- Email: [email protected]
Highly-oriented (104) polycrystalline α-Al2O3 transparent ceramics prepared by a templated grain growth method
Accepted Manuscript Title: Highly-oriented (104) polycrystalline ␣-Al2 O3 transparent ceramics prepared by a templated grain growth method Authors: Qinghua Yang, Huanping Wang, Shuilin Chen, Long Zhang, Shiqing Xu PII: DOI: Reference:
S0955-2219(18)30699-X https://doi.org/10.1016/j.jeurceramsoc.2018.11.033 JECS 12185
To appear in:
Journal of the European Ceramic Society
Received date: Revised date: Accepted date:
28 August 2018 17 November 2018 20 November 2018
Please cite this article as: Yang Q, Wang H, Chen S, Zhang L, Xu S, Highly-oriented (104) polycrystalline ␣-Al2 O3 transparent ceramics prepared by a templated grain growth method, Journal of the European Ceramic Society (2018), https://doi.org/10.1016/j.jeurceramsoc.2018.11.033 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highly-oriented (104) polycrystalline α-Al2O3 transparent ceramics prepared by a templated grain growth method
IP T
Qinghua Yanga, Huanping Wanga, Shuilin Chenb,c, Long Zhangb, Shiqing Xua, *
College of Materials Science and Engineering, China Jiliang University, Hangzhou 310018, China
b
Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics,
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a
University of Chinese Academy of Sciences, Beijing 100049, China
Corresponding authors; Tel.: +86 571 86836061; Fax: +86 571 86836061.
M
*
A
N
c
U
Chinese Academy of Science, Shanghai, 201800, China.
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Abstract
PT
ED
E-mail addresses: [email protected] (Shiqing Xu).
Highly-oriented (104) polycrystalline α-Al2O3 transparent ceramics were prepared
A
by a templated grain growth (TGG) method using A-plane single-crystal sapphire as the template. This is the first report, to best of our knowledge, on polycrystalline αAl2O3 ceramics oriented to non-optical axes. XRD and SEM results indicate that samples with high oriented grains and high-density structure are prepared. Compared
1
to random samples, the oriented samples exhibited an increase in the real in-line transmittance (at 650 nm) from ~23% to ~62%. Moreover, the transmission remains a high level as the wavelength shifts toward the UV range (< 300 nm). This method can be extended to other uniaxial materials without optical axes in order to produce
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polycrystalline ceramics with excellent optical transparency.
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Key words: polycrystalline α-Al2O3, transparent ceramics, grain growth, oriented
1. Introduction
U
Polycrystalline α-Al2O3 transparent ceramics have been widely studied for half a
N
century [1-3], since the first polycrystalline α-Al2O3 transparent ceramics were
A
developed by Coble in the 1960s [4]. However, due to the presence of randomly
M
oriented crystal in polycrystalline α-Al2O3, birefringence is inevitable when light is
ED
transmitted through a grain boundary [5]. Therefore, it is impossible to prepare highly transparent α-Al2O3 ceramics with randomly oriented grains due to the presence of
PT
numerous grain boundaries.
CC E
Numerous efforts have been devoted to prepare polycrystalline α-Al2O3 transparent ceramics with oriented grains. Orientation techniques primarily consist of magnetic orientation [6-10] and TGG [11-14]. During the magnetic orientation, a strong
A
magnetic field is applied to assist the slip-casting green bodies, and after a suitable sintering process, α-Al2O3 ceramics with parallel optical axes (c axes, (100)) are obtained. In the process of TGG, large and anisotropically shaped templates are homogeneously aligned in a fine matrix powder during compaction. During sintering, 2
oriented grains with a high-density structure are obtained by grain growth in the anisotropic direction of the aligned templated particles. All reports on the magnetic orientation of polycrystalline α-Al2O3 ceramics with orientated optical axes were prepared using strong magnetic field (＞10T) [6-10], and the orientation process is
IP T
conducted in an alumina suspension with a large amount of organic polymers. Obviously, organic materials used in ceramic green bodies inhibit the density
SC R
sintering of ceramics. Thus, some microspores are observed in the as-prepared
ceramics. Furthermore, strong magnetic fields are harmful to human health. In the
U
process of TGG, grain orientation is carried out in the process of density sintering.
