简介概要

Microstructure and thermal properties of copper matrix composites reinforced with titanium-coated graphite fibers

来源期刊：Rare Metals2013年第1期

论文作者：Hao-Ming Zhang Xin-Bo He Xuan-Hui Qu Qian Liu Xiao-Yu Shen

文章页码：75 - 80

摘要：Milled form of mesophase pitch-based graphite fibers were coated with a titanium layer using chemical vapor deposition technique and Ti-coated graphite fiber/Cu composites were fabricated by hot-pressing sintering. The composites were characterized with X-ray diffraction, scanning/transmission electron microscopies, and by mea-suring thermal properties, including thermal conductivity and coefficient of thermal expansion （CTE）. The results show that the milled fibers are preferentially oriented in a plane perpendicular to the pressing direction, leading to anisotropic thermal properties of the composites. The Ti coating reacted with graphite fiber and formed a continuous and uniform TiC layer. This carbide layer establishes a good metallurgical interfacial bonding in the composites, which can improve the thermal properties effectively. When the fiber content ranges from 35 vol% to 50 vol%, the in-plane thermal conductivities of the composites increase from 383 to 407 Wá（máK） -1 , and the in-plane CTEs decrease from 9.5 9 10-6 to 6.3 9 10 -6 K-1 .

Rare Metals 2013,32(01),75-80

Microstructure and thermal properties of copper matrix composites reinforced with titanium-coated graphite fibers

Hao-Ming Zhang Xin-Bo He Xuan-Hui Qu Qian Liu Xiao-Yu Shen

School of Materials Science and Engineering, University of Science and Technology Beijing

State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing

摘要：

Milled form of mesophase pitch-based graphite fibers were coated with a titanium layer using chemical vapor deposition technique and Ti-coated graphite fiber/Cu composites were fabricated by hot-pressing sintering. The composites were characterized with X-ray diffraction, scanning/transmission electron microscopies, and by mea-suring thermal properties, including thermal conductivity and coefficient of thermal expansion (CTE). The results show that the milled fibers are preferentially oriented in a plane perpendicular to the pressing direction, leading to anisotropic thermal properties of the composites. The Ti coating reacted with graphite fiber and formed a continuous and uniform TiC layer. This carbide layer establishes a good metallurgical interfacial bonding in the composites, which can improve the thermal properties effectively. When the fiber content ranges from 35 vol% to 50 vol%, the in-plane thermal conductivities of the composites increase from 383 to 407 Wá(máK) -1 , and the in-plane CTEs decrease from 9.5 9 10-6 to 6.3 9 10 -6 K-1 .

作者简介：Xin-Bo He e-mail:xb_he@163.com;

收稿日期：22 February 2012

基金：financially supported by the National Natural Science Foundation of China(No.51274040);the Fundamental Research Funds for the Central Universities(FRF-TP-10-003B);

Microstructure and thermal properties of copper matrix composites reinforced with titanium-coated graphite fibers

Abstract：

Keyword：

Metal matrix composites; Titanium coating; Microstructure; Thermal conductivity; Coefficient of thermal expansion;

Received： 22 February 2012

1 Introduction

Improved performance and smaller scale are two features for new applications in the electronics industry.Both of these targets result in increased heat flux densities within electronic devices,and thereby,the effective therma management becomes a very important issue for packaging of high-performance semiconductors[1].The electronic packaging materials should have a high thermal conductivity to effectively dissipate heat and a low coefficient of thermal expansion(CTE)to minimize thermal stresses[2]This is of vital importance to enhance the performance,life cycle,and the reliability of electronic devices.Traditiona packaging materials like Kovar,Cu/W,Cu/Mo or Si C/Al which suffer from certain limitations of their relatively low thermal conductivity(no more than 250 W˙(m˙K)^-1),are no longer sufficient to fulfill the requirements of heat removal of the most recent power electronic devices[3,4]Recently developed diamond/metal composites family despite featuring very high thermal conductivity[5,6],has so far been limited to niche markets,due to its poor machinability and high cost[7–9].

With the advancement of graphite fiber technology,the milled form of mesophase pitch-based graphite fibers can offer a thermal conductivity in excess of 900 W˙(m˙K)^-1,a CTE to-1.45 9 10^-6K^-1,as well as a low price.Combining such graphite fibers with high thermal conductive copper in an appropriate way,it is expected to obtain the graphite fiber/Cu composites with high thermal conductivity,low CTE,good machinability,and reasonable cost However,this research is rarely done at present.It has been suggested that the major problem in the development of the Cu matrix composites containing carbon-based fillers is the absence of chemical reaction between carbon and copper which will lead to a weak interface and a bad transfer of properties between reinforcement and matrix[10–13].In the present work,titanium,a carbide-forming element,is coated on the surface of such milled graphite fibers to improve the interfacial bonding.The Ti-coated graphite fiber reinforced Cu matrix composites(Ti-coated graphite fiber/Cu)are fabricated by hot-pressing.The microstructure and thermal properties in terms of thermal conductivity and CTE are characterized.

