Dielectric and thermal properties of GFs/PTFE composites with hybrid fillers of Al2O3 and hBN for microwave substrate applications

The alumina/hexagonal boron nitride/glass fibers cloth/Polytetrafluoroethylene (Al₂O₃–hBN/GFs/PTFE) composites were prepared by blending-impregnation followed by hot compression method, and the dielectric, thermal properties of the composites with various hBN fillers content (0–20 wt.%) were investigated. The results show that the thermal conductivity of the composites increase significantly, while the coefficient of thermal expansion (CTE) decrease gradually, with the hBN content increasing. Composite substrates with 20 wt.% hBN exhibited high thermal conductivity as 1.05 W m^?1 K^?1, which is 5.3 times that of pure PTFE. Such compositions may be a promising material in high thermal conductivity copper clad laminate.

     

Effect of titanium carbide coating on the microstructure and thermal conductivity of short graphite fiber/copper composites

Graphite fiber–Cu composites have drawn much attention in electronic packaging due to its excellent machinability and thermal properties. However, the weak interface bonding between graphite fiber and copper resulted in low thermo-mechanical properties of composites. In this work, a titanium carbide coating with thickness of 0.1 μm or 1 μm was synthesized on the surface of graphite fiber through molten salts method to strengthen interfacial bonding. The enhanced composites present 24–43 % increase in thermal conductivity and achieve the thermal conductivity of 330–365 W m^?1 K^?1 as well as the coefficient of thermal expansion of 6.5 × 10^?6–14 × 10^?6 K^?1. A Maxwell–Garnett effective medium approach on the anisotropic short fiber reinforcement with interfacial thermal resistance was established. The obtained enhancement was in good agreement with the estimates. The results suggest that the major factor that influences the thermal conductivities is not the interfacial thermal resistance but the low thermal conductivity of fiber in transversal direction when a well interfacial bonding is obtained.     

Effect of copper content on the thermal conductivity and thermal expansion of Al–Cu/diamond composites

Al–Cu matrix composites reinforced with diamond particles (Al–Cu/diamond composites) have been produced by a squeeze casting method. Cu content added to Al matrix was varied from 0 to 3.0 wt.% to detect the effect on thermal conductivity and thermal expansion behavior of the resultant Al–Cu/diamond composites. The measured thermal conductivity for the Al–Cu/diamond composites increased from 210 to 330 W/m/K with increasing Cu content from 0 to 3.0 wt.%. Accordingly, the coefficient of thermal expansion (CTE) was tailored from 13 × 10⁻⁶ to 6 × 10⁻⁶/K, which is compatible with the CTE of semiconductors in electronic packaging applications. The enhanced thermal conductivity and reduced coefficient of thermal expansion were ascribed to strong interface bonding in the Al–Cu/diamond composites. Cu addition has lowered the melting point and resulted in the formation of Al₂Cu phase in Al matrix. This is the underlying mechanism responsible for the strengthening of Al–Cu/diamond interface. The results show that Cu alloying is an effective approach to promoting interface bonding between Al and diamond.     

Effect of chromium carbide coating on thermal properties of short graphite fiber/Al composites

A chromium carbide coating was synthesized onto graphite fibers by molten salts method to improve the interfacial bonding and thermal properties of short graphite fiber/Al composites which were fabricated by vacuum pressure infiltration technique. The graphite fiber/Al composites with different thicknesses of chromium carbide coatings were prepared through varying plating times to investigate the influence of chromium carbide layer on the microstructures and thermal properties of the composites. The combined Maxwell–Garnett effective medium approach and acoustic mismatch model schemes were used to theoretically predict thermal conductivities of the composites. The results indicated that the chromium carbide coating formed on graphite fiber surface in molten salts consists mainly of the Cr₇C₃ phase. The Cr₇C₃-coating layer with plating time of 60 min and thickness of 0.5 μm was found to be most effective in improving the interfacial bonding and decreasing the interfacial thermal resistance between graphite fiber and aluminum matrix. The 40 vol% Cr₇C₃-coated graphite fiber/Al composite with Cr₇C₃ thickness of 0.5 μm exhibited 45.4 % enhancement in in-plane thermal conductivity of 221 W m^?1 K^?1 compared to that of uncoated composite, as well as the coefficient of thermal expansion of 9.4 × 10^?6 K^?1, which made it as very interesting material for thermal management applications.     

