MoltenSaltSynthesisofCalciumCopper.doc

精品论文大集合molten salt synthesis of calcium coppertitanate powderkepi chen1，cuiwei li21. key laboratory of condition monitoring and control for power plant equipment (north chinaelectric power university), ministry of education, beijing 102206, china2. institute of materials science and engineering, school of mechanical, electronic and controlengineering, beijing jiaotong university, beijing 100044, chinaemail：abstractcacu3ti4o12 (ccto) with perovskite-like structure ceramic powder was synthesized by heating a stoichiometric composition of caco3, cuo and tio2 in molten nacl-kcl and na2so4-k2so4 respectively. phase compositions and particle morphology of the resultant powders have been investigated by x-ray diffraction (xrd) and scanning electron microscope (sem). the results indicate that synthesis temperature could be decreased from 1000 oc (required by conventional solid-state reactions) to 750oc in nacl-kcl or to 850oc in na2so4-k2so4, and the flux type plays more important role on the phase compositions and morphology of resultant powders than synthesized temperature.keywords: calcium copper titanate, high dielectric constant, molten salt synthesis, perovskite-like1 introductionmaterials with high dielectric constant are widely used in technological applications like wireless communication systems, such as cellular phones, and global positioning systems, namely as capacitors, resonators, or filters. high dielectric constants allow smaller capacitive components, thus offering theopportunity to decrease the size of electronic devices1. recently, cacu3ti4o12 (ccto) has attractedsignificant attention as a nonferroelectric and lead-free ceramics due to its high permittivity 105 and weak temperature dependence of permittivity over a wide range 100-500 k2, 3. so far, several models have been proposed to explain the dielectric behavior4. sinclair et al. carried out an impedance spectroscopy measurement to demonstrate that ccto ceramics is electrically heterogeneous and consists of semiconducting grains with insulating grain boundaries and asserted that the giant dielectric phenomenon is attributed to a grain boundary internal barrier layer capacitance (iblc) rather than an intrinsic property associated with the crystal structure5, 6. to date, iblc explanation ofextrinsic mechanism is comparatively widely accepted7-10.various methods have been developed to prepare ccto ceramics. it is usually made by conventional solid-state reactions (ssr) from the metal oxides at high temperatures (typically 1000oc for 20 h) with several intermediate grindings5, 6. this method requires tedious works, relatively long reaction times, and high-temperature conditions and may still result in unwanted phases because of-6-limited atomic diffusion through micrometer-sized grains5. synthesis techniques other than solid-state reactions have been reported recently. hassini et al. prepared ccto by using an organic gel-assisted citrate process11. jha et al. prepared ccto by a polymeric citrate precursor route, but this method is still relatively complex and needs a long heat-treatment time12. liu et al. prepared ccto by pyrolyzing an organic solution containing stoichiometric amounts of the metal cations, which is done at lower temperature with shorter reaction time than the conventional solid-state reaction13. jin et al prepared nano-ultrafine ccto powders by the sol-gel method and citrateauto-ignition method14.molten salt synthesis (mss) is a well established low temperature synthesis technique that has recently attracted increasing interest. it has been used to synthesize ceramic powders, such as pb(mg1/3nb2/3)o315, bi4ti3o1216, laalo317 and mgal2o418. as far as know, however, there are hardly to use on the synthesis of ccto powders by mss. in the present study, mss method was applied to synthesize ccto powder using na2so4-k2so4 and nacl-kcl eutectic mixtures as the flux respectively. the effects of processing factors, such as heating temperature and salt type, on the formation of ccto have been investigated.2 experimentalcaco3, cuo and tio2 were used as the starting materials. salts used were nacl, kcl, na2so4, andk2so4. in this study, nacl-kcl salt with a 1:1 eutectic composition was used, its melting point is about657c; na2so4-k2so4 salt with a 0.75:0.25 eutectic composition was used, its melting point is about832c. a stoichiometric composition of caco3, cuo and tio2 was mixed in acetone using an agate mortar. the mixed powder was then combined with different salts in the salt/oxide weight ratio =1:1. the mixtures were heated in a high purity alumina crucible for 2 h at a temperature between 750 and 1000 oc. after cooling to room temperature, the reacted mass was washed for 2 h in hot deionized water to remove the salts. this washing process was repeated about twenty times. the resultant powders were oven-dried at 105 oc for 4 h prior to characterization.phases in the resultant powders were identified by x-ray diffractometry (xrd, xpert pro 2004, panalytical, netherlands). patterns were recorded at 40 ma and 40 kv using cu k radiation ( = 1.54056). the scan rate (2) was 3o/min at a step size of 0.02 o. icdd cards used to identify phases present were75-2188 (cacu3ti4o12), 48-1548 (cuo) and 75-2100 (catio3). the morphology of as-synthesized powder was observed using scanning electron microscopy (sem, jeol, jsm6490lv, japan).3 experiment results and analysisfig. 