JPH11277515A - High density boron nitride and manufacture of high density boron nitride particle - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce boron nitride flocs having a density, which is nearly equal to a mean density attained through a hot pressing by a method wherein one or more stages of cold pressing and granulating operations of boron nitride particles prepared by crushing a high purity hexagonal system boron nitride into particles each having a specified particle diameter are executed. SOLUTION: A high purity hexagonal system boron nitride is crushed into boron nitride particles, which have the range of the particle diameter of at least 100 μm and the major portion of which has a particle diameter exceeding 50 μm. The crushed particles are cold-pressed into a compressed state and the compressed particles are granulated into a granular powder. Further, through a method containing one or more stages of cold pressing and granulating operations so as to turn the granules into ones having the size suitable for the use of a filling material having a high heat conductivity, boron nitride flocs, each having a high density and the high heat conductivity are formed from a cold-pressed hexagonal system boron nitride.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

FIELD OF THE INVENTION The present invention relates to a method for producing high density boron nitride for use as a precursor feedstock in the conversion of hexagonal boron nitride to cubic boron nitride, and to a method for producing such high density boron nitride. High density agglomerated boron nitride particles for use as high thermal conductivity fillers.

[0002]

The two common forms of hexagonal boron nitride are turbostratic boron nitride and ideal or graphitic boron nitride. The hexagonal form of boron nitride is used for conversion to cubic boron nitride and as a filler for many other applications, especially where high thermal conductivity and high electrical resistance are required. Generally, layered (turbostratic) boron nitride is
By treating at elevated temperatures, typically about 1800 ° C. to 1900 ° C., to remove volatile impurities and surface oxide contaminants, a first step is to add what is conventionally referred to as “high-purity hexagonal boron nitride”. Purify. Such high temperature treatment causes the boron nitride to aggregate very tightly and must be ground for suitable commercial use. So in practice, high purity boron nitride is first milled into a fine powder, then
Cold-press and remove boron nitride for easy handling.
Granulate in two or more stages. The milling operation typically results in 99.9% of the milled powder being -3%.
A fine powder with a small particle size of less than 25 mesh is formed. The average particle size of the milled powder is between 5 and 11 microns. The density of the boron nitride pellets formed from the cold compaction operation averages about 1.80 g / cc, or the theoretical density of hexagonal boron nitride, regardless of the number of repeated granulation and cold compaction steps. Less than 80%.

In the conversion of high purity hexagonal boron nitride to cubic boron nitride, compacts or pellets of boron nitride formed by compression are subjected to extremely high pressures within the stable region of the cubic boron nitride phase diagram. And work temperature. The density of the boron nitride pellets is important to the economics of the cubic boron nitride conversion process.

According to the invention, the particle size distribution of the boron nitride particles is adjusted by adjusting the particle size distribution before compression so that the particle size distribution is as wide as possible, preferably such that most of the particles have a particle size of more than 50 microns. The density of the inter-compressed boron nitride powder can be substantially increased to at least about 1.86 g / cc, a density close to 1.9 g / cc, or about 85% of theory. The preferred particle size range for the majority of the particles should be between 20 and 500 microns. Further, according to the present invention, agglomerated particles of boron nitride formed from a broad boron nitride particle size distribution following cold compaction and granulation,
It has a density close to the average density achieved by hot compaction. Furthermore, its thermal conductivity is enhanced so that it can be used as a filler, especially as a filler in polymer composites.

[0005]

[Summary of the Invention] The method of the present invention comprises the steps of forming a high-purity hexagonal boron nitride;
The high-purity hexagonal boron nitride having a particle diameter of at least 1
Crushing into boron nitride particles, extending beyond the 00 micron particle size range, and the majority of the particles having a particle size greater than 50 microns, cold compressing the crushed particles into a compressed form Granulating the compressed particles into a granular powder, and comprising performing the operations of cold compaction and granulation in one or more stages.

[0006] Also according to the present invention, namely high purity hexagonal boron nitride, a boron nitride having a particle size extending beyond the particle size range of at least 100 microns and a majority of the particles having a particle size of more than 50 microns A step of crushing the particles, cold-pressing the crushed particles into a compressed form, granulating the compressed particles into a granular powder, and a filler having high thermal conductivity Cold compaction and granulation operations in one or more stages to obtain a high density and high thermal conductivity from cold compacted hexagonal boron nitride to a size suitable for use as Aggregated boron nitride particles having

[0007]

DETAILED DESCRIPTION OF THE INVENTION The advantages of the present invention will be described in detail below with reference to the accompanying drawings.

