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The goads of this thesis: Successfully fabricated ceramic piezoelectric samples (Ba1-xCax) TiO3 (BCT) and BZT-BCT by solid phase synthesis method. BZTBCT materials must be good quality, high piezoelectric coefficient (500-600 pC / N). Studying the relationship between morphological competition and dielectric ferroelectric properties, especially with the high piezoelectric properties of materials.
MINISTRY OF EDUCATION AND VIET NAM ACADEMY OF SCIENCE TRAINING AND TECHNOLOGY GRADUATE UNIVERSITY OF SCIENCE AND TECHNOLOGY …………… *****…………… NGUYEN VAN KHIEN Fabrications of ferroelectric materials not contain Pb on BaTiO3 substrate and study their electricity and piezoelectricity properties Specialized: Electronic materials Numerical code: 62.44.01.23 SUMMARY SCIENCE Ha noi, 2018 OF DOCTORAL IN MATERIALS The work is completed at: INSTITUTE OF MATERIALS SCIENCE - VIET NAM ACADEMY OF SCIENCE AND TECHNOLOGY Science supervisor: PGS.TS Le Van Hong PGS TS Nguyen Van Dang PhD dissertation reviewer 1: PhD dissertation reviewer 2: PhD dissertation reviewer 3: The thesis will be protected under supervisory board academy level at: Academy at … hours… day … month … 2018 People can find this thesis at: - National library - Graduate university of Science and Technology library LIST OF PROJECTS PUBLISHED Articles in the ISI directory Le Van Hong, Nguyen Van Khien and Truong Van Chuong, “Dielectric Relaxation of Ba1¹xCaxTiO3 (x = 0.00.3)”, Materials Transactions, Vol 56, No (2015) pp 1374 to 1377 Van Khien Nguyen, Thi Hong Phong Le, Thi Kim Chi Tran, Van Chuong Truong and Van Hong Le, “Influence of Ca Substitution on Piezoelectric Properties of Ba1xCaxTiO3” Journal of electronic materials, DOI: 10.1007/s11664-017-5332-0 (2017) Nguyen Van Khien, Than Trong Huy, Le VanHong, “AC conduction of Ba1-xCaxTiO3 and BZT-BCTx”, Physica B, S0921-4526(17)30193-X (2017) Articles published in domestic magazines Nguyễn Văn Khiển, Vũ Đình Lãm Lê Văn Hồng, “Ba1xCaxTiO3 tính chất điện mơi chúng”, Tạp chí Khoa học Cơng nghệ 52(3C) (2014) 725-730 Nguyen Van Khien, Vu Dinh Lam and Le Van Hong, “Ba1xCaxTiO3 AND THE DIELECTRIC PROPERTIES”, Communications in Physics, Vol 24, No (2014), pp 170-176 Nguyễn Văn Khiển, Trương Văn Chương, Đặng Anh Tuấn, Lê Văn Hồng, “Ảnh hưởng thay Ca cho Ba lên tính sắt điện hệ Ba1-xCaxTiO3”, Hội nghị Vật lý chất rắn Khoa học vật liệu toàn quốc lần thứ - SPMS2015 Nguyen Van Khien and Le Van Hong, “ Effect of Ca concentration substituting for Ba on structure and ferroelectric properties of BZTBCT material”, Vietnam Journal of Science and Technology 56 (1A) (2018) 86-92 Related articles T D Thanh, P T Phong, D H Mạnh, N V Khien, L V Hong, T L Phan, S C Yu, Low-field magnetoresistance in La0.7Sr0.3MnO3/BaTiO3 composites, J mater SCI (2013) 24: 13891394 Nguyễn Văn Khiển, Trịnh Phi Hiệp, Nguyễn Thị Dung Nguyễn Văn Đăng, Nghiên cứu ảnh hưởng biên pha nano BaTiO3 lên tính chất điện từ vật liệu La0.7Sr0.3MnO3, Tạp chí Khoa học Công nghệ Đại học Thái Nguyên, tập 118 số 4, 2014, trang 197202 Introduction Piezoelectric materials is a material that can can generate a Voltage corresponding to mechanical stess change Although it was discovered in 1880, it was not widely used until the 1950s Over the past half decade, PZT ceramics materials (PbZr1-xTixO3) have been studied and demostrated by researchers and It has a relatively large piezoelectric coefficient (d33 = 220 ÷ 590 pC / N) That‟s why most piezoelectric applications, both telephone batteries and high-tech scanning-tunneling microscopes use PZT piezoelectric materials.However, Pb is a radioactive element It not only is very dangerous to humans but also it is one of the causes of global environmental pollution if used extensively Therefore, it is imperative for scientists to find out that piezoelectric materials not contain Pb with a high piezoelectric coefficient which can be use instead of traditional PZT materials Some piezoelectric materials not contain Pb have recently been publish and have shown good results Special, material systems not contain Pb on (K,Na)NbO3 and BaTiO3 substated However, in our understanding, piezoelectric material systems not contain Pb have not been adequately researched There are some publications published in international journals, but a few and sporadically The physical mechanism to explain the cause of the high piezoelectric coefficient and the properties of the material is still a lot of unsoud, need to focus more research, deeper In the country, piezoelectric material systems are studied by many scientists in centers, scientific institutes and universities such as Ha Noi unviersity of Science and Technology, University of Science-Hue University In order to promote the research activities on the family of piezoelectric materials not contain Pb and based on the actual situation as well as research conditions such as experimental equipment, references, research collaboration capabilities with domestic