Field of Invention
This invention is related to a powder consisting of finely
divided single crystalline diamond particles and, in particular, an abrasive powder
that is especially adapted to the use in high-precision machining processes. The
invention is also related to a method for the production of such powder.
With the progress in high precision machining technology,
demands for diamond abrasives have been shifting to increasingly smaller particle
sizes, to an extent that, in some cases, a surface roughness of 1 Å is required.
The smallest diamond particles ever produced for abrasive applications are of "detonation"
type, consisting of a mass of secondary particles that is an agglomeration of smaller,
primary particles, which average 5 to 10 nm. It has been observed that this type
of diamond, synthesized in a process where an explosive is combusted incompletely,
has a lot of defects within the crystal and shows, when observed by transmission
electron microscopy (TEM), rather round appearance commonly, as a result of the
growth duration too short, being on the order of one-digit microseconds.
Individual diamond particles as yielded in a detonation
technique are, as described above, very small and thus have a very active surface.
They agglomerate readily to form secondary particles, firmly joined by means of
non-diamond carbon or other substances that come in from the synthesis process and
atmosphere. So this type of diamond behaves apparently as agglomerated particles
with a size of 100 nm or more. It is also known that such secondary particles can
be disintegrated into primary particles in a rigorous acid treatment.
Among the products of a technology based upon the extreme
compression powered by chemical explosion or detonation, commonly known is the DuPont
polycrystalline type diamond, which is produced by conversion from graphite under
extremely high pressure by means of energy of chemical explosion. This type of diamond
also is in a secondary particle structure: such that primary particles, commonly
having a size of 20 to 30 nm, are fused in part and joined to each other under the
extreme compression in excess of 30 GPa during the conversion process, trapping
some graphite left unused. Consisting of firmly joined secondary particles of 100
nm or more, DuPont diamond also behaves as such; however this type, in contrast,
cannot be disintegrated even by rigorous acid treatment. TEM microscopy shows that
the primary particles do not exhibit idiomorphic faces but a somewhat spheroidal
overall appearance, which is considered as an evidence of the limited conversion
Either of the above described processes is not adequate
for the production of idiomorphic crystals, since they depend, for the compression
of the starting material of low pressure phase of carbon, upon a chemical detonation,
which, if extremely high in magnitude, lasts only for one-digit microseconds, a
duration too short for the product to grow into such desired abrasive particles
with sharp edges and points. So when used as an abrasive, the diamond products,
which have few sharp edges or points, of the both techniques are short of achieving
an efficient grinding rate, although the abrasive grits leave fine polishing marks
in accordance with such small size of the primary particles.
On the other hand, static compression techniques can control
properties such as shape, hardness and brittleness of the diamond product by operating
at properly chosen pressure, temperature and time parameters to be applied. Further
so produced diamond crystals can be readily crushed into very fine particles by
impact-milling with steel balls.
It is observed under TEM microscopy that most of such fine
particles, which have a size of tens of nanometers, are idiomorphic and have sharp
edges, as a result of the crushing process that mostly takes place on the basis
of the cleavage of diamond crystals. In some instances there are even flat triangular
fragments of crystal observed with about 5 nm sides.
The Inventors have found that very fine powder or mass
of very minute single crystalline diamond particles can be produced in a properly
combined process of micro-crushing and precision grading. Based upon this finding
we developed a technology for the production of 100 to 50 nm D50 diamond
powder, for which we filed a patent application (published under
Japan Kokai 2002-035636
In the invention processed are particles of single crystalline
diamond size-reduced by impact-crushing described above. The particles so crushed
have commonly sharp edges and points to an extent that they often include some particles
with a flat, regular triangular shape, as a result of well-known cleavage on (111)
For crushing of the invention there are available such
techniques as a handy process of ball mill with steel balls, while vibration mill
and planetary mill can load more powerful impacts. A preferable crushing medium
is steel balls for they have a sufficiently high density. Coarse diamond particles
may also be used for the purpose of minimizing the contamination originating from
the medium material.