N
Therefore, transparent ceramics can be prepared with oriented grains and a high-
A
density structure.
M
Template alignment in the green compact is a critical factor in TGG method since it
ED
determines the final orientation. To the best of our knowledges, all polycrystalline αAl2O3 samples have been oriented to optical axes in previous studies on TGG
CC E
reported.
PT
techniques [11-14]. Polycrystalline α-Al2O3 oriented to non-optical axes has not been
In this work, A-plane single-crystal sapphire was used as the template, and highly-
oriented (104) polycrystalline α-Al2O3 transparent ceramics were prepared by TGG.
A
The as-prepared highly-oriented (104) polycrystalline α-Al2O3 transparent ceramics show a real in-line transmittances of ~ 62% in the visible regions (at 650 nm). This method can be extended to other uniaxial materials without optical axes to produce polycrystalline ceramics with excellent optical transparency. 3
2. Experimental Procedure High-purity powders of α-Al2O3 (99.95%, 0.25-0.45 μm, Alfa Aesar) were used as starting materials, and 500 ppm MgO (99.99%, 50 nm, Aladdin) was used as a
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sintering aid. First, 40 g α-Al2O3 and 0.02 g MgO were mixed by ball-milling. Anhydrous alcohol and alumina balls with a diameter of 5 mm were used as the
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dispersion medium and milling medium, respectively. The milling time was 12 h, and
the alcohol-ball-powder ratio was 1:6:1. The rotational speed was 200 rpm. After ballmilling, the mixtures were dried in an oven at 70 oC for 4 h and sieved using a 200-
U
mesh sieve. Then, the as-sieved powders were coated on A-plane single-crystal
A
N
sapphire (φ10×4mm, polished on both sides, Ra ≤ 5 Å, Shanghai Institute of Optics
M
and Fine Mechanics, Chinese Academy of Science), dry-pressed under 100 MPa into φ20 mm×10 mm discs and finally cold-isostatically pressed under 250 MPa to
ED
dimensions of ~ φ18 mm×9 mm. The green bodies were vacuum hot-press sintered at
PT
1770 oC and 12.5 MPa for 5 h and then annealed at 1450 oC for 20 h in air. The dimensions of the sintered samples were ~ φ16 mm×7 mm. During the sintering
CC E
process, samples adhering to the A-plane single-crystal sapphire induced oriented growth, and the dimensions of the oriented samples were ~ φ7.5 mm×1 mm. The
A
original single-crystal was removed and the oriented samples were polished to 0.8 mm for testing. Samples without A-plane single-crystal sapphire induced were prepared simultaneously for comparison. The orientation of the α-Al2O3 grains in the sintered samples was determined by Xray diffraction (XRD, D8 ADVANCE, Bruker, Germany). The XRD patterns were 4
acquired for locations in which the sample adhered to the A-plane single-crystal sapphire. The diffraction data were collected over a 2θ range of 20-80° with a scanning step of 5 °/min. Fractured samples were investigated by scanning electron microscopy (SEM, SU 8010, HITICHI, Japan). The real in-line transmittance of
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nm using a spectrophotometer (UV 3600, SHIMADZU, Japan).
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optically polished samples with a thickness of 0.8 mm was recorded for 200 nm-800
3. Results and Discussion
U
Fig. 1 shows the XRD patterns of (a) random transparent polycrystalline α-Al2O3
N
ceramics, (b) oriented transparent polycrystalline α-Al2O3 ceramics and (c) A-plane
A
single-crystal sapphire. For the random samples, all of the diffraction patterns match
M
well to that of the trigonal α-Al2O3 phase (JCPDS No. 10-0173), as shown in Fig.
