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ISSN : 1225-0562(Print)
ISSN : 2287-7258(Online)
Korean Journal of Materials Research Vol.31 No.9 pp.488-495
DOI : https://doi.org/10.3740/MRSK.2021.31.9.488

# Single Crystal Growth Behavior in High-Density Nano-Sized Aerosol Deposited Films

Ji-Ho Lim1, Seung-Wook Kim1, Samjung Kim1, Eun-Young Kang1, Min Lyul Lee2,3, Sneha Samal4, Dae-Yong Jeong1
1Dept. of Materials Science and Engineering, Inha University, 100 Inha-ro, Michuhol-gu, Incheon, 22212 Republic of Korea
2Program in Metal·Materials Process Engineering, Inha University, 100 Inha-ro, Michuhol-gu, Incheon, 22212 Republic of Korea
3DAHA SYSTEMS, 166 LS-ro, Gunpo-si, Gyeonggi-do, 15807 Republic of Korea
4Institute of Physics of the Czech Academy of Sciences, Na Slovance 1999/2, 182 00 Praha 8, Czech Republic
Corresponding author E-Mail : dyjeong@inha.ac.kr (D.-Y. Jeong, Inha Univ.)
August 10, 2021 September 2, 2021 September 2, 2021

## Abstract

Solid state grain growth (SSCG) is a method of growing large single crystals from seed single crystals by abnormal grain growth in a small-grained matrix. During grain growth, pores are often trapped in the matrix and remain in single crystals. Aerosol deposition (AD) is a method of manufacturing films with almost full density from nano grains by causing high energy collision between substrates and ceramic powders. AD and SSCG are used to grow single crystals with few pores. BaTiO3 films are coated on (100) SrTiO3 seeds by AD. To generate grain growth, BaTiO3 films are heated to 1,300 °C and held for 10 h, and entire films are grown as single crystals. The condition of grain growth driving force is ΔGmax < ΔGc ≤ ΔGseed. On the other hand, the condition of grain growth driving force in BaTiO3 AD films heat-treated at 1,100 and 1,200 °C is ΔGc < ΔGmax, and single crystals are not grown.

## 초록

This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

## 1. Introduction

Solid state grain growth (SSCG) is a method of growing single crystals by heat treatment at temperatures lower than grain melting points. Because no liquid phase is involved in the process, single crystals of complex composition can be grown without stoichiometric changes.1,2) For example, ferroelectric single crystals with complex compositions have been grown by SSCG and utilized in piezoelectric sensors and accelerometers and for energy harvesting.3-5)

For SSCG, pre-sintering is performed at lower than normal sintering processes before single crystal growth occurs and increases adhesion between seed crystals and matrix grains. Pores trapped in pre-sintered materials remain inside single crystals after SSCG and adversely affect the physical properties of single crystals.6-8) To grow single crystals with fewer pores, several approaches have been attempted, such as applying uniaxial pressure using a hot press and thermal treatment or growing single crystals using fully dense polycrystals formed at high temperature under vacuum.9,10)

SSCG provides a means of growing single crystals by abnormal grain growth in seeded materials and of controlling grain growth behavior.7,11,12) Grain growth behavior is determined by the differences between growth driving force of other nearby grains under specific conditions. Because growth driving force is directly affected by grain diameter, it is important to control grain size in presintered materials. In the case of SSCG of BaTiO3, after forming a pre-sintered body composed of 1 μm diameter grains, abnormal grain growth is induced by seed crystals and a single crystal is grown.6,13)

AD films are produced by strong collision between powder particles and substrates at room temperature.14-18) Particles are injected into an evacuated coating chamber in a carrier gas and are accelerated to high speeds.15,18-20) These particles are fractured on collision a substrate and thermal energy is generated. As a result, high density films comprised of nanocrystals with collapsed lattices are formed.17,21)

AD films are composed of nanocrystal grains at room temperature and have dense microstructures, and can be used to grow single crystals with few pores by inducing solid-state growth. (Fig. 1) However, it is considered difficult to control grain size in AD films. In this study, a single crystal BaTiO3 film was grown using AD and subsequent SSCG. Before inducing single crystal growth, optimum heat treatment conditions were selected. The cross-sections of BaTiO3 AD films were observed by SEM, and their crystallinities were confirmed by XRD. The sizes of the crystal grains were confirmed using AFM images.

