Structural and Electrical Properties of Nanocrystalline Barium Strontium Titanate

Structural and Electrical Properties of Nanocrystalline Barium Strontium Titanate

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ScienceDirect Materials Today: Proceedings 4 (2017) 3842–3851

www.materialstoday.com/proceedings

5th International Conference of Materials Processing and Characterization (ICMPC 2016)

Structural and Electrical Properties of Nanocrystalline Barium Strontium Titanate Nisha D. Patel1, M. H. Mangrola2, Krishna G. Soni3, V. G. Joshi1* 1*, 2, 3

1 Department of Physics, Sir P.T. Science collage, Surat, India Department of Physics, Veer Narmad South Gujarat University, Surat

Abstract Barium strontium titanate, π΅π‘Žπ‘₯ π‘†π‘Ÿ1βˆ’π‘₯ π‘‡π‘–π‘œ3 (BST) exhibits paraelectric or ferroelectric properties depending upon the specific composition and temperature. Nowadays various methods are presented for synthesis of BST nano-powders with different morphology and properties that makes it suitable for special applications. We have prepared π΅π‘Žπ‘₯ π‘†π‘Ÿ1βˆ’π‘₯ π‘‡π‘–π‘œ3 (BST) using solid state reaction method with different Sr/Ba ratio of Barium carbonate (Ba𝐢𝑂3 ), strontium carbonate (Sr𝐢𝑂3 ), and titanium dioxide (Ti𝑂2 ) (x = 0.5 and 0.4). The perovskite structure has been confirmed via X-Ray Diffraction (XRD). The results showed that BST (0.5) and BST (0.4) had peaks at around 32. 2Β°, 46. 2Β° and 57. 4Β°. The crystalline size was found to be 11 to 22 nm using Scherrer’s formula. SEM results for cubic and spherical particles were obtained for both samples. Electrical properties measurements using LCR meter show that for the both BST materials and for increasing Sr content as the temperature and frequency increases capacitance decreases Β©2017 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of Conference Committee Members of 5th International Conference of Materials Processing and Characterization (ICMPC 2016). Keyword: BST nano, XRD, SEM, Dilectric.

1.

Introduction

In recent years, increasing attention has been paid to the synthesis and characterization of nano-materials because of their novel chemical and physical properties arising from the large surface–volume ratios and also the quantum size effect. Similarly barium strontium titanate [π΅π‘Žπ‘₯ π‘†π‘Ÿ1βˆ’π‘₯ π‘‡π‘–π‘œ3 (BST)] ferroelectric materials have attracted considerable attentions due to their chemical stability, high permittivity, high tunability and low dielectric losses [1,2]. The physicochemical properties of these nano-materials are highly sensitive to their size, shape, and composition [3]. It is well-known that BST has a variety of electronic applications in multilayer and voltage-tuneable capacitors, dynamic random access memories (DRAM), * Corresponding author. Tel.: 09426746135 E-mail address:[email protected]

2214-7853Β©2017 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of Conference Committee Members of 5th International Conference of Materials Processing and Characterization (ICMPC 2016).

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microwave phase shifters, tuneable filters, oscillators, un-cooled infrared sensors, etc. due to the high dielectric constant, nonlinear variation of dielectric constant with the electric field, ferroelectricity, and pyroelectric properties and so on. Ferroelectric materials have been extensively studied in thin film form, mainly for the application as multilayer ceramic capacitors (MLCCs) and the dynamic random access memories (DRAM) [4]. Among the many types of ferroelectric materials, the barium strontium titanate (BST) has been the most intensively investigated because of its high dielectric constant, low dielectric loss and good thermal stability. Moreover, the temperature range in which the ferroelectric behaviour is reflected can be easily controlled by adjusting the barium-to-strontium ratio [4]. 2. Synthesis of𝑩𝒂𝒙 π‘Ίπ’“πŸβˆ’π’™ π‘»π’Šπ’πŸ‘ Using Solid State Method The starting materials used in the synthesis of Bax Sr1βˆ’x Tio3 (BST) powders with different Sr/Ba ratio (x = 0.5 and 0.4) were Barium carbonate (BaCO3 ), strontium carbonate (SrCO3 ), titanium dioxide (TiO2 ). The chemicals were calculated and weighed, corresponding to the stoichiometrical composition of Ba0.5 Sr0.5 TiO3 and Ba0.4 Sr0.6 TiO3 . Raw materials were taken according to above calculated values and mixed well. Mixed powder then calcined at 800℃ for 10 h to remove CO2 . The heating rate was set such as 8h were given to increase the room temperature to 800℃ after that temperature was kept stable at 800℃ for 10 h and then 6 h were given to decrease the temperature from 800℃ to room temperature. Then the powder was milled for 1 h and again the heat cycle was given. This process of milling and heating was repeated for 5 times to obtain fine powder. The chemical equation leading to BST is xBaC𝑂3 + (1-x)SrC𝑂3 + Ti𝑂2 β†’ π΅π‘Žπ‘₯ π‘†π‘Ÿ1βˆ’π‘₯ π‘‡π‘–π‘œ3 + 𝐢𝑂2 . 3. Structural Analysis For the structural analysis and to study surface morphology of the prepared Barium Strontium Titanate (BST) samples, methods of X-ray diffraction (XRD) and scanning electron microscopy (SEM) have been used. The Energy dispersive x-ray spectroscopy has been used to identify the composition of prepared samples. 3.1 Energy-dispersive X-ray spectroscopy (EDS) Result In order to investigate the composition of the BST, EDX analysis was used. Figure (1, 2) below shows the EDX spectra of BST (for π΅π‘Ž0.5 π‘†π‘Ÿ0.5 Ti𝑂3 and π΅π‘Ž0.4 π‘†π‘Ÿ0.6 Ti𝑂3 respectively). Also the tables (table-1, 2) along with it show element proportion.