ED
1(a). However, for orientation samples, only (104) peak is still strong, other peaks are weak and nonexistent. The XRD results reveal that the grains in the polycrystalline α-
PT
Al2O3 ceramics have been successfully orientated by the TGG method with A-plane
CC E
single-crystal sapphire as the template. Interestingly, the A-plane single-crystal sapphire shows oriented growth along the (110) crystal plane [15], as shown in Fig. 1(c), while the induced polycrystalline α-Al2O3 ceramics show oriented growth along
A
the (104) crystal plane. The mechanism governing this behavior will be investigated in future studies. Fig. 2 shows the real in-line transmission curves for (a) random and (b) oriented transparent polycrystalline α-Al2O3 ceramics (0.8 mm). It is clear that the 5
transmission of the oriented sample is much higher than that of the random sample. Furthermore, the transmission remains high as the wavelength shifts toward the UV range (< 300 nm), implying that the birefringence is greatly decreased [16]. For example, the transmission of the oriented sample is ~ 62% and ~ 43% at 650 nm and
IP T
300 nm, respectively. In contrast, the transmission is only ~ 23% and ~ 10% at the same wavelengths for the random sample. In the work of Mao et al [8], transparent
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polycrystalline alumina ceramics with oriented optical axes were demonstrated using
a magnetic-field-assisted slip-casting method, with a transmission (0.8 mm) of ~ 55%
U
and ~ 44% at 650 nm and 300 nm, respectively. These experimental results indicate
N
that compared to samples with oriented optical axes, transparent polycrystalline
A
alumina ceramics with oriented non-optical axes have a higher transmission.
M
The differences in transmission resulting from grain orientation are also evident in
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the macroscopic photographs shown in Fig. 3. The distance between the samples and the paper is 5 mm. The letters under the oriented sample (placed in the right) can be
PT
seen clearly, while the letters under the random sample (placed in the left) are fuzzy.
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Fig. 4 shows SEM graphs of the fracture surfaces of (a) random and (b) orientated transparent polycrystalline α-Al2O3 transparent ceramics. It can be seen that both of the samples have a high-density structure with no obvious pores. The average gain
A
size of the ceramic (D) is determined using the linear intercept method [17, 18], and the grain structure is bimodal comprising coarse grains (~ 30 μm in diameter) and small grains (~ 10 μm in diameter). The results indicate that polycrystalline α-Al2O3 transparent ceramics with oriented grains and a high-density structure are prepared 6
from TGG techniques. The orientation factor is widely used in the literature to characterize the texture degree of hexagonal α-Al2O3 and can be obtained from the XRD pattern [19]. The f factor varies between 0 and 1. A large f value implies a highly textured material; thus,
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since f = 0 for a random sample and f = 1 for a fully oriented material. Table 1 summarizes the calculated data of the orientation factor f for random and oriented
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transparent polycrystalline α-Al2O3 ceramics prepared in the current work and other published literatures [7, 8, 11, 14]. It is clear that compared to the random samples
U
both the f value and the real in-line transmission increase with orientation. Compared
N
to other TGG methods, oriented samples induced by A-plane single-crystal sapphire
A
have higher f value. And although the magnetic orientation method (f > 0.95) can
M
receive higher f value, their transmissions are lower due to low density. In the present
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work, samples with high oriented grains and high-density structure are prepared, and
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their transmissions are higher than 62% (at 650nm, 0.8 mm).
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4. Conclusion
Highly-oriented (104) polycrystalline α-Al2O3 transparent ceramics have been
A
prepared by TGG method using A-plane single-crystal sapphire as the template. XRD results reveal that grans in the polycrystalline α-Al2O3 ceramics have been orientated successfully to (104) crystal plane, and the orientation factor f is higher than 0.65. SEM results indicate that oriented samples have high dense structure with no obvious 7
pores. The as-prepared oriented and dense samples have higher optical transmission than random samples. In addition, the transmission remains high as the wavelength shifts toward the UV range (< 300 nm). This method can be extended to other uniaxial materials without optical axes to produce polycrystalline samples with excellent
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optical transparency.
Acknowledgments
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This work was financially supported by the project of the Zhejiang Provincial
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Natural Science Foundation (LY15F050005).
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References [1] R. Apetz, M. J. Bruggen. Transparent Alumina: A Light-Scattering Model. J. Am. Ceram. Soc. 86(3) (2013) 480-86. [2] M. Trunec, K. Maca, R. Chmelik. Polycrystalline alumina ceramics doped with
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nanoparticles for increased transparency. J. Eur. Ceram. Soc. 35 (2015) 10011009.
