Spherical Aluminum Nitride Fillers for Heat-Conducting Plastic Packages

It is necessary for encapsulants to have not only a suitable coefficient of thermal expansion (CTE) compatible to IC devices and a low dielectric constant to reduce the device propagation delay, but also a high thermal conductivity to dissipate large amounts of heat from power-hungry, high-speed IC and high-density packages. Fillers such as silica have been mixed with polymers to improve their properties. Aluminum nitride (AlN) is considered as an alternative one, because it has a higher theoretical thermal conductivity of ∼320 W/mK¹, a compatible CTE with silicon chips and a low dielectric constant. Commercial AlN fillers are angular in shape, because they are prepared via grinding coarse AlN powders synthesized by direct nitridation of aluminum metal and classification. The angular AlN are not expected to have high fluidity when mixed with polymers and hence low packing density. Recently, we successfully obtained single-crystalline spherical AlN fillers. Furthermore, polymer composites filled with the spherical AlN showed excellent thermal conductivity (>8 W/mK) as encapsulants for dissipating the heat generated in electronic devices.     

Effects of doping Al-metal powder on thermal,mechanical and dielectric properties of AlN ceramics

In this work, the influence of Al-metal powder addition upon that thermal, mechanical and dielectric properties of aluminium nitride (AlN) ceramic was studied. The findings show that adding Al-metal powder improves not only the mechanical and thermal properties of the AlN ceramic but also has no negative impact on its dielectric properties. Based on Y₂O₃ as sintering aid, the AlN ceramic with 1.0 wt% Al doping were 14.35% higher thermal conductivity, 11.73% higher flexural strength and 59.50% higher fracture toughness than those doped without Al, respectively. This study showed that the addition of Al-metal powder may favor the purifying of the AlN lattice and the formation of homogenous and isolated second phase, which would increase the AlN–AlN interfaces and improve the thermal conductivity. Furthermore, the grain boundaries of AlN ceramics might be strengthened by the isolated second phases due to the thermal mismatch between the second phases and AlN grains, thus strengthening and toughening the AlN ceramic doped with Al. However, the large additive amount of Al powder (>1.0 wt%) was not help the isolation and homogenization of the second phase, giving a deterioration in an AlN ceramic's mechanical and thermal properties. These results suggest that the introduction of an appropriate dose of aluminium metal powder is a simple method that can be used to improve the AlN ceramic's mechanical and thermal properties simultaneously.     

Effect of nano aluminum nitride filler on mechanical,thermal, and dielectric properties of the glass/mullite composites for low-temperature co-fired ceramic applications

Mullite/glass/nano aluminum nitride (AlN) filler (1–10 wt% AlN) composites were successfully fabricated for the low-temperature co-fired ceramics applications that require densification temperatures lower than 950°C, high thermal conductivity to dissipate heat and thermal expansion coefficient matched to Si for reliability, and low dielectric constant for high signal transmission speed. Densification temperatures were ≤825°C for all composites due to the viscous sintering of the glass matrix. X-ray diffraction proved that AlN neither chemically reacted with other phases nor decomposed with temperature. The number of closed pores increased with the AlN content, which limited the property improvement expected. A dense mullite/glass/AlN (10 wt%) composite had a thermal expansion coefficient of 4.44 ppm/°C between 25 and 300°C, thermal conductivity of 1.76 W/m.K at 25°C, dielectric constant (loss) of 6.42 (0.0017) at 5 MHz, flexural strength of 88 MPa and elastic modulus of 82 GPa, that are comparable to the commercial low temperature co-fired ceramics products.     

Mechanical,thermal and electrical properties of multi‐walled carbon nanotube/aluminium nitride/polyetherimide nanocomposites

A series of polyimide‐based nanocomposites containing polyimide‐grafted multi‐walled carbon nanotubes (PI‐g MWCNTs) and silane‐modified ceramic (aluminium nitride (AlN)) were prepared. The mechanical, thermal and electrical properties of hybrid PI‐g MWCNT/AlN/polyetherimide nanocomposites were investigated. After polyimide grafting modification, the PI‐g MWCNTs showed good dispersion and wettability in the polyetherimide matrix and imparted excellent mechanical, electrical and thermal properties. The utilization of the hybrid filler was found to be effective in increasing the thermal conductivity of the composites due to the enhanced connectivity due to the high‐aspect‐ratio MWCNT filler. The use of spherical AlN filler and PI‐g MWCNT filler resulted in composite materials with enhanced thermal conductivity and low coefficient of thermal expansion. Results indicated that the hybrid PI‐g MWCNT and AlN fillers incorporated into the polyetherimide matrix enhanced significantly the thermal stability, thermal conductivity and mechanical properties of the matrix. Copyright © 2012 Society of Chemical Industry     