research team We think that studying and solving the problems mentioned above is useful and will give many positive results Especially finding the relationship between the big piezoelectric coefficient and the dielectric recovery time of the object piezoelectric This is why we choose this thesis “Fabrication of ferroelectric materials not contain Pb on BaTiO3 substrate and study their electricity and piezoelectricity properties” we believe that our work will be sussces and will be useful for the understand about the interaction electric mechanism in the ferroelectric material systems, piezoelectric not contain Pb, also open application capacibility of these material systems in fabrication of pin, senso… contributory on the environment reduction The main contents of my thesis is present in chapters: Chapter Theoritical overview Chapter Experiment Chapter Effected of Ca substitution for Ba on the structure and magnetic properties of BCT and BZT-BCT Chapter 4.The relationship between time of restore dielectric and piezoelectric properties of BCT and BZT-BCT The goads of this thesis: Successfully fabricated ceramic piezoelectric samples (Ba1-xCax) TiO3 (BCT) and BZT-BCT by solid phase synthesis method BZTBCT materials must be good quality, high piezoelectric coefficient (500-600 pC / N) Studying the relationship between morphological competition and dielectric ferroelectric properties, especially with the high piezoelectric properties of materials In addition, based on the results of the synchronized studies about the material phase structure, the electric polarization of the material depends on temperature, electric field and frequency which will provide the analysis and general discussion contribute Demonstrate the physical mechanism of the phenomenon of high piezoelectri coefficient in ferroelectric material systems Research object of my thesis Research object: Piezoelectric materials Area of research: Piezoelectric materials not contain Pb on BaTiO3 substrate Research methods: The ceramic bulk is fabricated by solid phase reaction Structure of materials, morphological phase, particle size, The morphologic form of the material was investigated and analyzed on the basis of X-ray diffraction pattern, Raman spectra and Scanning Electron Microscope SEM After obtaining the necessary information on the phase structure, phase material cleanliness, morphology and supporting information as mentioned above we perform electrical measurements such as resistant R (T), capacity C (T), D (E) Measurement of C (T) will be made under the effect of high electric field to evaluate the maximum polarization of the material In addition, C (f) frequency-dependent measurements of polarization are also performed to evaluate the dielectric recovery characteristics and to indirectly evaluate the piezoelectric coefficient of the material Colecting all the results of the study will help us to evaluate the dielectric polarization mechanism in the material, the correlation between the morphological phase and the piezoelectricity ferroelectric properties of the materials In the process of working and writing this thesis, although the author has tried hard but still can not avoid the errors I wishes to receive the comments, the reviewer of the scientists as well as the people interested in the topic It can help me complete the thesis with good result Chapter Overview Chapter 2.Experiment Chapter Effected of Ca substitution for Ba on the structure and elctrical properties of BCT and BZT-BCT BZT-BCT is a material which is the largest piezoelectric property in the announced in piezoelectric material systems not contain Pb Before analyzing and investigating the cause of the piezoelectricity effect in the BZT-BCT systems Firstly we studied the BCT system (BZT system, there were many publications of the authors in the world) The structure and physical properties of the BCT system will change when Ba are substituted by Ca Does the morphological phase exist in the BCT material? And when the Ca substitution for Ba, the piezoelectric properties of the material is improved? We are going to disscus about this in the next chapters (222) (013) (031) (113) (311) (112) (121) (020) (022) (220) (112) * (012) (021) BCT30 (002) (001) (010) (111) (011) 3.1 Effected of Ca substitution for Ba on the structure of BCT and BZT-BCT For convenience in the sample analysis, we call Ba1-xCaxTiO3 is BCTx ( x = 0, 10, 12, 14, 16, 18, 20 and 30:Atomic percentage of Ca concentration) and Ba(Ti0.8Zr0.2)O3 – Ba1-yCayTiO3 system is BZT-BCTy (y = 15, 20, 25, 28, 28.8, 29.2, 29.6, 30, 30.4 and 35, Atomic percentage of Ca concentration in this system is y/2) BCT16 BCT20 BCT15.2 BCT16 BCT15.2 BCT15 BCT15 BCT14.8 BCT14.8 BCT14.6 BCT14.6 BCT14.4 BCT14 BCT14.4 BCT12 BCT14 BCT10 BCT12 BCT0 20 30 40 50 60 o 2 ( ) 70 80 90 82 84 86 Figure 3.1 X- ray diffraction pattern of BCTx samples The XRD patterns of all the samples are presented in Fig.3.1 It is easy to recognize that all the samples