晶粒尺寸效应对极化BaTiO3陶瓷的介电压电物性的影响

晶粒尺寸效应对极化BaTiO3陶瓷的介电压电物性的影响 晶粒尺寸效应对极化BaTiO3陶瓷的介电压电物性的影 响 Grain-Size Effects on Dielectric and Piezoelectric Properties of Poled BaTiO3 Ceramics ZHANG Jialiang ZHENG Peng TAN Yongqiang WANG Chunlei 5 10 15 20 25 30 35 40 School of physics State Key Laboratory of Crystal Materials Shandong University JiNan 250100 Abstract Barium titanate BaTiO3 is known historically as the first ceramic piezoelectric material and shows the high possibility to be revitalized as a popular lead-free piezoelectric material However systematical studies about the grain-size effects on the dielectric and piezoelectric properties of poled BaTiO3 ceramics have been rarely carried out so far In this work a series of dense BaTiO3 ceramics with uniform grain- size distribution were successfully prepared by the conventional solid-state reaction and the grain-size effects on the dielectric and piezoelectric properties were explored in their poled states An interesting physical phenomenon that dielectric permittivity ε and the piezoelectric constant d33 increase significantly at room temperature with the reduction of the average grain size g and reach the ima at g 094 μm has been found It was revealed that domain structure and relative density play the substantial roles in determining the ε and d33 behaviors The average 90?-domain width decreases monotonically with g Both the 90?-domain wall density and the area dimension of domain wall are considered as the important factors that greatly influence the d33 value Keywords Poled BaTiO3 ceramics Piezoelectric property Grain-size effect Domain Structure 0 Introduction Barium titanate is one of the most basic and wildly applied ferroelectric oxide materials with the perovskite-type crystalline structure With decreasing the temperature it undergoes three successive phase transitions from a cubic phase to a tetragonal phase then to an orthorhombic phase and finally to a rhombohedral phase The cubic phase is paraelectric and the other phases are ferroelectric 1 It has been well known since the 1950s that the grain size has a significant effect on the relative dielectric permittivity ε of BaTiO3 ceramics Coarse-grained ceramics with the average grain size larger than 10 μm show their ε in the range of 1500 2000 at room temperature The ε value of BaTiO3 ceramics increases with decreasing the average grain size passes through a imum over 5000 around 08 11 μm and decreases rapidly with further decreasing the average grain size2-5 This physical phenomenon is extremely important for BaTiO3 as a dielectric material to fabricate dielectric devices like the ceramic capacitors and the multilayered ceramic capacitors Correspondingly numerous efforts have been made so far to explain the puzzling origin of this dielectric grain-size effect observed in the unpoled BaTiO3 ceramics46-8 A satisfied conclusive explanation has not been reached at present despite that domain structure is believed to play the substantial essential role Lead-free piezoelectric ceramics are of much interest from the viewpoint of environmental concern910 Researches for lead-free piezoelectric ceramics with excellent properties have been actively carried out in recent years in order to replace the current lead-based ones which are mainly represented by lead zirconate titanate PZT and are extensively used for piezoelectric actuators sensors and transducers On the other hand BaTiO3 ceramics are known historically as the first polycrystalline piezoelectric material and were once practically used before the discovery of PZT For several decades it had been believed that BaTiO3-based ceramics show the comparatively low piezoelectric activity with the piezoelectric constant d33 in a level of 191 pCN Foundations the Doctoral Program of Higher Education Grant No 20090131110015 the Natural Science Foundation of Shandong Province Grant No ZR2010EM00 the National Natural Science Foundation of China Grant No 51172128 Brief author introductionZHANG Jialiang 1966- Male Prof Dr Ferroelectric Materials E-mail zhangjialiangcom -1- 45 50 55 60 65 70 75 80 85 90 or lower and are thus much inferior to PZT 1 Nevertheless amazingly high d33 values 350 pCN 460 pCN and 788 pCN respectively have been obtained recently in some BaTiO3 ceramics that were prepared from hydrothermally synthesized fine BaTiO3 powders by some special fabrication techniques of microwave sintering two-step sintering or templated grain growth11–13 More recently our group has succeeded in fabricating a BaTiO3 ceramic with its d33 value of 419 pCN through the conventional solid-state reaction route using ordinary BaCO3 and TiO2 powders as the starting raw materials1415 These progresses bring a great hope that BaTiO3-based ceramics might be revitalized as a popular lead-free piezoelectric material with the important low-cost