour should not be exceeded. Control of the heat exchanger
â
can be achieved in several ways, when its area cannot be varied. The easiest
and most convenient are to carefully control coolant flow rate through it
(generally a gas, for example helium for high melting point materials) and to
place insulation between it and the base of the crucible.
~, 20 It is also preferable to use an initial cooling period during which
,7 only the base of the crucible is cooled, whilst the walls are maintained at
.~` a substantially constant temperature. Conveniently this constant temperature
~t is at least 50C above the melting point of the material.
: `
After solidification is complete, it is desirable to cool the crystal
~, boule to room te~perature in such a way as to minimise thermal stresses there-
in. Generally this is achieved by cooling the whole crucible, by decreasing
the heat input to the walls, whilst decreasing (until finally stopped) heat
~ removal through the exchanger, whereby a temperature about 50C below melting
`~ point is obtained. The boule is then held at this temperature for a suitable
time to anneal it, generally for a few hours. The boule is then cooled to
roo~ temperature at a rate of about 50C per hour by decreasing the furnace
heat input to the crucible walls.
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~ ~038268
The furnace can be operated either evacuated, for example when
growing sapphire crystals, or under pressure, for example when growing
gallium phosphide crystals.
` In some growth processes, it is desirable to use a seed crystal.
It has been found that a seed crystal can be used in the process of this
invention by varying it slightly.
Thus in an alternative embodiment this invention provides a process
for growing single crystals including the steps of placing material to be
crystallized in a crucible, together with a seed crystal located in said
crucible on its bottom surface adjacent a heat exchanger applied to the out- ~`
side surface of the bottom of the crucible, the seed crystal being not smaller
than the area of the crucible bottom covered by the heat exchanger, melting
the material by heating the crucible walls, whilst maintaining the seed
s crystal at a temperature below its melting point by means of the heat exchanger
~ subsequently solidifying the melted material both by extracting heat from the
-s bottom portion of the crucible by means of the heat exchanger and simultane-
'~.`J~ ously maintaining at least those portions of the crucible side walls that are
~ in contact with the material within the crucible at a temperature above the
-' melting point of the material, until substantially all the material within
the crucible has solidified.
-~ The extent to which the crucible side walls are super heated above
-~ the melting point of the material to be crystallized depends on several
factors, particularly the thermal conductivity of the material, the desired
,.
i growth rate, and the ratio of crucible bottom area to heat exchanger area.
-;
`~ Typically the super heating is about 50C for processes involving relatively
,~ slow growth rates, materials having higher thermal conductivities, and/or
:i crucible-heat exchanger combinations with low area ratios, a higher level of
super heat, to about 100C, or sometimes more, might be found necessary.
The level of super heat used will in form determine how much cooling of the
walls - bearing in mind that the wall is kept above melting point - is
required. If the wall is kept too hot, thermal balance will result with
nok all the material solidified, unless an excessive amount of heat is removed
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103B268
~ through the heat exchanger. If only a small amount of super heat is used,
;~ no cooling of the walls, prior to the final cooling of the boule to remove it
`~ from the apparatus, might be needed in order to produce an adequate boule.
Generally, some cooling of the crucible walls is desirable.
The invention will now be described by way of reference to a typical
apparatus wherein the process may be operated, the manner of operation of the
process in this apparatus, and Examples of its use. Reference is also made
~ to the attached drawings in which:
; Figure 1 is a plan view, somewhat schematic, of a system used in
. ~ 10 the practice of the present invention;
; ~ Figure 2 is a perspective view, partially in section, of portions
; of the system of Figure l; and
- Figures 3a-3d are diagrammatic views illustrating various stages
in the growth of a large single crystal using the system of Figures 1 and 2,
according to the present invention.
Referring more particularly to the drawings, there is shown in
Figure 1 a vacuum graphite resistance furnace 10 ~manufactured by Advanced
` Vacuum Systems of Woburn, Mass.) connected to a vacuum pump 12. Within
furnace 10 is a double-walled heating chamber, generally designated 14. As
' 20 shown, ~he outer walls ~peripheral side; top and bottom) of heating chamber
14 are of stainless steel and are spaced from the adjacent walls of vacuum
- furnace 10. Heating chamber 14 is supported within the vacuum furnace by
- an annular flange 16 projecting inwardly from the cylindrical wall 11 of
furnace 10 and engaging the outer rim of the bottom 15 of chamber 14.
? The inner walls of heating chamber 14 are defined by a cylindrical
graphite sleeve 18, a top cover plate 20, and a bottom plate 22. The volume
between the inner and outer walls is filled with graphite felt insulation 24.