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[3] Q. H. Yang, B. X. Jiang, S. L. Chen, Y. G. Jiang, P. D. Zhang, X. J. Mao, J. Wang, L. Zhang. Influence of dopant concentration on the transparent and thermal
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properties of Nd2O3-doped alumina translucent ceramics. J. Rare Earths, 35(9)
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(2017) 883-886.
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[4] R. L. Coble. Transparent alumina and method of preparation. US Patent 3026210.
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[5] M. E. Thomas, S. K. Andersson, R. M. Sova, R. I. Joseph. Frequency and
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temperature dependence of the refractive index of sapphire. Infrared Phys. Thechn. 39 (1998) 235-49.
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[6] X. J. Mao, S. W. Wang, S. Shimai, J. K. Guo. Transparent polycrystalline alumina
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ceramics with orientated optical Axes. J. Am. Ceram. Soc. 91(10) (2008) 34313433.
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[7] H. L. Yi, X. J. Mao, G. H. Zhou, S. Chen, X. Q. Zou, S. W. Wang, S. Shimai. Crystal plane evolution of grain oriented alumina ceramics with high transparency. Ceram. Int. 38 (2012) 5557-5561. [8] X. J. Mao, S. Shimai, M. J. Dong, S. W. Wang. Gelcasting and pressureless sintering of translucent alumina ceramics. J. Am. Ceram. Soc. 91(5) (2008) 17009
1702. [9] N. Terada, H. S. Suzuki, T. S. Suzuki, H. Kitazawa, Y. Sakka, K. Kaneko, N. Metoki. Neutron diffraction texture analysis for α-Al2O3 oriented by high magnetic field and sintering. J. Phys. D: Appl. Phys. 42 (2009) 105404-8.
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[10] A. Pringuet, T. Takahashi, S. Baba, Y. Kamo, Z. Kato, K. Uematsu, S. Tanaka. Fabrication of Transparent Grain-Oriented Polycrystalline Alumina by Colloidal
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Processing. J. Am. Ceram. Soc. 99(10) (2016) 3217-3219.
[11] L. Zhang, J. Vleugels, O. V. Biest. Fabrication of textured alumina by orienting
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template particles during electrophoretic deposition. J. Eur. Ceram. Soc. 30
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(2010) 1195-1202.
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[12] K. Oishi, D. Akai, M. Ishida. Integration of crystalline orientated c-Al2O3 films
M
and complementary metal–oxide–semiconductor circuits on Si(100) substrate.
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Solid-State Electron. 103 (2015) 110-114. [13] Y. You, A. Ito, T. Goto. Highly (001)-oriented α-Al2O3 films prepared by laser
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chemical vapor deposition.
Mater. Lett. 106 (2013) 11-13.
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[14] E. Suvaci, G. L. Messing. Critical factors in the templated grain growth of textured reaction-bonded alumina. J. Am. Ceram. Soc. 83 (2000) 2041-2048.
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[15] Y. N Buzynin, O. I. Khrykin, P. A. Yunin, M. N. Drozdov, A. Y. Luk'Yanov. InN Layers Grown by MOCVD on a-Plane Al2O3. Phys. Status Solidi A 215(11) (2018) 1700919. [16] A. Krell, T. Hutzler, J. Klimke. Transparent Ceramics for Structural Applications II. Ceram. Forum Int. 84(4) (2007) 41-50. 10
[17] B. Yao, T. Sun, V. Kumar, K. Barmak, K. R. Coffey. Grain growth and void formation in dielectric-encapsulated Cu thin films. J. Mater. Res. 23(7) (2008) 2033-2039. [18] A. Makiya, S. Tanaka, D. Shoji, T. Ishikawa, N. Uchida, K. Uematsu. A
powder compacts. J. Eur. Ceram. Soc. 27(12) (2007) 3399-3406.
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quantitative evaluation method for particle orientation structure in alumina
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[19] K. Koganei, T. Oyama, M. Inada, N. Enomoto, K. Hayashi. C-axis oriented β″-
alumina ceramics with anisotropic ionic conductivity prepared by spark plasma
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M
A
N
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sintering. Solid State Ionics 267 (2014) 22-26.