## 2. Experimental Procedure

### 2.1. Aerosol deposition process

BaTiO3 powder (> 99 % Shanghai Dianyang Industrial Co., Ltd., China) was sprayed on a (100) SrTiO3 single crystal seed (Crystal bank, Korea) substrate by AD. The powders were sprayed onto substrates through a nozzle (orifice size 1 × 5 mm) in a coating chamber maintained at 1.0 torr at a spraying distance of 5 mm. The substrate moved repeatedly (5 times) with 1 mm/s along x-axis.

### 2.2. Heat-treatment process

To select the heat-treatment condition suitable for growing single crystals, specimens were heat-treated at various temperatures for 5 h at a heating rate of 5 °C/ min. Based on considerations of average grain sizes, a temperature of 1,300 °C was selected for single crystal growth. BaTiO3 single crystals were grown by heat treating AD films on seed at 1,300 °C for 7, 10, or 50 h. To analyze the grain growth behavior of the BaTiO3 AD films under different conditions, they were heat-treated at 1,100 or 1,200 °C for 10 h.

### 2.3. Analysis process

The particle size of as supplied BaTiO3 powder and microstructures of BaTiO3 AD film images were observed using a field emission scanning electron microscope (FESEM, Hitachi S-4300SE, Japan). Cross-sectioned specimens were polishing and chemically etched with a 4M HCl/1M HF mixture for 1 minute. Samples were Pt sputter-coated to prevent electric charge accumulation on the surface. Film surfaces after heat-treatment were observed using an atomic force microscope (AFM, BRUKER Multimode IVa, America). Crystallinities of AD films after heat-treatment were determined by X-ray diffraction (Philips X'pert MRD diffractometer, Netherlands).

## 3. Results and Discussion

SEM images of supplied BaTiO3 powder and surfaces of BaTiO3 AD films are shown in Fig. 2. The particle size of supplied BaTiO3 was ~ 1 μm, but grain sizes in AD films were not clearly observed, presumably due to the consolidation caused by high energy collision during AD.22)

BaTiO3 AD films were heat-treated at various temperatures for 5 h and then observed by AFM [Fig. 3(a)]. The average grain sizes of BaTiO3 AD films heat-treated at various temperatures for 5 h were calculated using the Scherrer equation (D: grain size, k: grain size factor (~0.9), λ: X-ray wavelength, β: half width, θ: angle of the X-ray diffraction pattern).23)

(1)

The grain size of as-deposited film was 44 nm, and after heat-treatment at 600, 700, 800, 900, or 1,000 °C for 5 h grain sizes were 76, 102, 126, 137, and 143 nm. The grain size of as-deposited AD films was around 44 nm, and after heat-treatment at 1,300, 1,310, 1,320, 1,350, or 1,500 °C for 5 h grain sizes were 0.8, 2.4, 3.3, 6.5, and 9 μm, respectively. Plots of average grains size versus heat-treatments are presented in Fig. 3(b). Grains grew slowly when heat-treated at temperatures under 1,300 °C but grew rapidly at higher temperatures, especially between 1,300 and 1,350 °C. Previous SSCG was conducted in two steps, that is, sintering to increase density followed by heat- treatment to induced single crystal growth. It has been reported6,13) that grain size after sintering is around 1 μm for the SSCG of BaTiO3. Based on the above findings 1,300 or 1,310 °C were selected for growing single crystals. However, because abnormal grain growth was observed in films heat-treated at 1,310 °C, 1,300 °C was finally selected for single crystal growth in BaTiO3 AD films.