Thermal expansion coefficient and thermal fatigue of discontinuous carbon fiber-reinforced copper and aluminum matrix composites without interfacial chemical bond

Fully dense carbon fiber-reinforced copper and aluminum matrix (Cu–CF and Al–CF) composites were fabricated by hot press without the need for an interfacial chemical compound. With 30 vol% carbon fiber, the thermal expansion coefficients (TECs) of pure Cu and Al were decreased to 13.5 × 10^?6 and 15.5 × 10^?6/K, respectively. These improved TECs of Cu–CF and Al–CF composites were maintained after 16 thermal cycles; moreover, the TEC of the 30 vol% Cu–CF composite was stable after 2500 thermal cycles between ?40 and 150 °C. The thermal strain caused by the TEC mismatch between the matrix and the carbon fiber enables mechanical enhancement at the matrix/carbon fiber interface and allows conservation of the improved TECs of Cu–CF and Al–CF composites after thermal cycles.     

Effect of sintering temperatures on properties of BaO–B<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> glass/silica composites for CBGA package

BaO–B₂O₃–SiO₂–Al₂O₃ (BBSA) glass/silica composites synthesized by solid-state reaction method were developed for CBGA packages, and the effects of sintering temperature (900–950 °C) on the phase transformation, microstructure, thermal, mechanical and electrical properties were investigated. XRD results show that the major phases quartz and cristobalite, and the minor phase BaSi₂O₅ are detected in BBSA composites. Furthermore, it was found that the quartz phase transforms to cristobalite phase at 930–940 °C. The formation of cristobalite phase with higher coefficient of thermal expansion (CTE) led to the increase of CTE value of BBSA composites. However, excessive cristobalite phase content would degrade the mechanical properties and the linearity of thermal expansion of the ceramics. BBSA composites sintered at 920 °C exhibited excellent properties: low dielectric constant and loss (ε_r = 6.2, tanδ = 10^?4 at 1 MHz), high bending strength (179 MPa), high CTE (12.19 ppm/°C) as well as superior linearity of the thermal expansion.     

Investigation of thermal properties of Al–Si matrix reinforced fine SiCp composites

Abstract

The characterisation of thermal expansion coefficient and thermal conductivity of Al–Si matrix alloy and Al–Si alloy reinforced with fine SiC_p (5 and 20 wt-%) composites fabricated by stir casting process are investigated. The results show that with increasing temperature up to 350°C, thermal expansion of composites increases and slowly reduces when the temperature reaches to 500°C. The values of both thermal expansion and conductivity of composites are less than those for Al–Si matrix. Microstructure and particles/matrix interface properties play an important role in the thermal properties of composites. Thermal properties of composites are strongly dependent on the weight percentage of SiC_p.     

Effect of Ba(Zn1/3Ta2/3)O3 and SiO2 ceramic fillers on the microwave dielectric properties of butyl rubber composites

Butyl rubber–Ba(Zn_1/3Ta_2/3)O₃ (BR–BZT) composites and butyl rubber–silica (BRS) composites were prepared by sigma mixing. The dielectric properties at 1 MHz and 5 GHz of BR–BZT and BRS composites were investigated as a function of ceramic loading and were found to be improved with filler loading. For a optimum BZT loading of 0.26 v_f, the BR–BZT composite have ε_r = 4.88, tanδ = 0.0022 (at 5 GHz), coefficient of thermal expansion (CTE) = 112 ppm/°C, thermal conductivity (TC) = 0.30 Wm^?1 K^?1 and water absorption = 0.047 vol%. The BRS composites attained ε_r = 2.79, tanδ = 0.0039 (at 5 GHz), CTE = 102 ppm/°C, TC = 0.40 Wm^?1 K^?1 and water absorption = 0.078 vol% for the same loading of silica. The stress–strain curves of both composites showed good flexibility of the composite. The measured relative permittivity and TC of both composites were compared with different theoretical approaches.     