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Figure captions
Fig. 1 XRD patterns of (a) random transparent polycrystalline α-Al2O3 ceramics, (b)
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oriented transparent polycrystalline α-Al2O3 ceramics and (c) A-plane single-crystal
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A
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sapphire
Fig. 2 Real in-line transmission curves of (a) random and (b) oriented transparent
A
polycrystalline α-Al2O3 ceramics (0.8 mm)
12
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Fig. 3 Macroscopic photographs of (left) random and (right) oriented transparent polycrystalline α-Al2O3 ceramics (0.8 mm) (samples are placed 5 mm above the
A
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PT
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paper)
Fig. 4 SEM results showing fracture surface of (a) random and (b) oriented 13
A
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PT
ED
M
A
N
U
SC R
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transparent polycrystalline α-Al2O3 ceramics
14
Table 1 Comparison of the orientation factor f and real in-line transmittance (at 650nm, 0.8 mm) of random and oriented transparent polycrystalline α-Al2O3 ceramics
[7]
TGG TGG Magnetic orientation Magnetic orientation
f
T (at 650 nm)
(104)
~0
~ 23%
~ 0.65
~ 62%
(001) (001)
~0 ~0
-
~ 0.49 ~ 0.60
-
(006)
~0
~ 22%
~ 0.97
~ 58%
(006)
~0
~ 18%
A
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PT
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M
A
N
U
[8]
TGG
Orientation T (at 650 f nm)
15
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This work [11] [14]
Method
Random
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Samples
Oriented plane
~ 0.95
~ 55%
S0955-2219(18)30699-X https://doi.org/10.1016/j.jeurceramsoc.2018.11.033 JECS 12185
To appear in:
Journal of the European Ceramic Society
Received date: Revised date: Accepted date:
28 August 2018 17 November 2018 20 November 2018
Please cite this article as: Yang Q, Wang H, Chen S, Zhang L, Xu S, Highly-oriented (104) polycrystalline ␣-Al2 O3 transparent ceramics prepared by a templated grain growth method, Journal of the European Ceramic Society (2018), https://doi.org/10.1016/j.jeurceramsoc.2018.11.033 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highly-oriented (104) polycrystalline α-Al2O3 transparent ceramics prepared by a templated grain growth method
IP T
Qinghua Yanga, Huanping Wanga, Shuilin Chenb,c, Long Zhangb, Shiqing Xua, *
College of Materials Science and Engineering, China Jiliang University, Hangzhou 310018, China
b
Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics,
SC R
a
University of Chinese Academy of Sciences, Beijing 100049, China
Corresponding authors; Tel.: +86 571 86836061; Fax: +86 571 86836061.
M
*
A
N
c
U
Chinese Academy of Science, Shanghai, 201800, China.
CC E
Abstract
PT
ED
E-mail addresses: [email protected] (Shiqing Xu).
Highly-oriented (104) polycrystalline α-Al2O3 transparent ceramics were prepared
A
by a templated grain growth (TGG) method using A-plane single-crystal sapphire as the template. This is the first report, to best of our knowledge, on polycrystalline αAl2O3 ceramics oriented to non-optical axes. XRD and SEM results indicate that samples with high oriented grains and high-density structure are prepared. Compared
1
to random samples, the oriented samples exhibited an increase in the real in-line transmittance (at 650 nm) from ~23% to ~62%. Moreover, the transmission remains a high level as the wavelength shifts toward the UV range (< 300 nm). This method can be extended to other uniaxial materials without optical axes in order to produce
IP T
polycrystalline ceramics with excellent optical transparency.
SC R
Key words: polycrystalline α-Al2O3, transparent ceramics, grain growth, oriented
1. Introduction
U
Polycrystalline α-Al2O3 transparent ceramics have been widely studied for half a
N
century [1-3], since the first polycrystalline α-Al2O3 transparent ceramics were
A
developed by Coble in the 1960s [4]. However, due to the presence of randomly
M
oriented crystal in polycrystalline α-Al2O3, birefringence is inevitable when light is
ED
transmitted through a grain boundary [5]. Therefore, it is impossible to prepare highly transparent α-Al2O3 ceramics with randomly oriented grains due to the presence of
PT
numerous grain boundaries.
