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Effect of processing on microstructure and optical properties of Dy-α-sialon
Materials Letters 58 (2004) 3340 – 3344 www.elsevier.com/locate/matlet
Effect of processing on microstructure and optical properties of Dy-a-sialon Xinlu Sua, Peiling Wanga,*, Weiwu Chena, Yibing Chengb, Dongsheng Yana a
The State Key Lab of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Ding Xi Road, Shanghai 200050, China b School of Physics and Materials Engineering, Monash University, Clayton, Victoria 3800, Australia Received 30 March 2004; received in revised form 17 May 2004; accepted 26 May 2004 Available online 26 July 2004
Abstract A Dy-a-sialon with a composition of Dy0.33Si9.3Al2.7O1.7N14.3 was prepared by hot pressing at 1800 8C with an intermediate dwelling at 1550 8C or 1650 8C during the sintering. The microstructures and optical properties of the sintered specimens were studied. The results showed that an intermediate dwelling stage in the hot pressing was beneficial to densification of the specimens and could suppress the development of elongated a-sialon grains, but it had a negative effect on hardness and fracture toughness. Besides, the intermediate dwelling process also improved the optical transmittance and the maximum infrared transmission, which is attributed to the increase in bulk density and the development of a homogeneous equi-axed microstructure. D 2004 Elsevier B.V. All rights reserved. Keywords: a-sialon; Microstructure; Processing; Optical property
1. Introduction In the past several decades, the reaction behavior, microstructure, phase assemblages of a-sialon (abbreviated as aV) materials and the transformation of aVto hV(h-sialon) were widely studied to optimize the final mechanical properties of the material [1,2]. Our previous study of RaV(R=Nd, Sm, Gd, Dy, Y, Er and Yb) systems indicated that the hot pressing temperature for achieving fully dense samples decreased with increasing the atomic number of rare earth stabilizing elements [3]. For the Dy-aVcomposition, the maximum bulk density could be achieved by hot pressing at 1600 8C for 1 h (Fig. 1(a)). The reaction sequence of the Dy-aVcomposition revealed that the amount of aV phase increased with the increase of sintering temperature, attaining 40 and 60 wt.% of the aV phase in the samples at 1550 and 1650 8C, respectively (Fig. 1(b)).
* Corresponding author. Tel.: +86 21 52412324; fax: +86 21 52413122. E-mail address: [email protected] (P. Wang). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.05.077
In recent years, attention has been paid to the functional properties of sialon ceramics, especially to their optical properties. It is noted that the first paper dealing with the preparation of translucent hVceramics was published more than 20 years ago, which reported transmittance of the material in the range of 1–7 Am [4]. Most researchers focused on the studies of optical properties of aVand aV–hV in the visible range lately [5–7]. It has been found that some of the aVceramics and aV–hVcomposites have relative high optical transmittances, showing great potential for broadening these structural ceramics to functional applications. Although some studies have pointed out the importance of processing conditions to the optical properties of aV materials [6], few have reported in detail the effect of sintering procedures on its final optical transmission. For transparent polycrystalline ceramics, microstructure and phase assemblage are very important factors that affects the optical transmittance of the material [8–10]. On the other hand, the microstructures and phase assemblages of aV ceramics can be tailored by sintering process [11]. In this work, Dy-aV materials were prepared with dwellings at
X. Su et al. / Materials Letters 58 (2004) 3340–3344
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Fig. 1. (a) Densification behavior of R (R=Nd, Sm, Gd, Dy, Y, Er and Yb)-a-sialon compositions prepared by hot pressing; (b) reaction sequences of Dy-asialon reported in Ref. [3]. Note: The hot pressing time is 1 h for all the samples showed here (a: a-Si3N4; aV: a-sialon; h: h-Si3N4; hV: h-sialon; M: melilite; A: AlN).
different temperatures before reaching the final sintering temperature to understand the effects of the sintering process on the materials optical transmittance. The microstructures, infrared transmittance and mechanical properties of Dy-aVceramics were studied.