merit Moreover the primary investigation results revealed that those non-textured BaTiO3 ceramics with high d33 values show commonly the fine-grained microstructure and the d33 values are seemingly to relate closely with the corresponding domain structure in the fine grains111215-18 However further systematical investigations are necessary in order to better disclose the grain-size dependence of piezoelectric property and clarify the related underlying mechanism It is worth noting that almost all previous studies on the grain-size dependence of ε have been carried out in the unpoled BaTiO3 ceramics Investigations on the grain-size dependence of physical properties in the poled BaTiO3 ceramics are still rare Also it has been well known for a long time that the physical properties of BaTiO3 ceramics could be affected largely by factors such as the raw materials used stoichiometry and processing11920 However practically there always exist some experimental difficulties to prepare a series of dense BaTiO3 ceramics with the various desired values of average grain size using a single source of BaTiO3 powder and a single sintering technique Accordingly different kinds of BaTiO3 powders and sintering techniques usually had to be adopted together such as in Ref 4 This may obstruct the reliability of the experimental results due to the different characters of the obtained BaTiO3 ceramics On the other hand it is highly desirable for studying the grain-size effects to use a group of BaTiO3 ceramics that possess the high relative density and the uniform grain-size distribution in microstructure In our previous study a fine BaTiO3 powder with an average particle size of about 05 μm was first synthesized from ordinary BaCO3 and TiO2 powders via optimized procedures of calcination and ball-milling and a number of BaTiO3 ceramics with different average grain sizes were then prepared under various sintering temperature conditions It was found that d33 increases significantly from 185 pCN to 419 pCN with decreasing the sintering temperature from 1300oC to 1210oC14 Our late detailed investigation with scanning electron microscope SEM revealed that these BaTiO3 ceramics show a large change in microstructure with varying the sintering temperature While those sintered at temperatures below 1230oC show a bimodal grain-size distribution with both large and small grains the ones sintered at 1250oC or above this temperature possess the relatively large grain sizes and uniform grain-size distributions in their microstructure The characteristic of bimodal grain-size distribution in microstructure disappears gradually as the sintering temperature is raised For a BaTiO3 ceramic sintered at 1210oC it contains roughly one-half volume fraction of large grains with an average size of about 70 μm and one-half volume fraction of small grains with an average size of about 08 μm It is speculated that the part of small grains has made a great contribution to the observed high d33 value in this BaTiO3 ceramic Thus the piezoelectric grain-size effect similar to the one known as the dielectric grain-size effect in the unpoled BaTiO3 ceramics may be expected in the poled BaTiO3 ceramics The good understanding of the grain-size effects on the dielectric and piezoelectric properties of the poled BaTiO3 ceramics is considered to be significantly important from both the scientific and technological viewpoints In this work we have made the attempt preparing a series of dense BaTiO3 ceramics with uniform grain-size distribution and a systematical investigation of the -2- grain-size effects on ε and d33 in the poled BaTiO3 ceramics It has been found that ε and d33 show the strong grain-size dependences like ε in the unpoled BaTiO3 ceramics Further possible mechanism that results in the newly found phenomenon was examined from the aspects of crystalline structure phase transition microstructure and domain structure 95 100 105 110 115 120 125 130 1 Experimental procedure The BaTiO3 ceramics were prepared by the conventional solid-state reaction technique starting from the properly choosing raw materials of commercial BaCO3 powder purity ?990 Sinopharm Chemical Reagent Co Ltd and TiO2 powder purity?998 Xiantao Zhongxing Electronic Material Co Ltd The preparation procedures are basically the same as those in the previous studies141521 except the adoption of a different TiO2 powder The raw materials were weighed according to the stoichiometric ratio and ball-milled for 12 h on a planetary ball mill The milled slurry mixture was dried crushed in an agate mortar and then pressed into large circular plates at 30 MPa Calcination was performed at 1050oC for 4 h Followed is a second ball milling procedure with the milling time of 12 h After the second ball milling a fine BaTiO3 powder that has particle sizes less than 05 μm