Note: Descriptions are shown in the official language in which they were submitted.
63~ ~
This invention relates to a process for the manu-
facture of hexagonal boron nltride.
Hexagonal boron nitride, also known as "~hite i`
graphite", has a number of valuable properties. It has a low
density, a low dielectric constant, a good resistance to
ternperature changes, and a good corrosion resistance. In a
- hot~-pressed form, it can be worked in a way similar to grap-
hite. It is also a good conductor of heat and an excellent
; electrical insulator. These properties make it a very useful
material for a number of purposes. It is useful in electronics,
~ecause of its outstanding dielectric properties. It is used
up to a temperature of about 2500C as a crucible material for
many fused non~Ametallic substances; which do not wet it. It is
chemically very inert, and in particular is more resistant than
` graphite to oxidation,, and it can therefore be considered as
one of the most valuable refractory nitrides.
Many processes for the manufacture of hexagonal boron
- nitride have been described, and a review of these is given in
Gmelin's Handbuch der anorganischen Chemie, supplement to 8th
edltion, vol. 13, part 1, pages 1~6 (Springer Verlag, 1974).
One commonly used industrial proces~ mentioned by
Gmelin is to cause horic acid or boron trioxide to react with
~ania at ahout 900C in the presence of tertiary calcium
phosphate. The calcium phosphate serves as a carrier to pre-
vent the boron oxide, which is liquid at the reaction tempera-
ture of - 700C, from fusing together whereby an approximate~
ly complete reaction with -NH3 becomes feasable. The steam
formed during the reaction according to the equation
B203 + 2NH3--~ 2 BN + 3 H20
is permitted to escape unimpeded.
In a British compilation in "Special Ceramicsl' by
Popper, London Heywood & Company, Ltd., 1960, p. 146~the re-
1- ' ~ .
~5~3fl~ ~
action is described as follows: "The reaction occurs at 800 ~
1200C. A solid filler is used to prevent f~sion of the
B203- -
Thus, without a carrier the B203, which is liquid
at that temperature, would form large agglomerations or lumps
- so that only at the surlace could some minor reaction take
place, but not in the interior. However, when a solid carrier
is present, the liquid ~23 is distributed in a thin sheet and
provides a larger surface for NH3 to react; moveover, hetter
permeability for gas is obtained in this manner, that facili
tates the removal of steam together with excess of gaseous
ammonia, such removal being necessary to prevent reversal of
the reaction illustrated by the above equation. This process,
however, has a nu~ber of disadvantages.-
Pirst, care has to be taken regarding the purity,and especially the moisture content, of the ammonia. Because
of the large volume of the reactants, the space~time yield is
poor. When operating in a continuous manner, it is necessary
to perform the reaction with a~monia in at least two separate
20- heating steps in order to achieve as quantitative as possible
a conversion of the B203 compound. The boron nitride obtained
after washing out the calcium carrier with dilute mineral acid,
generally has a purity of only 80 to 90%, thus necessitating
a further purification step. This may be carried out either in
a stream of ammonia at about 1200C or above, or in a stream
-of nitrogen or ar~on at about 1800C. Any boron oxide still re--
maining r then has to be washed out or removed in a further re-
action with ammonia. Furthermore, various comminution, homog-
enization, filtration, granulation, and drying steps are neces-
sary between the several reaction steps, and the chemical com-
position and particle size of the products has to be monitored.
` '
`
:-
3~ .
These steps are, of course, both time consuming and expensive.
Finally, t~e use of gaseous ammonia necessitates specia]. atten-
tion being paid to safety regulations and environmental pol-
lution. In general, the processes listed in Gmelin are un-
satisfactory ~or large scale industrial use : in many cases,
the yields are poor, whereas in other cases, the products have
to be purified by a number oE time consuming and expensive
stepsj as set forth.
It is an object of the present invention to provide
a process for preparing hexagonal boron nitride, which over-
comes the disadvantages of the known processes and permits
hexagonal boron nitride of high purity and good yield to be
obtained by simple operations without the use o~ additional .
carrier materials and the inconvenience of incurring environ-
-mental pollution.
The present invention provides a process for the manu
facture of hexagonal boron nitride, which comprises causing a
- starting ~aterial containing an alkali metal boride or an alka-
line earth metal boride to react with nitrogen at a temperature
2V of at least about 900C, in the presence of impurities or addi-
tives in addition to the boride.
