Classifications

• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10W—GENERIC PACKAGES, INTERCONNECTIONS, CONNECTORS OR OTHER CONSTRUCTIONAL DETAILS OF DEVICES COVERED BY CLASS H10
  • H10W40/00—Arrangements for thermal protection or thermal control
  • H10W40/20—Arrangements for cooling
  • H10W40/25—Arrangements for cooling characterised by their materials
  • H10W40/254—Diamond
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C1/00—Making non-ferrous alloys
  • C22C1/10—Alloys containing non-metals
  • C22C1/1036—Alloys containing non-metals starting from a melt
  • C22C1/1073—Infiltration or casting under mechanical pressure, e.g. squeeze casting
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C26/00—Alloys containing diamond or cubic or wurtzitic boron nitride, fullerenes or carbon nanotubes
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T428/00—Stock material or miscellaneous articles
  • Y10T428/12—All metal or with adjacent metals
  • Y10T428/12181—Composite powder [e.g., coated, etc.]
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T428/00—Stock material or miscellaneous articles
  • Y10T428/12—All metal or with adjacent metals
  • Y10T428/12493—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
  • Y10T428/12771—Transition metal-base component
  • Y10T428/12861—Group VIII or IB metal-base component
  • Y10T428/12903—Cu-base component
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T428/00—Stock material or miscellaneous articles
  • Y10T428/25—Web or sheet containing structurally defined element or component and including a second component containing structurally defined particles
  • Y10T428/254—Polymeric or resinous material
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T428/00—Stock material or miscellaneous articles
  • Y10T428/25—Web or sheet containing structurally defined element or component and including a second component containing structurally defined particles
  • Y10T428/259—Silicic material
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T428/00—Stock material or miscellaneous articles
  • Y10T428/30—Self-sustaining carbon mass or layer with impregnant or other layer
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T428/00—Stock material or miscellaneous articles
  • Y10T428/31504—Composite [nonstructural laminate]
  • Y10T428/31678—Of metal

Definitions

• the invention relates, in general, to very high thermal conductivity metal matrix composite structures; and, more particularly, to such composite structures containing diamond particles which have a thin conversion surface layer of beta-SiC formed thereon by a chemical vapor reaction process.
• the SiC coating on these particles is present as a reaction zone, or graded layer, as opposed to a distinct SiC/coating build-up without a diffusion conversion interface, that would have been obtained by a coating process, such as chemical vapor deposition of SiC upon the diamond surface.
• the SiC is readily wet by molten aluminum and aluminum alloys when it comes in contact with the filler particles.
• the Al/SiC composites have been reported to achieve strengths of 400 MPa at a filler loading of >40 vol.%, indicating a good bond was formed between the SiC particles and Al.
• MMC's comprised of particulate SiC and an aluminum matrix have advantages over pure Al structures in terms of coefficient of thermal expansion (CTE), stiffness, and wear resistance. In general, however, the thermal conductivity of the Al/SiC MMC's do not meet desired expectations. Thermal conductivity of pure aluminum is ⁇ 200W/m.k, and the thermal conductivity of pure crystalline SiC particles is ⁇ 320W/m.k.
• thermal conductivity for Al/SiC MMC's are generally ⁇ 200W/m.k, and typically ⁇ 180 W/m.k (ref 1-5). These Al/SiC MMC's were consolidated by processes such as stir casting, powder metallurgy, or low pressure and pressureless melt infiltration. These methods are relatively slow, and have a considerable residence time when the aluminum is in the molten state, allowing the SiC to react with the molten Al forming aluminum carbide.
• these processes generally require the use of Al-Si alloys which decrease the activity of the Si and reduce the kinetics of the adverse carbide reaction during long contact times with the molten aluminum.
• These Al-Si alloys generally have a lower thermal conductivity than pure aluminum, thus reducing the thermal conductivity of the SiC/Al MMC.