was obtained This powder was mixed with a 05 wt polyvinyl alcohol PVA binder and was pressed into small disks with the dimension of 15 mm in diameter and 15 mm in thickness at 200 MPa The following procedure was to burn out the PVA binder at 650oC for 05 h Finally sintering was carried out at temperature conditions varied from 1180 to 1450oC for 2 h To obtain BaTiO3 ceramics with the particularly large grain sizes sintering time was extended to 10h or 20h while sintering temperature was maintained at 1450oC For a certain case the two-step sintering technique51217182223 was adopted in order to obtain a dense and fine-grained BaTiO3 ceramic where the temperature is first raised to 1380oC and held for 16 min is then lowered rapidly to 1150 oC and kept for 6 h To characterize dielectric and piezoelectric properties the ceramic specimens were coated with silver paint on the upper and bottom surfaces and fired at 575oC for 20 min Poling was accomplished at 105oC in silicon oil under 50 kVmm for 30 min Measurements of piezoelectric and dielectric properties were performed after 24 h The d33 value was measured by a Berlicourt-type d33 meter YE 2730A and the ε was measured using an Agilent 4294A precision impedance analyzer An Espec SU-261 chamber was used to meet the accurate temperature control requirement of dielectric and piezoelectric measurements Crystallographic structure was investigated by X-ray diffraction XRD on a D8 ADVANCE diffractometer using CuKα radiation To characterize microstructure and domain structure the ceramic specimens were mirror-polished after grinding off the surface layer of about 01mm thickness with a fine Al2O3 powder and chemically etched for 30 s in an aqueous solution of 5 HCl into which a few drops of HF was added For the coarse-grained BaTiO3 ceramics observation was performed under an Olympus BX51 polarizing microscope which has magnification up to 2000 and is equipped with the function of the differential interference contrast Optical images were recorded by an Olympus DP70 digital camera For those fine-grained BaTiO3 ceramics observation was carried out on a HITACH S-4800 scanning electron microscope SEM The average grain size g was determined from the observed images of chemically etched microstructure using the line intercept method The mass density ρ was obtained from measuring the mass and the dimensions and the ρ0 value was calculated on the basis of crystal lattice parameters obtained in XRD analysisSecond-order headline -3- 2 Results and discussion 135 140 145 150 Table 1 lists the ρ ρ0 and g data of the various BaTiO3 ceramics sintered under different conditions For those BaTiO3 ceramics sintered below 1250oC The ρ0 and g values increase with the enhancement of sintering temperature While the ceramic sintered at 1180oC shows the g and ρ0 values of 074 μm and 931 the one sintered at 1230oC has its g and ρ0 values of 225 μm and 975 For the BaTiO3 ceramics sintered at temperatures above 1250oC the ρ0 values are nearly a constant of 980 Further enhancing the sintering temperature or extending the sintering time can not improve the ρ0 value but increases the g value The BaTiO3 ceramic sintered at 1450oC for 20 h shows its g value as large as 115 μm Also it can been seen from Table 1 that preparing dense and fine-grained BaTiO3 ceramics with the ordinary sintering technique from the synthesized BaTiO3 powder of submicron-sized particles is practically very difficult Fortunately we have recently found that the two-step sintering technique is also applicable to such kind of BaTiO3 powder Using the two-step sintering technique a dense and fine-grained BaTiO3 ceramic with its g and ρ0 values of 167 μm and 982 has been successfully obtained Tab 1 Data of the density and the average grain size for the various BaTiO3 ceramics prepared under different sintering conditions Sintering Condition 1180oC 2 h 1190oC 2 h 1200oC 2 h 1210oC 2 h 1230oC 2 h 1250oC 2 h 1270oC 2 h 1300oC 2 h 1350oC 2 h 1400oC 2 h 1450oC 2 h 1450oC 10 h 1450oC 20 h ρ gcm3 559 561 569 580 585 591 589 587 586 592 588 589 589 ρ 0 931 934 948 965 975 983 981 977 975 985 978 981 980 g μm 074 094 121 163 225 395 837 203 382 564 743 912 115 Figure 1 shows the change of ε with g for the poled BaTiO3 ceramics For comparison those data of the unpoled BaTiO3 ceramics are also plotted in the same figure All the measurements 155 160 were carried out at room temperature of 25oC with the frequency of 1 kHz As can be seen the poled BaTiO3 ceramics shows a strong grain-size dependence of ε quite similarly to those unpoled BaTiO3 ceramics While the coarse-grained one with g 115 μm shows the ε value of 2640 the fine-grained one with g 094 μm displays the imum ε value of 4045 No significant changes of ε occur in the coarse-grained ceramics before and after the poling process but large changes