Table 1 Parameters and thermal properties of as-received milled mesophase pitch-based graphite fibers 下载原图

Table 1 Parameters and thermal properties of as-received milled mesophase pitch-based graphite fibers

2 Experimental

2.1 Raw materials

The milled form of mesophase pitch-based graphite fibers(XN100 type)used in this work was purchased from Nippon Graphite Fiber Corporation.Their basic parameters and thermal properties taken from the data sheet of the manufacturer are listed in Table 1.The Cu powders used are the gas-atomized spherical Cu powders,having a mean particle size of 14 lm and a purity of[99.9%,supplied by General Research Institute for Nonferrous Metals,Beijing,China.

2.2 Coating process

The surface of graphite fiber was coated with titanium through chemical vapor deposition(CVD)technique.Generally,the CVD technique is based on a chemical reaction leading to the deposition of metallic elements on the surface of target substance[14].In this case,Ti Cl₃powders were selected to provide titanium resource,and Ti H₂powders were chosen as reducing agent.After mixing graphite fibers with Ti Cl₃and Ti H₂powders at a weight ratio of 100:15:3,the mixture was placed into a deposition chamber of CVD apparatus.Then,the vacuum in the chamber was pumped to a level of 1 9 10^-2Pa.The deposition temperature and time were set to 680°C and90 min,respectively.After deposition,the vacuum was pumped to 1 9 10^-2Pa again.Finally,the Ti-coated graphite fibers were sieved from the resulting mixture using a shaking-sieve machine.The average thickness of the Ti coating on the fibers is about 120 nm,which is examined by scanning electron microscopy(SEM)method.The main reaction in current CVD process can be expressed as 2Ti Cl₃(g)+3Ti H₂(s)+5Ti(s)+6HCl(g).With the increase of temperature,Ti Cl₃gasified and reduced to Ti atoms by reacting with Ti H₂.These generated reactive gaseous Ti atoms deposited on the surface of graphite fibers forming Ti coating.The SEM images of the Ti-coated graphite fibers are shown in Fig.1.The Ti coating is successfully deposited on the surface of the fibers forming a continuous coverage,and it is seen to be smooth,compact and homogeneous.

2.3 Hot-pressing

The Cu powders with different contents of Ti-coated graphite fibers(35 vol%,40 vol%,45 vol%,and 50 vol%)were dry mixed at room temperature using a three-dimensional vibratory mill for 8 h with rotary speed of 2,500 r˙min^-1.Vacuum hot-pressing sintering system(Model High-Multi 5000,Fujidempa Kogyo,Co.,Ltd.,Japan)was used to synthesize the Ti-coated graphite fiber/Cu composites.The powder mixture was compacted into a cylindrical graphite die with an inner diameter of 30 mm.A sheet of graphite felt was placed between the punch and the powders as well as between the die and the powders for easy removal.The compact powders were sintered at 940°C under a uniaxial pressure of 35 MPa for40 min in high vacuum(1 9 10^-3Pa).The heating/cooling rates were about 10°C˙min^-1.After sintering,the samples with diameter about 28 mm and thickness about 12–14 mm were obtained after getting rid of the graphite felt left on the surface of the composites.For comparison purposes,a sintered copper sample and an uncoated graphite fiber(50 vol%)/Cu composite sample were also fabricated under the same conditions.

Fig.1 SEM images of the Ti-coated graphite fibers

2.4 Characterization

The bulk density of the composites was measured by the water immersion method based on Archimedes’law and compared with the theoretical density.The morphology and fracture surface of the composites were observed on LEO-1450 SEM.The phases in composite were determined by X-ray diffraction(XRD)conducted on Siemens D5000diffractometer using Cu Ka radiation.The observation for interface area was carried out on LEO JSM-7001F field emission(FE)-SEM and JEM-2100 transmission electron microscopy(TEM).