• the use of rapid high pressure metal infiltration (also referred to as squeeze casting) to consolidate the particulate reinforced aluminum composites results in a much faster consolidation of the composite. Exposure times of the particles to the molten aluminum are generally seconds as opposed to hours for the non-pressure processes described above.
• Coating the diamond particles with a protective layer before contacting the diamond particles with molten aluminum forming the aluminum composite can prevent the reaction to form AL t C . .
• the application of a distinct SiC coating on diamond particles and subsequent composite formation with Al has been described in the literature (ref 6).
• a loose bed of industrial diamond powder (Beta Diamond Products), with a particle size of 40-50 microns, was coated with SiC using a chemical vapor deposition process of the diamond particle array, which was termed chemical vapor infiltration, or CNI, by the authors. In this CNI process, a distinct SiC coating is applied or deposited on the surface of the diamond particles.
• Table I Physical properties of -50 vol. % diamond/ Al MMC composites described in reference 6.
• the present invention involves the provision of a composite structure comprised of a metal matrix having diamond particles dispersed therein, wherein the diamond particles are characterized by the presence of a layer of beta- SiC chemically bonded to the surface thereof and wherein the carbon of the SiC is derived from the diamond of the respective particle to which it is bonded.
• the matrix of the composite structure is comprised essentially of a metal selected from among aluminum, copper,magnesium and alloy of one or more of said metals.
• the matrix is comprised of aluminum.
• the matrix metal is comprised of copper.
• the instant invention further describes the formation of a metal matrix composite containing such diamond particles coated with SiC applied by a different process that results in a very high thermal conductivity for the composite. It is, therefore, a primary object of the present invention to provide a metal matrix composite with substantially improved thermal conductivity. It is another valuable object of the present invention to provide a SiC coated diamond particulate with the necessary characteristics to enable its use in the formation of such high thermal conductivity MMC's. It is another valuable object of the present invention to provide a novel process to produce the above-mentioned SiC coated diamond particulates.
• the above-mentioned high thermal, conductivity metal matrix compositions are produced by a method wherein the layer of chemically bonded SiC is produced in-situ on the diamond particles of an array thereof that is then embedded in the metal matrix, preferably by a rapid high pressure metal infiltration technique known as squeeze casting.
• the chemically bonded layer of SiC is produced on the respective diamond particles by a chemical vapor relation process (CVR) by contacting the diamond particles with SiO gas.
• CVR chemical vapor relation process
• FIG. 1 is a schematic of apparatus suitable to carry out the CVR process to coat diamond particles with SiC in accordance with the present invention.
• FIG. 2 is a schematic of apparatus suitable to perform the squeeze casting assembly to form the diamond metal matrix composites in accordance with the present invention.
• DETAILED DESCRIPTION OF PREFERRED EMBODIMENT Referring now to Figure 1, there is shown a schematic drawing of an apparatus suitable for preparing diamond particles for use as a component of a composite structure including a metal matrix in which the diamond particles are dispersed according to the present invention, wherein the prepared diamond particles have a conversion surface layer of beta-SiC formed thereon.
• FIG. 1 there is shown in side elevation a cross sectional view of a crucible 101 formed of SiC and which is divided into a lower chamber 102 and an upper chamber 103 by means of a lower ring 104 of Si and an upper ring 105 of SiC and having a web 106 of 100% SiC fabric disposed between the two rings.
• the 100% SiC fabric was formed by reacting graphite fabric with gaseous SiO, to produce essentially 100% conversion of the graphite to SiC.
• the lower chamber 102 houses a SiO generator.
• the SiO generator was prepared by mixing silicon (Si) and silica (SiO 2 ) in equimolar ratios.
• Si + SiO 2 2 SiO (g) (3)
• FIG. 2 shows a die assembly suitable for high pressure squeeze casting of metal matrix composites made in accordance with the present invention.
• This apparatus is fabricated from tool steel and consists of the die 110, die plug 111, and shot tube (or gate)l 12.