Diamond particles as taken out from the crushing mill are
first treated with chemical, in order to remove by dissolving debris of crushing
medium having mixed during the process. The diamond particles then are subjected
to a combined grading process of elutriation and centrifugation. In both processes
the diamond particles are held in suspension and processed in the water, it is needed
that the particles have affinity for water on the surface, in order to maintain
For this purpose a surface oxidization treatment is effective,
whereby diamond particles are oxidized to attach on the surface such hydrophilic
atom as oxygen or oxygen-containing group, as hydroxyl, carbonyl, and carboxyl,
for example. For the surface oxidization, while heating to 300° C or more in
air may be available with a certain effect, a more reliable process may consist
of a wet process, whereby diamond is treated in a bath comprising both one selected
from sulfuric acid, nitric acid, perchloric acid, and hydrogen peroxide, and one
selected from potassium permanganate, potassium nitrate and chrome oxide.
For preparing a good suspension of diamond particles in
water it is necessary to minimize the overall concentration of ions that coexist
in the water and, at the same time, to regulate the surface potential within the
range adequate for establishing a good suspension. It is known that in weak alkaline
condition diamond particles hold in suspension by repulsion each other of charges
on the particle surfaces, so it is necessary to regulate the hydrogen-ion concentration
and the zeta potential within the proper ranges, which are between pH 7.0 and 10.0
and between -40 and -60 mV, respectively.
While the elutriation technique is widely employed for
the size grading of small diamond particles, it has a difficulty in that they need
an extremely long precipitation time with such 100 nm or less diamond particles,
resulting in a poor productivity. A combined process with super high speed centrifuge
may somewhat increase the productivity; this approach is not necessarily realistic,
because such equipment itself can be expensive, while there will be some problems
in both maintenance and securing safety.
DISCLOSURE OF INVENTION
Problem to be solved by the invention
Therefore, one of the principal objects of this invention
is to provide an efficient method for the particle-size grading to produce finely
divided diamond particles having a D50 size less than 50 nm. Another
object is to provide a fine abrasive powder of diamond particles that have cleavage-based
sharp edges, specific to diamond crystal, as well as a close range of particle size
distribution, and thus can meet the high criteria for the precision and efficiency
demanded in the precision machining industry.
The present inventors have by now found that such problems
as described above can be solved, and a fine powder composed of such minute diamond
particles and having a D50 particle size less than 50 nm and a close
particle size range can be efficiently separated and recovered, by introducing to
the size grading (classification) process a common centrifugal machine that is commercially
available on market, and by optimizing the operation parameters of the process.
Means for solving the Problem
The method of this invention for the production of a fine
powder of single crystalline diamond particles, comprises:
- (1) a step of crushing, by an impact loading mechanical breaking means, a raw
material of single crystalline diamond particles that is a product of a conversion
from non-diamond carbon under static ultrahigh pressure, to prepare starting minute
- (2) a step of oxidizing surfaces of said starting minute diamond particles to
chemically attach hydrophilic atoms or functional groups to the surfaces of diamond
particles and thus to impart thereto hydrophilic quality,
- (3) a step of preparing a slurry by dispersing the hydrophilic diamond particles
in a water medium, wherein said slurry is set and kept weakly alkaline (by addition
of an alkaline substance thereto),
- (4) a step of subjecting said slurry to a wet-mode preliminary particle size
grading process, in order to remove by sedimentation from said slurry a top particle
size fraction of the diamond particles, said top particle size fraction having a
D50 size of 60 nm or more,
- (5) a step of adding de-ionized water to a remainder of said slurry, which has
been removed of said top particle size fraction, to a diamond concentration of 0.1
% by weight or less,
- (6) a step of subjecting said remainder of the slurry to a centrifugal grading
process, whereby a coarser particle size fraction of the diamond particles contained
in said slurry is condensed and removed from the slurry, while said water medium
is taken out of the centrifugal grading process as an effluent slurry that contains
a fraction of diamond particles of an average particle size that has been decreased
due to the removal of coarser diamond particles,
- (7) a step of repeating once or more an operation of the step (6) above, whereby
a coarser particle size fraction of the diamond particles is separated by condensation
and removed as a solid mass from the slurry, until a target set of parameters have
been achieved with the diamond particles contained in said mass: a D50
size of 50 nm or less and ratios of D10 size to D50 size and
D90 size to D50 size of 50 % or more and 200 % or less, respectively,
said slurry effluent from the centrifugal process containing a fraction of diamond
particles having a smaller average particle size; and
- (8) a step of recovering from said slurry the diamond particles as having a
minimal average particle size by precipitating into a solid mass.
