Summary of doctoral in materials science: Fabrications of ferroelectric materials do not contain Pb on BaTiO3 substrate and study their electricity and piezoelectricity properties

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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.

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Nội dung Text: Summary of doctoral in materials science: Fabrications of ferroelectric materials do not contain Pb on BaTiO3 substrate and study their electricity and piezoelectricity properties

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 do not contain Pb on BaTiO3 substrate and study their electricity and piezoelectricity properties Specialized: Electronic materials Numerical code: 62.44.01.23 SUMMARY OF DOCTORAL IN MATERIALS SCIENCE Ha noi, 2018
The work is completed at: INSTITUTE OF MATERIALS SCIENCE - VIET NAM ACADEMY OF SCIENCE AND TECHNOLOGY Science supervisor: 1. PGS.TS Le Van Hong 2. 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 1. Le Van Hong, Nguyen Van Khien and Truong Van Chuong, “Dielectric Relaxation of Ba1¹xCaxTiO3 (x = 0.00.3)”, Materials Transactions, Vol. 56, No. 9 (2015) pp. 1374 to 1377. 2. 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). 3. 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 4. Nguyễn Văn Khiển, Vũ Đình Lãm và Lê Văn Hồng, “Ba1- xCaxTiO3 và tính chất điện môi của chúng”, Tạp chí Khoa học và Công nghệ 52(3C) (2014) 725-730 5. Nguyen Van Khien, Vu Dinh Lam and Le Van Hong, “Ba1- xCaxTiO3 AND THE DIELECTRIC PROPERTIES”, Communications in Physics, Vol. 24, No. 2 (2014), pp. 170-176. 6. Nguyễn Văn Khiển, Trương Văn Chương, Đặng Anh Tuấn, và Lê Văn Hồng, “Ảnh hưởng sự thay thế Ca cho Ba lên tính sắt điện của hệ Ba1-xCaxTiO3”, Hội nghị Vật lý chất rắn và Khoa học vật liệu toàn quốc lần thứ 9 - SPMS2015 7. Nguyen Van Khien and Le Van Hong, “ Effect of Ca concentration substituting for Ba on structure and ferroelectric properties of BZT- BCT material”, Vietnam Journal of Science and Technology 56 (1A) (2018) 86-92 Related articles. 8. 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: 1389- 1394. 9. Nguyễn Văn Khiển, Trịnh Phi Hiệp, Nguyễn Thị Dung và Nguyễn Văn Đăng, Nghiên cứu ảnh hưởng của biên pha nano BaTiO3 lên tính chất điện từ của vật liệu La0.7Sr0.3MnO3, Tạp chí Khoa học và Công nghệ Đại học Thái Nguyên, tập 118 số 4, 2014, trang 197- 202
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 do 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 4 chapters: Chapter 1. Theoritical overview Chapter 2. Experiment Chapter 3. 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. BZT- BCT 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 do 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 1. Overview. Chapter 2.Experiment. Chapter 3. 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 do 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. 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). (011) (121) (020) (111) (112) (010) (220) (022) (031) (311) (001) (002) (222) (013) (021) (012) (112) (113) BCT30 * 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 70 80 90 82 84 86 o 2 ( )
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 1 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 BCT15 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 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 X- ray 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
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). (110) (200) (212) (111) (002) (211) (100) (220) (310) (210) (221) (311) (322) 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 60 70 80 90 100 44.4 45.6 o 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 0 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: I T200 + I T002 FT = , (1) I T200 + I R200 + I T002 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 2. 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%.