- ~ It is particuiarly surprising that boron nitride can
be obtained in good yields and good purity by the nitridation
of an alkali metal boride or alkaline earth metal boride, since
it had previously been thought that alkaline earth metal borides
were inert to nitrogen even at high temperatures, calcium hexa-
boride bein~ thought to be inert to nitrogen even at tempera-
tures of up to 2000C (cf. N.N. Greenwood, _omprehensive In-
o ganic Chemistry, Pergamon Press 1973, vol. 1, p. 729). Liter-
30) ally, Greenwood states "The alkaline earth hexaborides are un-
,. affecteA by nitrogen or ammonia at high temperatures and CaB6
. is stable to at least 2000C." ~ctually, Greenwood went back
,._ , . ~ 1
~5~63~
to an original publication by M.I,. Andrieux in Rev. Met 32,
pgs. 487 - 493 (1935) according to which the investigated
CaB6 had been obtained by electrolysis and had almost theore-
tical composition : 61.84% B; 38.16~ Ca. It was, therefore,
very pure. Pa~e 490 states "It did not react with ~l2 even
when heated to 2000C. At that temperature, a heavy atta~k of
the yraphite vessel took place, in which it was heated." Thus,
this literature did not disclose the fact that CaB6 reacts
with nitrogen at temperatures above 2000C with ~ormation o~
~N. Moreover, by analogy with silicon, it would have been ex-
pected that any reaction of an alkali metal boride or alkaline
earth metal boride with nitrogen would produce a stable ter~
nary metal-boron-nitrogen phase, and that there would be con-
tamination with carbon or oxygen thus preventing the manufac-
ture of pure boron nitride.
Surprisingly, these expected difficulties do not
occur, and the present process enables boron nitride to be pro-
- duced in a relatively simple manner, without the use of a
carrier material, causiny additional costs without the need
2~ for a large number o~ purification steps, and with a good
- yield and purity. A better space-time yield i5 obtained with
-
the present process than with the previously described process,
since the boron-containing reactant is less voluminous and
richer in boron. The use of nitrogen has the advantage that
it does not entail pollution problems as does the use of
ammonia~ At temperatures akove 1400C~BN is obtained, which
is very stable against moisture and acids and no heating to
1800C ~or stabilizatinn is required as recommended in low
temperature processes.
Of the various alkali metal borides and alkaline
earth metal burides, it is pre~erred to use hexaborides and
'~,
105~3~ .
dodecaborides. Calcium hexaboride is a particularly suit-
able starting material, especially technical grade calcium
hexaboride. Calcium hexaboride can be manufactured on a
large industrial scale from various cheap boron ores, such as
colemanite. Processes for the manufacture of this and other
borides are described in German Auslegeschriften Nos.
1,22~,236 and 1,229,505. Mixtures of borides or mixed crystals
of borides can be used as starting materials.
It is characteristic for the invention that the
borides used in the present process contain impurities or
additives. In fact, such impurities or additives will aid
~ in achieving an improved yield, a lowering of the nitridation
- temperature, or a purer final product. ~ome of the impuri-
ties may be present in the Ca~6 used. They may, moreover,
deliberately be added to the borides to the same end. Par-
ticularly suitable additlves and/or impurities are carbon,
boron carbide, boric acid, boron trio~ide, carbonates,
borates, hydroxides and oxides of alkali or alkaline earth
metals, as well as iron and iron oxide combinations of im-
purities and/or additlves can result in the formation of ad-
ditional boron nitride from boron or boride formed in sltl~,
by carbothermal reduction as described in greater detail below.
The presence of these impurities and/or additives
can, of course, result in the formation of products in addition
to boron nitride, but these can, in general be removed either
in the gas phase during the reaction or by wet chemistry with
dilute mineral acids, e.g., hydrochloric acid.
The starting material advantageously has a boride
content of at least 10~ by weight, and preferahly of from 40
to 98% by weight.
.
_r~_
jl
3~
Moreover, the starting material is advantageously .
,
in the form of particles having an average particle siæe of
less than 10 ~ and a specific surface area measured by the
BET method of greater than 1 m2/g~ Preferabiy, the average
~: particle size is less than 3 ~n and the specific surface area
is greater than 5 m2/g., especially greater than 20 m2/g.