To permit access into the interior of the heating chamber, the top 13 of
vacuum furnace 10 and the top 17 of heating chamber 14 ~including graphite
top plate 20, stainless steel top 19, and the insulation 24 between the two
top plates) are removable.
A cylindrical resistance heater 26 is mounted in the cylindrical
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1038268
cavity 28 within heating chamber 14. The electrical power and control leads
30 of the heater pass through the peripheral walls of the heating chamber 14
and furnace 10.
A helium-cooled, tungsten/molybdenum heat exchanger 32 is mounted
on the bottom of furnace 10 and projects into the furnace and then through
a graphite sleeve 33 extending through the bottom of heating chamber 14 up
; into cavity 28. This heat exchanger 32, is of the type described in U.S.
Patent No. 3,653,432, and includes a base segment 34 secured to the outside
of the bottom of furnace 10, and a hollow cylindrical rod segment 36 extending
from base segment 34 into cavity 28. The top 38 of rod segment 36 is flat.
A tungsten inlet tube 40 and a thermocouple 44 extend within heat exchanger
32 from below base 34, through rod segment 36 to closely adjacent top 38.
An outlet tube 42 extends from an outlet aperture ~communicating with the
interior of rod 36) in base 34. Inlet tube 40 and outlet tube 42 are bolh
connected to a helium source 45. Helium from source 45 can be either recir-
. .
. culated or, if desired, released into the atmosphere.
.. -. . i
. As shown most clearly in Figure 2, the refractory crucible 48 in
- which the crystals are grown is supported within cavity 14 by the top 38 of
~ the heat exchanger 32 and eight tungsten plates 50 mounted vertically in
--'; 20 radially extending grooves 52 in the upper surface of a graphite support
plate 54. Support plate 54 rests on bottom plate 22. Grooves 52 in plate
54 are regularly spaced at 45 intervals. Each tungsten plate 50 engages
. ~ ~
the outer annular portion of the bottom of crucible 48. Heat exchanger rod
segment 36 extends through a hole 55 in the center of support plate 54, and
the flat top 38 of the rod segment engages the center of the bottom 49 of
-l crucible 48.
The top of the crucible is covered by a cover plate 60, made of
~- the same material as crucible 48, having a sight hole 62, one inch in diameter,
- in the center thereof.
Sight holes 64, 66 extend through, respectively, the top 13 of
furnace 10 and the top 17 of heating chamber 14, and are axially aligned with
~ sight hole 62 in crucible cover 60. Sight hole 64 through furnace top 13 is,
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1038268
of course, vacuum tight and is defined by lens assembly 68. Sight hole 66
through heating chamber top 15 is defined by a cylindrical graphite sleeve
extending between the double walls 19, 20 of heating chamber top 17.
Two other sight hole assemblies, generally designated 70 and 72
respectively, permit the temperatures of heating element 26 and vertical side
wall 56 of crucible 48 to be monitored during crystal growth. Each assembly
includes three axially aligned sight holes--one through the peripheral wall
11 of furnace 10, defined by a vacuum tight lens assembly at the cylindrical
periphery of the furnace 10, and designated 74, 76 respectively; a second,
defined by a graphite sleeve extending through the cylindrical double side
wall of heating chamber 14 and designated 78, 80 respectively; and a third,
extending through heating element 26 and designated 82, 84 respectively. -
Pyrometers 71, 73 are mounted adjacent the exterior end of, respectively,
sight hole assemblies 70, 72. As shown, sight hole assembly 70 is located
so as to permit pyrometer 71 to view the interior surface of the far vertical
wall of heating element 26, just above the top of crucible 48. Sight hole
assembly 72 is below assembly 70 and permits pyrometer 73 to view the side
- wall 56 of crucible 48, about 1/2 inch above bottom 49 and just above thin
wall portion 58.
Pyrometers 71 or 73 and thermocouple 42 are connected to a con-
troller 85. One output of controller 85 is connected to the source of power
~` 86 for heating element 26. A second controller output is connected to
helium source 45. Controller 85 is responsive to the temperatures sensed by
pyrometers 71, 73 to vary the power supplied by heating source 86 as required
to maintain the temperatures of heating element 26 and crucible 48 at the
proper level; and to the temperature sensed by thermocouple 42 to vary the
flow from helium source 45 as required properly to vary the temperature of
heat exchanger top 38.