Microstructure and thermo-mechanical properties of SiCp/Al composites prepared by pressureless infiltration

In this paper, SiCp/Al composites with high reinforcement content are fabricated by pressureless infiltration with aluminum alloy into porous SiC preforms obtained by cold press forming. Microstructures and particulate distributions are analyzed with scanning electron microscope, X-ray diffraction and energy dispersive spectrometer. The reinforcement volume fraction reaches 65 % by using bimodal particle distributions. The bending strength ranges from 320 to 342 MPa, depending on particle sizes. Due to the intrinsically low thermal conductivity of the matrix, the thermal conductivity of SiCp/Al composites are in the range of 121–143 W m^?1 K^?1.     

Novel thermal interface materials: boron nitride nanofiber and indium composites for electronics heat dissipation applications

With increased power density and continued miniaturization, effective thermal dissipation is of significant importance for operational lifetime and reliability of electronic system. Advanced thermal interface materials (TIMs) with excellent thermal performance need to be designed and developed. Here we report novel TIMs consisted of boron nitride (BN) nanofibers and pure indium (In) solder for heat dissipation applications. The BN nanofibers are fabricated by electrospinning process and nitridation treatment. After surface metallization by sputtering, the porous BN film is infiltrated with liquid indium by squeeze casting to form the final solid composites. The new composites show the in-plane and through-plane thermal conductivity respectively of 60 and 20 W/m K. The direction dependence thermal properties of the TIM are due to the anisotropic thermal performance of BN nanofibers in the composite. A low thermal contact resistance of 0.2 K mm²/W is also achieved at the interface between this new composite and copper substrate. These competent thermal properties demonstrate the great potential of the BN–In TIMs in thermal management for electronic system.     

Microstructure and properties of Sip/Al–20 wt% Si composite prepared by hot-pressed sintering technology

In the present work, 50 vol% Sip/Al–20Si composite was prepared by hot-pressed sintering technology. Si particles were uniformly distributed in the Sip/Al–20Si composite, and only the presence of Si and Al phases were detected by XRD analysis. Dislocations, twins, and stacking faults were found in the Si particles. Several Si phases were found to be precipitated between Al matrix and Si particles. Si/Al interface was clean, smooth, and free from interfacial product. HRTEM indicated that the Si/Al interface was well bonded. The average CTE and thermal conductivity (TC) of Sip/Al–20Si composite were 11.7 × 10^?6/°C and 118 W/(m K), respectively. Sip/Al–20Si composite also demonstrated high mechanical properties (bending strength of 386 MPa). Thus, the comprehensive performance (low density and CTE, high TC, and mechanical properties) makes the Sip/Al–20Si composite very attractive for application in electron packaging.     

A novel silver–aluminium high-temperature die attach nanopaste system: the effects of organic additives content on post-sintered attributes

An Ag–Al die attach material having a fixed Ag–Al nanoparticles weight percent content (80–20 %), as well as varying organic additives weight percent content was formulated. The total nanoparticle weight percent content was varied between 84.7 and 87.0 %. As the organic additives content in the Ag₈₀–Al₂₀ die attach material decreased from 15.3 to 13.0 %, the nanopaste’s viscosity increased. The die attach material was sintered at 380 °C for 30 min to form Ag₂Al and Ag₃Al compounds. With decreasing organics content from 15.3 to 13.0 %, the porosity of the post-sintered samples also decreased from 30 to 19 %, while the density increased from 2.36 to 6.42 g/cm³. The highest melting point was recorded for the sample with the least organic weight percent content at 519 °C. The coefficient of thermal expansion and electrical conductivity values varied between 6.99–7.74 × 10^?6/ °C and 0.95–1.01 × 10⁵ (ohm-cm)^?1 respectively with decreasing organic content from 15.3 to 13.0 %. The electrical conductivity values recorded were higher than or equal to that of most solder alloy die attach materials. By changing the organic additives content in the Ag₈₀–Al₂₀ die attach material, suitable properties are obtained for high temperature die attach applications.     