2. Experimental procedure The composition of Dy-aVused is located on the aV-plane in the system of Dy–Si–Al–O–N with m=1.0 and n=1.7 according to the formula of Dym/3Si12 (m+n)Alm+n On N16 n . The starting materials are a-Si3N4 (SN-10, UBE Industries, Japan), AlN (Wuxi Chemical Plant, China), Dy2O3 (Yaolong Chemical Plant, China) and Al2O3 (Wusong Chemical Plant, China). In calculating the starting composition, the oxygen contents on the surface of the Si3N4 and AlN powders were taken into account. The powders weighed according to the designed composition were milled in absolute alcohol for 24 h, using sialon milling media. On the basis of the reaction sequence of Dy-aV[3], three batches of dried powders were hot-pressed at 1800 8C for 1 h under 20 MPa under nitrogen atmosphere in a graphite furnace, in which two batches were dwelled at 1550 and 1650 8C,
respectively, for 1 h before reaching 1800 8C, and the third one was hot-pressed at 1800 8C directly. The corresponding specimens were named as Dy155, Dy165 and Dy180, respectively. Phase assemblages were determined by XRD using a Guinier–H7gg camera with Cu Ka1 radiation and Si as an internal standard. The measurement of X-ray film and refinement of lattice parameters were completed by a computer-linked line scanner (LS-18) system [12] and the program PIRUM [13]. Microstructure observation was performed under field emissive Scanning Electronic Microscopy (SEM, JEOL JSE-6700 F). Optical transmittances in the range of 4000–1500 cm 1 (2.5–6.6 Am) were measured by FT-IR (NICOLET NEXUS). The Vickers hardness as well as indentation fracture toughness were determined at room temperature by indentation method using a Vickers diamond indenter and a load of 98 N for 10 min (AKASHI).
3. Results and discussion The bulk densities of Dy155, Dy165 and Dy180 as well as the cell dimensions of aVphases are listed in Table 1. It is found that the bulk densities of hot-pressed specimens with
Table 1 Phase assemblages, cell dimensions of aVand mechanical properties of the specimens Specimen
Dy155 Dy165 Dy180 a
s=Strong.
Bulk density (g/cm3)
Phase assemblagea
Cell dimensions of aV a (2)
c (2)
v (23)
Hardness, H v10 (GPa)
Fracture toughness, K 1c (MPad m1/2)
3.37 3.38 3.36
aV/s aV/s aV/s
7.8143 (4) 7.8152 (2) 7.8146 (3)
5.6954 (5) 5.6968 (3) 5.6944 (4)
301.19 301.33 301.16
18.78 18.87 19.16
3.83 4.24 4.75
Mechanical property
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X. Su et al. / Materials Letters 58 (2004) 3340–3344
dwelling at 1550 or 1650 8C are slightly higher than that of Dy180, and three specimens are all composed of singlecrystalline aV phase. The cell dimensions of aV phase in Dy155 and Dy180 are very close, but that in Dy165 is slightly larger. A Dy-aV composition (Dy0.36Si10.38Al1.62O0.54N15.46) studied in our previous work [3] has reached the maximum density at 1600 8C (Fig. 1(a)). That composition contained less amount of oxygen and thus would be relatively more difficult to be densified, in comparison to the one used in the present work. It is thus thought that when the composition is hot-pressed at either 1550 or 1650 8C for 1 h, most pores should have been removed from the sample and an additional sintering at 1800 8C after the dwelling would promote grain growth of aVand further improve densification of the material due to a high diffusion rate expected at 1800 8C. However, when the sample was heated to 1800 8C directly as for sample Dy180, the density was slightly reduced. It is thought that the mass diffusion rate at 1800 8C could be so fast that the grain boundary moved faster than pores. This would leave the pores trapped inside the aV grains to become closed pores, leading to a decrease in bulk density. The SEM micrographs of Dy155, Dy165 and Dy180 are shown in Fig. 2. It is noted that most of aVgrains in Dy155 has an equi-axed morphology with the grain size larger than 1.0 Am, whereas the grain size for most aVin Dy165 is less than 1.0 Am although the grains in both samples possess a similar morphology. This difference in grain size is possibly a result of the different sintering procedures employed. According to the reaction sequence of Dy-aV shown in Fig. 1(b), about 40 wt.% of aVis formed at 1550
Fig. 3. Optical transmittances of Dy155, Dy165 and Dy180 samples (1.0 mm in thickness).