CC E
Numerous efforts have been devoted to prepare polycrystalline α-Al2O3 transparent ceramics with oriented grains. Orientation techniques primarily consist of magnetic orientation [6-10] and TGG [11-14]. During the magnetic orientation, a strong
A
magnetic field is applied to assist the slip-casting green bodies, and after a suitable sintering process, α-Al2O3 ceramics with parallel optical axes (c axes, (100)) are obtained. In the process of TGG, large and anisotropically shaped templates are homogeneously aligned in a fine matrix powder during compaction. During sintering, 2
oriented grains with a high-density structure are obtained by grain growth in the anisotropic direction of the aligned templated particles. All reports on the magnetic orientation of polycrystalline α-Al2O3 ceramics with orientated optical axes were prepared using strong magnetic field (＞10T) [6-10], and the orientation process is
IP T
conducted in an alumina suspension with a large amount of organic polymers. Obviously, organic materials used in ceramic green bodies inhibit the density
SC R
sintering of ceramics. Thus, some microspores are observed in the as-prepared
ceramics. Furthermore, strong magnetic fields are harmful to human health. In the
U
process of TGG, grain orientation is carried out in the process of density sintering.
N
Therefore, transparent ceramics can be prepared with oriented grains and a high-
A
density structure.
M
Template alignment in the green compact is a critical factor in TGG method since it
ED
determines the final orientation. To the best of our knowledges, all polycrystalline αAl2O3 samples have been oriented to optical axes in previous studies on TGG
CC E
reported.
PT
techniques [11-14]. Polycrystalline α-Al2O3 oriented to non-optical axes has not been
In this work, A-plane single-crystal sapphire was used as the template, and highly-
oriented (104) polycrystalline α-Al2O3 transparent ceramics were prepared by TGG.
A
The as-prepared highly-oriented (104) polycrystalline α-Al2O3 transparent ceramics show a real in-line transmittances of ~ 62% in the visible regions (at 650 nm). This method can be extended to other uniaxial materials without optical axes to produce polycrystalline ceramics with excellent optical transparency. 3
2. Experimental Procedure High-purity powders of α-Al2O3 (99.95%, 0.25-0.45 μm, Alfa Aesar) were used as starting materials, and 500 ppm MgO (99.99%, 50 nm, Aladdin) was used as a
IP T
sintering aid. First, 40 g α-Al2O3 and 0.02 g MgO were mixed by ball-milling. Anhydrous alcohol and alumina balls with a diameter of 5 mm were used as the
SC R
dispersion medium and milling medium, respectively. The milling time was 12 h, and
the alcohol-ball-powder ratio was 1:6:1. The rotational speed was 200 rpm. After ballmilling, the mixtures were dried in an oven at 70 oC for 4 h and sieved using a 200-
U
mesh sieve. Then, the as-sieved powders were coated on A-plane single-crystal
A
N
sapphire (φ10×4mm, polished on both sides, Ra ≤ 5 Å, Shanghai Institute of Optics
M
and Fine Mechanics, Chinese Academy of Science), dry-pressed under 100 MPa into φ20 mm×10 mm discs and finally cold-isostatically pressed under 250 MPa to
ED
dimensions of ~ φ18 mm×9 mm. The green bodies were vacuum hot-press sintered at
PT
1770 oC and 12.5 MPa for 5 h and then annealed at 1450 oC for 20 h in air. The dimensions of the sintered samples were ~ φ16 mm×7 mm. During the sintering
CC E
process, samples adhering to the A-plane single-crystal sapphire induced oriented growth, and the dimensions of the oriented samples were ~ φ7.5 mm×1 mm. The
A
original single-crystal was removed and the oriented samples were polished to 0.8 mm for testing. Samples without A-plane single-crystal sapphire induced were prepared simultaneously for comparison. The orientation of the α-Al2O3 grains in the sintered samples was determined by Xray diffraction (XRD, D8 ADVANCE, Bruker, Germany). The XRD patterns were 4
acquired for locations in which the sample adhered to the A-plane single-crystal sapphire. The diffraction data were collected over a 2θ range of 20-80° with a scanning step of 5 °/min. Fractured samples were investigated by scanning electron microscopy (SEM, SU 8010, HITICHI, Japan). The real in-line transmittance of
SC R
nm using a spectrophotometer (UV 3600, SHIMADZU, Japan).