The flow diagram, labeled A in FIG. 1, illustrates the current method of forming a cold pressed boron nitride compact starting from a high purity boron nitride material. The high purity boron nitride material is converted to a very fine high purity hexagonal boron nitride (hBN) powder by pulverizing the BN into a fine powder using, for example, a conventional milling operation using a high speed impact mill. Typical properties of the fine high purity BN powder are as shown in Table I.

Table I Properties of Milled High Purity BN and Final Compressed Density (after 2 Cycles) Sample 1 2 3 4 Oxygen% 0.605 0.488 0.275 0.548 Surface Area (m ² / g) 5.67 5.48 5.26 6.39 Tap Density (g / (cc) 0.58 0.60 0.51 0.47 Soluble borate% 0.33 0.16 0.10 0.26 -325 mesh fractionation 99.9 99.9 99.9 99.9 Average particle size (μm) 10.62 10.05 10.25 9.59 Maximum particle size (μm) 42.2 29.9 29.9 29.9 Density (g / cc) ( 2X) 1.80 1.79 1.77 1.81 Theoretical density% 80.0 79.6 78.7 80.44

As is evident from Table I above in connection with column A of FIG. 1, the fine powder formed from the milling operation contains 9
9.9% is milled to less than -325 mesh (44 microns) with a particle size distribution such that the average particle size is 5-11 microns and the maximum particle size is about 42 microns or less. Powders of this size are difficult to handle and compress. The powder is so fine that it needs to be degassed (pre-compressed) before compaction. If not degassed, the powder will tend to flow out of the press die during compaction.
Degassing is typically performed by compressing the powder at relatively high pressure in a large uniaxial press. Next, the obtained cake is granulated. The feed is placed in the hopper of an automated single-screw press and the powder is continuously compressed to form small pellets or plugs of material, for example, 2.25 "diameter x 2" length. After the initial compression, the compressed boron nitride (BN) can be re-granulated and re-pressed in an automated uniaxial press. In fact, the degassing / compression / granulation procedure can be repeated any number of times. As can be seen from FIG. 4, the highest green density achieved for the compact formed from the milled powder was 1.74 g / cc or 77.3% of theory. By repeating the compression / granulation process, the density was 1.
Increases slightly to 81 g / cc. Additional compression / granulation steps do not affect this density, as seen in FIG. Also, the observation that the upper part of the compressed pellet has a tendency to exfoliate indicates rebound after compression.
By adjusting the particle size distribution of boron nitride by expanding the particle size range to at least over 100 microns prior to compression, the density of the compressed pellets can be reduced to about 1.9 g / cc.
, Ie, up to 85% of theory. A preferred method for achieving a broad particle size distribution is high purity B obtained from high temperature treatment of low purity boron nitride.
Crushing N hard masses. This results in a broad particle size distribution, as evident in Table II below and FIG. This is achieved by the method of the present invention by performing a crushing operation instead of a milling operation, as is apparent from the respective flow sequences B, C, and D of FIG. As shown in flow diagram B, the standard flow diagram A of FIG.
Following a crushing operation as used in Example 1 followed by a standard degassing / granulation-compression / granulation step, or as shown in the flow diagram sequence C or D in FIG.
It is also possible to perform only the compression / granulation step without performing the degassing step. The final step of the flow diagram sequence D in FIG. 1 is a crushing operation for obtaining a broad particle size distribution of the aggregated boron nitride particles. Agglomerated boron nitride particles are particularly useful as thermally conductive fillers, especially for filling polymers.