8C, whereas the relative aVcontent reaches about 65 wt.% for the same composition at 1650 8C. The aVgrains formed at the intermediate dwelling temperature become nuclei for grain growth at 1800 8C. Due to a less amount of aVnuclei formed at 1550 8C in Dy155 compared to that formed at 1650 8C in Dy165, larger aVgrains are expected in Dy155 after grain growth at 1800 8C. In contrast, sample Dy180 shows a typical aVceramic microstructure prepared by hot pressing, consisting of both equi-axed and elongated grains [14]. The optical transmittance curves of the three specimens (1.0 mm in thickness) in the range of 1500–4000 cm 1 (6.6– 2.5 Am) are shown in Fig. 3. The cut frequency of each specimen is located at about 2000 cm 1 (5.0 Am). Each curve
Fig. 2. SEM micrographs of (a) Dy155, (b) Dy165, (c) and (d) Dy180 samples.
X. Su et al. / Materials Letters 58 (2004) 3340–3344
has an absorb peak at about 3500 cm 1, corresponding to 6 H15/2Y6H11/2 electron transition of Dy3+. It is noted that two absorb peaks located at 2840 and 2920 cm 1, respectively, also appear in the transmittance curves, in which the intensities of the peaks in Dy155 and Dy165 samples are higher, while they are very weak in Dy180. The reasons for appearances of these two peaks are not clear. In the range of 1500–4000 cm 1, the optical transmittance of Dy155 and Dy165 is similar, but they are higher than that of Dy180. The maximum optical transmittances of the Dy155 and Dy165 are 59% and 57.5%, respectively, about ~12% higher than that of Dy180. It is well known that some characteristics of transparent polycrystalline ceramics are as follows: (1) High bulk density, as the pores in polycrystalline ceramics would decrease the transmittance greatly, because the difference in the refractive index between air and polycrystalline ceramic body would lead to increase of loss of light scattering. (2) No second crystalline phases, the refractive index of the second crystalline phase differs from the main crystalline phase would also lead to the loss of light scattering. (3) Microstructure, larger grain sizes, less grain boundary phase and homogenous distribution of grains are helpful to improve the optical transmittance. (4) The finish of surface, churlish surface will increase the reflection of light. All specimens in the present work have the same composition and are composed of single-crystalline aV phase; thus, the difference in optical transmittance among these specimens is attributed to their variation in bulk density, microstructures and the amount of grain boundary glass. Dy155 and Dy165 have slightly higher density than Dy180, although the difference in density between Dy155 and Dy180 is only 0.1. Both Dy155 and Dy165 consist of mostly equi-axed aVgrains with a homogenous distribution, while Dy180 has both equi-axed and elongated grains. It is difficult to have quantitative values of grain boundary phase present in these samples. However, considering the same starting composition and the same phase assemblage of aVas well as the similar cell dimensions of aVphase in Dy155, Dy165 and Dy180, in spite of little larger lattice parameters of aVin Dy165, we thought that the components and amount of grain boundary glass phase in these samples should be similar, According to the slightly higher bulk density of Dy165 than that of Dy155, Dy165 should have a higher optical transmittance. Larger grain sizes of aVphase and more homogeneous distribution of aVgrains with equiaxed morphology in Dy155 compared to that of Dy165 are positive factors that could improve the optical transmittance, however. All these factors contribute to the higher optical transmittance for Dy155 and Dy165 than for Dy180. It is therefore thought that the optical transmittance of Dy doped aV by could be related to the material’s sintering process. Table 1 also lists the hardness and fracture toughness of the specimens prepared. The hardness of the three specimens is very close, but Dy180 has slightly higher fracture
3343
toughness than Dy155 and Dy165. The minor difference in the mechanical properties of the three specimens is associated with their microstructures. The cell dimensions of the aV phases in all three specimens are very similar, suggesting the aVcompositions and the amounts of glassy grain boundary phase in these samples are comparable. This can explain the similar hardness observed in the three specimens. Moreover, the elongated aV grain morphology, although having a relatively small aspect ratio, should be responsible for the improvement of fracture toughness in Dy180 [15].