had the same tetragonal structure with c/a ratio close to unity but depending on the Ca concentration, changing from 1.0079 to 1.0083 as x was increased from zero to 0.16 (Table 3.1) Table 3.1 lattice spacing of samples BCT sample a b C α β γ c/a V BCT0 3,9866 3,9866 3,9866 90 90 90 63,36 BCT10 3,9877 3,9877 4,0178 90 90 90 1,00754 63,89 BCT12 3,9905 3,9905 4,0223 90 90 90 1,00796 64,05 BCT14 3,9910 3,9910 4,0239 90 90 90 1,00824 64,09 BCT14.4 3,9914 3,9914 4,0244 90 90 90 1,00826 64,11 BCT14.6 3,9917 3,9917 4,0248 90 90 90 1,00829 64,12 BCT14.8 3,9919 3,9919 4,0252 90 90 90 1,00834 64,14 3,9915 3,9915 4,0248 99 90 90 1,00834 64,12 BCT15.2 3,9897 3,9897 4,0232 99 90 90 1,00839 64,04 BCT16 3,9869 3,9869 4,0226 90 90 90 1,00859 63,91 BCT18 3,9860 3,9860 4,0212 90 90 90 1,00883 63,79 BCT20 3,9852 3,9852 4,0211 90 90 90 1,00901 63,66 BCT30 3,9651 3,9651 4,0021 90 90 90 1,00932 62,92 BCT15 Tetragonal symmetry was also identified from HRTEM images for the BCT14 sample, as presented in Fig 3.2a, which clearly shows parallel lattice faces with tetragonal structure having c/a ratio close to unity (supercubic structure) This result is consistent with the XRD analysis As shown in our previous report, Ca successfully substituted for Ba and induced a shift of the (222) diffraction peak toward higher angle (as shown inthe inset) This shift is due to the smaller ionic radius of Ca2+ (0.134 nm) compared with Ba2+ (0.161 nm) It is known that, at room temperature, BTO crystallizes in tetragonal structure and its (222) diffraction peak should be single In our case, the (222) diffraction line of the sample doped With x= 0.14 of Ca started to split into two peaks, indicating that this sample contained material with two structural symmetries Probably, both tetragonal and orthorhombic structures coexist due to the grain-size effect, as also reported by other authors for BTO materials with average grain size in theregion of 0.1µm to 1.0µm Karaki et al also observed the orthorhombic–tetragonal transition at a temperature TOT of around 24C for BTO with grain size of micrometers This may be evidence of the existence of a MPB in this ceramic compound Using the commercial Rietveld program X‟PertHighScore Plus, we fit the XRD data and estimated the contribution of tetragonal and orthorhombic phases in the samples The fitting results showed that tetragonal and orthorhombic phases coexisted at ratio of 93/7 in sample BCT14 On increasing x to 0.14, the (222) peak splitting increased, becoming triple with three small peaks for x= 0.148 For the samples doped with x higher than 0.148 (samples BCT15, BCT15.2, and BCT16) the (222) peak broadened, forming a wide single peak when x reached 0.16 This could be due to overlapping of the (222) peaks of BaTiO3 and CaTiO3 that started to coexist in these samples, as seen in their XRD patterns Such coexistence can also be seen in the HRTEM image with clear parallel lattice faces for BCT16 (Fig 3.2b) The fast Fourier transform (FFT) for this material region exhibits three diffraction points arranged in a linear line This suggests that, in this sample, there exists a region where the material phases are nested similar to a superlattice These lected-area diffraction (SAED) image (Fig 3.2c) shows ordered repetition of the diffraction points of the (220) face of the tetragonal crystal lattice with a= 3.9975 A˚ and c= 4.0094 A˚ The diffraction points appeared to be repeated periodically as for a superlattice The separation between lattice faces as estimated directly from the HRTEM images was about 2.6 A˚ to 2.7 A˚, in good agreement with the XRD analysis Figure 3.2 HRTEM images Figure 3.3 X- ray diffraction of sample systems BZT-BCT From Xray diffraction of sample systems: It is found that when Ca concentration is less than 14,8 % atoms (the Ba: Ca ratio is 85.2: 14.8 corresponding to the y = 29,6) The sample systems are single phase When the y concentration (322) (310) (311) (220) (221) (211) (212) (210) (002) (200) (100) (111) (110) is more than 30, the new spectral peak of the CaTiO3 component appears on the Xray diffraction (this result is quite siutable with the BCTx material systems) BZT-BCT35 BZT-BCT30.4 BZT-BCT30 BZT-BCT29.6 BZT-BCT29.2 BZT-BCT28.8 BZT-BCT28 BZT-BCT25 BZT-BCT20 BZT-BCT15 20 30 40 50 o 60 70 80 90 100 44.4 45.6 2 ) Figure 3.3 Xray diffration pattern of sample systems of BZT-BCT It is clear that diffraction peaks tend to shift toward 2θ when the concentration of Ca increases and some diffraction peaks tend to split vertices Particularly, we see that the diffraction peaks at 2θ= 44,70 It separates the peak when the concentration of Ca increases and when the concentration of 14.8% of the atoms (y = 29.6), it was split into three distinct vertices (These vertices can correspond to two different types of structures: the tetragonal and the irhombohedral) However, when the concentration of y is more than 30, it tends to incorporate into two vertices corresponding to the tetragonal structure The