The thermal conductivity,λ,of the composites was determined from measurements of composite density,q,thermal diffusivity,a,and specific heat capacity,C,using the relationship λ=aq C[15].Specific heat capacity was measured using differential scanning calorimetry(TA-Instruments Q100)on cylindrical discs4 mm in diameter and 0.5 mm thick.Thermal diffusivity was measured at room temperature by a laser flash apparatus(LFA447,Netzsch,Germany)on disc-shaped specimens with a diameter of 10 mm and a thickness of2.5 mm.In order to obtain reliable results,five tests were performed on each specimen,and the mean value was chosen for calculations throughout this paper.The CTE of the composites was measured by a Perkin-Elmer TMA7 dilatometer in temperature range 20–250°C at a nominal heating and cooling rate of 5°C˙min^-1.The dimension of the specimens was 5 9 5 9 2.5 mm.To diminish systematic errors,the dilatometer was calibrated by measuring an alumina specimen under identical conditions.The reported CTE was an average value in the temperature range between 30 and 100°C.The thermal diffusivity and CTE were measured in X–Y and Z direction(assuming that the Z axis is the pressing direction)of the composites,respectively.

3 Results and discussion

3.1 Microstructural

Density measurements show a post-sintering densification of[98%for all prepared Ti-coated graphite fiber/Cu composites samples.Typical SEM morphologies of the composites in perpendicular and parallel to hot-pressing direction are shown in Fig.2.The graphite fibers are homogeneously distributed in the Cu matrix.There is almost no specific degradation of the fibers,separated interfaces,or evident pores can be observed.Moreover,the densification process leads to a preferential orientation of the fibers in the plane(X–Y)perpendicular to the pressing direction(Z).Apparently,this planar orientation of the fibers points out that 2-D composites are obtained,and thus,their properties will be highly anisotropic.

Figure 3 shows the XRD pattern of the composite with50 vol%graphite fibers.Besides the main phases of copper and graphite,only the Ti C phase was detected in the composite.Therefore,it can be deduced that the prior Ti coating reacts with the graphite fibers and synthesizes the titanium carbides during sintering densification process.

Figure 4a presents FE-SEM micrograph of the interface area obtained from the composite with 50 vol%fibers.A continuous interlayer of a contrast(dark gray)different from those of graphite fiber(black gray)and Cu matrix(light gray),about 100 nm thick on the average,is clearly visible in the second electron(SE)mode.Combined the results from XRD analysis,it can be verified that this interlayer is the titanium carbide layer.At this scale of observation,the overall morphology of this interface region exhibits a neat profile,where the thin and uniform Ti C layer is tightly adhered to both the fiber and Cu matrix Moreover,the detailed observation of the Ti C/graphite fiber interface was determined by TEM in Fig.4b.Further TEM study also identifies the interlayer as Ti C by electron diffraction analysis.Moreover,it reveals that this Ti C layer is consisted of highly coherent nano-sized Ti C crystallites(10–50 nm),which are close coupled with the graphite fiber at the Ti C/fiber interface.

Fig.2 SEM morphologies of the Ti-coated graphite fiber(50 vol%)/Cu composite in(a)perpendicular,(b)parallel to hot-pressing direction,and(c)schematic of the fiber orientation

Fig.3 XRD pattern of the composite with 50 vol%graphite fibers

The uncoated graphite fiber(50 vol%)/Cu composite was also examined by TEM.It is evident that the adhesion between graphite fiber and Cu matrix is very weak,because some nano-sized interfacial gaps are often observed,as in Fig.5.According to the C/Cu phase diagram,the C solubility in Cu is negligible.In addition,copper is known to be chemically inert with respect to carbon.The chemical incompatibility of carbon and copper results in a weak mechanical interfacial bonding in the uncoated graphite fiber/Cu composite.

Figure 6 gives the SEM fracture images of the both uncoated and Ti-coated graphite fiber/Cu composites containing 50 vol%fibers.The fracture surfaces are parallel to the Z direction.Most of uncoated graphite fibers that keep their original appearances are separated from the Cu matrix(Fig.6a),whereas nearly all Ti-coated graphite fibers present fracture,bends,or fragment morphologies(Fig.6b).These findings indicate that a strong interfacia bonding can be achieved in the coated composites due to the formation of Ti C layer.

Fig.5 TEM micrograph of the interface between uncoated graphite fiber and Cu matrix

From the above observations,it can be concluded that the Ti C interlayer with a strong adhesion force with the graphite fiber and copper caused the interface structure of the graphite fiber/Cu composite to change from mechanical bonding into metallurgical bonding.Consequently,the heat and load transfer abilities between the graphite fiber and Cu matrix could be enhanced,which are beneficial to the improvement of the thermal properties.

