Thermal properties of polycrystalline cubic boron nitride sintered under high pressure condition

The excellent thermal and chemical properties of cubic boron nitride (cBN) indicate that it is potential materials to prepare the thermal dissipate substrate applied in the electronic packaging. The thermal properties of polycrystalline cBN ceramics, however, have not been fully investigated. We report the first sintering experiment on preparing polycrystalline cBN ceramics using cBN powder as starting material without any sintering aids. The microstructure and high bending strength show that the strong combination was achieved among the crystal grains. The measured results, including density, thermal conductivity and thermal expansion coefficient, reveal that the properties of this ceramics depend on the grain size of starting crystal cBN. The PcBN ceramics has low thermal expansion coefficient extremely matching to that of silicon and exhibits moderate thermal conductivity due to its low density and the existence of low thermal conductive phase of hexagonal boron nitride.


Introduction
As electronic devices become smaller, faster and more powerful, thermal management and thermal stresses are becoming critical issues in many packaging applications, including microprocessors, power semiconductors, high power RF devices, and light-emitting diodes.As a result, the low density materials with high thermal conductivity and low thermal expansion coefficient (matching to that of silicon) are extremely needed for reliable performance of electronic devices [1,2].Non-metallic materials has attracted enough attention due to its excellent thermal properties and chemical stability, such as diamond [3,4], cubic boron nitride (cBN) [5][6][7], aluminum nitride (AlN) [8,9], silicon nitride (Si 3 N 4 ) [10,11], silicon carbide (SiC) [12,13], etc.
The cBN, first synthesized by Wentorf [14] with the transformation of hexagonal boron nitride (hBN) to cubic form under high pressure and high temperature conditions, is usually utilized to prepare cutting tools for processing various hard steel works because of its super wear-resisting property.Moreover, cBN possesses excellent thermal properties.Theoretical model predicts its single crystal with very high thermal conductivity, of about 1300 W/mK at room temperature, only second to diamond [5].These properties indicate cBN is very likely to be applied in the uses of electronic packaging as thermal dissipate substrate.No crystals of cBN were available that were large enough to be implied directly as the thermal dissipate substrate.Therefore, the only form of this material that could be used in electronic packaging is the sintered polycrystalline ceramics.
However, the research on sintering this thermal ceramic and studying its related properties has not been fully investigated, because polycrystalline cubic boron nitride (PcBN) is hard to be sintered using traditional sintering process, such as the hot pressing and spark plasma sintering methods.The reason is that cBN is metastable phase in contrast to hBN under ambient pressure and high temperature condition.It tends to transform to the stable phase, hBN, under the high sintering temperature condition with the traditional sintering methods.The previous experiment revealed that the cBN entirely transform to hBN at the temperature of about 1200 o C under the ambient pressure [15], implying that the PcBN ceramics only can be sintered under the pressures of GPa order (1 GPa=10 9 Pa).
There are several experimental studies on the thermal conductivity of PcBN ceramics, which were synthesized by direct conversion of hBN to cBN at the pressure of approximately 10 GPa.The reported results, however, remain very scattered.The first research on the thermal conductivity of PcBN ceramics was conducted by Slack in 1972 [5].He reported the thermal conductivity of PcBN is 180 W/mK at room temperature.Subsequently, Corrigan reported a high thermal conductivity value range from 250 W/mK to 900 W/mK [6].Ohashi and co-workers synthesized PcBN ceramics with the conditions similar to Corrigan's, but reported relatively lower thermal conductivity value, range from about 200 W/mK to 600 W/mK [16].These studied PcBN ceramics were prepared at extreme conditions, leading it hard to be applied in industrial manufacturing.Moreover, these works mainly investigated the thermal conductivity.No research was conducted on other properties related to the uses of the electronic packaging, such as the thermal expansion and bending strength.
In this study, we carried out the sintering experiments to study the thermal properties of PcBN ceramics.We placed emphasis on thermal conductivity, thermal expansion coefficient and bending strength properties of the ceramics.We used pure cBN powder as starting material to prepare PcBN ceramics, without any sintering aids.Four grain size cBN crystals were employed to prepare high purity and large size ceramics under a same sintering condition.We contributed the initial work to exploring a potential material that could be implied to electronic packaging.