Various known methods of intensive co~inution and gxinding
may be used to obtain a starting material of the desired par-
ticle size; jet mills or agitator ball mills may, for example,
~- 10 be used.
In order to facilitate the circulation of the nitro-
gen through the boride powder, it is advantageous for the
powder to be made up into porous granules. Such granules
may suitably be in the form of cylindrical moldings having a .
diameter of from 3 to 15 mm and a length of from 10 to 50 m~.
The granules should (when dry) advantageously have a ratio of
bulk density of the granules to the density of the powder of
not more than 70%, preferably not more than 60%. A suitable
- r,lethod of preparing the granules is to work the boride powder
in~o a stif~ paste, uslng a binder (for example, 1% Polyviol
solution, (trademark for polyvinyl alcohol owned by Wacker
Chemie GmbH)or 3~ boric acid solution) if necessary, processing
the paste on an extruder to form cylindrical moldings, and
drying these at a temperature oE up to 400C, preferably about
250C. In some cases,for example, if the boride powder con-
not .
tains adhering boron oxide, the use of a binder may/be neces-
- sary~
The process is carried out at a temperature of at
least 900C. Advantageously, it is carried out ~n a stream of
nitrogen, in which case it is preferable to use a temperature
of at least 1400C, especially of from 1600 to 1~00C. It is
.
6--
105~3~ ~
also preferable for the nitrogen and the boride to be flowing
in counter--current to one another. The process may, alter-
natively, be carried out in a stationary manner, in which .
case the nitrogen may be used at an elevated pressure and in
- which case it is advantageous to use a temperature of from 900
to 1400C.
The gas used for the reaction with the boride should
contain at least 90~ by volume of nitrogen.. Nitrogen gas of
technical pur~ty may be used in the present process, since the
impurities present in conventional nitrogen gas do not inter-
fere with the reaction. In the case of nitrogen-containing
gases, the nitrogen may be admi~ed with, for exaniple, carbon
monoxide, air or oxygen. Industrial nitrogen-containing gas
mixtures, for example, generator gas ~nitrogen-carbon monoxide
mixture~ and cracked ammonia (nitrogen--hydrogen mixture), may
be used. The rate at which the nitridation reaction proceeds
increases with increasing temperature. An upper limit is im-
posed on the reaction temperature by the fact that boron ni-
tride begins to dissociate at about 3000C.
The manufacture and pur.ification of the boron ni-
tride can be carried out in various ways. For example, it is
` possible to carry out the manufacture and purification in one
process stage, that is to say without a separate purification
stage using wet chemistry subsequent to the nitridation. In
this case, the process is generally carried out at a tempera-
ture above the boiling point of the alkali metal or
- of the
alkaline earth metal~boride. In this method, it is advan-
*ageous, especially when using an alkaline earth metal boride,
.. either to use reactants (boride and nitrogen) that are free of
oxygen or to remove any oxygen present by doping with the
calculated a~ount ol carbon or a suitable carbon-containing
.~ ~
- -7` . ~1
7A ' . I i
'
~35~34
compound; thus forming carbon monoxide. This prevents the
formation of stable metal oxides or metal borates, in the
final product. In some cases, carbon or a carbon-containing
compound may be added to aid in the removal of the metal vla
the gas phase. When using calcium boride, for example, the
addition of carbon or boron carbide results in the formation
of calcium cyanamide, which can be sublimated without decom--
position in a nitrogen stream at a temperature of at least
- 1~00C.
This method of carrying out the process is advan-
tageously performed in a discontinuous, or batchwise, pro
duction. This is because long reaction times are generally
required for complete removal of by products vla the gas phase,
and because volatilised by-products may condense in the cooler
parts of ~e reactor, which, in a continuous method, could
cause blockages.
If, by a suitable choice of starting material,
nitrogen-containing gas, and/or reaction conditions the by
products will not, or not wholly, be removed _ia the gas
. ~.20 phase, they can be removed by wet chemistry. The solid by-
- products may suitably be removed by treating the product .
.~ ~ ~ith a dilute mineral acid, for example dilute hydrochloric
acid, thus dissolving the by-products and leaving the insoluble
boron nitride. This method of nitridation, lends itself to a
simple and economical production in a continuous manner.
One advantage of this method of operation is that
it enables the reducing action of the metal formed inter-
- mediately from the metal boride to be utilized in reducing
any oxygen-containing boron compounds, for example, boron-
trioxide, or alkali metal or alkaline earth metal borates,
that are present in the reaction mixture. This enables addi-
~.