By subjecting mechanically crushed single crystalline diamond
particles to the particle size grading process of the invention described above,
a fraction of diamond particles can be recovered from the final effluent slurry
that has a D50 average size as small as 50 nm or less and, at the same
time, a close ranging size distribution such that the ratios of D10 size
to D50 size and D90 size to D50 size are 50% or
more and not exceeding 200%, respectively, as evaluated with a Microtrac UPA particle
The hydrophilic atom or group to be used for and in the
method of this invention comprises one or more selected from hydroxyl group, carbonyl
group and carboxyl group.
In the method of the invention at step 3, dispersion of
diamond particles in the slurry can be promoted by addition of an alkaline substance
to set and keep the slurry weakly alkaline. So it is preferable in this context
that the hydrogen ion concentration in the slurry be set and kept between pH 7.0
and 10.0 at steps 3 and later.
Also, at step 3 the zeta (&zgr;) potential in the slurry
may preferably be controlled within a range of -40 to -60 mV.
Further in the method of the invention at step 6, a coarser
particle size fraction of diamond particles can be condensed and taken out from
the centrifugation process. The diamond particles, recovered as a cake, then can
be dispersed again in de-ionized water to form further slurry and can subsequently
be subjected to a centrifugal grading process as in step 6. From this process, an
effluent slurry can be recovered from the centrifugal process that contains a fraction
of diamond particles having a further decreased average particle size.
EFFECTS OF INVENTION
In accordance with the method of this invention, it is
possible to produce single crystalline diamond particles that have, in spite of
the extremely minute particles with a D50 average size of 50 nm or less,
cleavage-based sharp edges and sharp points, in contrast to the conventional and
commercially available diamond abrasives of corresponding minute particle sizes.
Minute diamond particles can be recovered at high yields
with minimum loss:
BRIEF DESCRIPTION OF DRAWING
- while minute diamond particles may be trapped to a degree in a cake of condensed
coarser diamond particles, they can be released back into the water medium when
the cake is dispersed again in de-ionized water for re-use as slurry to be processed
on a centrifugal machine at step 6, and the water medium is also taken out as effluent
slurry as containing diamond particles with a further decreased particle size.
BEST EMBODIMENT OF WORKNG OUT THE INVENTION
- Figure 1 shows schematically a flow diagram for an exemplar precision particle-size
grading process following the method of the invention, wherein the slurry is processed
with two centrifugal machines connected in series (Example 1, to be given); and
- Figure 2 shows schematically a flow diagram illustrating another exemplar precision
particle-size grading process following the method of the invention, wherein the
slurry is processed in a two-way flow on a single centrifugal machine (Example 2,
to be given);
In an embodiment of the method of the invention, a precision
particle-size grading system is used that comprises two slurry storage tanks connected
to a single centrifuge, or centrifugal machine. Fine diamond particles to be graded
are held in suspension in water or as slurry, which is stored in a first tank, and
fed to the centrifugal machine to be subjected to a centrifugal force, so that a
coarser particle size fraction of the diamond particles in the slurry is condensed,
separated and removed as a cake from the slurry, which leaves the centrifugal machine.
The slurry, which has been eliminated of the coarser diamond
particles and thus contains the finer particle size fraction at an increased proportion
accordingly, is flown out of the centrifugal machine and received and collected
in the second storage tank.
In the centrifugal grading process, grading efficiency
can be increased by (1) regulating the diamond concentration of slurry well below
a specific level by occasionally diluting the slurry with de-ionized water and,
at the same time, (2) stabilizing the conditions of diamond particles dispersed
in the slurry, by regulation of hydrogen ion concentration of the slurry and thus
securing a zeta (&zgr;) potential maintained within a proper range.
For the use in the grading process it is desired that centrifugal
machines have a capacity to generate a centrifugal acceleration of at least 2 x
104 g, which may vary to a degree depending upon the target quality in
particle size of diamond powder to be collected. Effluent slurry is passed to another
centrifugal process after conditioned as described below.
The slurry to be fed to the centrifugal machine preferably
should have a concentration of diamond as low as possible, so that the diamond particles
can behave effectively as individual particles, although favorable concentrations
can vary depending upon the particle size to be recovered. On the other hand, however,
higher concentrations of diamond particles in the slurry are desired in order to
secure a good productivity level with the centrifugal grading process.
Considering the both aspects, the proper concentration
is, it can be said, 0.1 (mass) % or less for the collection of a fraction of diamond
particles having a 40 nm D50 average size and 0.05 % or less but not
anyway less than 0.01 % for a fraction having a 20 nm D50 average size.