of ε happen in those fine-grained ones In general the unpoled BaTiO3 ceramics show higher ε values than the corresponding poled ones particularly when g are less than 10 μm When compared to the literature data in reported Ref 4 these unpoled BaTiO3 ceramics show the -4- much high ε values in the range of g 20 μm but the close ones around g 10 μm This together with the uniform grain-size distribution and comparatively high ρ0 indicate that our BaTiO3 165 ceramics possess the good quality and thus can guarantee the high reliability of the obtained experimental results 6000 5000 4000 3000 2000 1000 poled unpoled 0 01 1 10 100 g m 170 175 180 185 190 Fig 1 Grain-size dependences of ε observed at room temperature in the unpoled BaTiO3 ceramics and the poled BaTiO3 ceramics respectively Figure 2 presents the change of d33 with g measured at room temperature in the poled BaTiO3 ceramics It is clear that d33 shows a strong grain-size dependence in the range of g 40 μm A d33 imum is observed at g 094 μm similarly to the case observed in the grain-size dependence of ε However when g is larger than 40 μm d33 is almost independent of g For the coarse-grained BaTiO3 ceramic with g 115 μm it exhibits the d33 value of 153 pCN which is close to the earlier reported value of 191 pCN1 In contrast the fine-grained one with g 094 μm shows a considerably high d33 value of 338 pCN The result is generally consistent with those obtained previously in other investigations111214-18 and indicates that it is possible to obtain high piezoelectric property by properly controlling the ceramic microstructure On the other hand bearing the significant similarity between the grain-size dependences of ε and d33 in mind it seems reasonable to assume that they possibly result from the same physical origin Detailed discussion on this point will be made below Here we note that the largest d33 value of 338 pCN obtained in the present study is actually a little lower than the previously obtained value of 419 pCN in a BaTiO3 ceramic prepared using a different TiO2 powder as raw material1415 As mentioned earlier the latter BaTiO3 ceramic contains one-half volume fraction of small grains approximately with the grain sizes of 08 μm in microstructure which should have made the determinant contribution to the observed high d33 value The result suggests that further improving the ρ0 values of fine-grained BaTiO3 ceramics should be a challenge in our future work to obtain the better piezoelectric properties The idea has been confirmed very recently in our another experiment where a dense and fine-grained BaTiO3 ceramic with its g and ρ0 values of 167 μm and 982 was successfully prepared by the two-step sintering technique and a high d33 value of 390 pCN was obtained -5- 400 300 200 100 0 01 1 10 100 g m 195 200 205 210 Fig 2 Grain-size dependence of d33 observed at room temperature in the poled BaTiO3 ceramics Figure 3 describes the temperature dependence of d33 for some typical poled BaTiO3 ceramics with the typical g values The measurement was carried out during cooling process On the whole d33 increases first reaches a peak and then decreases with decreasing the temperature The peak temperature which corresponds to the d33 imum increases slightly with the reduction of g For the poled BaTiO3 ceramic with g 837 μm it exhibits the d33 value of 193 pCN at 25oC and a d33 imum of 222 pCN at 15oC By comparison the one with g 094 μm and the large d33 value of 338 pCN at 25oC shows a d33 imum of 366 pCN at 175oC As will be discussed below the peaks observed around 15oC in the d33 vs temperature curves shown in Fig 3 are closely related to the tetragonal-orthorhombic polymorphic phase transition Figure 4 demonstrates the temperature dependence of ε for some BaTiO3 ceramics with the typical g values before poling Measurement was made at 1 kHz during cooling process with the rate of 1oCmin As shown in Fig 4 two peaks are seen in each curve within the measured temperature range They are considered to correspond to the tetragonal-cubic phase transition TC and the orthorhombic-tetragonal phase transition TO-T respectively While the peak corresponding to TC is very sharp the peak corresponding to TO-T is rather broad 400 300 200 100 094m 163m 837m 743m 0 -60 -40 -20 0 20 40 60 o Fig 3 Temperature dependence of d33 observed in some poled BaTiO3 ceramics with the typical g values Data were measured during cooling process -6- 16000 12000 6000 5000 4000 094m 163m 837m 743m TC 3000 2000 215 8000 1000 -30 -20 -10 0 10 20 30 o 40 50 TO-R 4000 0 -60 -30 0 30 60 90 120 150 o FIG 4 Temperature dependence of ε observed in the typical BaTiO3 ceramics before poling Data were measured at 1 kHz during cooling process at a rate of 1?Cmin The inset shows a partially enlarged view around TO-T Figure 5 gives representatively the ε vs temperature