XRD results of as supplied powder and of BaTiO3 AD films heat-treated at 1,300 °C for 7 h are presented in Fig. 4. The degree of the (h00) orientation of the BaTiO3 powders, AD film heat-treated at 1,300 °C for 7 h were evaluated from the intensity of the XRD peaks [Fig. 4(a),(b)] using the Lotgering factor f which is defined by the following equations. [I: relative intensities of each reflection peak of the (hkl) planes of our XRD data, I0: relative intensities of each reflection peak of the (hkl) planes of standard XRD data, ρ: value calculated from the XRD data measured for our samples, ρ0: value calculated from the standard XRD data (JCPDS #05- 0626)]24)

$f = ρ - ρ 0 1 - ρ 0$
(2)

$ρ = Σ I ( h00 ) Σ I ( hk1 )$
(3)

$ρ 0 = Σ I 0 ( h00 ) Σ I 0 ( hk1 )$
(4)

The Lotgering factor of BaTiO3 raw powder and AD film were 0.022 and 0.134, respectively, and this increased to 0.112 after thermal treatment, which suggested BaTiO3 AD film had oriented in the seed orientation during heattreatment.

AFM images of BaTiO3 AD films heat-treated at 1,300 °C for 7 or 10 h are shown in Fig. 5(a),(b), respectively. Spherical and rectangular 10 μm sized grains were evident after treatment for 7 h [Fig. 5(a)], but no grain boundaries were observed after treatment for 10 h [Fig. 5(b)]. The Rms values of AD films heat-treated at 1,300 °C for 7 or 10 h were 284.95 nm and 114.84 nm, that is, roughness decreased as heat treatment duration increased. The XRD patterns of BaTiO3 AD films heat-treated at 1,300 °C for 10 h are shown in Fig. 5(c). Only (100), (200) XRD patterns were observed, indicating that the films were perfectly oriented. To explain the single crystal growth behavior of BaTiO3 AD films, schematic diagrams of single crystal growth process are provided in Fig. 5(d). At a temperature of 1,300 °C, BaTiO3 grains oriented as dictated by the (100) SrTiO3 seed and gradually grew to grow a single crystal. Our results can be explained as follows: BaTiO3 AD films heat-treated for 7 h at 1,300 °C were weakly oriented (100) and rectangular and spherical grains were observed on film surfaces, which suggests the BaTiO3 AD film grew as a large grain based on seed but that it did not grow perfectly enough to grow to a single crystal. Portions of single crystal were exposed on film surfaces and appeared as rectangular grains. BaTiO3 AD film heat-treated at 1,300 °C for 10 h was perfectly oriented (100), and no surface grain boundaries were observed, which indicated a perfect single crystal had been produced.

Cross-sectional SEM images of BaTiO3 AD films heattreated at 1,300 °C for 7, 10, or 50 h are shown in Fig. 6. Pore numbers and sizes in films varied with holding time, and as holding time increased, pore sizes and numbers gradually decreased. After heat-treatment at 1,300 °C for 50 h, pores were almost disappeared. After the AD process, a film which had a dense microstructure composed of collapsed lattices with oxygen defects was formed.17,19,26,27) The grains grow slowly by heat treatment, and pores are formed in the film due to the collapsed lattice.28) As the heat treatment temperature increases, the pores formed therein gradually disappear, but it is estimated that some remain as single crystals. (Fig. 7)

To analyze grain growth behaviors in BaTiO3 AD films during heat-treatment, films were heat-treated at 1,100 or 1,200 °C for 10 h, cross-sectioned, and subjected to SEM (Fig. 8). Normal 1 ~ 3 μm sized grains were observed in BaTiO3 AD films heat-treated at 1,100 °C [Fig. 8(a)]. Meanwhile, large 20 μm sized grains were observed in BaTiO3 AD films heat-treated at 1,200 °C [Fig. 8(b)]. Schematic diagrams of grain growth behavior during heat treatment are shown in Fig. 9. Normal grain growth was observed in BaTiO3 AD films heat-treated at 1,100 or 1,200 °C for 10 h [Fig. 9(a)], whereas single crystals grown in BaTiO3 AD films heat-treated at 1,300 °C for 10 h [Fig. 9(b)].