particularity in this structure may be the reason for the highest piezoelectric coefficient, at y = 29.6 which will be explored in detail later When the y component is still small (less than 29.2), the material has a irhombohedral structure characteristic of BZT, whereas when the y component is higher, the material has a tetragonal structure characteristic of BCT At y=29.6, two types of tetragonal and irhombohedral are exist This assertion is confirmed by the separation of the special diffraction peaks corresponding to at 2θ= 44,70 and the Gaussian fitting of the components around y = 29.6 BCT-BZT30.4 BCT-BZT30 BCT-BZT29.6 BCT-BZT28.8 BCT-BZT28 44 44.5 45 45.5 46 2 ) Figure 3.4 XRD in the 44o-46o area of the samples is fitted to the Gaussian function From the result shown in Figure 3.4: at y = 29.6, the material exits two phase: tetragonal (corresponding to (002)T , (200)T at 45,11o and 45,36o) and irhombohedral phase (peak (200)R at 45,21o) According to W Wersing, W Heywang et al., The proportion of tetragonal components is determined by: FT = I T200 + I T002 I T200 + I R200 + I T002 , (1) where:I T200 ,I T002 , I R200 are the intensity of the diffraction peaks at (200), (002) corresponding to the tetragonal and irhombohedral respectively In the case of BZT-BCT material system for y = 29.6, we calculated the tetragonal and irhombohedral ratio to be around 69% This result also shows that the formation of the morphological boundary with the surrounding components y = 29.6% 3.2 Effected of Ca substitution for Ba on AC conductivity of BCT and BZT-BCT As known BTO is an isolate ferroelectric material as oxygen deficiency in material is small In this case the localized reorientation is a main contribution in AC conduction of BTO For analysis the mechanism of AC conduction we applied the power law equation (Eq 3.2) to fit the conduction data of the BCTx samples.The experimental data of conductivity of the BCT samples and fitting curves are presented in Fig 3.5 This fit provides a very good description of the data in the whole measured frequency and the obtained parameters are presented in table As displayed in Fig 3.6 the AC conductivity of the samples decreases with increasing the Ca concentration It may be related with a pinning effect of electrical dipoles in material that induced a depression of dielectric relaxation time as Ca concentration lower than 16 at% way we have analyzed the frequency dependence of AC conductivity of the BZT-BCTy samples As wellknown the change of AC conductivity in different frequency ranges can be attributed to different contributions of migration of charges (charges jump and/or ion migration) and polarization (ion or atomic) in low and high frequency region, respectively Basically δac in whole frequency range can be analyzed by using UDR model as presented in Eq = dc + ac = dc + os (3) s n = dc + ac = dc + o + 1 (4) The dependence of the AC conductivity on frequency of the BZT-BCTx samples is shown in Fig 3.6 Using Jonscher UDR model the experimental data were fitted and the fitting results are presented in Fig 3.6 and table It can be seen from Fig 3.6 that the UDR model fitted well the experimental data of the BZT-BCTy samples The δ0 is three orders larger than δ1, and thefrequency exponent s changed in a range of 0.6 – 0.85 confirm that the short range single polaron hoping is dominated in the low frequency range in the BZT-BCTy samples This may be related with lattice defects and/or oxygen vacancies formed in material samples due to the large number of constituents in BZT-BCTy The hopping of charge carriers over a potential barrier between charged defects is applicable for analyzing of δac in the low frequency range , and the relation between the frequency exponent s and the potential barrier can be expressed by the relation: s = 4kBT/Ws (4) where Ws is the maximum barrier height According to equation (4) the barrier height of all the BZT-BCTx samples were estimated and presented in table It is clear that the barrier height changed abnormally, has a minimal value at the sample BZT-BCT28 that has maximal piezoelectric parameter d31and d33 corresponding the Ca concentration in the Morphology Phase boundary (MPB) region In the high frequency range the localized polarization dominated with the frequency exponent larger than 1.5 This is suggested to be related with the polarization of dipole and/or atomic polarization in material samples 3.3 Effected of Ca concentration on dieclectric properties of materials 3.3.1 Effected of Ca concentration on dieclectric properties of BCT materials Figure 3.7 is the dependence of the dielectric constant on the temperature at kHz of the BCT model We see that the dielectric constant increases with temperature and increases rapidly in the vicinity of the 4.5 10 4 10 3.5 10 10 2.5 10 10 1.5 10 10 BCT0 BCT10 BCT12 BCT14 BCT16 BCT18 BCT20 5000 40 60 3.5 10 10 2.5 10 ' ' Curie TC phase - transition temperature Dielectric constant increases with temperature