[0011] Table II crushing, high purity BN properties and ultimate compression density (after 2 cycles) Sample 1 * 2 3 4 oxygen% 0.571 0.275 0.426 0.60 surface area ^{(m 2 / g) 3.16 5.26} 2.51 3.02 tapped density ( g / cc) 0.76 0.85 0.92 0.89 Soluble borate% ---- 0.10 0.09 0.14 Screen sizing +40 ---- 11.12 71.64 67.51 -40 + 80 ---- 41.46 12.04 13.72 -80 + 100 ---- 6.16 1.08 1.82- 100 + 150 ---- 8.36 2.20 3.01 -150 + 200 ---- 5.76 1.44 2.42 -200 + 325 ---- 7.32 2.32 2.90 -325 11.16 19.82 9.28 8.62 Density (g / cc) (2X) 1.86 1.89 1.91 1.84 Theoretical density% 82.70 84.07 84.90 81.78 Note: Sample 1 had fine powder removed by crushing and screening. Samples 2 and 3 were not processed.

Table II above shows the typical properties of roll-crushed high purity boron nitride. Table II
As can be seen, crushing the high-purity boron nitride material results in a much wider particle size distribution, a higher tap density, and both of which result in a higher compression density. The tap density of the crushed particles is at least 0.76 g / cc. Sample 1 lot was screened to remove fine powder, whereas Samples 2, 3, and 4 were not screened. Sample 1 has a higher tap density and pre-compressed density than the milled sample, but less than Samples 2, 3, and 4. In samples 1 to 3 of the roll-crushed lot, the deaeration step was eliminated, and the uniaxial compression was performed as described in the step of milling the material.
Compressed using granulation and uniaxial compression procedures. The compaction density is significantly higher than that of Table I. In addition, no peeling occurred in these samples, indicating that there was little tendency to rebound. The high compaction density and low tendency to rebound in roll crushed materials is effective for conversion to cubic boron nitride. Flow diagram D of FIG.
Can be used to produce crushed boron nitride aggregated particles as a filler with high thermal conductivity.

[0013]

Examples of the present invention will be described below.

Example 1 A highly agglomerated high-purity boron nitride powder was crushed in a roll mill to obtain a powder having the particle size distribution shown in FIG. 3 and the properties shown in the third column of Table II. did. Using a horizontal press, the powder is
Compressed at a pressure of 9,000 psi. Open the compressed piece about 1 /
The granulation was forced through a 2 inch screen.
The granulated particles were compressed again at 19,000 psi. This procedure is schematically illustrated in FIG. The density of the obtained compressed product was 1.91 g / cc.

For comparison, a highly agglomerated high purity boron nitride powder was milled to a fine powder using the standard milling procedure shown in Flow Sequence A of FIG.
The properties of this powder sample 4 are shown in the fourth column of Table I. The particle size distribution of this powder is shown as the curve of “milled fine powder 4” in FIG. The fine powder was degassed at 2500 psi as schematically shown in the flow diagram of FIG. 1 and granulated, followed by recompression, granulation and compression. The density of the resulting compact was only 1.81 g / cc.

Example 2 A highly agglomerated high-purity boron nitride powder was crushed in a two-roll mill to give a powder having a particle size distribution shown as "crushed powder 4" in FIG. The properties of this powder are shown in column 4 of Table II.
The powder was compressed using a horizontal press at a pressure of 19,000 psi. In addition, four additional granulation / compression steps were performed as described in Example 1. The resulting compressed density was 1.91 g / cc. The relationship between the density of the compact and the compression cycle is shown in FIG.
Is shown by the curve.

In a comparative test, highly agglomerated high purity boron nitride powder was milled into a fine powder. The properties of this powder are shown in the fourth column of Table I. The particle size distribution of this powder is shown as the curve of “milled fine powder 4” in FIG. The fine powder was degassed at 2500 psi, granulated, and then subjected to the same five cycles of granulation and compression as were performed on the crushed powder. The density of the resulting compact is 1.84
It was just g / cc. The relationship between the density of the compact and the compaction cycle is shown by the curve of "Precompressed and milled powder 4" in FIG.

Example 3 A highly agglomerated high-purity boron nitride powder was crushed in a roll mill to give a powder having a particle size distribution shown as "crushed powder 2" in FIG. The properties of this powder are shown in the second column of Table II. The powder was compressed using a horizontal press at a pressure of 19,000 psi. The pressed pieces were granulated by forcing through a screen having an opening of about 1/2 inch. Re-granulate the particles 1
Compressed at 9,000 psi. The density of the resulting compact is
It was 1.89 g / cc. This density is the compressed density of the four different milled powders shown in Table 1, 1.8 each.
0, 1.79, 1.77 and 1.81 g / cc.