4. Summary Compared with the one-step hot pressing, an intermediate dwelling at 1550 or 1650 8C during hot pressing of DyaV compositions is beneficial to grain growth and homogenous distribution of aV with equi-axed morphology and improvements in densification. The two-step sintered Dy-aV ceramics also show higher optical transmittance than the one-step sintered sample. The maximum infrared transmission for both the two-step sintered specimens (1 mm in thickness) reaches ~60% in the range 2600–3000 cm 1, about 12% higher than that for the one-step sintered sample. This difference is attributed to the higher density and the homogeneous equi-axed grain microstructure. Combining the improved transmittance in infrared range with its good mechanical and thermal as well as chemical stability properties, it is expect that the new produced aV ceramics could be used as window material under harsh conditions.
Acknowledgements This work was supported by the Outstanding Overseas Chinese Scholars Fund of Chinese Academy of Sciences and the National Natural Sciences Foundation of China.
References [1] T. Ekstrfm, M. Nygren, J. Am. Ceram. Soc. 75 (1992) 259 – 276. [2] V.A. Izhevskiy, L.A. Genova, J.C. Bressiani, F. Aldinger, J. Eur. Ceram. Soc. 20 (2000) 2275 – 2295. [3] P.L. Wang, W.Y. Sun, T.S. Yen, J. Solid State Inorg. Chem. 31 (1994) 93 – 104. [4] M. Mitomo, Y. Moriyoshi, T. Sakai, T. Ohsaka, M. Kobayashi, J. Mater. Sci. Lett. 1 (1982) 25 – 26. [5] H. Mandal, J. Eur. Ceram. Soc. 19 (1999) 2349 – 2357. [6] B.S.B. Karunaratne, R.J. Lumby, M.H. Lewis, J. Mater. Res. 31 (1996) 2790 – 2794. [7] M.I. Jones, H. Hyuga, K. Hirao, J. Am. Ceram. Soc. 86 (2003) 520 – 522. [8] I.-J., Bae, S. Baik, J. Am. Ceram. Soc. 80 (1997) 1149 – 1156. [9] C. Greskovich, K.N. Woods, 52 (1973) 473–478.
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[10] N. Saito, S.-I. Matsuda, T. Ikegami, J. Am. Ceram. Soc. 81 (1998) 2023 – 2028. [11] Z. Shen, Z. Zhao, H. Peng, M. Nygren, Nature 417 (2002) 266 – 269. [12] K.E. Johansson, T. Palm, P.-E. Werner, J. Phys. E. Sci. Instrum. 13 (1980) 1289 – 1291.
[13] P.-E. Werner, Arkiv fur Kemi 31 (1964) 513 – 516. [14] Y.W. Li, P.L. Wang, W.W. Chen, Y.-B. Cheng, D.S. Yan, J. Eur. Ceram. Soc. 21 (2001) 1273 – 1278. [15] P.F. Becher, E.Y. Sun, K.P. Plucknett, J. Am. Ceram. Soc. 81 (1998) 2821 – 2830.
Effect of processing on microstructure and optical properties of Dy-a-sialon Xinlu Sua, Peiling Wanga,*, Weiwu Chena, Yibing Chengb, Dongsheng Yana a
The State Key Lab of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Ding Xi Road, Shanghai 200050, China b School of Physics and Materials Engineering, Monash University, Clayton, Victoria 3800, Australia Received 30 March 2004; received in revised form 17 May 2004; accepted 26 May 2004 Available online 26 July 2004
Abstract A Dy-a-sialon with a composition of Dy0.33Si9.3Al2.7O1.7N14.3 was prepared by hot pressing at 1800 8C with an intermediate dwelling at 1550 8C or 1650 8C during the sintering. The microstructures and optical properties of the sintered specimens were studied. The results showed that an intermediate dwelling stage in the hot pressing was beneficial to densification of the specimens and could suppress the development of elongated a-sialon grains, but it had a negative effect on hardness and fracture toughness. Besides, the intermediate dwelling process also improved the optical transmittance and the maximum infrared transmission, which is attributed to the increase in bulk density and the development of a homogeneous equi-axed microstructure. D 2004 Elsevier B.V. All rights reserved. Keywords: a-sialon; Microstructure; Processing; Optical property
1. Introduction In the past several decades, the reaction behavior, microstructure, phase assemblages of a-sialon (abbreviated as aV) materials and the transformation of aVto hV(h-sialon) were widely studied to optimize the final mechanical properties of the material [1,2]. Our previous study of RaV(R=Nd, Sm, Gd, Dy, Y, Er and Yb) systems indicated that the hot pressing temperature for achieving fully dense samples decreased with increasing the atomic number of rare earth stabilizing elements [3]. For the Dy-aVcomposition, the maximum bulk density could be achieved by hot pressing at 1600 8C for 1 h (Fig. 1(a)). The reaction sequence of the Dy-aVcomposition revealed that the amount of aV phase increased with the increase of sintering temperature, attaining 40 and 60 wt.% of the aV phase in the samples at 1550 and 1650 8C, respectively (Fig. 1(b)).