Fig 1. EDS spectra of BST (0.5)

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Fig 2. EDS spectra BST (0.4) Table 1. Element table for π΅π‘Ž0.5 π‘†π‘Ÿ0.5 Ti𝑂3 Element OK Ti K Sr L Ba L Total

Weight % 45.32 18.83 15.64 20.21 100.00

Atomic % 79.77 11.07 5.03 4.14

Table 2. element table for π΅π‘Ž0.4 π‘†π‘Ÿ0.6 Ti𝑂3

Element OK Ti K Sr L

Weight % 39.87 24.35 16.91

Atomic % 74.82 15.26 5.80

Ba L Total

18.87 100.00

4.12

3.2 X-RAY Diffraction analysis Room temperature (25℃) XRD analysis (BRUKER AXS, GERMANY; model: D8 advance; XRAY Tube: Tungsten Filament with Copper Target, 40kv, 3.3kw,40 mA) with step time 2 sec, step size 0.050Β° ; conducted on the samples. It was seen in literature study that for polycrystalline structure materials, sharp peaks at different angle would be produced. For non-polycrystalline structure materials or amorphous materials, broad peaks with low intensity would be produced.π΅π‘Žπ‘₯ π‘†π‘Ÿ1βˆ’π‘₯ π‘‡π‘–π‘œ3 is a continuous solid solution between BaTi𝑂3 and SrT𝑖𝑂3 over the whole concentration range.

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BST (0.5) d=2.77944

1300

1200

1100

1000

800

d=1.09138

200

d=1.18592 d=1.18080

d=1.21479

d=1.26299

d=1.33051

d=1.38682

d=1.47771

d=2.12899

d=2.43013 d=2.37480

d=1.69735

d=2.30312 d=2.26728

d=3.50742 d=3.12333 d=3.00500

d=6.38808

d=8.48843

300

d=3.45488

d=3.96414

500

400

d=1.96277

600

d=1.63144 d=1.60398

700

d=1.88732

Lin (Counts)

900

140 5

10

80

70

60

50

40

30

20

9

2-Theta - Scale BST (0.5) - File: BST.raw - Type: 2Th/Th locked - Start: 5.000 Β° - End: 90.000 Β° - Step: 0.050 Β° - Step time: 2. s - Temp.: 25 Β°C (Room) - Time Started: 16 s - 2-Theta: 5.000 Β° - Theta: 2.500 Β° - Chi: 0.00 Β° - Phi: 0.00 Β° - X: 0.0 mm - Y: Operations: Smooth 0.150 | Import

Fig.3, shows the XRD data ofπ΅π‘Ž0.5 π‘†π‘Ÿ0.5 Ti𝑂3

d=2.76869

BST (0.4) 1000

d=2.81787

900

d=3.50652

800

d=1.19610

d=1.26244

d=1.23841

d=1.41215 d=1.38707

d=1.46219

d=1.69799 d=1.66320 d=1.63209 d=1.59920

d=1.83567

d=1.98925 d=1.95551 d=1.88799

d=2.12142 d=2.08625 d=2.05219

d=2.55216 d=2.48290 d=2.45921 d=2.37393 d=2.30349 d=2.26176

d=3.97972 d=3.88867

d=8.41779

d=3.65002

400

300

d=3.45334

500

d=3.13248 d=3.05844 d=3.00939 d=2.88337

600

d=16.03329

Lin (Counts)

700

200 170 5

10

20

30

40

50

60

70

80

2-Theta - Scale BST (0.4) - File: BST - 0.4.raw - Type: 2Th/Th locked - Start: 5.000 Β° - End: 90.000 Β° - Step: 0.050 Β° - Step time: 2. s - Temp.: 25 Β°C (Room) - Time Started: 14 s - 2-Theta: 5.000 Β° - Theta: 2.500 Β° - Chi: 0.00 Β° - Phi: 0.00 Β° - X: 0.0 mm Operations: Smooth 0.150 | Smooth 0.150 | Import

Fig.4, shows the XRD data ofπ΅π‘Ž0.4 π‘†π‘Ÿ0.6 Ti𝑂3 .