In practice, crucible 48 is first washed with, for example, nitric
acid and chlorox, to remove impurities. In those growth processes where it is
desirable, a seed crystal 100, shown in dashed lines in Figure 3a and having
an overall diameter slightly greater than the diameter of the top 38 of heat
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1038Z68
exchanger 32, is placed in the center of the bottom 49 of thc crucible. The
crucible is then filled with small pieces of the material to be melted. If
, a seed crystal is used, the first pieces are placed tightly around the
crystal to hold it in place. To obtain maximum loading, the pieces are all
placed in the crucible one-by-one and are fitted closely together.
The loaded crucible is then placed in heating chamber 14 with
' crucible bottom 49 seated on heat exchanger top 38. The height of the heat
exchanger, i.e., the distance it protrudes into the heating chamber, is
` determined by experimentation. The heat exchanger is positioned so that,
~- 10 when the crucible side walls are superheated over the melting point of the
material therein (typically about 50C), a relatively small flow of helium
` through the heat exchanger will prevent a seed crystal, if used, from melting.
,:, . .
~ As shown, the seed crystal slightly overhangs all sides of heat exchanger
: .~
~; top 38. Slots 52 are cut into support plate 54 to such a depth that, when
the crucible is cold, the tops of tungsten plates 50 are slightly below the
--~ crucible bottom. When the temperature of the crucible is increased, the
~ crucible slightly sags and its bottom 49 rests on plates 50.
--, Cover plate 60 is placed on the crucible with its sight hole 62
-~ axially aligned with the crucible and heat exchanger, and the tops 17, 13 of
~, 20 heating chamber 14 and furnace 10 are replaced. Vacuum pump 12 is then
-~ started and the furnace is evacuated to a pressure of about 0.1 torr.
-i When the furnace pressure has reached the desired level, heat source
:~ 86 is actuated. The power supplied to heating element 26 is gradually
. increased, typically so that the temperature within heating chamber will rise
at a rate not over about 250C per hour. The power supplied to the heating
element is increased until, as observed through sight holes 62, 64, 66, the -
material within the crucible begins to melt.
The first material to melt is the pieces adjacent the outer
cylindrical wall of the crucible. As soon as such melting is observed, the
temperature of heating element 26 (TF), crucible side wall 56 (Tw), and heat
.
; exchanger 36 (THE), as indicated by, respectively, pyrometers 71 and 73 and
thermocouple 42, are measured and recorded. Although the actual temperature
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1038Z6B
at which any particular material melts does not change, the melting point,
Tmp, indicated by the different instruments may vary somewhat depending on
such things as the contact between the thermocouple and crucible, size and
length of the sight hole assemblies, cleanliness of the windows, and the like.
For accurate control of the process it is important that the instruments be
calibrated.
When a seed crystal is used, it is important that it be kept from
melting. Therefore, helium source 46 is actuated to cause an initial flow
of room temperature helium, typically at a flow rate of about 40 c.f.h., as
soon as melting of pieces within crucible 36 begins.
The amount of power from source 86 applied to heating element 26 is
further increased to superheat the crucible side walls to above, typically
about 50C, the initial melting point. The power input is then held constant
until all temperatures within the furnace have stabilized.
At this stage, conditions are substantially as shown in Figure 3a.
.:~; .
The temperatures of the heating element, TF, and crucible side walls, Tw, are
substantially equal, and above (typically about 50C) the melting point of
the material in the crucible. All material within the crucible, with the
exception of seed crystal lO0 assuming one is provided, has melted to form
20 a liquid 102. The liquid has melted the edges of seed crystal lO0 (to the
extent shown by solid lines) to promote nucleation, but melting of the major
portion of the seed crystal above the Heat Exchanger has been prevented by
the flow of helium through heat exchanger 32. Because of the helium flow,
-~ the temperatures of the top 38 of heat exchanger 32, THE, and hence of the
adjacent engaged portion of crucible bottom 49, are below the melting point
- of the seed crystal, even though the temperatures of the heating element and
- crucible cylindrical wall are above the melting point.
Initial crystal growth is commenced by gradually increasing the
rate of helium flow through heat exchanger 32, typically at a rate of about
30 10 to 15 c.f.h. per hour, to slowly decrease the temperature of the heat
exchanger and increase the rate at which heat is drawn from the center bottom
of the crucible. Simultaneously, the amount of power from source 86 applied
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103826~
to heating element 26 is increased as required to maintain the temperatures
of heating element 26 and of crucible vertical wall 56 (as observed by
pyrometers 71, 73) constant.
. The duration of this initial period of crystal growth depends on the
size of the crucible and on the particular material being crystallized.
