GdAl2塊材在磁熱效應的應用上個很好的材料。近幾年來，一些研究認為GdAl2微粒可能存在很好的磁熱效應。然而相反地，與GdAl2結構相同的CeAl2奈米微粒的磁性被觀察到比塊材來的低[1]，則此將減弱材料的熱效應。在本論文中，我們成功利用放電加工法(EDM)製作出GdAl2的奈米微粒，並由X光繞射確認這些GdAl2微粒為單一相。利用TEM觀察發現，GdAl2微粒樣品的粒徑約為20~40 nm，並觀察到有些許微米顆粒摻雜其中。本論文中兩個微粒樣品的鐵磁-順磁轉變溫度均接近170 K，當溫度低於170 K，樣品呈現超順磁性(superparamagnetism)行為。利用「居禮-外斯」定律對塊材及微粒樣品在溫度高於170 K的磁化率數據擬合，發現微粒的居禮常數數值比塊材小，暗示這些樣品中磁性Gd3+濃度明顯減少，這行為與CeAl2微粒樣品相似。利用不同溫度下的磁滯曲線可計算磁熵變化-ΔS，發現塊材在TC附近有一極大值。但對於奈米微粒而言，TC附近的磁熵變化被抑制，且在溫度低於TC，隨著溫度降低，磁熵變化量逐漸上升，這行為與GdAl2/Al2O3奈米膠囊行為相似[2]。但本論文中的樣品因其磁性Gd3+的濃度較低，-ΔS約降為塊材的三分之一，其現象與文獻[2]不同，顯示奈米化是否能提升GdAl2的磁熱效應，仍需進一步的研究。 Bulk GdAl2 is good candidate for application of magnetocaloric effect. Recently, some studies predicated a larger cryogenic magnetocaloric effect can be found in nano-structure GdAl2. On the contrary, the magnetization was reduced in CeAl2 nanoparticles. In this report, the good quantity GdAl2 nanoparticles were fabricated by electron discharge machine (EDM) to verify the magnetic behaviors of GdAl2 nanoparticles. The X-ray diffraction pattern shows that the GdAl2 nanoparticles are single-phase. The nanoparticles with size about 20~40nm and a few μm-particles can be observed in our specimen by HRTEM. Two nano-scale specimens for this research exhibit ferromagnetic behavior with ferromagnetic temperature near 170 K. Below 170 K, the superparamagnetic behavior coming from nanoparticles can be recognized by magnetic susceptibility for these specimens. The smaller Curie constant of nanoparticles as compared to that of bulk can be obtained the Curie-Weiss fit about 170 K. This imply that the concentration of magnetic Gd3+ decreases when the size decrease. The behavior is similarly to the GdAl2 nanoparticles. The magnetic entropy change -ΔS were calculated from variable magnetic field M-H curve for variable temperature. The maximum value of -ΔS is near TC for bulk GdAl2. But for GdAl2 nanoparticles, the magnetic entropy was suppressed neat TC and increased with temperature decreasing below TC. The behavior is similarity to the GdAl2/Al2O3 nanocapsules.