• a cavity 113 is machined in the die corresponding to the required geometry for the squeeze cast part 113a.
• the die plug 111 has a 0.005" clearance to the die cavity 113 to allow air to be vented from the casting as it fills with molten aluminum.
• the inside diameter (ID) of the shot tube 112 is sufficiently large such that it completely covers the die cavity 113.
• the powder is placed in the die cavity 113, and shaken to provide a uniform loading of maximum density.
• Ceramic paper 114 is placed in the shot tube 112 to cover the powder filled die 110. A quantity of molten aluminum sufficient to fill the die cavity 113 plus part of the shot tube 112 is then poured in the shot tube 112.
• the preferred route to prevent this heat buildup is to mount the chip on a high thermal conductivity substrate which will conduct the heat away from the chip to a large heatsink therefore limiting the maximum temperature of the chip during operation to an acceptable value.
• the chip is generally attached to the substrate by means of a high thermal conductivity solder or some polymeric adhesive. Solder is generally preferred over the adhesives because the solders generally have a higher thermal conductivity than the organic adhesive. There is an additional mechanism by which the chip can be damaged, and that relates to the relative coefficient of thermal expansion of the chip and the substrate. When a chip is attached to a substrate with solder, the components must be heated to a temperature sufficiently high to melt the solder (180 degrees centigrade - 425 degrees centigrade).
• the solder solidifies providing a bond between the chip and substrate.
• differential thermal expansion of the chip and substrate can lead to the buildup of substantial mechanical stresses in the chip. This cool down, plus continued thermal cycling of the package can cause fatigue in either the substrate, chip, or interface, affecting performance and lifetime.
• stresses exceed the tensile strength limits of the chip, they can cause cracking (delamination) to occur.
• the materials commonly used for electronic chips are either Si with a CTE -3 p.p.m./degrees centigrade, or GaAs which has a CTE -6 p.p.m./degrees centigrade.
• a substrate with a CTE that is closely matched to that of the chip. It would be even more desirable to have a substrate with such thermal expansion characteristics combined with a very high thermal conductivity.
• One type of substrate used in the electronic packages described above is a ceramic substrate.
• an aluminum oxide substrate is commonly used, which has a thermal conductivity of -20-40 W/m.k and a CTE of -7 p.p.m./degrees centigrade.
• an aluminum nitride ceramic substrate with a thermal conductivity of -150-200 W/m.k and a CTE of ⁇ 4 p.p.m./degrees centigrade may be a better choice.
• Aluminum nitride ceramic substrates generally represent the highest thermal conductivity alternative in ceramic substrates other than the toxic BeO. These aluminum oxide and aluminum nitride ceramic substrates have excellent electrical insulating characteristics, which is desirable for many electronic packages, including those based on bipolar chips. However, for electronic packages based on LDMOS chips wherein the bottom side of the device becomes the source contact, a substrate is required which has good electrical conductivity between the front and back faces or surfaces. While this may be accomplished by fabricating electrically conductive vias in the ceramic substrate, which would electrically connect those two surfaces, it would be generally be simpler to have the substrate body itself be electrically conductive, in addition to having properties of high thermal conductivity and a good CTE match to the chip.
• MMC substrates can often meet the CTE and electrical conductivity requirements, but are limited in the thermal conductivity they can achieve.
• the Al SiC particulate reinforced MMC described above has a thermal conductivity of -180-225 W/m.k, CTE of ⁇ 7p.p.m/degrees centigrade and good electrical conductivity. A much higher thermal conductivity would be desirable given the tendency for LDMOS chips to operate at very high power densities.
• a metal matrix composite manufactured as described in the present invention with an aluminum matrix and containing particulate diamond prepared in accordance with the present invention can meet all the needs of those LDMOS electronic packages.
• Al has a thermal conductivity of -200 W/m.k.