This invention intends to a precise grading of minute particles
of diamond, which, as particles of size less than 50 nm, have very active surfaces
and thus tend to aggregate and readily form secondary particles. Thus it is essential
to a successful grading process that the diamond particles are well dispersed in
the slurry in the state of primary particles. For this purpose it is effective,
as described above, to use a slurry containing as low a concentration of diamond
particles as possible, in order to maintain a low frequency of collision of diamond
particles that lead to their aggregation.
In the invention the centrifugal grading process can be
realized based on and by using a common centrifugal machine available on market,
with a low centrifugal performance of at least 1 x 104 g acceleration,
for example, though several cycles of the centrifugal grading process may be carried
out, in order to secure a higher grading performance. In this case the diamond particles
condensed and collected as a cake in the centrifugal machine is dispersed again
in de-ionized water medium to form further slurry, and then fed to the same or another
machine for a further centrifugal grading.
In the precision grading process of the invention, which
involves several cycles of collecting diamond particles as a cake and dispersing
them in water medium, coarser agglomerated, or secondary, particles can increasingly
break down over those cycles into constituent smaller particles of varying sizes,
among which larger particles can preferentially be condensed to a cake, while smaller
particles preferentially remain in the slurry.
The apparent particle size of the diamond particles contained
in a cake can vary from cycle to cycle, as aggregated particles increasingly break
down into constituent particles of various sizes. More commonly cakes from a second
grading process tends to have an apparent D50 size increased over the
In the method of this invention, it is possible and effective
for the production of diamond particles of average particle size of 30 nm or less,
to operate the grading process with an arrangement comprising two slurry tanks,
tank 1 and tank 2, which are connected on the inlet (influent) side and outlet (effluent)
side of the said centrifugal machine, as diagrammatically shown in Fig. 2. Thereby,
a reciprocal cycle or cycles of the centrifugal grading process is made possible
to be done in a manner that the slurry is made to flow in two reciprocal by traversed
stream directions from the first tank to the second tank via the centrifugal machine
and next from said second tank to the first one tank via the centrifugal machine.
In the invention it is effective for the collection of
diamond of average particle size under 30 nm to arrange two slurry tanks: tank 1
and tank 2, in connection on the inlet (influent) and outlet (effluent) sides of
a centrifugal machine, as shown schematically in the flow diagram in Fig. 1. Slurry
can be flown in two directions by switching, either from tank 1 to tank 2 or reversely.
Here slurry is fed from one tank at the inlet, subjected to a centrifugal process,
removed of a coarser fraction of diamond particles, and received as effluent slurry
in the other tank at the outlet, which is held in suspension by fully agitating.
In a subsequent centrifugal process, the direction of slurry flow is reversed: starting
slurry is fed from slurry tank 2 to the centrifugal machine and the effluent slurry
is received in slurry tank 1.
Several repeated processes of centrifugation substantially
increase the time over which the slurry stays in the centrifugal machine. A similar
effect can be achieved by decreasing significantly the feeding rate of slurry to
the centrifugal machine.
Slurry to be fed to the centrifugal machine is prepared
by disintegrating agglomerated particles into individual constituent (primary) crystals,
removal of smaller particles attached to larger particles, and holding such minute
diamond particles in apparently stable suspension. Such disintegration and smaller
particle removal can be effectively conducted by irradiation of ultrasonic waves.
Surfactant may be used, when necessary, for stabilizing and maintaining the dispersion
of the slurry.
In the grading system of the invention as constructed above,
such fine diamond powder with a D50 size under 50 nm and, in an optimized
condition, under 20 nm can be recovered from a process with a centrifugal machine
available on market for common purposes. Stable dispersion of diamond particles
in the slurry is essential to achieving a powder of close particle size range, and
can be realized by properly regulating the hydrogen ion exponent and zeta potential.
Thus it is possible, for example, at a pH of 7.0 to 10.0 and a zeta potential of
-40 to -60 mV, to achieve ratios of D10 and D90 to D50
average value of or over 50 % and not exceeding 200 %, respectively, and in a better
conditioned process, of or over 60 % and not exceeding 190 %.
A diamond powder of class zero, which had a nominal particle
size of less than one micrometer, synthetic single crystalline diamond particles
was used as a raw material for the precision grading of the invention. The diamond
powder was a product of a synthetic process whereby non-diamond carbon had been
converted on a hydraulic press, which had been significantly size reduced by crushing
process in a mill with steel balls. The diamond powder also had passed a rough size-grading,
in order to obtain raw diamond powder ready for the particle size grading process
in accordance with the invention.