curves measured in the BaTiO3 ceramic 220 225 230 235 with its g value of 163 μm during the successive cooling and heating processes Thermal hysteresis is observed around both phase transitions which indicates that these phase transitions are the first-order ones However the thermal hysteresis width around TO–T is about 6?C being broader than the value of about 2?C around TC The TC and TO–T values measured during the successive cooling and heating processes for the various BaTiO3 ceramics with different g values are collectively plotted in Fig 6 It is obvious from this figure that TC decreases but TO–T increases with the reduction of g For the BaTiO3 ceramics with the g values of 743 837 163 and 094 μm the TO–T values measured during cooling process are 159?C 197 201 and 205 while the corresponding TO–T values measured during heating process are 222 243 262 and 272?C respectively The result basically agrees with those reported data312 Here we note that the TO–T values presented in Fig 6 were obtained from the ε vs temperature curves of unpoled BaTiO3 ceramics However it has been experimentally confirmed in this study that no substantial difference in the TO–T values occur before and after poling It should also be pointed out that the present BaTiO3 ceramics show a quite different TO–T behavior from the one obtained in our previous study which BaTiO3 ceramics were prepared with a different TiO2 powder as raw material and their TO–T value were almost independent of g and roughly a constant of 17?C when measured upon heating process1421 The mechanism why different TiO2 powders could lead to such a distinct difference in the TO–T behaviors is still not clear enough at present -7- 14000 5000 12000 4000 TC 10000 3000 Cooling Heating 8000 2000 -20 -10 0 10 20 30 40 50 60 6000 4000 2000 TO-R o 0 -40 0 40 80 o 120 240 Fig 5 ε vs temperature curves measured during the successive cooling and heating processes for the BaTiO3 ceramic with g 163 μm The inset shows a partially enlarged view around TO-T 128 a 126 124 122 heating cooling 120 1 10 100 g m 32 b 28 heating 24 20 cooling 16 12 1 10 100 g m 245 250 Fig 6 Variations of TC and TO-T with g Data were obtained from the ε vs temperature curves measured during the successive cooling and heating processes Figure 7 displays the XRD profiles of the various BaTiO3 ceramics with the typical g values which were taken at room temperature For all of these ceramics diffraction peaks appearing in the XRD profiles could be indexed by a tetragonal crystal structure However the diffraction peaks corresponding to tetragonal planes 110 002 and 112 become weak gradually with the reduction of g The phase transition from tetragonal structure to orthorhombic structure takes place gradually Nevertheless tetragonal phase still dominates even in the crystal structure of the fine-grained BaTiO3 ceramics In other words all the investigated BaTiO3 ceramics at room temperature are practically in tetragonal phase although some of the fine-grained BaTiO3 ceramics -8- 255 260 show their TO–T values slightly higher than room temperature when measured upon heating process On the basis of XRD analysis the crystal lattice parameters were then obtained It is found that the lattice parameter a expands the lattice parameter c shrinks and the ratio ca decreases continuously with the reduction of g The result agrees with the previously reported data4 For those coarse-grained BaTiO3 ceramics with g 10 μm their a c and ca are nearly the constant values of 0399 and 0403 and 1009 respectively By comparison the fine-grained one with g 094 μm has a 0400 c 0402 and ca 1005 respectively a 094m b 094m 002 200 163m 163m 837m 837m 743m 743m 20 30 40 50 60 70 44 45 46 2 degree 2 degree 265 270 275 280 Fig 7 XRD profiles of some BaTiO3 ceramics with the typical g values b c and d are the partially enlarged views of a Figure 8 exhibits representatively the SEM images observed in the various poled BaTiO3 ceramics with small g values Figure 9 reveals some optical images observed in the poled coarse-grained BaTiO3 ceramics with a large g value of 743 μm The evolution of microstructure and domain structure could be clearly seen from these figures Stripes and herringbone patterns inside the grains are recognizable inside the grains It has been reported that such stripes and herringbones are the typical feature of domain configuration for BaTiO3 ceramics in the tetragonal phase124-28 The stripes are taken to correspond to the 90?-domain patterns where polarization vectors in adjacent domains adopt a head-to-tail arrangement across the domain boundaries25 The physical origin of the formation of 90?-domains is considered to associate with the release of the internal stress12627 The herringbone patterns are the consequence of the combination of two sets of such alternating 90?