摘要
铬酸钴(CoCr_2O_4)是尖晶石型材料的一种,广泛应用于高性能颜料、催化和磁性材料等众多领域。本文以碳酸氢铵反滴定共沉淀法和低温燃烧法(LCS)合成铬酸钴材料,用热重-差热分析(TG-DTA)、示差扫描量热仪(DSC)、傅里叶变换红外(FTIR)、X-射线衍射仪(XRD)、激光粒度仪、扫描电镜(SEM)、(高倍)透射电镜(HRTEM/TEM)、紫外可见分光光度计(Uv-Vis Spectrometer)和分光色差仪等分别表征了前驱体的热性能、组成及样品的结构、粒度分布、形貌、显微结构、紫外可见吸收性能和颜色参数。
在共沉淀法中,主要研究了不同沉淀剂、滴定方式、水浴温度、碳酸氢铵浓度和烧结温度等因素对前驱体结构、组成、转晶温度、晶粒尺寸和晶格常数和颜色性能等的影响。结果表明,用碳酸氢铵做沉淀剂、浓度为0.8mol/L,采用反滴定方式,在75℃水浴合成前驱体,然后在800-C保温1h能获得形貌规整,粒径为35nm左右的晶粒。Uv-V曲线上可见575、615和670nm左右有较强的吸收,575和615nm左右的吸收可归于Co~(2+)的自旋允许跃迁~4A_2(F)→~4T_1(P),670nm处的吸收可能是Co~(2+)的自旋允许跃迁和Cr~(3+)在八面体中的跃迁~4A2g→~4T_(2g)吸收发生重叠。随着煅烧温度的升高,颜料的亮度提高,绿色调和蓝色调逐渐加深,色饱和度逐渐增加,表现为由暗绿色趋向于鲜艳的蓝绿色。
在低温燃烧法中主要研究了不同金属硝酸盐(氧化剂)和柠檬酸(CA,作还原剂)之比、烧结制度、水浴温度、不同pH值等因素对前驱体的热性能、样品的粒径、晶格畸变和晶格常数、凝胶形成时间、凝胶燃烧性能和Uv-Vis光谱的影响。用Scherrer公式和Williamson-Hall method(W-H法)分别估算了粒径的大小,用W-H法计算了不同升温速率对样品晶格畸变和晶粒大小的影响。用LCS法,以Zn~(2+),Ni~(2+)和Al~(3+)等金属离子分别取代部分Co~(2+)和Cr~(3+),形成了A位和B位取代的复合尖晶石材料,研究了样品结构和性能的变化。
结果表明,当CA/NO_3=0.278时,前驱体具有最佳的反应性能,400℃点燃,在800℃保温2h即可合成单相、形貌规整的纳米级铬酸钴。水浴体系温度选择在90～95℃,约2.5h形成凝胶,调整pH=5能得到结晶度高、单相的铬酸钴。红外谱图上可以明显的看到527和630cm~(-1)左右对应于八面体基团[CrO_6]和四面体基团[CoO_4]的振动吸收。Uv-V曲线在约578、618和670nm左右有较强的吸收,随着温度升高,吸光度增大,对应Co~(2+)的自旋允许跃迁~4A_2(F)→~4T_1(P)和Cr3+在八面体中发生~4A_(2g)→~4T_2g跃迁。从趋势上看,W-H法估计本样品晶粒大小更合理,样品晶格畸变率和升温速率有直接关系,这在HRTEM的观察中得到证实,随着温度升高,晶格畸变率基本呈线性上升。掺杂Zn~(2+)和Al~(3+)的复合型样品能保持结构不变而有更好的颜色性能,具有经济性和实用性。
Cobalt chromate (CoCr2O4) is one of spinel materials with wide application in various fields such as high performance pigment, catalysis and magnetic material, etc. In this paper, single-phase nanocrystalline CoCr2O4 has been synthesized by co-precipitation and low-temperature combustion synthesis(LCS) method using ammonium hydrogen carbonate(AHC) as precipitant and citric acid as reductant/fuel and metal nitrates as oxidants, respectively. TG-DTA, DSC, FTIR, XRD, Laser Particle Distribution Analyzer, SEM, (HR)TEM, Uv-V Spectrometer and Color-Eye 7000A Spectrophotometer were applied to characterize the thermo-performance and the composition of the precursors, particle size distribution, structure, morphology, micro-structure, Uv-V absorbance curve and the color parameters of the samples, respectively.
In co-precipitation method, different precipitants, the manner of titration, water bath temperature, the concentration of AHC and calcination program were studied in terms of effect on the structural and component of the precursor, the crystallite formation temperature, the crystallite size, the lattice constant and color performance. The results show that, AHC is a good precipitant in this system with the concentration of 0.8mol/L at 75℃and using the reverse-strike method (i.e. adding salt solution to the precipitant solution). Crystalline with uniform morphology and about 35nm in size can be obtained by calcining the precursor at 800℃for 1h. Absorbtion bands at 575,615 and 670nm can be seen in the Uv-V curve due to the spin allowed transition 4Ar (F)→4T1 (P) of Co2+in tetrahedron and 4A2g→4T2g of Cr3+in octahedron. As the increase of the calcination temperature, the pigment exhibit deeper green and blue hue and higher color saturation, indicating better color performance.
In LCS, the impact of different reductant/oxidant ratio (CA/NO3'), calcination program, water bath temperature and different pH on the thermo performance of the precursor, lattice distortion ratio, gel formation time and the absorbtion curve were studied. The lattice distortion and crystalline size of the samples prepared by different calcination rates were estimated by Williamson-Hall method and Scherrer method. CoCr2O4 was also treated as a host to accept other metal ions, i.e. Zn2+, Ni2+and Al3+. The structure and Uv-V performance of the complex oxides were investigated.
Results show that CA/NO3'=0.278 exhibits best thermo reaction performance, single phased and uniform cobalt chromate can be obtained by igniting the precursor at 400℃and then soaking at 800℃for 2h. Gel can be formed in the water bath at 90～95℃in 2.5h. Adjust the pH to 5 is suitable to gain single-phased samples with high crystallinity. Two absorbtion bands at 527 and 630cm-1 were witnessed in IR spectrum which were ascribed to the vibration of [CrO6] and [CoO4], respectively. Absorbtion bands at 578,618 and 670nm can be seen in the Uv-V curve due to the spin allowed transition 4A2(F)→4Ti(P) of Co2+ in tetrahedron and 4A2g→4T2g of Cr3+ in octahedron. The W-H method was rekoned to be better to estimate the crystalline size obtained by different calcination rates and different soaking time and the lattice distortion is dependent on the calcination rate, which is evidenced by the HRTEM observation. Zn2+and Al3+ doped complex oxides were found to have better color performance without change in structure, which is meaningful both in economy and application.
引文
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