Typically it extends through about six to eight hours. At the end of the
` initial period, conditions are substantially as shown in Figure 3b. The
temperature of the top 38 of the heat exchanger, THE, has decreased to well
below the melting point, Tmp. The temperatures of the heating element 26 and
... .
- 10 cylindrical side wall 56 of crucible 48 are still at the initial superheated
level, typically 50C above the observed melting point. Crystal growth
~` (solidification of the liquid in the melt) has progressed to a stage where
-~ the solidified crystal or boule 104 is more or less ovoid in shape. The
', entire boule, with the exception of that portion overlying the top 38 of heat
exchanger 32, is surrounded by molten material and its exact size and shape
cannot directly be observed. The general shape of the boule is known from
~ the facts that the material at the top of the crucible is liquid, the entire -
side wall of the crucible is well above the melting point, and the portions
of the crucible wall adjacent the top and bottom of the crucible are even
hotter ~due to heat reflected from the top and bottom of heating chamber 14).
For crystal growth to proceed further, it is necessary not only
::
to continue to increase the rate of helium flow through the heat exchanger,
but also to decrease the temperature of the crucible vertical wall. During
the next crystal growth period, therefore, helium flow is further increased,
typically at the same rate of 10 to 15 c.f.h. per hour, and the observed
temperature of the top 38 of the heat exchanger continues to drop. Addition- -
ally, the power applied to heating element 26 is reduced at such a rate that
the temperatur~ of the heating element 26 and cylindrical wall of crucible
48 will slowly decrease, typically at a rate of less than 15C per hour and
preferably less than 5C per hour, until the observed temperatures have
reached a level about 5C above the observed melting point.
At about this time, solidification has advanced to the point shown
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1038~6~
in Figure 3c. The top of the solidified crystal boule 104 has just broken
through the top of the melt, as observed through sight holes 62, 64, 66.
Except for a thin annulus 106 of liquid between the boule 104 and the verti-
cal cylindrical wall of the crucible, which is still above the melting point,
the boule fills substantially the entire crucible. Annulus 106 is thickest
near its top 108 and bottom 110, which as indicated previously, are adjacent
the hottest points of the crucible.
- , To complete crystallization, the slow increase in helium flow and
slow decrease of furnace temperature are continued until it is observed
~through sight holes 62, 64, 66) that the only liquid left in the crucible
48 is a very thin film or miniscus, which runs back and forth over the top
of the solid crystal boule 104 and creeps over the side of the crucible. At
.. .
. this point, the temperature of the crucible side wall, except for the slightly
warmer, extreme top and bottom, is approximately equal to the melting tempera-
ture and solidification is substantially complete. The final miniscus is
solidified by further decreasing the power supplied to heat element 26, to
~ drop temperature of the heating chamber and crucible to slightly below the
-~- melt temperature.
`` EXAMPLE I
.
.:.
~ 20 A sapphire seed crystal was placed in a molybdenum crucible and
. .
` the crucible filled with cracked pieces of Verneuil sapphire. The filled
crucible was placed in the furnace, the furnace evacuated, and the power
, source turned on.
The power was increased at such a rate that the furnace temperature
2 increased at a rate of 250C per hour, and after about eight hours the sapp-
hire at the crucible side walls began to melt. When melting was observed,
the instruments were calibrated, and helium source was turned on to force
: helium through the heat exchanger at an initial rate of 40 c.f.h. The temper-
ature of the furnace was then furthQr increased until it was 50C. above the
observed initial melting point, and was held at this temperature for four
-. hours to permit conditions within the crucible to stabilize.
~ To commence crystal growth, the rate of helium flow through the
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1a3B268
heat exchanger was then increased from the initial flow of 40 c.f.h. per
hour, at a rate of about lO c.f.h. per hour, until the flow reached 100 c.f.h.
This period of flow increase extended through about 6 hours, during which
.
time the power applied to the furnace was adjusted as required to hold the
observed temperature of the crucible side walls constant, at 50C above the
` observed initial melting temperature.
For the next stage of crystal growth, which extended through
approximately 18 hours, the power applied to the furnace was decreased as
required to drop the observed temperature of the crucible side walls at a
rate of 3C per hour, and the rate of helium flow through the heat exchanger -~
was further increased, still at the rate of 10 c.f.h. per hour. When the -
temperature of the crucible side walls fell to a level only slightly above
the observed initial melting temperature, substantially all liquid in the
crucible had solidified. The only remaining liquid was a very thin and dis-
- continuous miniscus that ran back and forth from side to side over the top
of the boule. The miniscus was solidified by continuing to drop the crucible
side wall te~perature until it was slightly below the initial melting temper-
ature.