Grain growth by 2D nucleation is determined by the relative magnitude of ΔGc (critical driving force which is no grain growth and shrinkage) and ΔGmax (grain growth driving force of maximum sized grain in film). When ΔGc = 0, grain growth rate is linearly proportional to the driving force and all grains grow, and when ΔGc << ΔGmax, most of grains with ΔG > ΔGc grow in a pseudonormal manner. In the case of ΔGc ≤ ΔGmax, some grains with ΔGc ≤ ΔG grow abnormally while absorbing surrounding grains with lower ΔG values. However, when ΔGc >> ΔGmax, there is insufficient driving force for growth, and grains grow slowly. Plots of grain growth driving force versus grain growth rate of BaTiO3 AD films heat-treated at 1,100, 1,200, or 1,300 °C for 10 h are shown in Fig. 10. Normal grains in BaTiO3 AD films heat-treated at 1,100 °C and 1,200 °C. Based on these results, it appears that the grain growth driving force of BaTiO3 AD films heat-treated at 1,100 and 1,200 °C were ΔGc < ΔGmax. Abnormal grains were grown on seed surfaces with no grain growth in matrix during heat-treatment at 1300 °C, and abnormal grains grew with heat treatment duration. Based on these results, the grain growth driving force of BaTiO3 AD films heat-treated at 1,300 °C was ΔGmax < ΔGc ≤ ΔGseed.28)

BaTiO3 single crystals can be fabricated by using AD films, but due to the collapsed lattices formed during AD, pores are formed in single crystals. In this study, numbers of pores were reduced by increasing heattreatment time but were not eliminated. To solve this problem, we believe lattice relaxation must be achieved to enable single crystal growth.

## 4. Conclusion

To fabricate single crystals with few pores, a highdensity BaTiO3 film was coated on (100) SrTiO3 seeds by AD and heat-treated using various conditions. Preliminary experimentation showed heat treatment at 1,300 °C was optimal for single crystal growth. In addition, the effects of heat treatment times on BaTiO3 AD films were also investigated. As compared with the BaTiO3 powder, the (100) orientation (α(100)) of the BaTiO3 AD film heattreated for 7 h increased by 2.84 %, and heat treatment for 10 h (100) resulted in single crystal growth. The grain growth driving forces of BaTiO3 AD films heattreated at 1,100 and 1,200 °C for 10 h were considered to be ΔGc < ΔGmax, and heat-treated at 1,300 °C for 10 h was considered to be ΔGmax < ΔGc ≤ ΔGseed. On the other hand, pores were observed in all heat-treated specimens, which we suppose were formed by lattice distortions. Therefore, we consider that if the single crystal growth process is preceded by complete relaxation of the collapsed lattice, single crystals without pores could be manufactured.

## Acknowledgements

This study was supported by the National Research Foundation of Korea (Grant No. NRF-2021R1F1A1062334).

## Figure

The schematic images of conventional SSCG and SSCG of AD process.

SEM images of (a) as supplied BaTiO3 powder, (b) surface of a BaTiO3 AD film.

(a) AFM images of BaTiO3 films (as-deposited, 1,300, 1,310, 1,320, 1,350 and 1,500 °C for 5 h) and (b) plots of grain size versus heat-treatment temperature.

X-ray diffraction patterns of (a) as supplied BaTiO3 powder, and (b) BaTiO3 AD film heat-treated at 1,300 °C for 7 h.

AFM images of BaTiO3 films heat-treated at 1,300 °C for (a) 7 or (b) 10 h, (c) XRD patterns of BaTiO3 AD films heat-treated 1,300 °C for 10 h, and (d) schematic images of single crystal growth process.

SEM cross-section images of BaTiO3 films heat-treated at 1,300 °C for (a) 7, (b) 10 or (c) 50 h.

Schematic images of grain growth behaviors during various heat treatment conditions.

SEM cross-section images of BaTiO3 AD films heat-treated at (a) 1,100 or (b) 1,200 °C for 10 h.

Schematic images of grain growth behaviors of (a) poly crystal (< 1,300 °C) and (b) single crystal (1,300 °C).

Plots of grain growth rate by driving force under various heat-treatment conditions.

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