• Diamond has a thermal conductivity between about 700-2000 W/m.k. Furthermore, the relatively low CTE of diamond (-1 p.p.m./degrees centigrade) will result in a CTE for the composite which is a better match to either Si or GaAs chips than the unfilled metals.
• the particulate diamond reinforced MMC's described in the literature have demonstrated only a marginal improvement in thermal conductivity over the Al SiC MMC's readily available.
• the MMC's described in the instant invention have unexpectedly high thermal conductivities, with values up to -650 W/m.k demonstrated to date, and with the potential for even higher levels of thermal conductivity, e.g. values of 800 W/m.k and above.
• a thin, uniform, surface conversion, adherent coating of SiC must be present on the diamond prior to composite formation.
• the purpose of this coating is to provide an interfacial coupling layer between the diamond and the aluminum, which in addition to being readily wet by the aluminum, provides graded acoustical impedance properties at the interface for enhanced phonon transfer.
• this SiC coating must be integrally bonded to the diamond in such a way that a distinct, discrete interface conversion layer between the diamond and the coating is produced to prevent any thermal barrier to heat transfer.
• the coatings obtained by a CVI or CVD process are expected to have an abrupt interface between the SiC coating and the substrate.
• the CVI/CVD deposition temperature of SiC are below the temperature required to effect a conversion of the diamond surface to SiC and results in only a built-up coating free of any interface reaction with an abrupt interface between the diamond and CVI/CVD SiC coating. It is believed this CVI interface contributed to the poor thermal conductivity of the prior diamond/aluminum composite described by Johnson and Sonuparlac in ref 6. Furthermore, this chemical vapor reaction (CVR) process ensures that a uniform surface layer of SiC is obtained at very low levels of conversion coating thickness.
• the thermal conductivity of the SiC acts as a thermal barrier between the very high thermal conductivity diamond particles and the metal matrix, limiting the maximum thermal conductivity that can be achieved for the MMC.
• a prediction of composite thermal conductivity can be made using a rules of mixture calculation. While thermal conductivity is a transport property, and the rules of mixture calculation is not exact, it can provide insight into a comparison of the CVI coating on diamond described by Johnson and Sonuparlac in ref 6, with the CVR conversion coating of the present invention. The typical relation for simple mixtures would be:
• K m , Kp, and Kc are the thermal conductivities of the matrix, particle, and composite, respectively (ref 7).
• the above equation requires perfect heat transfer across the particle-matrix interface. This is limited by phonon scattering at the interface between dissimilar components. Calculations were carried out to estimate Kp in order to judge the effectiveness of the thermal interface. For an aluminum composite with 50 vol.% of diamond particles coated with the CVR process, a value of 200W/m.k was used for Km, and 650W/m.k for Kc (which was obtained for a composite produced using the CVR coating technology of this invention).
• FIG. 1 A preferred process to coat the particulate diamond powder is illustrated in FIG. 1.
• the diamond particles are placed on a supporting SiC fabric, which is then placed in a crucible and heated to the reaction temperature.
• the crucible is placed in a furnace, in which a SiO partial pressure is generated by a reaction between SiO 2 and a reducing agent such as Si or C.
• a predetermined time at temperature for the SiO-diamond conversion reaction to be completed the furnace is cooled, and the fabric supporting the coated diamonds removed from the crucible.
• the SiC conversion coated diamond particles are removed from the fabric by peeling off the fabric from the coated diamonds. Any agglomerates are then broken up by mechanical agitation.
• the reaction between the diamond and SiO to form SiC coated diamond will generally proceed more rapidly as the process temperature is increased. However at a certain temperature threshold, the diamond can be converted to graphite.
• Diamond particulate Synthetic Diamond from Oshmens Corp.