Ba(Zn1/3Ta2/3)O3 ceramics reinforced high density polyethylene for microwave applications

The effect of Ba(Zn_1/3Ta_2/3)O₃ (BZT) ceramic filler on the dielectric, mechanical and thermal properties of high density polyethylene (HDPE) matrix have been investigated. The dispersion of BZT particles in the matrix was varied up to 0.45 volume fraction (V_f)_. The SEM images confirmed the increase in connectivity between the filler particles with the increase in filler loading. All the composites showed excellent densification (>99 %) with relatively low moisture absorption (<0.04 wt%). The dielectric properties of the composites were investigated at 1 MHz, 5 GHz and at 10 GHz. The relative permittivity and the dielectric loss were found to increase as a function of BZT loading. Different theoretical models were used to predict the relative permittivity at 10 GHz. Effective medium theory gave the best correlation with the experimental results. An enhancement in the thermal conductivity (TC) and a reduction in the coefficient of linear thermal expansion (CTE) were achieved with filler loading. A slight decrease in the tensile strength was also observed with BZT loading. At 10 GHz, 0.45 V_f BZT reinforced HDPE showed a low relative permittivity (ε_r = 8.2) and a low dielectric loss (tanδ = 1.6 × 10^?3) with good thermal (TC = 1.4 W m^?1 K^?1, CTE = 92 ppm/°C) and mechanical (tensile strength = 18 MPa) properties.     

Enhanced thermoelectric properties in polyaniline composites with polyaniline-coated carbon nanotubes

Polyaniline (PANI)-coated multi-walled carbon nanotubes (PANI-CNTs) were firstly synthesized by in situ polymerization and then incorporated into the PANI matrix by hot pressing to fabricate bulk PANI-CNT/PANI composites. The composites showed homogeneously dispersed CNTs into the PANI matrix with a strong interface interaction. Thermoelectric measurements at room temperature showed a significant enhancement in both the electrical conductivity and Seebeck coefficient with the addition of PANI-CNTs. At the same time, relatively low thermal conductivity was also obtained. The maximum electrical conductivity and Seebeck coefficient of the composites were up to 2.8 × 10³ S/m and 21.6 μ/K, respectively, and the maximum figure of merit reached 1.0 × 10^?3 more than three orders of magnitude higher than that of neat PANI. This study proposed a novel and effective way to fabricate bulk PANI/CNT composites with enhanced thermoelectric properties.     

Microstructure and thermal properties of binary ceramic particle-reinforced aluminum matrix composites with SiC/LAS

Multiphase particle-reinforced strategy shows promise for efficiently improving the comprehensive properties of aluminum matrix composites (AMCs) such as thermophysical and mechanical properties. In this work, AMC reinforced with β-eucryptite (LAS), and silicon carbide (SiC) particles were successfully prepared via a powder forging process. The microstructure morphology, interface compatibility, and coefficient of thermal expansion (CTE) of these composites were evaluated. Microstructural characterization illustrated that the co-effect of SiC and LAS resulted in a discontinuous phase with a microporous and deformation-free structure. The microporous structure of these composites was conducive for inward expansion and the elimination of internal stress, effectively limiting the outward thermal expansion behavior of the Al alloys. Moreover, SiC and LAS exhibited tight interfacial bonds with the Al grains, enhancing interfacial bonding strength. These composites provided practical and robust tensile stress that limited the thermal expansion of the Al matrix under heating. A fine Al grain size (53.5 nm) and low micro-strain (0.4?×?10^–4) were obtained with increasing LAS content. Consequently, the composites achieved a low CTE of 17.27?×?10^–6 K^?1 at 500 °C. The experimental CTE values were also compared with theoretical values calculated by a rule of mixture model to confirm that the excellent interfacial bonding between the LAS and SiC reinforcements and the Al matrix imposed an effective constraint on matrix expansion.

Graphical abstract

     

Spark plasma sintering of Co-WC cubic boron nitride composites

25 vol.% cubic boron nitride (cBN) added tungsten carbide (WC) powders containing 6 wt.% Co (WC-6Co) were densified by spark plasma sintering (SPS) technique under different experimental conditions and the effect of cBN addition on the microstructure, mechanical properties and thermal conductivity were investigated. Over 99.5% theoretical density was achieved for WC-6Co-cBN composites sintered at 1300 °C, under 75 MPa pressure for 7.5 min. Under these conditions, cBN → hBN phase transformation was not observed.     

