Optical properties of transparent nanocrystalline yttria stabilized zirconia
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Author's personal copy
Optical Materials 32 (2009) 62–68
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Optical properties of transparent nanocrystalline yttria stabilized zirconia J.E. Alaniz, F.G. Perez-Gutierrez, G. Aguilar, J.E. Garay * Department of Mechanical Engineering, Materials Science and Engineering Program, University of California, Riverside, CA 92521, United States
a r t i c l e
i n f o
Article history: Received 10 December 2008 Received in revised form 21 May 2009 Accepted 15 June 2009 Available online 16 July 2009 Keywords: Nanocrystalline ceramics Grain boundaries Transparent ceramics Spark plasma sintering
a b s t r a c t The optical properties of transparent nanocrystalline zirconia produced using a current activated method were characterized over the entire visible spectrum. The resolutions of the samples were characterized using standard resolution targets. All of the samples produced were found to have as high a resolution as detectable from the test, i.e., they are transparent not translucent. Transmission, reflectance, and absorption coefficients are reported for various wavelengths. The absorption coefficients were found to be highly dependent on processing time. Annealing experiments helped determine that oxygen vacancies (with free electrons) are the primary absorption centers in the visible wavelengths. In addition it was found that grain boundary cores or their associated defects do not contribute significantly to light absorption in the visible range. The lack of an influence of the grain boundary regions is discussed in terms of low oxygen vacancy concentration in the grain boundary space charge layer. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Recently there has been increasing interest in transparent ceramics spurred by wide ranging applications such as high temperature window/viewports, optical electronics, and transparent armor. Transparent Al2O3 (alumina), MgAl2O4 (spinel), indium tin oxide (ITO), and Y3Al5O12 (yttrium aluminum garnet or YAG) have been the most studied systems. Since polycrystalline yttria stabilized zirconia (YSZ) is one of the most versatile engineering ceramics, introduction of optical transparency to this ceramic should yield an exceptionally versatile material. Well documented properties of YSZ are high hardness, toughness, and high oxygen diffusivity. In addition, low thermal conductivity makes it an excellent thermal barrier material. However, this material has traditionally been opaque with a white to grayish appearance in its polycrystalline form. The promise of excellent multifunctionality has promoted work in harnessing the light transmittance properties of polycrystalline YSZ. Recently the elusive goal of producing optically transparent polycrystalline YSZ has been attained by two groups both using spark plasma sintering (SPS) [1,2]. Anselmi-Tamburini et al. [1] used a high pressure version of the SPS (480 MPa), while Casolco et al. used a traditional die set up and two-step load application procedure [2]. Both groups produced large samples (10 mm [1] and 19 mm [2] diameters) with high degrees of transparency. In the latter case, the color of the samples was shown to change dramatically with processing conditions. Natural applications of these ceramics with high optical transparency coupled with very low thermal conductivity and good toughness are high temperature * Corresponding author. Tel.: +1 951 827 2449. E-mail address: [email protected] (J.E. Garay). 0925-3467/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2009.06.004
windows. They also have potential as varying color filters in extreme environments and impact/scratch resistant electronic displays. Although the optical properties of single crystal YSZ are well documented [3,4], the inherent optical properties (reflectivity, absorption coefficient) of bulk nanocrystalline YSZ have not been reported. There has been a resurgence of interest in polycrystalline optical ceramics and hence on the effects of grain boundaries on optical properties. The role of grain boundaries on optical properties becomes crucial in nanocrystalline ceramics that have a significantly higher concentration of grain boundaries than their microcrystalline counterparts. Grain boundaries can be envisioned to change light transmission for various reasons. In solids with an anisotropic refractive index, grain boundaries could serve as scattering sites since they form the boundary between crystallites with different orientation. This effect is likely responsible for the increased light transmittance in fine grained alumina [6]. Another mechanism is the possible absorption by point defects associated with grain boundaries. This effect has not been widely discussed in the literature. YSZ is an ideal material for exploring the latter case since it has known grain boundary associated defects. In this article we present experimental data on the transmission, reflection, and the absorption coefficient in the visible range for nanocrystalline YSZ with varying colors in order to elucidate the role of grain boundary defects on optical properties. Our samples were produced using spark plasma sintering (SPS). SPS is related to hot pressing in that the powder is heated while pressure is applied. However, in hot pressing, heating is accomplished externally by way of a heating coil while SPS uses a high density current flux through the sample and the die to cause Joule heating within the sample. This unique technique allows us to
Author's personal copy
J.E. Alaniz et al. / Optical Materials 32 (2009) 62–68
63
decrease the heating time and the cooling time of the sintering process. This minimizes the amount of grain growth in the material and maintains the nanometric grains of the powder. A nanostructure in ceramics has a number of benefits. The mechanical properties (hardness and toughness) are sensitive to the concentration of grain boundaries within the sample. In fact nanocrystalline YSZ has perhaps the highest fracture toughness of any monolithic transparent ceramic reported [2]. Here we explore the consequences of nanocrystallinity on the optical properties of YSZ. 2. Experimental procedure 2.1. Sample preparation Commercial (Tosoh Corporation, Tokyo, Japan) nanocrystalline 8YSZ powder with a reported grain size of 50 nm was densified using SPS. Each of the samples was prepared in a graphite die with 19 mm inner diameter. Temperature was measured using a grounded k type thermocouple placed in a hole drilled halfway through the thickness of the die. The sintering process was performed in a fabricated vacuum chamber (