.. 1,;
Si 16 ~3~ 4
ional boron nitride to be obtained from the boron in the
boron-oxygen compounds. A similar effect can be achieved by
the presence of carbon or boron carbide in the reaction mix-
,;
ture.
When using calcium hexaboriae as the boride withadhering boron oxide, an optimum starting composition (assum~
ing the calcium hexaboride to be stoichiometric and carbon-
free) is 82~ by weight calcium hexaboride and 18% by weigh'c
boron trioxide, in accordance with the equation
- --l 10 3 CaB6 ~ B203 ~ 10 N2 20 BN + 3 CaO.
If the boron trioxide content is higher than the optimum
~ amount, it lS advantageous to ~ix sufficient carbon with the
- boride to ensure that all the boron oxide is converted to
-~ boron nitride.
- It is, moreover, ~ossible to use a high proportion
of boron oxide and to allow the latter reaction to proceed
predominantly, alongside the nitridation of the boride. In
this case, in contrast to a process solely using boron trioxide
and carbon with no boride, carbon-free boron nitride is obtain-
ed in high yield.
The starting material may also comprise a combina--
tion of substances which, on heating , under
an inert gas or in vacuo, will give a boride. Such a com-
bination is, for example, calcium oxide, boron trioxide and
carbon in a molar ratio of 1 : 3 : 10 respectively. In batches
which are rich in boron trioxide, the boride or the boron nit-
ride produced therefrom, acts as a carrier or diluent for the
- boron trioxide thus preventing it from stic~ing and avoiding
the associated processing and engineering difficulties.
I~ was further found that calcium hexaboride con~
~ .
taminated with up to 10~ by weight of iron can be used quite
.
~9_
,
.
63~
successfully in the present process, since the iron can read-
ily be removed after nitridation from the boron nitride by
treatment with a dilute mineral acid, e.g. hydrochloric acid.
A suitable reactor for carrying out the present
process ln continuous manner at a temperature of up to 2000C
consists of a vertical graphite tube heated by the direct
passage of a current, through which granules of the boride
starting material may be passed downward under their own
weight while nitrogen may be passed upward. Such tubes are
known by the name Tamman furnace. If the nitridation is in~
complete after one passage of the material through such a
- furnace, for example because of too short a residence time in
the zone of maximum temperature, nitridation may be completed
by a second passage through the furnace, or by passage through
a second furnace. In this case, it is advantageous for the
product from the first passage to be homogenized by a short
grinding step and again formed into granules, pribr to the
second passage. The COmpQsitiOn of the mixture may also be
altered at this intennediate stage, if desired.
If it is desired to use temperatures exceeding
- 2000C in order to achieve higher reaction rates, the process
- may, for example, be carried out in a plasma jet furnace.
- The following examples illustrate the process of the
invention. Percentages are by weight unless stated otherwise.
Example 1
10 g of lithium dode~aboride powder, having an
analytical composition of 87.~5% B (total), 5.20% Li, 0.90%
C and 4.59-O B203, and a specific `(BET) surface area of 7.6
m2/g, was mixed with a little water to form a paste and then
pressed in an extruder to give cylindrical moldings of a di-
ameter of about 8 mm and a length of 15 to 30 mm. The moist
moldings were dried in a circulating-air over at 250C. The
--10--
," 1~.
11~516~4
dried granules were then introduced into the heatin~ zone
(zone of maximum constant temperature~ of a carbon tu~e-short
`~ circuit Tammann furnace; the inside of the heating tube o~
which had been lined with a thin protective layer of boron
nitride, in order to prevent as far as possible a reaction of
the granules with the carbon of the heating tube. After suf-
ficient flushing (300 litres/h) of the installation with
technical grade nitrogen (main impurity about 0.1% of oxygen)
from a cylinder, the furnace was heated up under a stationary
, 10 nitrogen atmosphere, to 2000 + 100C, and this temperature was
maintained constant for 60 minutes. The furnace was then
switched off and cooled to room temperature under a nitrogen
atmosphere. After con~inution of the product in a fixed-
~ hammer mill, 19 g of boron nitride ppwder having a nitrogen
content of 55.65% were obtained. An X-ray diffractometer
diagra~ of the material obtained showed only lines of the
hexagonal modification ~ boron nitride. The yield of BN was
94%.