• XRD X-ray diffraction
• the XRD pattern gave no indication of any Conversion of diamond to graphite. However, when the same powder was heated to 1800 degrees centigrade for 8 hours, the XRD pattern on this sample showed a large graphite peak, a broadened diamond peak, and a SiC peak from the SiC conversion reaction. The significant conversion of diamond to graphite after 8 hours at 1800 degrees centrigrade led to a decision to restrict the diamond-SiO reactions to lower time and/or temperature. A series of furnace runs were then carried out to react the diamond particulate with
• the SiO(g) generator for the conversion of diamond particles was made by adding equimolar ratios of silicon (Si) and silica (SiO 2 ).
• the silica used had a particle size of 150- 300 microns. The mixture was milled for 1 hour using alumina milling media. The reaction of the Si and SiO 2 proceeds faster as the particle size of the SiO 2 is reduced. To minimize this effect, silica of the same particle size (150-300 microns) was used and the milling time was fixed at 1 hour.
• the data in Table III indicates the variable with the largest impact on SiC formation is the particle size of the diamond.
• the peak height ratio of SiC to diamond is generally 0.01-0.04 for 100-120 micron diamond particles, 0.03-0.07 for 15-30 micron diamond, and 0.1-0.6 for 0.5 -2 micron diamond. Thus the largest diamond particles appear to have the lowest SiC content.
• FIG. 2 is a schematic of the squeeze casting assembly to form the diamond metal matrix composites.
• Diamond powder is placed directly into the die cavity of a tool steel die and shaken to achieve full tap density.
• the diamond powder bed is covered with a sheet of 1/32" alumina ceramic paper that acts as a filter to remove oxide particles from the melt during infiltration.
• a sheet of 1/32" alumina ceramic paper that acts as a filter to remove oxide particles from the melt during infiltration.
• an 0.005" gap is maintained around the bottom plug in the die to allow trapped air to escape during infiltration.
• Aluminum is heated above the melt temperature (660 degrees centigrade) under Argon cover gas to avoid oxidation.
• the die is heated separately to a temperature slightly below the melting point of the Al.
• the heated die is removed from the heating furnace and transferred into the casting machine and Al melt is poured into the heated shot tube of the die. Infiltration is completed within a few seconds under pressurization.
• the die is cooled by unforced cooling in the casting machine prior to removal of the casting, which consists of the cast part and attached biscuit.
• An aluminum metal matrix composite having diamond particles dispersed therein was formed by the following method.
• Loose uncoated diamond powder Synthetic Diamond from Oshmens Corp.
• Loose uncoated diamond powder (Synthetic Diamond from Oshmens Corp.) of size 15-30 microns was placed directly into the die cavity of a tool steel die ( as illustrated in Figure 2) and shaken to achieve full tap density of 50-55%.
• Dimensions of the die cavity were 2" x 2" x 0.25"
• the diamond powder bed was covered with a sheet of 1/32" thick high alumina ceramic paper which acts as a filter to remove oxide particles from the melt during infiltration.
• a 0.005" gap was maintained around the bottom plug in the die to allow trapped air to escape during infiltration.
• Aluminum metal with a purity of 99.8% was heated to 850 degrees centigrade under Ar cover gas to avoid oxidation.
• the die was heated separately to a temperature of 650 degrees centigrade, slightly below the melting point of the Al (660 degrees centigrade).
• the heated die was removed from the heating furnace and transferred into the casting machine within 2 seconds and the Al melt was poured into the heated shot tube of the die maintained at 200 degrees centigrade. Infiltration was completed with 2 seconds, using a ram speed of 0.1 inch/sec. under pressurization and a maximum pressure of 15 ksi .
• the die was cooled to 300 degrees centigrade by unforced cooling in the casting machine prior to removal of the casting.
• Laser flash measurement of thermal conductivity on a number of samples produced by a number of casting runs returned values of conductivity in the range of 55-60 W/m.k. Measurement of strength of the composite using 4-point bend rupture yielded an average rupture strength of 22 ksi (average of 6 measurements).
