銅粉與銅-氧化鋁複合粉應用於散熱元件之研究

Abstract

目前粉末燒結式熱導管應用於散熱零件上遭遇到兩個瓶頸，第一是如何利用銅粉之粉末特性提升熱導管的散熱性能，可惜目前這方面的研究較少，所以業界在提升散熱性能方面仍有改進之空間；第二是如何改善熱導管在裝配時容易發生變形，此問題的原因是所用之無氧銅管雖經過冷加工，強度與硬度均佳，但經過高溫燒結後因晶粒成長造成無氧銅管的強度與硬度大幅下降。 在提升散熱性能方面，本研究發現選用之銅粉的氧含量越低、平均粒徑越大、.粒度分佈越窄以及粉末形狀越趨近球形時會有越佳的散熱性能，並且證實以毛細速度可以取代滲透率及毛細壓力作為評估銅粉是否適用於粉末燒結式熱導管之最佳檢測方法。 在改善強度與硬度方面，希望藉由散佈強化銅的高導電性、高熱傳導性、高硬度以及高強度等特性來解決，所以本研究先利用粉末冶金法製作出銅粉中能含有少量的奈米級Al2O3顆粒，可藉由散佈強化來增加硬度與強度，有望將此銅-氧化鋁複合粉經由加工而製作成銅管或銅板來取代目前散熱零件所用之無氧銅管及無氧銅板。此製程在高溫燒結時少量的奈米級Al2O3顆粒可以阻礙晶界成長有助於晶粒細化，能避免硬度與強度的大幅下降。但由於奈米級氧化鋁容易團聚不易均勻分佈在銅的基地中，因此在銅粉中直接加入奈米氧化鋁粉有其困難度。為了解決此問題，本研究利用金屬置換法、添加含有金屬元素之潤滑劑以及高能量球磨法等三種方式來製作銅-氧化鋁複合粉。 在金屬置換法方面，將鋁片放入氯化銅溶液中以置換出銅鋁複合粉，並以水洗滌法將所含之氯含量降低至100 ppm以下，再經由空氣乾燥以及高溫氧化等過程將鋁氧化成氧化鋁，接著以氫氣氣氛還原即可得到銅-氧化鋁複合粉，若要使此粉之粉末粒度分佈與形狀適合應用於粉末冶金或射出成形，則可利用球磨法在粉末氧化後將其粒度分佈變窄，並可調整粉末形狀，最後再將粉末還原即可。實驗結果發現鋁含量為0.108 wt% (0.456 vol%Al2O3)的粉末燒結體，其相對密度高達99.0 %，硬度為45.4 HB，遠高於純銅(A635)之相對密度(91.8 %)及硬度(32.4 HB)。而鋁含量為0.263 wt% (1.111 vol%Al2O3) 的粉末燒結體，其相對密度為95.4 %，硬度為60.2 HB。而在導電度方面會隨著氧化鋁含量增加而遞減，但上述兩種鋁含量的粉末燒結體之導電度皆為84 %IACS仍稍高於純銅粉末燒結體之導電度(82.5 %IACS)。因此，綜合實驗結果得知最佳的鋁含量為0.108 wt% (0.456 vol%Al2O3)，Cu-Al2O3粉不僅有散佈強化的效果，也可以提高密度。 在添加含有金屬元素之潤滑劑方面，利用濕式混合法將潤滑劑與銅粉混合均勻，再藉由高溫將潤滑劑燒除，可留下奈米級的金屬氧化物顆粒在銅之基地中，而達到散佈強化之效果。實驗結果顯示添加硬脂酸鋰之效果最佳，硬脂酸鋁次之，而硬脂酸鋰的最佳添加量為1.0 wt%，其燒結後之相對密度可由94.6 %提升至97.5 %，但在射出成形方面，只有添加0.5 wt%之硬脂酸鋰才有助於提高燒結密度。 在高能量球磨法方面，使用銅-鋁母合金粉經由高能球磨法將所含之鋁氧化成氧化鋁，再經過氫氣還原後而製得銅-氧化鋁的粉末。結果顯示當鋁含量為0.117 wt% (0.498 vol%Al2O3)的粉末燒結體，相對密度與導電度為最佳，分別為96.5 %與83.1 IACS%。而當鋁含量為0.587 wt% (2.453 vol%Al2O3)的粉末燒結體時，硬度則為最高可達到33.7 HRB。但綜合比較上述三項性質，則以鋁含量為0.352 wt% (1.483 vol%Al2O3)的粉末燒結體為最佳，其相對密度為96.4 %，硬度為27.3 HRB，導電度為82.3 IACS%。

Heat pipes have been widely applied in thermal management devices for notebook computers and light emitting diodes. But, there is still room for improvement. The first subject studied in this work is to improve the performance of heat dissipation by adjusting the characteristic of Cu powders. The other subject is to resolve the defects encountered during mechanical deformation during heat pipe assembly. These defects are mainly caused by the coarse grains, low hardness, and low strength of the sintered copper tubing. To improve the performance of heat dissipation, a copper with a low oxygen content, larger mean particle size, narrow particle size distribution, and spherical powder shape is preferred for preparing the porous Cu wicks. It is also noticed during evaluation of the heat dissipation performance that the permeability and the capillary pressure, which have been widely used in the past, can be replaced by the capillary speed, which is simpler and more accurate. For increasing the strength and the hardness, the oxide dispersion strengthened Cu (ODS Cu) has been used in rods, plates, or bars, which have simple cross-sections. In this study, the development of the ODS Cu is aimed for press-and-sintered and powder injection molded parts, which are net-shaped. To attain good distribution of fine alumina particles in the Cu matrix, three methods are used. The first one is to use the cementation process to form alumina in the powder. The second approach is to add metal-containing lubricant, which forms alumina during sintering. The third method is to form alumina using the high energy milling process and Cu-Al pre-alloyed powders. The results of the cementation method show that the relative density up to 99 % and the hardness of 45.4 HB can be attained with the addition of 0.108 wt% aluminum (0.456 vol% Al2O3). These properties are better than the 91.8 % and 32.4 HB of the alumina-free copper. For 0.263 wt% aluminum (1.111 vol%Al2O3), the density is 95.4 % and the hardness is 60.2 HB. The electrical conductivity of Cu-Al2O3 sintered compacts decreased with increasing amount of alumina. The electrical conductivities of these two Cu-Al2O3 sintered compacts are about 84 %IACS, slightly higher than the 82.5 %IACS of the alumina-free copper. The results of adding metal-containing lubricant reveal that adding 1.0 wt% lithium stearate is effective and increases the relative density of sintered compacts from 94.6% to 97.5%. In high energy milling process, the results exhibit that compact containing 0.117 wt% aluminum (0.498 vol%Al2O3) has the highest relative density of 96.5% and electrical conductivity of 83.1 %IACS. The highest hardness of compact is 33.7 HRB when the aluminum content in the compact is 0.587 wt% (2.453 vol%Al2O3). Considering the mechanical and electrical properties, the suggested optimum aluminum content is 0.352 wt% (1.483 vol%Al2O3), which gives a sintered density of 96.4 %, a hardness of 27.3 HRB, and an electrical conductivity of 82.3 %IACS.