proving that surface polarization increases in the BCT material 80 100 120 140 10 1.5 10 10 5000 40 BCT14 BCT14.4 BCT14.8 BCT15.2 BCT16 60 80 100 120 140 o o T ( C) T ( C) Figure 3.7 Real part of dielectric constant depends on temperature of BCT samples It is clear that at x = or concentration of Ca2+ substitution for Ba2+ is low, the peak of transition phase ferroelectric – paraelectricity is sharp Then the Tc phase transition temperature follows Curie-Weiss's law: ' = C/(T - TC) (5) When the concentration of Ca increases, the phase shift peak is no longer sharp, they gradually blur and the peak expands Then the phase transition is spread over a temperature range and the dielectric constant reaches the maximum at Tm In this case, when matching the Curie-Weiss law above is not appropriate, we must use the Curie-Weiss law of extension 1 T Tm max C' or 1 log max (6) logT Tm log C ' (7) where, C‟: Curie – Weiss constant extension, : is the coefficient representing the level blur of the phase transition (1 2) The change in temperature Tm and the maximum permittivity constant ‟max by component in the sample groups are list in table 3.2 The relatively large dielectric constant values of the samples initially met some of the requirements of the power material in practical applications Table Tc, Tm temperature maximum dielectric constant of samples Samples BCT0 BCT10 BCT12 BCT14 BCT14.4 BCT14.6 BCT14.8 BCT15 BCT15.2 BCT16 BCT18 BCT20 Tc 118 114 113 113 113 112 112 112 112 112 111 111 Tm 118 114 112 110 110 110 109 109 109 109 108 106 ε'max 10221 19665 25667 30767 31583 31943 32400 32543 32944 34556 38110 42556 3.3.2 Effected of Ca concentration on dieclectric properties of BZT-BCT materials To understand the change structure phase depends on temperature of BZT-BCT systems We measured temperature-dependent dielectric spectra with different Ca concentrations (Fig 3.8) In case of BZT, it is not easy to distinguish three phase transformations for BZT-BCT systems We use the peak of the dielectric constant at different temperatures to determine the transition temperature of O-T and T-C structures (Fig 3.8) For samples with a substitution of less than 30% Ca, a polymorphism phase transition occurs, where the peak of the dielectric constant near the room temperature is the phase transition from the orthorhombic to the tetragonal At Ca> 30% concentration no more polymorphism phase transition occurs We cannot calculate the phase-transition temperature structure from the orthorhombic to the tetragonal This may be due to the ferroelectric phase competition of the BZT-BCT material system and the dielectric phase CTO which is generated at Ca> 30% concentration This result further demonstrates why the highest piezoelectricity value at 29.8% Ca doped Because piezoelectricity properties is most often expressed at the morphological boundary 2.5 10 4 1.5 10 10 10 BZT-BCT15 BZT-BCT20 BZT-BCT25 BZT-BCT28 BZT-BCT28.8 BZT-BCT29.2 BZT-BCT29.6 BZT-BCT30 BZT-BCT30.4 BZT-BCT35 5000 30 40 50 60 70 80 90 100 t ( C) Figure 3.8 Depending on the dielectric constant according to the temperature of the BZT-BCT samples 3.4 Effected of Ca substitution for Ba ferroelectric properties of BCT and BZT-BCT First of all, we consider the effect of substituting Ca for Ba on the ferroelectric properties of the material system We have measured the hysteresis electric curve by using the Sawyer-Tower (S-T) method for samples with different Ca concentrations to investigate the effect of substitution Ca on the ferromagnetic property of the material 20 15 P (C/cm2) 10 -5 20 15 BCT10 10 -5 -5 -10 -10 -15 -15 -15 -20 -30 -20 -10 10 20 30 E (kV/cm) -20 -30 -20 -10 10 20 E (kV/cm) 20 20 15 15 -5 -20 -30 -20 -10 10 20 30 E (kV/cm) 30 20 15 BCT14.4 10 -5 P (C/cm2) BCT14 P (C/cm2) P (C/cm2) -10 10 BCT12 10 BCT0 P (C/cm ) 15 P (C/cm ) 20 BCT14.6 10 -5 -10 -10 -10 -15 -15 -15 -20 -30 -20 -10 10 20 30 E (kV/cm) -20 -30 -20 -10 10 20 30 E (kV/cm) -20 -30 -20 -10 10 20 30 E (kV/cm) 20 20 BCT14.8 15 P (C/cm2) P (C/cm2) 10 -5 20 BCT15.2 15 BCT15 10 P (C/cm2) 15 -5 10 -5 -10 -10 -10 -15 -15 -15 -20 -30 -20 -10 10 20 30 E (kV/cm) -20 -30 -20 -10 10 20 30 E (kV/cm) -20 -30 -20 -10 10 20 30 E (kV/cm) 15 12 10 10 -5 BCT20 0 -5 -4 -10 -8 -10 -15 -20 -30 -20 -10 10 20 30 E (kV/cm) -15 -30 -20 -10 10 20 30 E (kV/cm) BCT30 BCT16 P (C/cm2) P (C/cm2) 15 P (C/cm ) 20 -12 -30 -20 -10 10 20 30 E (kV/cm) Fig 3.9 Hysteresis electric loop of BCT 16 16 BZT-BCT15 12 -4 -4 -8 -8 -12 -12 -16 -10 -5 E (kV/cm) -16 -10 10 12 BZT-BCT25 15 P (C/cm2) -4 E (kV/cm) 10 -5 -8 -10 -15 -20 -10 10 P (C/cm2) 20 BZT-BCT28.8 15 10 -5 -10 -15 -20 -15 -10 -5 E (kV/cm) 20 15 10 -5 -10 -15 -20 10 15 -5 E (kV/cm) 20 BZT-BCT29.2 P (C/cm2) E (kV/cm) P (C/cm ) -5 BZT-BCT28 10 -12 -16 -10 -5 20 16 P (C/cm ) BZT-BCT20 P (C/cm ) P (C/cm ) 12 -15 -10 -5 E (kV/cm) 10 BZT-BCT29.6 10 -10 -20 10 15 -15 -10 -5 E (kV/cm) 10 15 15 15 BZT-BCT30.4 P (C/cm2) P (C/cm2) -5 -10 -5 -10 -15 10 15 BZT-BCT35 10 10 P (C/cm2) 20 BZT-BCT30 15 10 -5 -10 -15 -20 -15 -10 -5 E (kV/cm) -10 -5 E (kV/cm) -15 -10 10 -5 E (kV/cm) 10 Fig 3.10 Hysteresis electric loop of BCT-BZT Fig 3.9 and 3.10 show the hysteresis electric loops for all samples of the BCT and BZT-BCT materials Fig 3.9 and 3.10 show the hysteresis electric loop for all samples of the BCT and BZT-BCT materials Based on the hysteresis electric loop, we can clearly see the effect of Ca on ferroelectric properties of both BCT and BZT-BCT materials For the BCT system, in the Ca concentration less than 14.8%, when the remanence power of the samples increase, the coercive electric force (Ec) decreases from 1.94 kV / cm to 1.66 kV / cm for the sample has a concentration of Ca 10% atoms and 1.19 kV / cm for samples with a concentration of 14% atoms This suggests that the material was softened by substituting Ca for Ba in this concentration range When the Ca concentration is greater than 14.8% of the atoms, the material is hardened, the coercive electric force increases and it is proportional to the concentration of Ca doped (Ec = 2.35 kV / cm, 6.86 kV / cm, 9.32 kV / cm, corresponding to x = 16%, 20% and 30%) The graph of Ec depends on the concentration of Ca shown in Fig 3.11 10 7.5 E P C r P C 1.6 r c E (kV/cm) 1.2 7.8 7.6 0.8 12 16 x (%) 20 24 28 5.5 32 0.6 7.4 15 20 25 y (%) 30 35 7.2 Fig 3.11 The dependence of Ec, Pr on the x, y component of the BCT and BZT-BCT system respectively 2 8.2 r 6.5 1.4 P (?C/cm ) r 8.6 8.4 P (?C/cm ) c 8.8 E E (kV/cm) 1.8 The same phenomenon occurs with the BZT-BCT material system The obtained values of Ec are relatively small which suggests that the material exhibits soft ferroelectric properties The electric remanence power Pr and the coercive electric force Ec are inversely proportional to the concentration of Ca The Ca / Ba ratio increases, the coercive electric force Ec initially decreases to the minimum value (corresponding to x = 29.6%), then it increases again The remanence power Pr value increases to the maximum value ( with x = 29.6%) but then it decrease Chapter Relationship between structure, dielectric recovery time and piezoelectric To understand the relation between the relaxation time and piezoelectric parameters, we carried out impedance versus frequency measurements on disk-shaped samples before and after electrical polarization It is generally accepted that the permittivity does not depend on the measurement method and should be the same in measurements on disk- versus cylinder-shaped samples In „„Experimental Procedures‟‟ section, it was shown that the impedance depends linearly on the S/d ratio Inour case, the S/d ratio was evaluated to be about 7.98 cm and 0.118 cm for the disk- and cylinder shaped samples, respectively Therefore the S/d ratio of the disk-shaped samples is much larger than that of the cylinder-shaped samples, so the capacitance data obtained from disk-shaped samples are larger and more precise This is the main reason why we used only the permittivity data obtained from disk-shaped samples to estimate there laxation time in this work The dielectric relaxation time was estimated by fitting the frequency dependence of the real and imaginary parts of the dielectric permittivity measured for samples, using the following modified Debye formula: ε*= ε∞ + (εs - ε∞)/[1 + (jωη)1-β] (6) with real and imaginary parts ε‟= ε∞ + (εs - ε∞)/[1 + (ωη)2(1-β)] (7) ‟‟ 1-β 2(1-β) ε = (εs - ε∞)(jωη) /[1 + (ωη) ] (8) * where ε is the complex dielectric permittivity, εs and ε∞ are the static and high frequency dielectric permittivity, respectively, η is the relaxation time and1 >β > is an empirical parameter concerning with the distribution function of the relaxation time, which was accepted first time by K.S Cole and R.H Cole 900 60 BCT0 BCT10 BCT14 BCT16 850 800 BCT18 BCT20 BCT30 40 BCT18 BCT20 BCT30 '' 750 ' BCT0 BCT10 BCT14 BCT16 50 700 30 650 20 600 550 500 10 500 1000 1500 2000 2500 f (kHz) 500 1000 1500 2000 2500 f(kHz) Figure 4.1 The dielectric constant dependence on frequency and matching lines Fig.4.1 exhibits a good fit for the real part of dielectric permittivity in dependence of frequency from Hz to 2.5 MHz by using the equation (6) From the fitting parameters we have estimated relaxation time values for all the sample material The dependence of the estimated relaxation time on Ca doped concentration can be seen in Fig 4.1 It is clearly seen that the dielectric relaxation time decrease to a minimal value of 1,8021.10-6 s as Ca doped concentration increases to14,8 at% After that it increases in dependence of the Ca concentration The decrease of the dielectric relaxation time as the Ca doped concentration increased up to 14,8 at% may be concerned with that the ion radius of Ca2+ion is smaller than that of Ba2+ ion However this explanation induces a question that why the recorded value of dielectric relaxation time at the critical Ca concentration of 14 at% is minimal? Turning back to X-ray diffraction we have known that the substitution of Ca for