• Example 2 The procedure for forming an aluminum metal matrix composite with uncoated diamond powder described in example 1 was repeated using uncoated 100-120 micron diamond powder. Laser flash measurement of thermal conductivity on a number of samples produced by a number of casting runs returned values of conductivity in the range of 150-200 W/m.k. Measurement of strength of the composite using 4-point bend rupture yielded an average rupture strength of 17 ksi (average of 6 measurements).
• Example 4 A process for producing an array of diamond particles having a conversion coating of SiC on the respective particles was performed as followed.
• Diamond particles Synthetic Diamond from Oshmens Corp.
• a chemical vapor reaction process as follows.
• the crucibles were loaded with 30 grams of diamond particles, and the crucibles were placed in a high temperature furnace where SiO was caused to flow through the array of diamond particles in contact with the surface of each particle and the particles were heated in the furnace.
• the furnace was heated to 1550 degrees centigrade in a non oxidizing atmosphere using a flowing argon atmosphere, and held at that temperature for 3 hours. After the furnace was cooled to room temperature, the diamond particles with the conversion coating of SiC were removed from the SiC fabric.
• the coated diamond powder comprising an array of conversion coated diamond particles from example 4 (15-30 micron particle size) was placed directly into the die cavity of a tool steel die (as illustrated in Figure 2) and shaken to achieve full tap density of 50-55%. Dimensions of the die cavity were 2" x 2" x 0.25"
• the diamond powder bed of SiC conversion coated particles was covered with a sheet of 1/32" thick high alumina ceramic paper which acts as a filter to remove oxide particles from the melt during infiltration. To avoid entrapment of gases in the die casting, a 0.005" gap was maintained around the bottom plug in the die to allow trapped air to escape during infiltration.
• Aluminum metal with a purity of 99.8% was heated to 850 degrees centigrade in a non-oxidizing atmosphere, under Ar cover gas to avoid oxidation.
• the die was heated separately to a temperature of 650 degrees centigrade, slightly below the melting point of the Al (660 degrees centigrade).
• the heated die was removed from the heating furnace and transferred into the casting machine within 2 seconds and the Al melt was poured into the heated shot tube of the die maintained at 200 degrees centigrade. Infiltration was completed within 2 seconds, using a ram speed of 0.1 inch/sec under pressurization and a maximum pressure of 15 ksi .
• the die was cooled to 300 degrees centigrade by unforced cooling in the casting machine prior to removal of the casting.
• Diamond particles of size 100-120 microns were reacted with SiO by repeating the chemical vapor reaction process described in example 4 to produce an array of SiC conversion coated diamond particles of larger particle size than those of Example 4.
• the die casting procedure for forming an aluminum metal matrix composite described in example 1 was repeated using the 100-120 micron chemical vapor reaction treated diamond powder array described in example 6.
• Laser flash measurement of thermal conductivity on a number of samples of the composite of SiC conversion coated diamond particles in the aluminum metal matrix produced by a number of casting runs gave values of conductivity in the range of 575-620 W/m.k.
• Measurement of strength of the composite using 4-point bend rupture yielded an average rupture strength of 51 ksi (average of 6 measurements).
• the coefficient of thermal expansion for the composite was measured at 7.1x10 " * in/in/degrees centigrade.
• Example 8 Diamond particles of size 0.5-2 microns were reacted with SiO by the chemical vapor reaction process described in example 4 to produce an array of conversion coated diamond particles of different size.
• the die casting procedure for forming an aluminum metal matrix composite described in example 1 was repeated using the array of 0.5-2 micron diamond particle chemical vapor reaction treated diamond powder described in example 8.
• Laser flash measurement of thermal conductivity on a number of samples of the composite of SiC conversion coated diamond particles in the aluminum matrix produced by a number of casting runs gave an average value of conductivity of 150 W/m.k.
• Measurement of strength of the composite using 4-point bend rupture yielded an average rupture strength of 51 ksi.