Example 4 A highly agglomerated, high-purity hexagonal boron nitride powder having the characteristics shown in the fourth column of Table 2 was crushed using a roll mill, and FIG. A powder having a particle size distribution shown as "4" was obtained. The crushed powder was compressed using a powder compression press (uniaxial press) at a pressure of 19,000 psi and then using a granulator to form granules of 1/16 inch or less. The granules were compressed again at 19,000 psi. The compacts were crushed using a saw blade and a roll crusher and screened through a 120 mesh screen to give a powder with a tap density of 0.68 m ² / g. The screened particles were added to a cresol novolak thermosetting epoxy composition containing a phenol novolak curing agent to a BN concentration of 65% by weight and then molded in a transfer press. The thermal conductivity of the molded mixture was 7.4 W / m ° C.

The pellets or compact can be re-crushed into a powder mixture of agglomerates having a broad particle size distribution. These agglomerates are denser than the corresponding agglomerates formed from milled powder that have gone through the same process.

[Brief description of the drawings]

FIG. 1 shows four flow diagrams A comparing a standard prior art compression method A and inventive methods B, C and D, respectively, for cold pressing high purity boron nitride. , B, C, and D.

FIG. 2 shows a typical particle size distribution curve of high purity boron nitride formed from the milling operation used in the standard compression method A of FIG.

FIG. 3 shows particle size distribution curves corresponding to high purity boron nitride particles formed from high purity boron nitride crushing in each of methods B, C and D shown in FIG.

FIG. 4 is a graph showing the relationship between density and powder compaction cycle, ie, the number of powder compaction steps, for milled and crushed powder.

 ──────────────────────────────────────────────────の Continued on the front page (72) Inventor Richard, F. Hill, Ohio, United States, Chagrin, Falls, Elliott, Drive, 17982

Claims (8)

[Claims] 1. A process for forming high purity hexagonal boron nitride, wherein said high purity hexagonal boron nitride is crushed so that the particle size extends beyond a particle size range of at least 100 microns and a majority of the particles Forming boron nitride particles having a particle size of greater than 50 microns, cold-compressing the crushed particles into a compressed form, granulating the compressed particles into a granular powder, Perform the operation of compression and granulation in one or more stages,
A method for producing high-density boron nitride pellets or aggregates, comprising forming boron nitride pellets or high-density boron nitride aggregates. 2. The method of claim 1 further comprising the step of crushing the compressed boron nitride into agglomerated boron nitride particles having a broad particle size distribution for use as a filler. 3. The method of claim 1, wherein a majority of the particles have an average particle size greater than 200 microns. 4. The particle size range of less than 20 microns to 40 microns.
4. The method of claim 3, wherein the method extends to greater than 0 microns. 5. The method of claim 4, wherein said particle size range extends to 500 microns. 6. Crushing a relatively large agglomerated mass of high purity hexagonal boron nitride so that the minimum to maximum particle size extends beyond a particle size range of at least 100 microns, with a majority of the particles having a particle size of 50 microns. Forming boron nitride particles having a particle size of more than microns, cold-pressing the crushed particles into a compressed form, granulating the compressed particles into granular powder, granulating Cold compacting the powder into a final compacted form, crushing the final compacted form into agglomerated filler particles of highly thermally conductive particles, cold compacting and compacting. The lashing step is repeated until the density becomes 1.86 g / cc or more. The lashing step is performed by a cold forming step performed at room temperature, and is used as a highly thermally conductive filler. High density agglomerated particles of hexagonal boron nitride having a density of at least greater than 1.86 g / cc for use. 7. When mixed into a polymer, at least 7.
The high-density agglomerated particles of boron nitride according to claim 6, which provide a thermal conductivity of 4 W / m ° C. 8. Crushing the high-purity hexagonal boron nitride to form boron nitride particles having a particle size extending beyond a particle size range of at least 100 microns, with a majority of the particles having a particle size of greater than 50 microns. Cold-compressing the crushed particles into a compressed form;
A step of granulating the compressed particles into a granular powder, and a step of cold-pressing the granulated powder into pellets, and performing one or two steps of cold-pressing and granulating. The density for use in converting the hexagonal boron nitride formed by the method comprising the above into cubic boron nitride has a density of at least 1.86 to 1.91 g /
Boron nitride pellets that are cc.