Electric current assisted sintering of AlN ceramics: thermal conductivity and transparency

Abstract

Densification, thermal conductivity and transparency of aluminium nitride ceramics densified by electric current assisted sintering are reviewed. Aluminium nitride powders could be densified without any sintering additives by electric current assisted sintering. Sintering additives, Y₂O₃ or CaF₂, and others are effective for densification, increase in thermal conductivity and transparency. As a result of the reaction with the sintering additives and oxide on the surface of the AlN powder, a secondary phase forms between AlN grains, oxygen impurity between aluminium nitride grains decreases and thermal conductivity increases. These mechanisms are effective in the sintering of aluminium nitride ceramics densified by electric current assisted sintering, though sintering time is short, within 5 min. The secondary phase formation does not always give good effect for transparency.     

Novel in-situ constructing approach for vertically aligned AlN skeleton and its thermal conductivity enhancement effect on epoxy

As one of the key components of electronic devices, thermal management materials (TMMs) with high thermal conductivity are essential to ensure their safety and long service life. For polymer-based TMMs, AlN is one of the preferred fillers, but it has some drawbacks such as high cost and easy hydrolysis. Herein, a controllable and continuously oriented three-dimensional AlN skeleton (3D-AlNNS) was in-situ transformed from a low-cost 3D Al-containing skeleton (3D-AlNS) by combining the ice-templating and nitriding reaction sintering. Subsequently, AlN/epoxy composites were obtained by a vacuum infiltration. The composite containing 39.69 vol% AlN had the highest thermal conductivity of 4.29 W m^?1·K^?1, which was 21.45 times higher than that of pure epoxy. The composite substrates showed excellent heat dissipation performance in practical applications due to their high thermal conductivity. The continuous directional alignment of AlN powders in the 3D skeleton and intersection of AlN whiskers between the skeleton walls produced in-situ contributed to the formation of effective multichannel heat transferring paths and improvement in thermal conductivity. This novel approach has the advantages of low-cost, short processing time, simple operation and repeatability, and provides a new idea for developing heat-conducting polymer composites, which can also be extended to the preparation of similar TMMs.     

Preparation and thermal effects of polyarylene ether nitrile aluminium nitride composites

Particulate‐filled polyarylene ether nitrile (PEN) composites were prepared using methyltriethoxy‐silane‐treated aluminium nitride (AlN) as the filler for thermal modification. The effects of AlN fraction, particle size and surface treatment on the thermal performance of PEN were investigated. The thermal conductivities of the composites increased when the AlN filler concentration was increased, as well as with decrement of the filler size. The thermal conductivity value of the composites increased up to 0.779 W m^?1 K^?1 when the AlN weight loading was 60 wt%. The trend of the thermal conductivities of the composites can be more efficiently predicted by theoretical models than empirical models. The composites exhibited stable performances of thermal decomposition and thermal expansion when AlN filler faction in the composites increased. © 2013 Society of Chemical Industry     

New composites with high thermal conductivity and low dielectric constant for microelectronic packaging

Three composites based on cyanate (CE) resin, aluminum nitride (AlN), surface‐treated aluminum nitride [AlN(KH560)], and silicon dioxide (SiO₂) for microelectronic packaging, coded as AlN/CE, AlN(KH560)‐SiO₂(KH560)/CE, and AlN‐SiO₂/CE composite, respectively, were developed for the first time. The thermal conductivity and dielectric constant of all composites were investigated in detail. Results show that properties of fillers in composites have great influence on the thermal conductivity and dielectric constant of composites. Surface treatment of fillers is beneficial to increase the thermal conductivity or reduce dielectric constant of the composites. Comparing with binary composite, when the filler content is high, ternary composites possess lower thermal conductivity and dielectric constant. The reasons leading to these outcomes are discussed intensively. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers     