-domains124 For the fine-grained BaTiO3 ceramics domain patterns are quite simple Stripes are the dominant domain patterns Usually no more than two herringbones patterns are observed in each individual grain Thus the 90?-domains generally show their lengths comparable to the corresponding grain sizes The number of herringbone-type domain patterns increases gradually with the increase of g For those coarse-grained BaTiO3 ceramics as shown in Fig 9 diverse and very complicated domain patterns are more frequently observed while very regular herringbone patterns consisting of parallel bands of stripes are also seen occasionally at some locations -9- 285 290 295 300 Fig 8 SEM images of domain patterns observed in the various poled BaTiO3 ceramics sintered under different temperature conditions Sintering temperatures are a 1180?C b 1210?C c 1250?C and h 1270?C respectively Fig 9 Optical microscopy images of domain patterns observed in the poled BaTiO3 ceramics sintered at 1450?C for 2h Figure 10 illustrates the change of the average domain width w with g for the various poled BaTiO3 ceramics The w value of each BaTiO3 ceramic was statistically calculated by precisely determining the 90?-domain widths observed in the different domain patterns at a large number of locations For comparison those w data obtained in the unpoled BaTiO3 ceramics and reported in Ref 4 are also plotted in the same figure As shown a quite similar grain-size dependence of w to that obtained in the unpoled BaTiO3 ceramics is observed in the poled BaTiO3 ceramics The poled BaTiO3 ceramics show only the slightly larger w values than their corresponding unpoled BaTiO3 ceramics This is especially true for those fine-grained ones with g ? 163 μm Further w changes with g more rapidly in the range of g 837 μm For the BaTiO3 ceramics with the typical g values of 094 163 837 and 743 μm the w values before and after poling are 115 169 280 and 324 μm and 118 177 298 and 361 μm respectively As shown in Fig 10 the fine-grained BaTiO3 ceramics with g ? 163 μm show their w values close to - 10 - 305 the literature data whereas the coarse-grained BaTiO3 ceramics exhibit the much lower w values than the reported ones4 The result shown in Fig 10 qualitatively agree with the earlier study result29 It was reported that the amount of 90?-domain reorientation in a BaTiO3 ceramic is only 17 during poling and drops to 12 upon removal of the poling field while 180?-domain reorientation is virtually perfect 400 unpoled poled 300 1000 Ref 4 310 200 100 100 10 01 1 10 100 0 1 10 g m 100 g m Fig10 Grain-size dependences of average 90o-domain width observed in both the poled and the unpoled BaTiO3 ceramics For comparison data reported in Ref 4 are plotted in the inset where the log-log scales are adopted As mentioned above we have found the large grain-size effects on ε and d33 in the poled 315 320 325 330 335 340 BaTiO3 ceramics The ε and d33 values increase significantly at room temperature with the reduction of g and reach their ima at g 094 μm Since the poled BaTiO3 ceramics show the strong grain-size dependence of ε much like the unpoled BaTiO3 ceramics it is speculated that origins resulting in the grain-size dependence of ε should be the same in the two systems Therefore we will focus below only on a discussion about the possible mechanism that leads to the observed strong grain-size dependence of d33 It is known that the piezoelectric properties of a piezoelectric ceramic material could be generally resolved into intrinsic and extrinsic contributions30-33 The intrinsic one is attributed to the relative ion shift that preserves the ferroelectric crystal structure whereas the extrinsic one is ascribed mainly to the motion of domain walls Obviously as shown in Figs 3 and 4 neither the tetragonal-orthorhombic phase transition nor the shift of TO-T can explain the significant increase of d33 with the reduction of g Further the ratio ca decreases with the reduction of g can also hardly interpret the strong grain-size dependence of d33 since it is related directly to the decrease of spontaneous polarization Accordingly an intrinsic mechanism explanation seems to be difficult In contrast extrinsic factors particularly the one that associates with domain walls are considered to very likely make the substantial contribution The domain wall density has been taken previously as the origins for the strong dielectric grain-size dependence of ε observed in the unpoled BaTiO3 ceramics4518 and for the large enhancement of d33 observed in the poled fine-grained BaTiO3 ceramics 111215-18 In addition to the 90?-domain wall density we speculate that the area dimension of 90?-domain wall should be also an important factor that greatly influences d331421 As shown in Fig 8 and 9 the experimentally observed 90?