9

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From above figures 3 and 4, it can be observed that samples, BST0.5 and BST0.4 had peaks at around 32. 2Β°, 46. 2Β° and 57. 4Β° and which are corresponding to the diffraction from BST (011), (002) and (112) crystal planes, respectively. That means they both are BST materials. Also, it can be observed that the strongest BST peaks for both samples were occurred at around 32Β° . These peaks are also called perovskite peaks. We had got maximum 100% intensity at around 32Β°(shown in data-table below). It is the evidence that BST0.5 and BST0.4 had perovskite structure. 3.3 Scanning Electron Microscope (SEM) Here are the data that we have got from the SEM. Figures below (fig-5and6) show the images of sample -1 i.e. π΅π‘Ž0.5 π‘†π‘Ÿ0.5 Ti𝑂3 .

Fig.5 show the SEM images of π΅π‘Ž0.5 π‘†π‘Ÿ0.5 Ti𝑂3 powder sample.

Fig.6 show the SEM images of π΅π‘Ž0.4 π‘†π‘Ÿ0.6 Ti𝑂3 powder sample.

Finally, we can estimate that average grain diameter for BST0.5 is around 200-250 nm and for BST0.4 it is around 100-150 nm. Particle shape is cubic and spherical and somehow shows tetragonal

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distortion. This result matches with the calculated crystalline size and shape that we have got from X-ray diffraction study. In the case of BST0.4 powder more particles seem to be spherical. Particles seem to be inhomogeneous in shape and size it may be due to manual milling that we had done. Actually by using proper milling instrument one can achieve more accurate result. 4. Analysis of Dielectric Properties For electrical measurements we have prepared pellets from the powder samples. The same heat cycle treatment that we had used to prepare powder sample was repeated for two pellets, also. After that silver coating was done on the both pellets and finally those prepared pellets were used for dielectric measurement. We have used LCR meter to study the electrical properties. Measurements have been conducted over wide range of frequency (1 Hz to 1MHz) and temperature (room temperature to 300°𝐢). 8.00E-11

Capacitance(Farad)

7.00E-11 6.00E-11 5.00E-11 T-25

4.00E-11

T-50

3.00E-11

T-100

2.00E-11

T-150

1.00E-11 0.00E+00 5.00E+01

2.00E+05

4.00E+05

6.00E+05

8.00E+05

Frequency(Hz)

Fig 7. Shows the capacitance (in Farad) vs frequency (HZ) for BST0.5

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4.00E-11

Capcitance (farad)

3.50E-11 3.00E-11 2.50E-11

T-20

2.00E-11

T-100

1.50E-11

T-150

1.00E-11

T-200

5.01E-12 1.00E-14 5.00E+01

2.00E+05

4.00E+05

6.00E+05

8.00E+05 Frequency (Hz)

Fig 8. graph of capacitance (in Farad) vs frequency (HZ) for BST0.6

We have studied the capacitance as a function of frequency, for various temperature ranges. The graphs of capacitance vs frequency for sample 1 and 2 (BST0.5 and BST0.6) are shown in fig 7 and 8. From the graph we can see that with increase in frequency capacitance decreases for both the samples. That is because a typical property of ferroelectric capacitors, the measured capacitance decreases with increasing frequency because of dielectric relaxation, which causes a delay in polarization decreasing the relative permittivity linearly on a logarithmic frequency scale due to the Curie-von Schweidler time-behaviour [6 to 8]. When properly measured and analyzed, resistive electrodes can also reduce the capacitance [5,9]. The capacitance decrease due to relaxation is small for BST [7].