• Example 10 One gram of phenolic resin was mixed with 1.2 grams of Si powder and ball milled overnight. Four grams of diamond powder with a particle size of 100-120 microns was added to the mixture, which was thoroughly mixed. The mixture was pressed at 5 ksi pressure in a 0.5" diameter die and heated to 160 degrees centigrade to provide green strength. The resulting small pellet was pyrolyzed at 1600 degrees centigrade for 1 hr to produce a rigid preform with strength of >15 ksi. The pellet was placed in a steel die and infiltrated with Al using the same conditions as described above in example 1. Laser Flash measurement of thermal conductivity on a number of samples of the composite of SiC conversion coated diamond particles in the aluminum matrix produced by a number of casting runs produced values of conductivity in the range of 300-320 W/m.k.
• Example 12 The process of example 10 was repeated using a reduced amount of phenolic and Si: 0.5 gm. of phenolic resin and 0.6 gm. of Si powder. Diamond powder weight was the same as in example 10 at 4 gm. Laser Flash measurement of thermal conductivity on a number of samples of the composite of SiC conversion coated diamond particles in the aluminum matrix produced by a number of casting runs produced an average value of conductivity of 475 W/m.k.
• a composite structure comprising diamond particle dispersed in a magnesium metal matrix was formed using a process generally as described in example 1.
• Loose uncoated diamond powder with a particle size of 100-120 microns was placed directly into the die cavity of a tool steel die of dimensions 2" x 2" x 0.25" and shaken to achieve full tap density of 50-55% .
• the diamond powder bed was then covered by a sheet of 1/32" thick high alumina ceramic paper which acts as a filter to remove oxide particles from the melt during infiltration.
• a 0.005" gap was maintained around the bottom plug in the die to allow trapped air to escape during infiltration.
• the 99.8% purity Mg melt was heated to 830 degrees centigrade under Ar-2.5% SF6 cover gas to avoid oxidation.
• the die was heated separately to a temperature of 620 degrees centigrade, slightly below the melting point of the Mg (647 degrees centigrade ).
• the heated die containing the diamond powder was removed from the heating furnace and transferred into the casting machine within 2 seconds and pure Mg melt was poured into the heated shot tube of the die maintained at 200 degrees centigrade. Infiltration was completed within 2 seconds, using a ram speed of 0.1 inch/sec. under pressurization and a maximum pressure of 15 ksi.
• the die was cooled to 300 degrees centigrade by unforced cooling in the casting machine prior to removal of the casting. Laser flash measurement of thermal conductivity on a number of samples of the composite produced by a number of casting runs returned values of conductivity in the range of 120-250 W/m.k.
• the procedure for forming a magnesium metal matrix composite as described in example 12 was repeated using the 100-120 micron chemical vapor reaction treated diamond powder having an integral conversion coating of SiC as described in example 6.
• Laser flash measurement of thermal conductivity on a number of samples of the composite of SiC conversion coated diamond particles dispersed in the magnesium metal matrix produced by a number of casting runs returned values of conductivity in the range of 520-550 W/m.k.
• Measurement of strength of the composite using 4-point bend rupture yielded an average rupture strength of 28 ksi (average of 6 measurements).
• Example 14 A quantity of diamond particles with a size of 100-200 microns were placed in a column with an argon gas purge which was pulsed in order to jolt and move the diamond particles.
• the column of diamond particles was heated to 1200 degrees centigrade and a SiC organometallic (many are suitable for depositing SiC such as, methyltrichloro silane, dimethyldichlorosilane, triethylsilane, etc.) gas was passed through the diamond particles such that a thin coating of SiC was deposited on the diamond particles.
• a SiC organometallic manufactured are suitable for depositing SiC such as, methyltrichloro silane, dimethyldichlorosilane, triethylsilane, etc.
• the quantity of SiC coated diamond particles was heated to 1600 degrees centigrade for 1 hour to cause diffusion of the SiC into the diamond surface and produce a conversion coating of SiC on the diamond particles, which were then suitable for inclusion as the dispersed diamond particles in a matrix of magnesium metal as in example 13.