* Corresponding author. Tel.: +86 21 52412324; fax: +86 21 52413122. E-mail address: [email protected] (P. Wang). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.05.077
In recent years, attention has been paid to the functional properties of sialon ceramics, especially to their optical properties. It is noted that the first paper dealing with the preparation of translucent hVceramics was published more than 20 years ago, which reported transmittance of the material in the range of 1–7 Am [4]. Most researchers focused on the studies of optical properties of aVand aV–hV in the visible range lately [5–7]. It has been found that some of the aVceramics and aV–hVcomposites have relative high optical transmittances, showing great potential for broadening these structural ceramics to functional applications. Although some studies have pointed out the importance of processing conditions to the optical properties of aV materials [6], few have reported in detail the effect of sintering procedures on its final optical transmission. For transparent polycrystalline ceramics, microstructure and phase assemblage are very important factors that affects the optical transmittance of the material [8–10]. On the other hand, the microstructures and phase assemblages of aV ceramics can be tailored by sintering process [11]. In this work, Dy-aV materials were prepared with dwellings at
X. Su et al. / Materials Letters 58 (2004) 3340–3344
3341
Fig. 1. (a) Densification behavior of R (R=Nd, Sm, Gd, Dy, Y, Er and Yb)-a-sialon compositions prepared by hot pressing; (b) reaction sequences of Dy-asialon reported in Ref. [3]. Note: The hot pressing time is 1 h for all the samples showed here (a: a-Si3N4; aV: a-sialon; h: h-Si3N4; hV: h-sialon; M: melilite; A: AlN).
different temperatures before reaching the final sintering temperature to understand the effects of the sintering process on the materials optical transmittance. The microstructures, infrared transmittance and mechanical properties of Dy-aVceramics were studied.
2. Experimental procedure The composition of Dy-aVused is located on the aV-plane in the system of Dy–Si–Al–O–N with m=1.0 and n=1.7 according to the formula of Dym/3Si12 (m+n)Alm+n On N16 n . The starting materials are a-Si3N4 (SN-10, UBE Industries, Japan), AlN (Wuxi Chemical Plant, China), Dy2O3 (Yaolong Chemical Plant, China) and Al2O3 (Wusong Chemical Plant, China). In calculating the starting composition, the oxygen contents on the surface of the Si3N4 and AlN powders were taken into account. The powders weighed according to the designed composition were milled in absolute alcohol for 24 h, using sialon milling media. On the basis of the reaction sequence of Dy-aV[3], three batches of dried powders were hot-pressed at 1800 8C for 1 h under 20 MPa under nitrogen atmosphere in a graphite furnace, in which two batches were dwelled at 1550 and 1650 8C,
respectively, for 1 h before reaching 1800 8C, and the third one was hot-pressed at 1800 8C directly. The corresponding specimens were named as Dy155, Dy165 and Dy180, respectively. Phase assemblages were determined by XRD using a Guinier–H7gg camera with Cu Ka1 radiation and Si as an internal standard. The measurement of X-ray film and refinement of lattice parameters were completed by a computer-linked line scanner (LS-18) system [12] and the program PIRUM [13]. Microstructure observation was performed under field emissive Scanning Electronic Microscopy (SEM, JEOL JSE-6700 F). Optical transmittances in the range of 4000–1500 cm 1 (2.5–6.6 Am) were measured by FT-IR (NICOLET NEXUS). The Vickers hardness as well as indentation fracture toughness were determined at room temperature by indentation method using a Vickers diamond indenter and a load of 98 N for 10 min (AKASHI).