~` After solidification was complete, the flow of helium gas through
the heat exchanger was decreased at the rate of 100 c.f.h. per hour. At the
same time, the furnace power was decreased at such a rate that, when helium
flow through the heat exchanger terminated, the observed temperature of the
crucible side walls was about 50C below the initial melting point. The
furnace was then held at this temperature for two hours, after which the
power supplied to the furnace was again decreased, at the rate of about 50G
per hour, until it reach room temperature. The furnace was then opened and
~ the crucible and boule removed therefrom.
? EXAMPLE II
;j Single crystal sapphire was grown without a seed crystal. The
process of Example I was followed, with the following changes: -
; a. Sintered alumina pellets were used in lieu of cracked pieces
of Verneuil sapphire;
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~ 103826~
b. In place of a seed crystal, a molybdenum washer having a
restricted upper opening was placed in the center of the crucible bottom;
c. The heat exchanger was not activated until the crucible had
- been superheated to 50C above the initial melting point; and
d. The heat exchanger was then activated at a flow rate of 50
c.f.h.
EXAMPLE III
- To grow a germanium single crystal, the inside of a high purity
graphite crucible was machined very smooth. Since germanium is lighter in
the solid than in the liquid, the inside center bottom of the crucible was
formed in such a way as to hold the seed crystal in place over the heat
exchanger and prevent it from floating away when the germanium was melted.
A thin wafer of metal was placed on the top of the heat exchanger,
the seed and pieces of germanium placed în the crucible, and the crucible
placed in the furnace.
. . .
The furnace was then closed and heated, as in Example I, to 50C
over the observed melting point. Crystal growth was accomplished as in
- Example I, except that the rate of helium flow through the heat exchanger
was increased at a slower rate, about 5 c.f.h., since the thermal conductivity
J 20 and diffusivity of germani~m are much greater than those of sapphire. The
helium flow through the heat exchanger and power input to heating element
26 were both varied so that the rate of temperature decrease of the heat
exchanger did not exceed 50C per hour, and of the furnace did not exceed
5C per hour.
EXAMPLE IV
It is often desirable to grow single crystals of so-called 3/5
compounds. One of the most difficult to grow is gallium phosphide, which is
extremely unstable at its melting point. To prevent it from dissociating,
an inert gas pressure of 35 atmospheres and a llquid encapsulant of B2O3 are
required.
Because of the high pressure requirements, the apparatus of
Figures 1-2 was somewhat modified. The heating element and heating changer
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were placed in a high yressure graphite resistance furnace. In lieu of
sight hole assemblies 70, 71, Platinum/Platinum Rodium thermocouples were
provided at the crucible wall near its bottom, and centralized in the furnace
heat zone. ~eat exchanger 32 was made thick walled to withstand the high
pressure.
The gallium phosphide seed crystal was attached to the bottom of a
quartz crucible, the crucible filled with gallium phosphide pieces in a manner
similar to the loading of sapphire in Example I, and a B203 encapsulant
placed in the crucible.
The entire system was placed in a block house, and the thermocouple
- wires and control leads were taken to a remote control station. Since visual
~ inspection is not necessary, remote TV monitoring is not needed.
- The material in the crucible was melted and subsequently solidified
- as in Example I. The rate of temperature decrease of the furnace and heat
exchanger were controlled so as not to exceed, respectively, 10 and 75C
s per hour.
EXAMPLE V
Stoichiometric MgAl204 spinel single crystals were grown from the
i melt. A single crystal disc 2.5 cm. in diameter was used as a seed crystal,
and a molybdenum crucible was filled with mixture of high purity alumina
crackle and high purity magnesia chips in the correct proportions.
The crucible and material were heated to 1500C in a vacuum of
0.02 Torr, and the furnace was then backfilled with inert gas at an over- -
pressure of 0.3 atmospheres. The overpressure was maintained throughout the
crystal growth process, which proceeded as in Example I.
EXAMPLE VI
- A seed crystal was placed in a crucible as in Example I. A
' molybdenum screen and a molybdenum plate were placed vertically in the
crucible, on opposite sides of the seed crystal, with the plate extending
generally radially and the screen generally perpendicular to the plate. The
crucible was then filled with cracked pieces of sapphire, and crystal growth
accomplished as in Example I.
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The sapphire grew through the screen without change in crystal
orientation. A good bond existed between the sheet and the sapphire crystal,
and there was no cracking or change in orientation.
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