The raw diamond powder particles were first oxidized by
heating in a bath of mixture of concentrated sulfuric acid and concentrated nitric
acid, at a temperature between 250° and 300° C for more than 2 hours,
so that hydrophilicity was imparted to the diamond particle surfaces. The resulting
hydrophilicity was observed by IR spectroscopic analysis, which indicated absorption
by such oxygen-containing hydrophilic functional groups as carboxyl, carbonyl and
hydroxyl groups. The diamond particles were then washed fully to eliminate the residual
acid and, further, dispersed in de-ionized water to form slurry. Ammonia water was
added to the slurry to adjust the pH at 8.2. The slurry in this condition was observed
to have a zeta-potential of about -55 mV.
The slurry was passed to an elutriation grading system,
where a fraction of coarser diamond particles, which had a D50 size of
150 nm or over, was removed from the slurry. The remaining slurry, that is as eliminated
of such coarser particle fraction, was then passed into a centrifugal machine for
a preliminary grading, whereby another fraction having a D50 size over
60 nm was condensed into cake by the centrifugation and separated from the slurry.
The slurry as further eliminated of this fraction was taken out of the centrifugal
machine, then passed into and stored as raw slurry ready for the precision grading
in a raw slurry reservoir of 5 m3 capacity.
Said raw slurry was then subjected to the precision grading
process with a centrifugal machine, as diagrammatically shown in Fig. 1, in order
to recover a fraction of diamond particles having a D50 size less than
50 nm. Here two centrifugal machines were arranged in connection with each other
on a continuous single channel for the flow of slurry, and an intermediate slurry
tank (tank 2) was provided between the two centrifugal machines, while two slurry
tanks (tanks 1 and 3) were provided outward at the upstream and downstream ends
of the channel, respectively, as supply tank for the preparation and temporary storage
of slurry to be fed to the centrifugal machine and as slurry sedimentation tank
for receiving the effluent slurry and eventually recovering diamond particles by
In the centrifugal precision grading process, two centrifugal
machines are arranged on and in connection via a single continuous channel as a
passage for the slurry containing diamond particles to be graded. The machines were
referred as primary (upstream) and secondary (downstream) in relation to the direction
of slurry flow. An agitator was equipped on the slurry supply tank and intermediate
tank, so that the slurry held therein were steadily stirred and diamond particles
were fully in suspension in the water.
For the process, two high-speed centrifugal machines (both
of Model H2000 product of Kokusan Co., Japan) were used, which had a 200 mm diameter
rotor for collecting the cake of condensed particles and could generate a centrifugal
acceleration of 2.89 x 104 g at a rotation of 16,000 r.p.m. The three
slurry tanks each had a capacity of 0.5 m3.
In preparations for the centrifugal grading process, the
raw slurry containing the diamond particles, as eliminated in advance of top, or
oversize, fractions of diamond particles by the elutriation and preliminary centrifugal
grading, was transferred from the raw slurry reservoir (not shown) to the slurry
supply tank (tank 1). The raw slurry was added with de-ionized water to prepare
250 liters of starting slurry containing about 0.1 % (by mass) of diamond particles
to be fed to the centrifugal machine for further grading.
In the precision centrifugal grading system, the primary
centrifugal machine, or first-step centrifuge, was operated at a rotation speed
of 15,000 r.p.m. (generating an acceleration of 2.54 x 104 g), to which
the slurry containing about 0.1% (mass) of diamond particles was fed at a rate of
The effluent slurry from the centrifugal machine was received
and collected in the intermediate tank and, after fully stirred, was fed for a further
grading at a rate of 20 ml/min. to the secondary centrifugal machine, which was
driven at 16,000 r.p.m. (producing an acceleration of 2.89 x 104 g).
The effluent slurry from the outlet of this centrifugal machine was passed to and
collected in the sedimentation tank.
The fraction of diamond particles that was condensed and
removed as cake from the primary centrifugal machine was placed in the slurry supply
tank (tank 1), and added with de-ionized water to form 250 liters of slurry, and
likewise fed again to said primary centrifugal machine at a rate of 50 ml/min.;
which was driven at 15,000 r.p.m., generating an acceleration of 2.54 × 104
In the operations described above, the effluent slurry
from the second cycle of grading process with the primary centrifugal machine was
collected in the intermediate slurry tank (tank 2), to which added the cake of diamond
particles condensed and separated from the slurry in the previous cycle of process
with the secondary centrifugal machine. The slurry thus formed is then fed to the
secondary centrifugal machine at a rate of 20 ml/min. to the centrifugal machine
in operation at 16,000 r.p.m. generating a 2.89 x 104 g acceleration.