IP T
optically polished samples with a thickness of 0.8 mm was recorded for 200 nm-800
3. Results and Discussion
U
Fig. 1 shows the XRD patterns of (a) random transparent polycrystalline α-Al2O3
N
ceramics, (b) oriented transparent polycrystalline α-Al2O3 ceramics and (c) A-plane
A
single-crystal sapphire. For the random samples, all of the diffraction patterns match
M
well to that of the trigonal α-Al2O3 phase (JCPDS No. 10-0173), as shown in Fig.
ED
1(a). However, for orientation samples, only (104) peak is still strong, other peaks are weak and nonexistent. The XRD results reveal that the grains in the polycrystalline α-
PT
Al2O3 ceramics have been successfully orientated by the TGG method with A-plane
CC E
single-crystal sapphire as the template. Interestingly, the A-plane single-crystal sapphire shows oriented growth along the (110) crystal plane [15], as shown in Fig. 1(c), while the induced polycrystalline α-Al2O3 ceramics show oriented growth along
A
the (104) crystal plane. The mechanism governing this behavior will be investigated in future studies. Fig. 2 shows the real in-line transmission curves for (a) random and (b) oriented transparent polycrystalline α-Al2O3 ceramics (0.8 mm). It is clear that the 5
transmission of the oriented sample is much higher than that of the random sample. Furthermore, the transmission remains high as the wavelength shifts toward the UV range (< 300 nm), implying that the birefringence is greatly decreased [16]. For example, the transmission of the oriented sample is ~ 62% and ~ 43% at 650 nm and
IP T
300 nm, respectively. In contrast, the transmission is only ~ 23% and ~ 10% at the same wavelengths for the random sample. In the work of Mao et al [8], transparent
SC R
polycrystalline alumina ceramics with oriented optical axes were demonstrated using
a magnetic-field-assisted slip-casting method, with a transmission (0.8 mm) of ~ 55%
U
and ~ 44% at 650 nm and 300 nm, respectively. These experimental results indicate
N
that compared to samples with oriented optical axes, transparent polycrystalline
A
alumina ceramics with oriented non-optical axes have a higher transmission.
M
The differences in transmission resulting from grain orientation are also evident in
ED
the macroscopic photographs shown in Fig. 3. The distance between the samples and the paper is 5 mm. The letters under the oriented sample (placed in the right) can be
PT
seen clearly, while the letters under the random sample (placed in the left) are fuzzy.
CC E
Fig. 4 shows SEM graphs of the fracture surfaces of (a) random and (b) orientated transparent polycrystalline α-Al2O3 transparent ceramics. It can be seen that both of the samples have a high-density structure with no obvious pores. The average gain
A
size of the ceramic (D) is determined using the linear intercept method [17, 18], and the grain structure is bimodal comprising coarse grains (~ 30 μm in diameter) and small grains (~ 10 μm in diameter). The results indicate that polycrystalline α-Al2O3 transparent ceramics with oriented grains and a high-density structure are prepared 6
from TGG techniques. The orientation factor is widely used in the literature to characterize the texture degree of hexagonal α-Al2O3 and can be obtained from the XRD pattern [19]. The f factor varies between 0 and 1. A large f value implies a highly textured material; thus,
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since f = 0 for a random sample and f = 1 for a fully oriented material. Table 1 summarizes the calculated data of the orientation factor f for random and oriented
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transparent polycrystalline α-Al2O3 ceramics prepared in the current work and other published literatures [7, 8, 11, 14]. It is clear that compared to the random samples
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both the f value and the real in-line transmission increase with orientation. Compared
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to other TGG methods, oriented samples induced by A-plane single-crystal sapphire
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have higher f value. And although the magnetic orientation method (f > 0.95) can
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receive higher f value, their transmissions are lower due to low density. In the present
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work, samples with high oriented grains and high-density structure are prepared, and
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their transmissions are higher than 62% (at 650nm, 0.8 mm).