Materials and Experimental Procedures 2.1. Starting materials, high pressure apparatus and sample preparation
Commercial cBN powder (supplied by Zhongnan Jete Superabrasives Co. Ltd., Henan, China) of different grain size (0-2μm, 2-4μm, 4-8μm and 8-12μm) was used in this study.The cBN powder was first pressed into molybdenum container under the pressure of 20 MPa then assembled into synthetic block as showed in Fig. 1.

Fig. 1. Sample assembly for sintering experiments. For clearly, only four anvils of the cubic
press were showed in the picture [19].
We used a large volume cubic press to sinter the PcBN ceramics.This press can generate the high pressure condition up to 6 GPa and the high temperature up to 2000 o C. As shown in Fig. 1, the WC anvils were connected with the pistons which were driven by hydraulic oil, thus the six anvils move toward the cell center from three dimensions and generate high pressure in the synthesis chamber.The computer-controlled hydraulic system allows a vary pressure in the chamber within the pressure-generating capability of the apparatus.More detailed descriptions of the apparatus have been reported by the literature [19].The temperature in the chamber was controlled by controlling the heating power applied to the graphite heater.The temperature was measured with W-Re thermocouple.The relationship between the hydraulic oil pressure and the chamber pressure was calibrated by metal melting point method which has been described in the literature [20].
We sintered the PcBN ceramics for 5 minutes under a same condition.In order to avoid transformation of cBN to hBN, we selected the sintering condition of nearly 6 GPa and temperature of 1500 o C, which entirely locates in the cBN stable region (see the Fig. 4 in reference [15]).The sintered PcBN ceramics were ground with a diamond wheel and subsequently polished with diamond paste less than 2 μm.For the thermal expansion coefficient and the bending strength measurements, some polished samples were cut by laser to the size of 12mm×2.5mm×2.5mm.The samples used for the thermal conductivity measurements were ground to the size of Φ12mm×2.5mm.

Characterization
The densities of the sintered samples were measured using the Archimedes method.The relative densities were determined using the measured densities divided by the theoretical density of cBN crystal (3.486 g/cm 3 ).Crystalline phases of PcBN ceramics were analyzed by X-ray diffraction (XRD, D8 Advance, Bruker, Germany).The microstructures of the ceramics were observed by scanning electron microscopy (SEM, JSM-IT300, JEOL, Japan).The thermal expansion coefficients of the ceramics were measured by the differential method using an Al 2 O 3 rod as standard for the temperature range from room temperature to 300 o C (DIL402C, NETZSCH, Germany).Thermal conductivity was measured using the laser flash method (TC-7000H, ULVAC-RIKO, Japan) at room temperature, 200 and 300 o C, respectively.The bending strength was determined by the three points bending test (Instron-5800, US) using 12mm×2.5mm×2.5mmbars (span 10mm).

Crystalline phase and microstructure of PcBN
Fig. 2 shows the X-ray diffraction patterns of the starting crystal cBN powder and the sintered PcBN ceramics.Although the sintering condition locates in the cBN stable region, the sintered PcBN ceramics nevertheless contain a certain amount of hBN reversely transformed from the cBN, especially for the ceramics sintered with the 0-2 μm cBN powder.We attributed the reversed transformation to the voids among the cBN grains in the process of sintering.Because the cBN is a super hard material, the pressure around the voids could be much lower than that in the area where the cBN crystal faces contact.As the temperature increased to 1500 o C, the sintering conditions of pressure and temperature around the voids could locate in the hBN stable region.This situation is more severe when using the more small size cBN powder as starting materials.Therefore, the ceramics sintered with 0-2 μm CBN powder exhibits a significant content of HBN that is indicated by the diffraction peak of X-ray.The SEM images of sintered PcBN ceramics are shown in the Fig. 3.The cBN grains still keep their regular crystal shape and the strong combinations among the grains seem to be realized.Some obvious pores can be observed among the cBN grains, especially in the ceramics sintered with the large size cBN grains.The tiny particles also can be observed around the pores, which are believed to be the hBN.When using the smaller cBN grains as the starting materials the hBN particles seem to be more obvious around the pores.This resulted in that the crystal shape became obscure in the ceramics that was sintered with the 0-2μm cBN powder.