The effluent slurry from this cycle of grading process with the secondary centrifugal
machine was received and collected in the sedimentation tank (tank 3). Hydrochloric
acid was added to the collected slurry to adjust the hydrogen ion index to a pH
of 2, thus causing aggregation and sedimentation of the diamond particles from the
slurry for recovery.
In the primary and secondary centrifugal grading processes
described above, the cakes of diamond particles recovered from each centrifugal
machine, as well as the sediments recovered from the sedimentation tank were separately
dried at 130° C, weighed and evaluated in terms of their particle size distribution
by means of Microtrac UPA particle size analyzer.
As seen in Table 1, a fraction of diamond particles could
be achieved in the cakes recovered from both the secondary centrifugal machine and
sedimentation tank (tank 3).
Collected Cake mass (g)
Ratio (%) of
D10 or D90 to D50
In this example, a centrifugal machine was used that had
a performing capacity corresponding to the machines in Example 1.
The inlet (influent) side and outlet (effluent) side of
the centrifugal machine were each connected with a slurry supply tank of 0.5 m3
capacity equipped with an agitator. As shown in Fig. 2, a slurry supply tank 1 was
arranged upstream on the inlet side and connected to the centrifugal machine, to
be used as the slurry supply tank for feeding a starting slurry to the centrifugal
machine. First the sediment of diamond particles (Table 1), which had been collected
in Example 1, was placed in the slurry supply tank. Then de-ionized water is added
to the sediment to form 500 liters of slurry having a diamond concentration of 0.05
%, with the pH adjusted by addition of ammonia water at 9.7 and a zeta potential
of about - 48 mV.
The centrifugal machine was operated at a 16,000 r.p.m.
(with a 2.89 x 104 g acceleration), while the starting slurry was fed
at a rate of 30 ml/min. to the centrifugal machine. The effluent slurry from the
outlet of the centrifugal machine was received in the slurry tank 2 that was provided
downstream on the outlet side of the centrifugal machine.
As the feeding and grading of whole slurry volume left
the slurry tank 1 empty, while the slurry tank 2 was loaded with slurry, the slurry
flow was switched to a reverse direction, so that the slurry stored in the tank
2 was run via the centrifugal machine to the tank 1, for subjecting another cycle
of the grading process.
The two-way slurry flow centrifugal grading process was
repeated in three cycles, and at the end about 50 grams of cake was recovered when
caused sedimentation by addition of hydrochloric acid to the slurry stored in the
As evaluated in terms of particle size distribution with
a Microtrac UPA Particle Size Analyzer, the cake was found to contain diamond particles
having a D50 average size of 26 nm, with D10 and D90
sizes of 19 nm and 44 nm, respectively.
This cake was again added with and dispersed in 450 liters
of de-ionized water to prepare a slurry, which then was graded further in two cycles
of the centrifugal grading process that was operated with the two way slurry flow
as described above. At the end of the whole processes above, the effluent slurry
as collected was pH adjusted with acid so the diamond particles were condensed to
cause sedimentation as a cake.
The cake was again dispersed in 450 liters of de-ionized
water to form further slurry, which was then graded in two cycles of the centrifugal
process with a two-way slurry flow of this Example. The diamond particles recovered
from the collected effluent slurry had a D50 average size of 17 nm, D10
and D90 sizes of 12 nm and 29 nm.
While the grading process of the invention has been described
in reference with particular cycle numbers, that is slurry was passed to a centrifugal
machine twice in example 1, and processed, in Example 2,in those three sets of two-way
slurry flow grading with a machine that can correspond to six centrifugal processes,
the numbers are not given for the purpose of limiting the centrifugal grading process
to such numbers of cycles. Instead diamond particles carried in the effluent slurry
at step 7, may be passed to the centrifugal machine to operate any number of repeated
grading processes, whereby a powder may be obtained having a D50 size
under 50 nm, with the ratios of D10 and D90 to D50
being over 50 % and not exceeding 200 % respectively.
Further, although the above examples refer as raw diamond,
only to synthetic single crystalline diamond produced under static compression on
a hydraulic press, natural diamond may be similarly employed since they are single
crystalline, as produced by conversion from non-diamond carbon under static ultrahigh
compression and exhibiting similar physical properties.