• the SiC conversion coated diamond particles described in examples 14 and 15 were squeeze cast with aluminum as described in example 1 to produce a composite SiC coated diamond particles in an aluminum metal matrix.
• a laser flash measurement of thermal conductivity gave values in the range of 550-620 W/m.k.
• graphite tooling for a casting die was prepared from graphite stock which was premachined with a shaped cavity corresponding to the shape and thickness of the required net shape part and covered with a graphite lid. Tooling tolerances were maintained at +/-0.005" to plates and round parts. The completed graphite tooling and lid was then perforated with a series of 1/16" holes drilled through it to allow air to escape from the diamond filler during casting.
• the graphite tooling could also be patterned with a series of grooves to correspond to the fins on a heat sink design for instance or with a stud at a location corresponding to a through hole in a heat spreader diamond/Al part to allow an attachment point.
• the graphite tooling was wrapped in ceramic paper which was fixed in place by continuous graphite fiber in a hoop winding configuration to cover all the through drilled holes.
• the diamond powder was poured into the mold and tapped to full tap density corresponding to 45-55% of the volume of the mold.
• the graphite tooling was placed in a steel permanent pressure casting die and infiltrated with molten aluminum metal at a pressure of 3-10 ksi.
• the overlying Al metal was machined away and the graphite tooling surrounding the fully infiltrated composite piece was removed by grit blasting with 325 mesh glass beads.
• the remaining 1/16" metal feeders or gates corresponding to the holes drilled through the original tooling to facilitate Al flow were sheared away using a hand tool to leave a net shape composite diamond-aluminum casting.
• Four point bend strength of the castings was 46-51 ksi.
• the gate tabs were left attached such that this geometry provided a high surface area to provide for exceptionally high heat transfer with the passing of air over the tabs.
• a diamond/Al composite plate of dimensions 2 x 2 x 0.1 was made by the method of graphite tooling described above in example 17 and was hot rolled at 550 degrees centigrade to a final thickness of 0.05", using multiple roll passes of 0.010" thickness reduction per pass with cross rolling after every other pass to maintain flatness of the part. Some minor edge cracking was observed after rolling which extended 0.1" into the part. These edges were removed by shearing. Thermal conductivity of the part was measured before and after rolling and remained unchanged at 520 W/m.k. Strength of the piece was 51 ksi measured in 4-point bending.
• a diamond/Al composite plate of 0.05" thickness was produced by the rolling operation described above in example 18. This composite plate was hot stamped in a closed die at 500 degrees centigrade to form a 0.125 x 0.25 x 0.03" thickness piece with a raised rim of 0.05" height surrounding it. Thermal conductivity of the piece was maintained at 520 W/m.k after stamping.
• a diamond Al composite plate of size 1 x 1 x 0.2" was produced by squeeze casting as described above in example 7. This composite plate was then open die hot forged at 500 degrees centigrade with applied pressure of 15 ksi at a strain rate of 0.0001/sec. After forging, the thickness of the composite plate was reduced from 0.25 to 0.05" which corresponded to a forging strain of 150%. Measurement of thermal conductivity of the composite plate after forging showed that the thermal conductivity of the material remained unchanged from the starting value of 585 W/m.k before forging.
• Example 21 100-120 mesh size (100 micron) coated diamond powder, as described in example 6, was fully mixed with 99.8% purity Al powder of 45 micron size by tumbling in a ceramic ball mill. The weight ratio of the two powders was chosen to correspond to a 40% by volume diamond Al composite. The powder mixture was transferred into a 1" diameter casting graphite die and hot pressed at a temperature close to the Al melting point for 1 hr at 640 degrees centigrade at a pressure of 6 ksi. The resulting composite diamond and aluminum billet was sectioned for strength and thermal conductivity measurement. Thermal conductivity measurements were in the range of 400-430 W/m.k and four point strength measurements were 46-52 ksi.
• FAST field activated sintering techniques
• PAS plasma activated sintering

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