Thermal,dielectric, and rheological properties of aluminum nitride/liquid crystalline copoly(ester amide) composite for the application of thermal interface materials

In this article, thermally conductive and relatively low dielectric constant polymer matrix composites of an aluminum nitride filler (AlN) and a novel liquid crystalline copoly(ester amide) (LCP) were prepared via a solution blending method in the presence of a phosphate containing dispersant. The viscosities, thermal conductivities, and dielectric properties of the prepared AlN/LCP composites were investigated as a function of AlN loading. Our experimental results demonstrated that the AIN/LCP composite with AlN concentration of 50 wt% had 2.5 times higher thermal conductivity than pure LCP (2.020 and 0.817 W/mK for composite with 50 wt% of AlN and pure LCP, respectively), but its dielectric constant remained at low level, i.e., < 9.0 at frequency of 900 Hz. In addition, viscosities of AlN/LCP pastes in the N‐methyl‐2‐pyrrolidinone solvent remained at acceptable levels with the high AlN loading of 50 wt%. The morphologies of the prepared composites were also investigated by scanning electron microscopy. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers     

Effects of starting powder on microstructure and thermal conductivity of pressureless-sintered fully ceramic microencapsulated fuels

The effects of the starting SiC powder (α or β) with the addition of 5.67 wt% AlN–Y₂O₃–CeO₂–MgO additives on the residual porosity and thermal conductivity of fully ceramic microencapsulated (FCM) fuels were investigated. FCM fuels containing ～41 vol% and ～37 vol% tristructural isotropic (TRISO) particles could be sintered at 1870 °C using α-SiC and β-SiC powders, respectively, via a pressureless sintering route. The residual porosities of the SiC matrices in the FCM fuels prepared using the α-SiC and β-SiC powders were 1.1% and 2.3%, respectively. The thermal conductivities of FCM pellets with ～41 vol% and ～37 vol% TRISO particles (prepared using the α-SiC and β-SiC powders, respectively) were 59 and 41 Wm^?1K^?1, respectively. The lower porosity and higher thermal conductivity of FCM fuels prepared using the α-SiC powder were attributed to the higher sinterability of the α-SiC powder than that of the β-SiC powder.     

Enhanced thermal conductivity of low-temperature sintered borosilicate glass–AlN composites with β-Si3N4 whiskers

The effects of β-Si₃N₄ whiskers on the thermal conductivity of low-temperature sintered borosilicate glass–AlN composites were systematically investigated. The thermal conductivity of borosilicate glass–AlN ceramic composite was increased from 11.9 to 18.8 W/m K by incorporating 14 vol% β-Si₃N₄ whiskers, and high flexural strength up to 226 MPa were achieved along with low relative dielectric constant of 6.5 and dielectric loss of 0.16% at 1 MHz. Microstructure characterization and percolation model analysis indicated that thermal percolation network formation in the ceramic composites led to the high thermal conductivity. The crystallization of the borosilicate microcrystal glass also contributed to the enhancement of thermal conductivity. Such ceramic composites with low sintering temperature and high thermal conductivity might be a promising material for electronic packaging applications.     

纳米氮化铝填充聚四氟乙烯复合材料的性能研究

采用高速混合、冷压烧结成型的方法制备了聚四氟乙烯/纳米氮化铝（PTFE/nano AlN）复合材料,考察了nano AlN含量对复合材料结晶性能、力学性能与摩擦性能的影响,并采用扫描电子显微镜对样品磨损表面进行分析。结果表明：随着nano AlN含量的增加,复合材料的结晶度呈现先增大后降低的趋势;nano AlN可显著提高复合材料的力学性能与耐磨损性能,当其含量为4 %时,耐磨损性能与纯PTFE相比提高了2个数量级。     