Example 2
The procedure followed was as in Example 1, ~ut with
the difference that magnesium dodecaboride powder, having an
analytical composition of 80.01% B (total), 14.Z4% Mg, 0.40%
C and 1.71% B203, and a specific surface area ~ 14.0 m2/g,
was employed as the starting material and a stream of nitrogen
was used. 17 g of B~, which according to X-ray examination
- was a single phase and contained 54.30% of nitrogen, were ob- tained. Yield of BN 93%.
Example 3
74 g of a pulverulent mixture, consisting of 96~ of
-30 CaB6, having an analytical composition of 58.31% B (total~,i
35.47~ Ca, 4.17% C and 3-03% ~23' and a specific (BET) sur-
face area of 15.1 m2/g, and 4~ af carbon in the form of car~on
~ o5~G3~ , ,
black, were formecl into granules using.a 1% solution of
Polyviol. After nitriding the granules in a stream of ni-
trogen at 1800 + 50C for 3 hours, 85 g of boron nitride
~- with a nitrogen content of 55.15% were obtained. Yield of BN:
92%.
., , ~ .
57 g of calcium bori~e powder, having an analytical
composition of 62.14% B (total), 36.15% Ca, 0.75% C and 1.29
B203, and a specific (BET) surface area of 13.8 m2/g, were
granulated using a 1% sol.ution of Polyviol. After nitriding
the clried granules in a stream of nitrogen at 1800 + 50C for
one hour, 86 g of a light grey reaction product were obtained,
having an analytical composition of 46.45% N, 37.17% B ~total),
8.84% Ca, 0.18% C and 1.28% B203. An X-ray examination showed
the presence of BN as well as small amounts of calcium hydrox-
ide. After treating the powdered nitridation product with
.
dilute hydrochloric acid, 75 g of white BN, having an analyti
-~ cal composition of 55.51% Nj 44.31% B, C0.1% Ca and 0.1% C,
were obtained. Yield of BN: 93%.
Example 5
50 g of a pulverulent mixture, consisting of 85% of
; CaB6 of the same composition as in Example 4 and 15% of car-
bon in the form of carbon black, were formed into granules
using a 1% solution of Polyviol~ The drie~ granules were then
heated at 1600 + 50C in an atmosphere of flowing nitrogen
for 60 minutes. Obtained were 80 g oF a light grey product,
having an analytical composition of 45.50% N, 30,73% B (total),
15.85% Ca, 2.73% C and 1.95% B203V After treating the nitri-
dation product with dilute hydrochloric acid, 54 g of BN,
having an anal.ytical composition of 54.40% N, 43.44% B, 0.37%
Ca and 0.50~ C, were obtained. Yield of BN: 88~.
-12-
1~51634
~ le 6
The procedure followed was as in Example 5, with the
difference that 50 g of a pulverulent mixture, of 65~ of CaB6,
of the composition as in Example 4, and 35% of ~13B03, were
employed 63 g of a white nitridation product, having an
analytical composition of 40.20~ N, 33.75% B (total), 14.0%
Ca, 2.58% B203 and 0.1% C, were obtained. 47 g of BN, having
an analytical composition of 55.70% N, 43.87% B, ~ 0.1% Ca
and C0.1% C, resulted from treatment with dilute hydrochloric
acid. Yield of BN: 90%.
Example 7
The procedure followed was as in Example 5, with the
dlfference that 63 g of a pulverulent mixture of 65% of CaB6
and 35~ of colemanite were e~ployed. Af-ter treatment with
- dilute hydrochloric acid, 58 g of BN, having an analytical
composition of 55.65% N, 43.77% B, C 0.1% Ca and ~0.1% C,
were obtained. Yield of BN: 88%.
- Example 8
- - The procedure followed was as in Example 5, with the
- 20 difference that 50 g of a pulverulent mixture of 75% of CaB6
- and 25% of B4C (grade 1500) were employed. The nitridation-
was carried out at 1800 ~ 50C. After treatment with dilute
- hydrochloric acid, 68 g of BN, having an analytical composition
of 55.95~ N, 44.39% B, ~0.1% Ca and ~0.1~ C, were obtained.
Yield of BN: 87%.