Ba induces crystalline lattice deformation in structure of BTO and this deformation process enlarged when Ca doped concentration increased to a critical concentration so that induceda change in structure This crystalline deformation reasonably is a main reason to create a morphology phase competition and to pin the dielectric dipole state so that to prolong the dielectric relaxation process as well as to improve piezoelectric property of material This obtained result isan experimental evidence to believe that the material is a promising candidate for manufacturing the lead-free piezoelectric material having high piezoelectric constant t (s) 10 -5 1.6 10 -5 1.2 10 -5 10 -6 10 -6 10 15 20 25 30 x (%) Figure 4.2 The dependence of dielectric recovery time and concentration of Ca doping 4 1.2 10 10 ' 1.4 10 9500 BZT-BCT15 BZT-BCT20 BZT-BCT25 BZT-BCT28 BZT-BCT30 BZT-BCT35 BZT-BCT28 BZT-BCT28.8 BZT-BCT29.2 BZT-BCT29.6 BZT-BCT30 BZT-BCT30.4 9000 8500 8000 7500 ' 1.6 10 7000 6500 8000 6000 6000 5500 4000 500 1000 f (kHz) 1500 2000 500 1000 f (kHz) 1500 2000 Figure 4.3 The dependence of the real capacitance part on the frequency and the matching line 0.009 0.0085 (s) 0.008 0.0075 0.007 0.0065 0.006 15 20 25 30 35 x (%) Figure 4.4 The dielectric recovery time depends on y concentration 4.2 Effecting of Ca concentration the substitution for Ba on the piezoelectricity of the BCT and BZT-BCT material system 4.2.1 Effecting of Ca concentration the substitution for Ba on the piezoelectricity of the BCT material system Fig 4.5 is the dependence of the piezoelectric parameters and Qm qualities on Ca concentration the substitution for Ba When Ca concentration changes, the electromechanical coefficient k P also varies At first, the electromechanical coefficient increases dependence on x and the maximum value of x = 14.8% Then it decreases as x increases At x = 14.8%, the electromechanical coefficient achieved the maximum value We consider this to be due to a strong structural change as discussed in chapter three The structural symmetry of the material decreases, leading to a change in the mechanical and chemical properties of the material The variation of the electromechanical coefficient k31 is proportional to kp That is clearly expressed by the expression: 1 E k p k 31 Meanwhile, the coefficient d31, d33 is determined by the formula: d 31 k31 33T s11E T E d 33 k33 33 s33 The coefficient d33 has a sharp increase from 128 pC / N (corresponding to x = 0%) to 321 pC / N (corresponding to x = 14.8%) It is even comparable to lead materials that have been widely adopted in life and science This is shown in Tab 4.2 D33 attained the maximum value at x = 14.8% and it was further demonstrated that at this concentration there appeared morphological boundary between the phases of structure where we observed the interference peak in the X-ray diffraction pattern or minimum value of dielectric recovery time 12 350 d 250 g 33 33 0 10 x (%) 15 20 0.6 10 x (%) 15 20 15 20 120 k 0.5 k 0.4 k Q p m 100 31 33 0.3 80 Q 31 m 33 50 p 33 31 g ,g 31 d ,d 150 100 k ,k ,k 31 -3 200 g 10 31 (10 Vm/N) d 33 (pC/N) 300 0.2 60 0.1 40 -0.1 10 x (%) 15 20 10 x (%) Hình 4.5 The piezoelectric coefficients and qualities depend on the concentration x The value of d33 is quite large, which can be a very simple premise for lead free piezoelectric material that has good piezoelectric properties for practical application in place of conventional lead materials Conversely, when x = 14.8% of the piezoelectric coefficients are large but the quality of Qm is the minimum This is also a weakness of the sample model Small Qm leads to aging-sensitive materials which lose their piezoelectricity, making it difficult to put into practical applications Therefore, it is necessary to improve the piezoelectricity properties but the quality of Qm must be large enough for mechanical strength, heat as well as other peripheral effects 4.2.2 Effecting of Ca concentration the substitution for Ba on the piezoelectricity of the BZT-BCT material system The piezoelectric coefficients and Qm qualities of the BZT-BCT material system are the same as those of the BCT system (Fig 4.6) The electromechanical coefficients and the piezoelectric coefficients were increased by Ca concentration in place of Ba and it reached the maximum with respect to BZT-BCT 29.6 then decreased sharply despite the concentration of Ca continue to increase The maximum piezoelectric value is given for the sample BZTBCT29.6 because at this concentration there is competition between the two ferroelectric phases The boundary of the transition between two ferroelectric phases is called morphological boundary phase (MPB) The transition between the two ferroelectric phases at the morphological boundary phase results in the instability of the polarization state They can also be easily reversed in the direction of mechanical or electric field forces This results in a piezoelectric material and a high dielectric constant Although the