-domain walls are a large number of straight lines in the two-dimensional domain patterns However the domains are actually a group of lamellas and the domain walls are thus a collection of planes with certain limited areas in three dimensions2426 The area dimension of an individual domain wall should be proportional to the observed domain length in a two-dimensional domain pattern Correspondingly the shorter domain length that is - 11 - observed in a two-dimensional domain pattern and the smaller grain size the smaller domain-wall area is in the three dimensions For the BaTiO3 ceramics with small g values the 90?-domains inside grains show the comparable lengths to the grain sizes Thus a fact that the largeness of 90?-domain wall varies substantially with g could be deduced On the other hand the piezoelectric 345 350 355 360 365 370 375 380 385 effect in perovskite-type ferroelectric ceramics is known to be greatly influenced by the movement of the 90?-domain walls303134 Due to the strong anisotropic character of the 90?-domain walls the otion of a 90?-domain wall should be taken as a collective behavior Furthermore since domain walls with large areas should naturally have heavier inertia masses in motion34 domain walls of small area dimension will respond more actively to the external electrical or stress signal Consequently those BaTiO3 ceramics with smaller g show higher d33 values in the range of g 40 μm With respect to those coarse-grained BaTiO3 ceramics with g 40 μm as representatively shown in Fig 11 the lengths of domain walls are in the order of several micrometers to several tens of micrometers long Each individual domain wall has a large area and thereby heavier effective inertia mass Moreover a number of complex herringbone patterns often simultaneously exist in a single coarse grain The motion of a 90o-domain wall might be inhibited by the interactions from the neighboring domain walls that are included in the complex and conjoined herringbone domain patterns In such a case it might be more proper to consider the motion of domain walls as the joint behavior which will further increase the effective inertia mass for an individual domain wall This can reasonably explain why those coarse-grained BaTiO3 ceramics with g 40 μm show the low but nearly unchanged d33 values Regarding that the fine-grained BaTiO3 ceramic with g 074 μm shows a slight d33 decrease when compared to the one with g 094 μm the decrease of ρ0 could be considered at least as a major cause Actually this is well evidenced by the BaTiO3 ceramic prepared by the two-step sintering technique As mentioned above the latter BaTiO3 ceramic has its g and ρ0 values of 167 μm and 982 and shows the significantly high d33 value of 390 pCN while the fine-grained BaTiO3 ceramic with a close g value of 163 μm but a little lower ρ0 value of 965 exhibits only the d33 value of 308 pCN Meanwhile no substantial difference is found in the domain structure between the two BaTiO3 ceramics In addition to the ρ0 decrease two other causes may also be considered to contribute to the slight d33 decrease observed in the fine-grained BaTiO3 ceramic with g 074 μm Firstly the clamping forces on the domain walls from the corresponding terminating grain boundaries become sufficiently strong such that the effective inertia mass of a 90o-domain wall is greatly increased Secondly spontaneous polarization is largely reduced with the decrease of ca 3 Conclusion A series of high-quality BaTiO3 ceramics with various average grain sizes and relatively uniform grain-size distribution were successfully prepared with a proper TiO2 powder as raw material by the conventional solid-state reaction under different sintering conditions The grain-size effects on the dielectric and piezoelectric properties of poled BaTiO3 ceramics were investigated It has been found that ε and d33 increase significantly at room temperature with reducing the average grain size g Both ε and d33 reach their ima at g 094 μm The newly found phenomenon in the pole BaTiO3 ceramics is quite similar to the famous one known as the dielectric grain-size effect in the unpoled BaTiO3 ceramics Some analyses from the aspects of crystalline structure phase transition microstructure and domain structure were carried out to examine the possible mechanisms that result in the large grain-size effects on ε and d33 in the poled BaTiO3 ceramics The ε vs temperature and d33 vs - 12 - temperature measurements showed that the orthorhombic-tetragonal polymorphic phase transition and the TO-T shift are not sufficient to explain the newly found phenomenon For all the BaTiO3 ceramics the tetragonal phase is dominant in the crystalline structure at room temperature Domain structure and relative density are considered to play the substantial roles in determining 390 395 400 405 410 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