-5 -5 1.6 10 2.5 10 BCT0 1.4 10 -5 BCT14 -5 BCT10 BCT14.4 2 10 1.2 10 -5 BCT12 BCT14.6 BCT14 1 10 -5 BCT14.8 -5 (S/m) 1.5 10 (S/m) BCT16 BCT15 -6 BCT20 8 10 BCT15.2 -5 1 10 BCT30 6 10 -6 -6 -6 4 10 5 10 -6 2 10 0 0 5 6 6 6 6 5 6 6 6 6 0 5 10 1 10 1.5 10 2 10 2.5 10 0 5 10 1 10 1.5 10 2 10 2.5 10 f (kHz) f (Hz) Figure 3.5. AC conductivity depends on the frequency of the BCTx systems 0.012 0.0025 BZT-BCT15 BZT-BCT28 0.01 BZT-BCT20 BZT-BCT28.8 0.002 BZT-BCT25 BZT-BCT29.2 0.008 BZT-BCT28 BZT-BCT29.6 BZT-BCT30 0.0015 BZT-BCT30 S/m) S/m) 0.006 BZT-BCT35 BZT-BCT30.4 0.001 0.004 0.002 0.0005 0 0 0 200 400 600 800 1000 1200 1400 0 200 400 600 800 1000 1200 1400 f (kHz) f (kHz) Figure 3.6. AC conductivity depends on the frequency of the BZT-BCT systems Increasing Ca concentration the AC conductivity continuously decreased. It is assigned for existing of the CaTiO3 phase formed in the samples with Ca concentration higher than 16 at %. From the fitting parameters one can see the BCT sample material is an isolating ceramic with very low ζdc. It suggests that no oxygen deficiency appears in the samples material. This suggestion is confirmed again from values of frequency exponent n reported in table 2. All the obtained value of n is about 1.4. It means the localized reorientation is a main contribution in AC conductivity of all the BCT samples. The term of the polaron hopping in inhomogeneous electronic microstructure was not seen. It has supposed that the BCT lattice with less of the structure deformation and oxygen vacancy was manufactured. By the same
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. 3.  = dc + ac = dc + os (3)  = dc + ac = dc + o + 1 s n (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 3. 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 4. 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 1 kHz of the BCT model. We see that the dielectric constant increases with temperature and increases rapidly in the vicinity of the
Curie TC phase - transition temperature. Dielectric constant increases with temperature proving that surface polarization increases in the BCT material. 4 4 4.5 10 3.5 10 4 10 4 BCT0 4 BCT14 3 10 BCT14.4 4 BCT10 3.5 10 BCT12 4 BCT14.8 4 2.5 10 3 10 BCT14 BCT15.2 BCT16 BCT16 ' 4 4 ' 2.5 10 2 10 4 BCT18 2 10 4 BCT20 1.5 10 4 1.5 10 4 4 1 10 1 10 5000 5000 40 60 80 100 120 140 40 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 = 0 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 1 T  Tm     (6)   max C' 1     logT  Tm   log C ' 1 or log     max  (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 5. Tc, Tm temperature và maximum dielectric constant of samples Samples Tc Tm ε'max BCT0 118 118 10221 BCT10 114 114 19665 BCT12 113 112 25667 BCT14 113 110 30767 BCT14.4 113 110 31583 BCT14.6 112 110 31943 BCT14.8 112 109 32400 BCT15 112 109 32543 BCT15.2 112 109 32944 BCT16 112 109 34556 BCT18 111 108 38110 BCT20 111 106 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