C

10-10

sem-1

room temp.

sem-2

10-11 103

104

105

Frequency (Hz)

Fig 9. graph of capacitance (in Farad) vs frequency (in HZ)

room temp

106

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1.00E-10 Capacitance

8.00E-11 6.00E-11 4.00E-11

Capacitance

2.00E-11 0.00E+00 0

10

20

30

40

50

60

Temperature Fig 10. (sem-1) capacitance (farad) vs temperature (℃) graph for constant frequency

Capacitance

1.95E-11 1.85E-11 1.75E-11 1.65E-11 capa

1.55E-11 1.45E-11 1.35E-11 0

10

20

30

40

50

60

Temperature Fig 11. (sem-2) capacitance (farad) vs temperature (℃) graph for constant frequency

Above figure 9,10 and 11 shows the graph of capacitance vs temperature at constant frequency for sample-1(BST0.5) and sample-2(BST0.4). As we can see from the graph as the temperature increases capacitance peak is observed. One can notice that for BST0.5 sharp peak is observed and for BST0.4 obtain peak is somehow broad. As we can see from above figure.11, with increase in Sr content capacitance decreases. From graph it is clear that for frequency near about 50 Hz the curie temperature is around 30℃ for π΅π‘Ž0.5 π‘†π‘Ÿ0.5 Ti𝑂3 and 20℃for π΅π‘Ž0.4 π‘†π‘Ÿ0.6 Ti𝑂3 .So,one other result can be noted here that with increase in Sr content there is also decrement in curie temperature can be seen. The Curie temperature of BST is decreased with decreasing of the mole ratio of Ba:Sr, and the resistances are depend on the strontium substitution. The room-temperature resistivity decreases with the increasing of the π‘†π‘Ÿ +2 content, most probably due to the lower degree of crystallinity observed in the XRD analysis and the higher mobility of π‘†π‘Ÿ +2 ions, due to their smaller ionic radius. When comparing the resistance with various frequency-ranges for both samples we can find here increment in resistance with increase in strontium content. Also that increase may be partly due to the decrease in grain size and hence increase in grain boundary areas and resistance with Sr substitution. Grain boundary areas are highly resistive in oxide ceramics. Smaller grain sized ceramics has larger grain boundary areas and hence higher resistivity than bigger grain sized ceramics.

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1010

109

R

108

sem-2 room temp.

107

106 sem-1

room temp

5

10 103

104

105

106

Frequency (Hz)

Fig 12. Comparison of resistance values (in ohm)

Dielectric constant

1.00E+02 9.00E+01 8.00E+01 Series1

7.00E+01 6.00E+01 0.00E+00

2.00E+03

4.00E+03

6.00E+03

8.00E+03

1.00E+04

Frequency Fig 13.Graph of dielectric constant vs frequency (Hz)

Dielectric constant

The figure 12 and 13show graphs of dielectric constant vs frequency for different frequency ranges. The large change in dielectric constant with small change in frequency can be seen in the graph and that makes this material suitable for DRAM applications. It has been seen that the dielectric constant decreases exponentially with increase in frequency and after reaching at high frequency dielectric constant becomes constant. Moreover it was observed in XRD that BST0.5 having cubic structure which is suitable for DRAM application. 1.20E+03 1.00E+03 8.00E+02 6.00E+02 4.00E+02 2.00E+02 0.00E+00 0.00E+00

Series1

1.00E+01

2.00E+01

3.00E+01

4.00E+01

Frequency

Fig 14. Graph of dielectric constant vs frequency (Hz)

5.00E+01

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It is observed that the value of dielectric constant decreases as the frequency increases and attains a constant limiting value as shown in fig 14. This can be explained according to the behaviour of the dipoles moment, the dielectric permittivity related to free dipoles oscillating in the presence of an alternating electric field. At very low frequencies (f < 1/Ο„, Ο„ is the relaxation time), dipoles follow the electric field. As the frequency increases, dipoles begin to lag behind the field and dielectric constant slightly decreases. When the frequency reaches the characteristic frequency (f = 1/Ο„), the dielectric constant drops (relaxation process). At very high frequencies (f > 1/Ο„), dipoles can no longer follow the field and dielectric constant becomes constant [72] .The high values of dielectric constant at low frequencies can be explained as the accumulation of charges at the grain boundaries and at the interfaces. Conclusion Only important thing is that intimate mixing of the raw materials is required to produce highly homogeneous mixture in term of composition and particle size.XRD pattern of BST0.4 sample shows peak splitting at higher 2πœƒ which shows tetragonal distortion in BST. Also, the peak intensity decreases with increase in Sr content.From XRD patterns studies, it was confirmed that the crystallinity decreases with the increase of Sr content, a fact that was attributed to the difference in sizes of π΅π‘Ž+2 andπ‘†π‘Ÿ +2 .We have found some inhomogeneous particles, getting various particle sizes.The room-temperature resistance decreases with the increasing of the π‘†π‘Ÿ +2 content, most probably due to the lower degree of crystallinity observed in the XRD analysis and the higher mobility of π‘†π‘Ÿ +2 ions, due to their smaller ionic radius. The rapid change in dielectric constant with small change in frequency can be seen and that makes the material suitable for DRAM applications. It is observed that the value of dielectric constant decreases as the frequency increases and attains a constant limiting value. References [1] [2] [3] [4] [5] [6] [7] [8] [9]

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