燃烧合成AlN粉体的放电等离子烧结及其导热性能

利用放电等离子烧结(spark plasma sintering,SPS)工艺研究了燃烧合成法制备的2种具有不同形貌的AlN粉以及1种碳热还原氮化法制备的市售亚微米级AlN粉的烧结性能、致密化机理以及导热性能。结果表明:燃烧合成法制备的AlN纳米晶须状粉末具有与亚微米级标准市售AlN粉末同样优异的烧结性能,都能够在无烧结助剂情况下在1600℃的较低温度下烧结致密。在烧结过程中,由于燃烧合成AlN粉自身的高化学活性和SPS产生的等离子体活化作用,使得AlN粉以自身的分解-再结晶-凝聚机制进行致密化,导致晶界强度很高,断裂时以穿晶断裂为主;而在市售AlN粉末烧结过程中以表面扩散机制致密化,在晶界处形成了AlON相,降低了晶界强度,因此以沿晶断裂为主。AlN原料的氧含量对热导率的影响很大。由于燃烧合成AlN粉体的氧含量较碳热还原法制备的市售AlN粉体略高,导致其烧结试样热导率略低。     

Effect of nano fillers on the properties of polytetrafluoroethylene composites: Experimental and theoretical simulations

Polytetrafluoroethylene (PTFE) has excellent corrosion resistance and a low coefficient of friction; however, its high wear rate and low hardness severely limit its use. In the work, nano particles were used as fillers for PTFE. The composites were prepared by the homogeneous mixing of PTFE and other fillers and sintered at high temperatures. The work aimed to investigate the effect of various nano fillers (nanocarbon powders, graphene, fullerene, nano graphite powders, and nano copper powders) on the mechanical, thermal, and frictional properties of composites. The results of the experiments showed that the addition of graphene could improve the stress and strain values of the composites, and all the nano fillers could improve the thermal conductivity of the PTFE composites. The friction experiments showed that fullerenes could significantly improve the wear resistance of PTFE composites. In the theoretical simulation, the thermal conductivity of PTFE composites was predicted using ANSYS software, with the changes in the temperature and friction force in the friction process. The theoretical simulation results matched with the experimental values, which proved the accuracy of the theoretical simulations.     

High-Thermal-Conductivity AlN Filler for Polymer/Ceramics Composites

To increase the thermal conductivity of polymer/ceramic composites, aluminum nitride (AlN) granules were added as a ceramic filler. Granules, sintered at 1850°C for 24 h, showed a very high conductivity of 266±26 W (m·°C)⁻¹, as measured by a thermal microscope equipped with thermoreflectant and periodic heating techniques. This conductivity exceeds 80% of the theoretical value of AlN. Ceramic fillers consisting of the obtained AlN granules and commercially available hexagonal boron nitride particles (h-BN) powder plus polyimide resins were mixed and then molded at 100 MPa and 420°C in a vacuum. The resultant composite showed a high conductivity of 9.3 W (m·°C)⁻¹. This study demonstrates that a high-thermal-conductivity filler effectively enhances the conductivity of polymer/ceramic composites.     

The dielectric properties of epoxy/AlN composites

In this paper, AlN ceramic powder is chosen to be mixed with epoxy to form epoxy/AlN composites, the effects of the content of AlN filler on the physical and dielectric properties of epoxy/AlN composites are developed. From the SEM observation, the particles of self-synthesized AlN powder, obtained by using combustion method, is less uniformity and the average particle size is about 3.12 μm. Only the AlN phase can be detected in the XRD patterns of the epoxy/AlN composites. The more AlN powder is mixed with epoxy, the higher crystal intensity of AlN phase will exist in the XRD patterns. As the content of AlN powder in the epoxy/AlN composites increases from 5 to 40 wt.%, the dielectric constant increases from 6.52 to 7.28 (measured at 1 MHz). The loss tangent of epoxy/AlN composites is slightly increased as the measured frequency increases. Moreover, the epoxy/AlN composites in this investigation show less pores as compared to other literatures. The results indicate that the fabrication process has an apparent effect on the decrease of porosities, and the composites with a low porosity will lead to a low loss tangent.     

AlN/碳掺杂氮化硼纳米管复相陶瓷的制备及性能

将1%～10%(体积分数)碳掺杂氮化硼纳米管(carbon-doped boron nitride nanotubes,BCN-nt)引入到纳米AlN中,采用放电等离子烧结得到致密的AlN/BCN-nt复相陶瓷。结果表明:适当提高烧结温度能使AlN晶粒充分长大,提高AlN晶粒完整性并有效去除结构中的氧杂质,因而显著改善了引入BCN-nt对热导率的劣化。在Kα波段(26.5～40.0GHz),随BCN-nt含量的增加,材料的介电常数实部和虚部都呈现逐渐增大的趋势,损耗因子也逐渐增加。提高烧结温度对介电常数影响不大,而过高的温度使介电常数虚部明显下降。适当的BCN-nt含量和烧结温度能够在提供稳定的介电损耗同时兼顾较高的热导率。     