4 2.5 10 BZT-BCT15 BZT-BCT20 BZT-BCT25 4 2 10 BZT-BCT28 BZT-BCT28.8 BZT-BCT29.2 BZT-BCT29.6 4 BZT-BCT30 1.5 10 BZT-BCT30.4  BZT-BCT35 4 1 10 5000 30 40 50 60 70 80 90 100 0 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 20 20 15 BCT0 15 BCT10 15 BCT12 10 10 10 P (C/cm ) P (C/cm2) P (C/cm ) 2 2 5 5 5 0 0 0 -5 -5 -5 -10 -10 -10 -15 -15 -15 -20 -20 -20 -30 -20 -10 0 10 20 30 -30 -20 -10 0 10 20 30 -30 -20 -10 0 10 20 30 E (kV/cm) E (kV/cm) E (kV/cm) 20 20 20 15 BCT14 15 BCT14.4 15 BCT14.6 10 10 10 P (C/cm2) P (C/cm2) P (C/cm2) 5 5 5 0 0 0 -5 -5 -5 -10 -10 -10 -15 -15 -15 -20 -20 -20 -30 -20 -10 0 10 20 30 -30 -20 -10 0 10 20 30 -30 -20 -10 0 10 20 30 E (kV/cm) E (kV/cm) E (kV/cm)
20 20 20 15 BCT14.8 15 BCT15 15 BCT15.2 10 10 10 P (C/cm2) P (C/cm2) P (C/cm2) 5 5 5 0 0 0 -5 -5 -5 -10 -10 -10 -15 -15 -15 -20 -20 -20 -30 -20 -10 0 10 20 30 -30 -20 -10 0 10 20 30 -30 -20 -10 0 10 20 30 E (kV/cm) E (kV/cm) E (kV/cm) 20 15 12 15 BCT16 BCT20 10 8 BCT30 10 P (C/cm2) P (C/cm2) 5 4 P (C/cm ) 5 2 0 0 0 -5 -5 -4 -10 -10 -8 -15 -20 -15 -12 -30 -20 -10 0 10 20 30 -30 -20 -10 0 10 20 30 -30 -20 -10 0 10 20 30 E (kV/cm) E (kV/cm) E (kV/cm) Fig 3.9. Hysteresis electric loop of BCT 16 16 12 BZT-BCT15 12 BZT-BCT20 8 8 P (C/cm ) P (C/cm ) 2 2 4 4 0 0 -4 -4 -8 -8 -12 -12 -16 -16 -10 -5 0 5 10 -10 -5 0 5 10 E (kV/cm) E (kV/cm) 16 20 12 BZT-BCT25 15 BZT-BCT28 8 10 P (C/cm2) P (C/cm ) 2 4 5 0 0 -4 -5 -8 -10 -12 -15 -16 -20 -10 -5 0 5 10 -10 -5 0 5 10 E (kV/cm) E (kV/cm) 20 20 20 BZT-BCT28.8 BZT-BCT29.2 BZT-BCT29.6 15 15 10 10 10 P (C/cm2) P (C/cm ) P (C/cm2) 2 5 5 0 0 0 -5 -5 -10 -10 -10 -15 -15 -20 -20 -20 -15 -10 -5 0 5 10 15 -15 -10 -5 0 5 10 15 -15 -10 -5 0 5 10 15 E (kV/cm) E (kV/cm) E (kV/cm)
20 15 BZT-BCT30 15 BZT-BCT30.4 BZT-BCT35 15 10 10 10 P (C/cm2) P (C/cm2) P (C/cm2) 5 5 5 0 0 0 -5 -5 -5 -10 -15 -10 -10 -20 -15 -15 -15 -10 -5 0 5 10 15 -10 -5 0 5 10 -10 -5 0 5 10 E (kV/cm) E (kV/cm) E (kV/cm) 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 1.8 8.8 E P E P C r C r 8.6 1.6 8 7 8.4 1.4 8.2 P (?C/cm ) P (?C/cm ) E (kV/cm) 6 E (kV/cm) r r 6.5 1.2 8 c 4 c 2 7.8 2 1 6 7.6 2 0.8 7.4 0 5.5 0.6 7.2 0 4 8 12 16 20 24 28 32 15 20 25 30 35 x (%) y (%) Fig 3.11. The dependence of Ec, Pr on the x, y component of the BCT and BZT-BCT system respectively
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 4. 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 > 0 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 BCT18 850 BCT10 BCT20 BCT0 BCT18 50 BCT10 BCT20 800 BCT14 BCT30 BCT16 BCT14 BCT30 750 40 BCT16 '' ' 700 30 650 600 20 550 10 500 0 500 1000 1500 2000 2500 0 500 1000 1500 2000 2500 f (kHz) 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 0 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.
-5 2 10 -5 1.6 10 -5 t (s) 1.2 10 -6 8 10 -6 4 10 0 5 10 15 20 25 30 x (%) Figure 4.2. The dependence of dielectric recovery time and concentration of Ca doping 4 9500 1.6 10 BZT-BCT15 9000 BZT-BCT28 1.4 10 4 BZT-BCT20 BZT-BCT28.8 BZT-BCT25 8500 BZT-BCT28 BZT-BCT29.2 4 1.2 10 BZT-BCT30 8000 BZT-BCT29.6 BZT-BCT35 7500 BZT-BCT30 ' 4 1 10 ' 7000 BZT-BCT30.4 8000 6500 6000 6000 5500 4000 0 500 1000 1500 2000 0 500 1000 1500 2000 f (kHz) f (kHz) Figure 4.3. The dependence of the real capacitance part on the frequency and the matching line