1 shows xrd patterns of powders obtained after water-washing the reacted masses of stoichiometric composition of caco3, cuo and tio2 in nacl-kcl salts. after heat treatment at 750 c for2 h, ccto was the main phase with a small amount of cuo and catio3. the result indicates that cctocame into being at lower 750 c when using nacl-kcl salt. ccto contents almost keep constant with increasing temperature from 750 to 1000 c. even when the temperature was increased to 1000 c, peaks of cuo and catio3 still remained. it indicates that synthesized temperature had little effect on the purity of ccto between 750 oc and 1000 oc. in other words, ccto with high content could be synthesized in nacl-kcl salt at any temperature from 750 oc to 1000 oc. however, it is difficult to synthesize ccto byconventional solid-state reactions at lower 1000 oc due to low solid-state diffusion. the molten salt eutectic indeed accelerates the kinetics at this low temperature and facilitates the formation of ccto. this could be attributed to the enhanced diffusion coefficients in the molten chloride liquid phase compared to that in the solid state. furthermore, with increasing reaction temperature, the fwhm (full width at half maximum) of all the diffraction peaks is about 0.18o, which can be attributed to the growth and well development of crystallites. it indicates further that increasing the reaction temperature would have little effect on the formation and development of the ccto phase. moreover, at 850 oc, the nacl-kcl melts started to vaporize with small weight loss and this behavior was sharply enhanced above 900 oc19. therefore over high synthesized temperature would result difficultly to control the reaction.figure 1 x-ray diffractions of ccto samples heated in nacl-kcl for 2 h at(a) 750 oc; (b) 800 oc; (c) 850 oc; (a) 900 oc; (a) 950 oc; (a) 1000 octhe xrd patterns of the ccto powders prepared by the mms process in na2so4-k2so4 salt under different reaction temperatures for 2 h are shown in fig. 2(a)(d). in the powder obtained at 850 oc, ccto was the main phase, and cuo was the major impurities (fig. 2(a). with increasing temperature from 850 oc to 950 oc, cuo still remained (fig. 2(b) and fig. 2(c). further increasing temperature to 1000 oc, all the diffraction peaks can be indexed as body cubic ccto, and no trace of other phases can be detected (fig. 2(d). comparing fig. 2 with fig. 1, it seems that na2so4-k2so4 is more effective in forming ccto powder than nacl-kcl, which may attribute to the differences ofna2so4-k2so4 and nacl-kcl in melting point, viscosity and the solubility of the oxides in these molten salts. nevertheless, the exact formation mechanism in both molten salts is complex and further works is underway.figure 2 x-ray diffractions of ccto samples heated in na2so4-k2so4 for 2 h at (a) 850 oc; (b) 900 oc; (c) 950 oc; (d) 1000 octhe evolution of microstructure and particle sizes in the cacu3ti4o12 products were observed using sem. fig. 3(a) shows a typical sem image of the ccto synthesized in nacl-kcl molten salt. it can be seen that ccto powders show regular polyhedral morphology, and the average size is about 2 m. a typical sem image of the ccto synthesized in na2so4-k2so4 is shown in fig. 3(b). ccto particle morphology shown in fig. 3(b) is a little of different from that in fig. 3(a), which could be attributable to the differences of na2so4-k2so4 and nacl-kcl in melting point, viscosity and the solubility of the oxides in these molten salts. it is obvious that molten salt plays an important role in the development of ccto particle morphology. as far as know, those pictures can be the first time to show about ccto crystal growth in free surroundings, and will help us know about ccto growth mechanism better.fig. 3 a typical sem image of the ccto synthesized in (a) nacl-kcl, (b) na2so4-k2so4.4 conclusion and future workswell-crystallized ccto powders were successfully synthesized firstly by using na2so4-k2so4 and nacl-kcl eutectic mixtures as the flux. the synthesis temperature could be decreased from 1000 oc (required by standard solid-state reactions) to 750 oc in nacl-kcl or to 850 oc in na2so4-k2so4. the flux type plays more important role on the phase compositions and morphology of resultant powders than synthesized temperature.references1 renner b, lunkenheimer p, schetter m, et al. dielectric behavior of copper tantalum oxidej. j applphys, 2004, 96:4400-4404.2 subramanian m a, li d, duan n, et al. high dielectric constant in acu3ti4o12 and acu3ti3feo12phasesj. j solid state chem, 2000, 151:323-325.3 homes c c, vogt t, shapiro s m, et al. optical response of high-dielectric-constant perovskite-related oxidej science, 2001, 293:673-676.4 ramirez a p, subramanian m a, gardel m, et al. giant dielectric constant response in a copper-titanatej solid state commun, 2000, 115:217-220.5 sinclair d c, adams t b, morrison f d, west a r, cacu3ti4o12: one-step internal barrier layer capacitorjappl. phys. lett. 2002, 80:2153-2155.6 adams t b, sinclair d c, west a r, giant barrier layer capacitance effects in cacu3ti4o12 ceramicsj adv mater, 2002, 14:1 321-1 323.7 zhang l, tang z j, polaron relaxation and variable-range-hopping conductivity in the giant-dielectric-constant material cacu3ti4o12j phys rev b, 2004, 70:174306.8 chung s y, kim i d, kang s j l, strong nonlinear current-voltage behaviour in perovskite-derivativecalcium copper titanatej nat mater, 2004, 3:774-778.9 pan m j, bender b a, a bimodal grain size model for predicting the dielectric constant of calcium copper titanate ceramicsj j am ceram soc 2005, 88:2611-2614.10 hong s h, kim d y, park h m, et al. electric and dielectric properties of nb-doped cacu3ti4o12 ceramicsj j am ceram soc, 2007, 90:2118-2121.11 hassini a, gervais m, coulon j, et al. synthesis of ca0.25cu0.75tio3 and