Preparation and properties of transparent zinc oxide/silicone nanocomposites for the packaging of high‐power light‐emitting diodes

New transparent zinc oxide (ZnO)/silicone nanocomposites with outstanding integrated properties, including a high UV‐shielding efficiency and transparency, bigger thermal conductivity, and lower dielectric constant, were successfully developed; they were prepared by the uniform dispersion of organic modified nano‐ZnO in a silicone matrix through in situ polymerization. The ZnO precursor was prepared by a direct precipitation method, which was then calcinated at different temperatures to produce nano‐ZnO with various morphologies and sizes. The effects of the size, surface nature, and content of nano‐ZnO on the key properties (e.g., optical and dielectric properties, thermal conductivities) of the composites were systematically investigated. The results show that the organic nano‐ZnO prepared by 3‐methacryloxypropyltrimethoxysilane can increase the dispersion of nano‐ZnO in silicone resin, and the interfacial adhesion between inorganic and organic phases, and consequently improve the integrated properties of nanocomposites. The increase of the particle content and size of ZnO in composites can lead to high thermal conductivity and UV‐shielding efficiency but lower visible‐light transparency, so there is an optimum content and size of ZnO in composites to obtain the best integrated properties of the composites. Specifically, the nanocomposite containing 0.03 wt % organic nano‐ZnO with an average size of 46 ± 0.4 nm not only had a high visible‐light transparency, UV‐shielding efficiency, and thermal conductivity but also possessed a low dielectric constant and loss and met the requirements of high‐performance electronic packaging for high‐power light‐emitting diodes. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

High thermal conductivity ceramics from combustion synthesized AlN powder through microwave sintering and reheating

Combustion synthesized AlN powders were investigated for use as starting material for obtaining high thermal conductivity specimens by microwave sintering and reheating. Microwave sintering and reheating were carried out in a TE₁₀₃ single mode cavity with an adjustable microwave power in the range 0–3 kW at 2.45 GHz. Densification was found to be a primary requirement to obtain a high thermal conductivity AlN. AlN powders with good sinterability are required to achieve a high densification. AlN powder with D ₅₀ of ∼6 μm was found to have a poor sinterability that with D ₅₀ of ∼3 μm or finer, a good sinterability. Oxygen content was found to be another important factor determining the thermal conductivity. The thermal conductivity can be enhanced from ∼130 to ∼155 W/m K when the oxygen content is reduced from 2.3 to 1.4 wt %. The thermal conductivity can be significantly improved by microwave reheating of the sintered specimen under a reducing atmosphere. This is considered to be due to enhanced removal of the second phases by the reducing atmosphere.     

Synthesis of silica aerogel thin sheets and evaluation of its thermal,electrical, and mechanical properties

Silica aerogels are excellent thermal and acoustic insulators because of interconnected open nanopores with more than 90% porosity and higher surface area. Silica aerogel is derived by sol-gel process and dried under super-critical, sub-critical or ambient pressure conditions. Thin silica aerogel sheets could be effective thermal insulators but high fragility hinders the wider applications. We have successfully developed a synthesis method for thin, flexible, and non-fragile aerogel sheets with excellent hydrophobicity, lower thermal conductivity, and non-combustible properties via ambient drying method. The silica aerogel sheets prepared compose of silica aerogel powder, fiber glass chopped strands, and solvent-based binder. Aerogel thin insulation sheets of thickness 164 μm were prepared by pressing through rollers using aerogel paste composed of aerogel powder, fiber glass strands, and binders. The thermal conductivity values obtained were between 0.02~0.63 W/mK at temperature 25~400°C, contact angle θ = 121‘_, weight loss 3.91% when heated up to 800°C in air, dielectric voltage breakdown 3.67 kV, dielectric strength 6.37 kV/mm and tensile strength of 2.65 N/mm². The overall thermal, electrical, and mechanical evaluation of aerogel thin insulation sheet showed they have higher potential to replace existing thick and bulky aerogel composites as thermal and electrical insulators in aviation, automobiles, electronics, and high power batteries.