Density and bending strength of PcBN
The relative density and bending strength of the PcBN ceramics are shown in Fig. 4. Because the low density phase of hBN (2.29 g/cm 3 ) and pores exist, the sintered ceramics have lower density value than that of crystal cBN.The values of density increase when the size of starting cBN grains increase because the content of hBN decreases.This demonstrates that the reverse transformation becomes easier when using the small cBN grains as the starting materials.It is consistent with the results reflected by the X-ray patterns and SEM photos.The high bending strength value of the sintered PcBN ceramics shows that the strong combination among the cBN grains was realized.The bending strength, however, shows an opposite tendency to that of the density.The ceramics sintered from the small cBN grains exhibit a high bending strength value.We also attributed the result to the existence of hBN because it increases the combination strength among the crystal grains.The larger specific surface of the smaller cBN grain, of course, is another reason resulted in the tendency of the bending strength, because it enhances the combination opportunity of the crystal faces.Fig. 5 shows the thermal expansion coefficient of PcBN ceramics.The property of the ceramics is generally consistent with that of silicon and exhibits a decreasing tendency as the starting cBN grains increasing.The tendency demonstrates that the extent of purity, or, the content of hBN, dominates this thermal property of the PcBN ceramics.The ceramics sintered with 2-4 μm CBN powder has a thermal expansion coefficient value which is extremely matching to that of silicon.The thermal conductivity of the PcBN ceramics is shown in the Fig. 6.Generally, the ceramics have moderate thermal conductivity, although the starting material, crystal cBN, possesses an extreme high theoretical thermal conductivity.The thermal conductivities of the ceramics reach their maximum value near 200 o C.This temperature dependence of the thermal conductivity is consistent with the reports of Slack [5] and Corrigan [6].The thermal conductivity of the ceramics is also dependent on the starting cBN grain size.The ceramics sintered with the 2-4 μm cBN powder has the maximal thermal conductivity of 56 W/mK at room temperature.When using the too small grain size cBN as the starting material, the existence of considerate hBN content increases the phonon reflections between the crystal surfaces, thus obstructs the transmission of heat flux and results in a low thermal conductivity.The too large cBN grain size is also not good for the transmission of heat flux due to the weak combination of crystalline grains, which is reflected by the bending strength mentioned above.

Thermal properties of PcBN
Although the crystal cBN and the PcBN ceramics synthesized by direct conversion of hBN to cBN have excellent thermal conductivity, the PcBN ceramics prepared in this study have a moderate thermal conductivity.According to the measurements of XRD, SEM and bending strength, the reversed transformation and the obvious pores in the ceramics are thought to be the main reasons that results in the drastic decline of thermal conductivity.Adding sintering aids, such as the metal aluminum or cobalt, is an expected method to enhance the densification of the PcBN ceramics.And, improving the sintering temperature could be another means to increase the thermal conductivity because it is benefit to the strong combination among the cBN grains and preventing from the reversed transformation.

Conclusion
We conducted the experimental research on sintering polycrystalline cubic boron nitride thermal ceramics using only crystal cubic boron nitride powder as the starting material.The high bending strength and microscopic images show that the ceramics were sintered successfully.The sintered ceramics have low thermal expansion coefficient which is extremely matching to that of silicon.However, the reversed transformation of cubic boron nitride to the hexagonal form and the relative low density lead to a moderate thermal conductivity of the ceramics.Adding metal sintering aids or increasing the sintering temperature is the expected method to improve the thermal conductivity.This initial work indicated that the high purity polycrystalline cubic boron nitride ceramics can be prepared in a short time.And, the polycrystalline cubic boron nitride is a potential thermal ceramics that can be implied to electronic packaging as improving the sintering method.

Fig. 4 .
Fig. 4. The relative density and bending strength of the PcBN ceramics.

Fig. 5 .
Fig.5.The thermal expansion coefficient of PcBN.The line represents the thermal expansion coefficient of silicon, shown for comparison.