3.2 Thermal properties

Table 2 summarizes all experimental values of density specific heat,thermal diffusivity,CTE of the Ti-coated graphite fiber/Cu composites,the uncoated graphite fiber(50 vol%)/Cu composite,and the sintered copper.

Fig.4 a FE-SEM micrograph of the interface area in Ti-coated graphite fiber(50 vol%)/Cu composite using SE signal and b TEM micrograph of the carbide/graphite fiber interface and the corresponding electron diffraction pattern from interfacial carbide(Ti C)

Fig.6 SEM images of the fracture surfaces of a uncoated and b Ti-coated graphite fiber/Cu composites containing 50 vol%fibers

It can be clearly seen that the thermal properties of the Ti-coated graphite fiber/Cu composites in X–Y direction are evidently superior to that of those in the Z direction.From the data,the thermal conductivities in X–Y direction are1.5–2.5 times of that in Z direction,and the CTEs in X–Y direction are 30–60%lower than that in Z direction,depending on the fiber content.Obviously,these anisotropic thermal properties arise from the planar orientation of the fibers and large difference between their radial and axial thermal properties.

Also,it can be observed that the thermal conductivities of the composites in X–Y direction are all higher than that of sintered copper,and they are increased with the fiber content increasing.The higher thermal conductivities obtained suggests that graphite fibers contribute to enhancing thermal conductivity of the composites.However,it should be noticed that the increase extent of thermal conductivities is gradually decreased with the fiber content increasing.This can be attributed to the decreased densities of the composites.As we all know,pores are harmful to the thermal conductivity.Here,we point out that adding high volume fraction of the fibers will bring difficulties to the densification of the composites.In fact,when the fiber content exceeds 50 vol%,it is very difficult for us to obtain high density composites via the present fabrication route On the contrary,the thermal conductivities of the composites in Z direction are all lower than that of sintered copper,and they are decreased with increase of the fiber content.This can be easily understood because the major contribution of thermal conductivity in Z direction comes from Cu matrix.In addition,unlike the changing trend of thermal conductivity with fiber content,the CTEs of the composites in both X–Y and Z direction all decrease,while the fiber content increases;from that,it can be explained that the axial and radial CTEs of the graphite fibers are all much lower than the CET of copper.

Moreover,the experimental results demonstrate that coating of graphite fiber with titanium has a positive effect on thermal properties of the prepared composites.The thermal conductivity of the uncoated composite is only178 W˙(m˙K)^-1even in X–Y direction,which is229 W˙(m˙K)^-1lower than that of the coated composite The CTE values in X–Y and Z direction of the coated composite are about 3 9 10^-6and 2 9 10^-6K^-1lower than those of the uncoated composite,respectively.As the relative density difference of the both is less than 1%,i can be deduced that such remarkable thermal properties improvement mainly originated from the good metallurgical bonding between the graphite fiber and Cu matrix caused by the formation of Ti C layer.

Table 2 Measured values of density,specific heat,thermal diffusivity,CTE of Ti-coated graphite fiber/Cu composites,uncoated graphite fiber(50 vol%)/Cu composite,and sintered copper 下载原图

Table 2 Measured values of density,specific heat,thermal diffusivity,CTE of Ti-coated graphite fiber/Cu composites,uncoated graphite fiber(50 vol%)/Cu composite,and sintered copper

Overall,the in-plane thermal conductivities of the composites with 35 vol%–50 vol%fibers can reach to383–407 W˙(m˙K)^-1,which is much higher than that of traditional packaging materials.The in-plane CTEs of the composites are less than 10 9 10^-6K^-1.Due to the properties of high thermal conductivity,low CTE,and good machinability,the obtained composites are the suitable candidates as electronic packaging materials Although anisotropy could be an issue for some applications,it could also be a benefit,allowing designers the ability to make heat preferentially flow in one direction.

4 Conclusion

Ti-coated graphite fiber reinforced Cu matrix composites were successfully fabricated by hot-pressing sintering Densification process led to a preferential orientation of the fibers in a plane perpendicular to pressing direction.The composites are anisotropic materials.They have higher thermal conductivities and lower CTEs in 2-D plane direction.The Ti coating obtained by CVD technique,reacted with graphite fiber and formed a continuous and uniform Ti C layer during sintering process.This Ti C layer established a good metallurgical bonding between the fiber and Cu matrix,which can promote the enhanced thermal conductivity and reduced CTE of the composites,effectively The composites with 35 vol%–50 vol%fibers achieved in-plane thermal conductivity of 383–407 W˙(m˙K)^-1and in-plane CTE of 6.3–9.5 9 10^-6K^-1,making them suitable for being electronic packaging materials.