氮化硼复合材料在包装领域应用的研究进展

目的综述国内外氮化硼复合材料在包装领域的应用与进展,对未来氮化硼材料在包装领域的应用进行展望。方法整理归纳国内外文献,简单介绍氮化硼纳米片(BNNSs)的性质和制备方法,以及氮化硼复合材料的制备方法,重点整理分析氮化硼复合材料在包装领域的应用与进展。结果氮化硼具有独特的二维纳米片层结构和相互重叠的层层结构。添加BNNSs不仅可以明显提高复合材料的导热率、机械强度、绝缘性等,还可以改善复合材料的阻隔性能、力学性能、化学稳定性能、抗菌性能等。结论氮化硼复合材料具有热导率高、绝缘性好等优点,可应用于电子封装领域,并在阻燃、抗菌、防腐等包装材料领域具有不错的发展前景。     

Ice‐Templated Assembly Strategy to Construct 3D Boron Nitride Nanosheet Networks in Polymer Composites for Thermal Conductivity Improvement

Owing to the growing heat removal issue of modern electronic devices, polymer composites with high thermal conductivity have drawn much attention in the past few years. However, a traditional method to enhance the thermal conductivity of the polymers by addition of inorganic fillers usually creates composite with not only limited thermal conductivity but also other detrimental effects due to large amount of fillers required. Here, novel polymer composites are reported by first constructing 3D boron nitride nanosheets (3D‐BNNS) network using ice‐templated approach and then infiltrating them with epoxy matrix. The obtained polymer composites exhibit a high thermal conductivity (2.85 W m⁻¹ K⁻¹), a low thermal expansion coefficient (24–32 ppm K⁻¹), and an increased glass transition temperature (T_g) at relatively low BNNSs loading (9.29 vol%). These results demonstrate that this approach opens a new avenue for design and preparation of polymer composites with high thermal conductivity. The polymer composites are potentially useful in advanced electronic packaging techniques, namely, thermal interface materials, underfill materials, molding compounds, and organic substrates.     

Sandwiched epoxy–alumina composites with synergistically enhanced thermal conductivity and breakdown strength

Development of polymer-based composites with simultaneously high thermal conductivity and breakdown strength has attracted considerable attentions owing to their important applications in both electronic and electric industries. In this study, we successfully design novel epoxy-based composites with nano-Al₂O₃/epoxy composite layer sandwiched between micro-Al₂O₃/epoxy composite layers, which show synergistically and significantly enhanced thermal conductivity and breakdown strength. Compared with the traditional composites, the bottleneck that both thermal conductivity and breakdown strength cannot be simultaneously enhanced can be overcome successfully. An optimized sandwiched alumina–epoxy composite with 70 wt% micro-Al₂O₃ fillers in the outer layers and 3 wt% nano-Al₂O₃ in the middle layer simultaneously displays a high thermal conductivity of 0.447 W m^?1 K^?1 (2.4 times of that of epoxy) and a high breakdown strength of 68.50 kV mm^?1, which is 6.3 % higher than that of neat epoxy (64.45 kV mm^?1). The experimental results on the thermal conductivity of multi-layered alumina–epoxy composites were in well accordance with the theoretical values predicted from the series conduction model. This novel technique simultaneously improves thermal conductivity and breakdown strength, which is of critical importance for design of perspective composites for electronic and electric equipments.     

Material Based Structure Design: Numerical Analysis Thermodynamic Response of Thermal Pyrolytic Graphite /Al Sandwich Composites

Amine-grafted multiwalled carbon nanotubes (MWCNTs) based thermally conductive adhesive (TCA) was studied in the previous paper and applied here in thermal pyrolytic graphite (TPG)/Al radiator due to its high thermal conductivity, toughness and cohesiveness. In this paper, in an attempt to confirm the application of TCA to TPG/Al sandwich radiator, the thermodynamic response in TPG/Al sandwich composites associated with key material properties and structural design was investigated using finite element simulation with commercial available ANSYS software. The induced thermal stress in TCA layer is substantial due to the thermal expansion mismatch between Al plate and TPG. The maximum thermal stress is located near the edge of TCA layer with the von Mises stress value of 4.02 MPa and the shear stress value of 1.66 MPa. The reasonable adjustment of physical-mechanical properties including thermal conductivity, thermal expansion, Young^,s modulus and the thickness of TCA layer, Al plate and TPG are beneficial for reducing the temperature of the top surface of the upper skin and their effects on the reduction of thermal structural response in some ways. These findings will highlight the structural optimization of TPG/Al radiator for future application.