Author's personal copy
Optical Materials 32 (2009) 62–68
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Optical properties of transparent nanocrystalline yttria stabilized zirconia J.E. Alaniz, F.G. Perez-Gutierrez, G. Aguilar, J.E. Garay * Department of Mechanical Engineering, Materials Science and Engineering Program, University of California, Riverside, CA 92521, United States
a r t i c l e
i n f o
Article history: Received 10 December 2008 Received in revised form 21 May 2009 Accepted 15 June 2009 Available online 16 July 2009 Keywords: Nanocrystalline ceramics Grain boundaries Transparent ceramics Spark plasma sintering
a b s t r a c t The optical properties of transparent nanocrystalline zirconia produced using a current activated method were characterized over the entire visible spectrum. The resolutions of the samples were characterized using standard resolution targets. All of the samples produced were found to have as high a resolution as detectable from the test, i.e., they are transparent not translucent. Transmission, reflectance, and absorption coefficients are reported for various wavelengths. The absorption coefficients were found to be highly dependent on processing time. Annealing experiments helped determine that oxygen vacancies (with free electrons) are the primary absorption centers in the visible wavelengths. In addition it was found that grain boundary cores or their associated defects do not contribute significantly to light absorption in the visible range. The lack of an influence of the grain boundary regions is discussed in terms of low oxygen vacancy concentration in the grain boundary space charge layer. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Recently there has been increasing interest in transparent ceramics spurred by wide ranging applications such as high temperature window/viewports, optical electronics, and transparent armor. Transparent Al2O3 (alumina), MgAl2O4 (spinel), indium tin oxide (ITO), and Y3Al5O12 (yttrium aluminum garnet or YAG) have been the most studied systems. Since polycrystalline yttria stabilized zirconia (YSZ) is one of the most versatile engineering ceramics, introduction of optical transparency to this ceramic should yield an exceptionally versatile material. Well documented properties of YSZ are high hardness, toughness, and high oxygen diffusivity. In addition, low thermal conductivity makes it an excellent thermal barrier material. However, this material has traditionally been opaque with a white to grayish appearance in its polycrystalline form. The promise of excellent multifunctionality has promoted work in harnessing the light transmittance properties of polycrystalline YSZ. Recently the elusive goal of producing optically transparent polycrystalline YSZ has been attained by two groups both using spark plasma sintering (SPS) [1,2]. Anselmi-Tamburini et al. [1] used a high pressure version of the SPS (480 MPa), while Casolco et al. used a traditional die set up and two-step load application procedure [2]. Both groups produced large samples (10 mm [1] and 19 mm [2] diameters) with high degrees of transparency. In the latter case, the color of the samples was shown to change dramatically with processing conditions. Natural applications of these ceramics with high optical transparency coupled with very low thermal conductivity and good toughness are high temperature * Corresponding author. Tel.: +1 951 827 2449. E-mail address: [email protected] (J.E. Garay). 0925-3467/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2009.06.004
windows. They also have potential as varying color filters in extreme environments and impact/scratch resistant electronic displays. Although the optical properties of single crystal YSZ are well documented [3,4], the inherent optical properties (reflectivity, absorption coefficient) of bulk nanocrystalline YSZ have not been reported. There has been a resurgence of interest in polycrystalline optical ceramics and hence on the effects of grain boundaries on optical properties. The role of grain boundaries on optical properties becomes crucial in nanocrystalline ceramics that have a significantly higher concentration of grain boundaries than their microcrystalline counterparts. Grain boundaries can be envisioned to change light transmission for various reasons. In solids with an anisotropic refractive index, grain boundaries could serve as scattering sites since they form the boundary between crystallites with different orientation. This effect is likely responsible for the increased light transmittance in fine grained alumina [6]. Another mechanism is the possible absorption by point defects associated with grain boundaries. This effect has not been widely discussed in the literature. YSZ is an ideal material for exploring the latter case since it has known grain boundary associated defects. In this article we present experimental data on the transmission, reflection, and the absorption coefficient in the visible range for nanocrystalline YSZ with varying colors in order to elucidate the role of grain boundary defects on optical properties. Our samples were produced using spark plasma sintering (SPS). SPS is related to hot pressing in that the powder is heated while pressure is applied. However, in hot pressing, heating is accomplished externally by way of a heating coil while SPS uses a high density current flux through the sample and the die to cause Joule heating within the sample. This unique technique allows us to
Author's personal copy
J.E. Alaniz et al. / Optical Materials 32 (2009) 62–68
63
decrease the heating time and the cooling time of the sintering process. This minimizes the amount of grain growth in the material and maintains the nanometric grains of the powder. A nanostructure in ceramics has a number of benefits. The mechanical properties (hardness and toughness) are sensitive to the concentration of grain boundaries within the sample. In fact nanocrystalline YSZ has perhaps the highest fracture toughness of any monolithic transparent ceramic reported [2]. Here we explore the consequences of nanocrystallinity on the optical properties of YSZ. 2. Experimental procedure 2.1. Sample preparation Commercial (Tosoh Corporation, Tokyo, Japan) nanocrystalline 8YSZ powder with a reported grain size of 50 nm was densified using SPS. Each of the samples was prepared in a graphite die with 19 mm inner diameter. Temperature was measured using a grounded k type thermocouple placed in a hole drilled halfway through the thickness of the die. The sintering process was performed in a fabricated vacuum chamber (
