柠檬酸对钡铁氧体晶相变化的影响

Influence of citric acid on the crystal phase transformation of barium hexaferrite

采用柠檬酸溶胶．凝胶法制备超细晶粒钡铁氧体，选择柠檬酸与金属硝酸盐摩尔比（CA／MN）分别为1，1．2，1．4和2制得一系列钡铁氧体磁性粉末，通过热重分析（TGA）、X射线衍射（XRD）及扫描电镜（SEM）检测手段对产品的物相组分及显微结构进行了表征。结果表明，CA／MN在钡铁氧体晶相变化过程中起关键性作用，在430℃预烧、1280℃煅烧、同时保温2—5h的热处理条件下，随CA／MN增大，钡铁氧体的晶相由w相转变为M相，转变点在CA／MN=1．3左右。产生该现象的主要原因是，随着柠檬酸量的增加，自蔓延燃烧产物团聚程度增大，进而促使γ-Fe2O3向α—Fe2O3发生转化，导致w型铁氧体成相温度升高，在相同煅烧条件下，引起钡铁氧体晶相的转变。

A novel citric acid sol-gel process was used to synthesize barium hexaferrite. Four samples of barium hexaferrite were prepared with different molar ratio of citric acid to metal nitrates of 1,1.2,1.4 and 2 by sol-gel process. X-ray powder diffraction（XRD）, thermo-gravimelric analysis（TGA） and scanning electron microscopy（SEM） were used to characterize the crystal phase structures and microstructure of the products. The results revealed that the crystal phase transformation of barium hexaferrite is dependent significantly on the molar ratio of citric acid to metal nitrates （CA/MN）. Under heat treatment condition of 430~C and 1280~C for 2-5h, crystal phase of the products changes from W-type to M-type with increasing molar ratio of the citric acid to metal nitrates, with changing point at CA/MN=1.3. This is mainly because excessive citric acid makes the combustion product agglomerate, which promates the transformation from γ-Fe2O3 to α-Fe2O3, resulting in increase of formation temperature of the W-type barium hexaferrite.