mechanism of the piezoelectric effect at MPB is discussed primarily on the basis of lead piezoelectric materials The piezoelectric coefficient d33 is the maximum value for the component BZT-BCT29.6 We think that MPB appears in the same material as in PZT and PMN-PT This is demonstrated by analyzing the xray diffraction pattern in Chapter 3, where y = 29.6% of the diffraction peak (angle 2θ = 44.70) 600 11 500 (pC/N) d 10 g31 g33 31 (10 Vm/N) d 33 33 300 31 g ,g 31 d ,d 33 -3 400 200 100 15 20 25 30 35 15 20 0.7 35 200 k 0.6 k k p 190 31 180 33 170 m 33 0.5 Q 0.4 p 31 30 y (%) y (%) k ,k ,k 25 Q m 160 150 0.3 140 0.2 0.1 130 15 20 25 30 120 35 15 20 y (%) 25 30 35 y (%) Fig 4.6 The piezoelectric coefficients and qualities depend on the concentration y 4.3 Relationship between structure, dielectric recovery time and piezoelectric 1.6 10 -5 BCT15.2 (s) BCT15 10 -5 BCT14.8 BCT14.6 10 -6 10 -6 BCT12 82 84 86 10 -6 250 200 150 BCT14.4 BCT14 300 33 100 50 10 x (%) 15 20 (pC/N) -5 d 33 1.2 10 (s) 31 -5 BCT16 31 d ,d 1.4 10 350 d 0.009 BZT-BCT35 600 (s) 0.0085 d 31 d BZT-BCT30.4 500 33 0.0075 300 0.007 BZT-BCT28.8 BZT-BCT28 (pC/N) (s) 33 BZT-BCT29.2 31 400 BZT-BCT29.6 d ,d 0.008 BZT-BCT30 200 0.0065 BZT-BCT20 44 BZT-BCT15 44.5 0.006 45 45.5 46 15 20 25 30 35 100 y (%) Hình 4.7 Relationship between structure, dielectric recovery time and piezoelectric As a result of the analysis, both BCT and BZT-BCT have the highest piezoelectric coefficient corresponding to the minimum dielectric recovery time At these values, the structure of both systems is strongly variable In particular, the BZT-BCT system also observed the morphological boundary If this is a common feature for all sample systems, then there is an extra simple way to recognize whether the material is piezoelectric We just measure the dependence of the dielectric constant on the frequency and then calculate the dielectric recovery time Later, it can be deduced whether the material is piezoelectric or not Instead of expensive and complex methods to determine the structure and voltage polarization when measuring piezoelectricity, this method is simple, effective, easy and cheap Conclusion Successfully fabricated BCT and BZT-BCT materials having high quality, good enough to be used for deep studying on dielectric, piezo-electric properties of material Realized the changing in crystalline structure when Ba was substituted by Ca (based on X-ray Diffraction patterns and HRTEM images) The changing is described below: + For BCT composite: - With Ca doping concentration smaller than 14.8%, materials had a single crystalline phase of the tetragonal With Ca doing concentration higher than 14,8% there was a new crystalline phase of CaTiO3 existing HRTEM data suggested that there could be an existence of supper lattice in this material - Critical substitute concentration of Ca for Ba is 14.8 at.% + For BZT-BCT composite: - Ca doping concentration had a strong effect on crystalline structure of material In BZT-BCT29.6 composite, a boundary between two grains with different crystalline structure of tetragonal and orthohombic was detected Similar to BCT, critical concentration of Ca substituting for Ba in BZT-BCT is about 14,8 at.% - Effects of Ca doping on properties of BTC and BZT-BCT are listed below : Dielectric relaxation time of BCT and BZT-BCT is nonmonotonically depended on Ca concentration which substituted for Ba Dielectric relaxation time reduced to a minimum of 4.80.10-6 s (in BCT) and 6.43 10-3s (in BZTBCT) with Ca doping concentration of 14,8%, and then it increased for higher Ca doping concentration High piezo-electric constant was obtained in BCT and BZTBCT materials that have smallest dielectric relaxation time, corresponding to Ca concentration of 14,8%, (d 33 in BCT14.8 was 321pC/N; in BZT-BCT29.6 was 543pC/N) These materials with high piezo-electric constant can be applied to improve the performance of ultrasonic devices There could be a relationship between crystalline structure, relaxation time and piezo-electric constant This relation can be a consequence of a competition between different crystallographic phases near the critical point in ferro-electric and piezo-electric materials Based on these results, we proposed a new method to quickly evaluate the piezo-electric property without crystalline structure analysis by using expensive methods such as XRD or HRTEM ... coefficient and the dielectric recovery time of the object piezoelectric This is why we choose this thesis “Fabrication of ferroelectric materials not contain Pb on BaTiO3 substrate and study their electricity. .. piezoelectric not contain Pb, also open application capacibility of these material systems in fabrication of pin, senso… contributory on the environment reduction The main contents of my thesis... traditional PZT materials Some piezoelectric materials not contain Pb have recently been publish and have shown good results Special, material systems not contain Pb on (K,Na)NbO3 and BaTiO3