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4. Conclusion
Highly-oriented (104) polycrystalline α-Al2O3 transparent ceramics have been
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prepared by TGG method using A-plane single-crystal sapphire as the template. XRD results reveal that grans in the polycrystalline α-Al2O3 ceramics have been orientated successfully to (104) crystal plane, and the orientation factor f is higher than 0.65. SEM results indicate that oriented samples have high dense structure with no obvious 7
pores. The as-prepared oriented and dense samples have higher optical transmission than random samples. In addition, the transmission remains high as the wavelength shifts toward the UV range (< 300 nm). This method can be extended to other uniaxial materials without optical axes to produce polycrystalline samples with excellent
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optical transparency.
Acknowledgments
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This work was financially supported by the project of the Zhejiang Provincial
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Natural Science Foundation (LY15F050005).
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Processing. J. Am. Ceram. Soc. 99(10) (2016) 3217-3219.
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template particles during electrophoretic deposition. J. Eur. Ceram. Soc. 30
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(2010) 1195-1202.
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[12] K. Oishi, D. Akai, M. Ishida. Integration of crystalline orientated c-Al2O3 films
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and complementary metal–oxide–semiconductor circuits on Si(100) substrate.
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Solid-State Electron. 103 (2015) 110-114. [13] Y. You, A. Ito, T. Goto. Highly (001)-oriented α-Al2O3 films prepared by laser
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chemical vapor deposition.
Mater. Lett. 106 (2013) 11-13.
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[14] E. Suvaci, G. L. Messing. Critical factors in the templated grain growth of textured reaction-bonded alumina. J. Am. Ceram. Soc. 83 (2000) 2041-2048.
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[15] Y. N Buzynin, O. I. Khrykin, P. A. Yunin, M. N. Drozdov, A. Y. Luk'Yanov. InN Layers Grown by MOCVD on a-Plane Al2O3. Phys. Status Solidi A 215(11) (2018) 1700919. [16] A. Krell, T. Hutzler, J. Klimke. Transparent Ceramics for Structural Applications II. Ceram. Forum Int. 84(4) (2007) 41-50. 10
[17] B. Yao, T. Sun, V. Kumar, K. Barmak, K. R. Coffey. Grain growth and void formation in dielectric-encapsulated Cu thin films. J. Mater. Res. 23(7) (2008) 2033-2039. [18] A. Makiya, S. Tanaka, D. Shoji, T. Ishikawa, N. Uchida, K. Uematsu. A
powder compacts. J. Eur. Ceram. Soc. 27(12) (2007) 3399-3406.
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quantitative evaluation method for particle orientation structure in alumina
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[19] K. Koganei, T. Oyama, M. Inada, N. Enomoto, K. Hayashi. C-axis oriented β″-
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Figure captions
Fig. 1 XRD patterns of (a) random transparent polycrystalline α-Al2O3 ceramics, (b)
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oriented transparent polycrystalline α-Al2O3 ceramics and (c) A-plane single-crystal
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sapphire
Fig. 2 Real in-line transmission curves of (a) random and (b) oriented transparent
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polycrystalline α-Al2O3 ceramics (0.8 mm)
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Fig. 3 Macroscopic photographs of (left) random and (right) oriented transparent polycrystalline α-Al2O3 ceramics (0.8 mm) (samples are placed 5 mm above the
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paper)
Fig. 4 SEM results showing fracture surface of (a) random and (b) oriented 13
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transparent polycrystalline α-Al2O3 ceramics
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Table 1 Comparison of the orientation factor f and real in-line transmittance (at 650nm, 0.8 mm) of random and oriented transparent polycrystalline α-Al2O3 ceramics
[7]
TGG TGG Magnetic orientation Magnetic orientation
f
T (at 650 nm)
(104)
~0
~ 23%
~ 0.65
~ 62%
(001) (001)
~0 ~0
-
~ 0.49 ~ 0.60
-
(006)
~0
~ 22%
~ 0.97
~ 58%
(006)
~0
~ 18%
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M
A
N
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[8]
TGG
Orientation T (at 650 f nm)
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This work [11] [14]
Method
Random
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Samples
Oriented plane
~ 0.95
~ 55%