; Example 9 -
The procedure followed was as in Examr)le 4, with
the difference that 60 g of a technically pure calcium boride
powder, having an analy-tical composition of 58.31% B (total),
- 30 35.47~ Ca, 4.17% C and 3.03~ B203, and a specific (BET) sur-
face area of 15.1 m2/g, were employed. 85 g of a white nitri-
dation product with 44.85~ N and, after treatment with dilu-te ~l~
-13-
~!
!
~L~3S~i34 .
hydrochloric acid. 71 g of BN wi-th 55.90% N, were obtained.
Yield of BN: 88~.
Example 10
The procedure followed was as in Example 4, with
the difference that 80 g of a calcium boride powder, contami-
- nated with iron and having an analytical comvosition of 47.27
B (total) 28.38% Ca, 3.57% C, 5.82% B203 and 13.40~ Fe and a
specific (BET) surface area of 7.0 m2/g, were employed. After
treating the nitridation product with dilute hydrochloric acid,
81 g of BN, having an analytical composition of 55.41% N, 44.10%
B, C0.1% Ca, ~0.1% C and C0.01% Fe, were obtained. Yield
of BN: 94%.
Example 11
` 50 g of a mixture, consisting of 50% of CaB6 powder,
having an analytical composition of 58.24% B (total), 34.68%
Ca, 4.13% C and 4.00% B203, and a specific (BET) surface area
of 7.0 m2/g, 40% of H3B03 powder and 10% of C in the form of
carbon black, were formed into granules using a 1% solution of
Polyviol. After drying the granules at 300C, and nitriding
them at 1700 ~ 50C in a nitrogen stream for t~o hours, and
treatment of *he nitridation product with dilute hydrochloric
- acid, 36 g of B~ with a nitrogen content of 55.35% were obtain~
ed. Yield of B~l: 87%.
Example 12
- 70 g of calclum hexaboride, having an analytical com-
position of 55.97% B (total), 33.22% Ca, 3.86% C and 7.15%
; B203 and a specific (BET) surface area of 21.2 m2/g, were form-
- ed into granules without using an additional binder. The gran
ules I obtained were dried and then heated at 1600 ~ 50C in
a stream of nitrogen for 30 minutes. This gave 108 g of a
light grey nitridation product I, which was pulverized in a
.
j !;
~o~634
fixed-~har~er mill. The resulting fine powder was formed into
; yranules using a 3% solution of boric acid, to give, after
drying, 105 g of granules II. These were heated
in a second nitriding stage, at 1600 ~ 50C in a
stream of nitrogen for 30 minutes, analogously to the first
nitriding step. This gave lOS g of nitridation product II
and, after treatment with dilute hydrochloric acid, 77 g of
pure BN. Yield of BN: 86%.
Example l3 -
10 g of calcium boride powder, which passed through
a sieve with an open mesh width of 0.041 ~m ~-325 mesh) and
had an analytical composition of 59.90% B (total), 27.50% Ca,
6.03% C and 2.75% B203, and a specific (BET) surface area of
2.7 m2/g, were nitrided in loose form~ that is to say without
; granulacion, at 1400C under a nitrogen pressure of 100 atmos-
pheres gauge for 30 minutes. After treating the nitridation
product with dilute hydrochloric acid, 12 g of pure BN were
obtained. Yield of BN: 87%.
Example_l~
The procedure followed was as in Example 13, with the
difference that 15 g of calcium boride powder, having an ana-
lytical composition of 55.97% B (total), 33.22% Ca, 3~86% C
and 7.15% B203, and a specific (BET) surface area of 21.2
m /g, were employed. 17 g of BN were obtained. Yield of BN:
88%.
Example 15
.. .. _
-The procedure followed was as in Example 13, with
the difference that lO g of lithium dodecaboride powder,
having an analytical composition of 87.85~ B (total), S.20%
Li, 0.90% C and 4.5g% B203, and a specific (BET) surface area'
of 7.6 ~ /g, were employed. This gave 18 g of hexagonal BN.
Yield of BN- 8~.
1S~
r j.
j.
::
~1~5~3
:. ~rom the foregoing description, one skilled in the
art can easily ascertain the essential charac-teristics of this
invention, and without departing from the spirit and scope
thereof, can make various changes and modifications of the
invention to adapt it to various usages and conditions. Con-
sequently, such changes and modifications are properly,
equitably, and intended to be, within the full range of
eguivalen-e of the following claims.
: . ' .
... . ' .
:'` ' ' '' '' . '
, . . .
. ~16-