3. Results and discussion The bulk densities of Dy155, Dy165 and Dy180 as well as the cell dimensions of aVphases are listed in Table 1. It is found that the bulk densities of hot-pressed specimens with
Table 1 Phase assemblages, cell dimensions of aVand mechanical properties of the specimens Specimen
Dy155 Dy165 Dy180 a
s=Strong.
Bulk density (g/cm3)
Phase assemblagea
Cell dimensions of aV a (2)
c (2)
v (23)
Hardness, H v10 (GPa)
Fracture toughness, K 1c (MPad m1/2)
3.37 3.38 3.36
aV/s aV/s aV/s
7.8143 (4) 7.8152 (2) 7.8146 (3)
5.6954 (5) 5.6968 (3) 5.6944 (4)
301.19 301.33 301.16
18.78 18.87 19.16
3.83 4.24 4.75
Mechanical property
3342
X. Su et al. / Materials Letters 58 (2004) 3340–3344
dwelling at 1550 or 1650 8C are slightly higher than that of Dy180, and three specimens are all composed of singlecrystalline aV phase. The cell dimensions of aV phase in Dy155 and Dy180 are very close, but that in Dy165 is slightly larger. A Dy-aV composition (Dy0.36Si10.38Al1.62O0.54N15.46) studied in our previous work [3] has reached the maximum density at 1600 8C (Fig. 1(a)). That composition contained less amount of oxygen and thus would be relatively more difficult to be densified, in comparison to the one used in the present work. It is thus thought that when the composition is hot-pressed at either 1550 or 1650 8C for 1 h, most pores should have been removed from the sample and an additional sintering at 1800 8C after the dwelling would promote grain growth of aVand further improve densification of the material due to a high diffusion rate expected at 1800 8C. However, when the sample was heated to 1800 8C directly as for sample Dy180, the density was slightly reduced. It is thought that the mass diffusion rate at 1800 8C could be so fast that the grain boundary moved faster than pores. This would leave the pores trapped inside the aV grains to become closed pores, leading to a decrease in bulk density. The SEM micrographs of Dy155, Dy165 and Dy180 are shown in Fig. 2. It is noted that most of aVgrains in Dy155 has an equi-axed morphology with the grain size larger than 1.0 Am, whereas the grain size for most aVin Dy165 is less than 1.0 Am although the grains in both samples possess a similar morphology. This difference in grain size is possibly a result of the different sintering procedures employed. According to the reaction sequence of Dy-aV shown in Fig. 1(b), about 40 wt.% of aVis formed at 1550
Fig. 3. Optical transmittances of Dy155, Dy165 and Dy180 samples (1.0 mm in thickness).
8C, whereas the relative aVcontent reaches about 65 wt.% for the same composition at 1650 8C. The aVgrains formed at the intermediate dwelling temperature become nuclei for grain growth at 1800 8C. Due to a less amount of aVnuclei formed at 1550 8C in Dy155 compared to that formed at 1650 8C in Dy165, larger aVgrains are expected in Dy155 after grain growth at 1800 8C. In contrast, sample Dy180 shows a typical aVceramic microstructure prepared by hot pressing, consisting of both equi-axed and elongated grains [14]. The optical transmittance curves of the three specimens (1.0 mm in thickness) in the range of 1500–4000 cm 1 (6.6– 2.5 Am) are shown in Fig. 3. The cut frequency of each specimen is located at about 2000 cm 1 (5.0 Am). Each curve
Fig. 2. SEM micrographs of (a) Dy155, (b) Dy165, (c) and (d) Dy180 samples.
X. Su et al. / Materials Letters 58 (2004) 3340–3344
has an absorb peak at about 3500 cm 1, corresponding to 6 H15/2Y6H11/2 electron transition of Dy3+. It is noted that two absorb peaks located at 2840 and 2920 cm 1, respectively, also appear in the transmittance curves, in which the intensities of the peaks in Dy155 and Dy165 samples are higher, while they are very weak in Dy180. The reasons for appearances of these two peaks are not clear. In the range of 1500–4000 cm 1, the optical transmittance of Dy155 and Dy165 is similar, but they are higher than that of Dy180. The maximum optical transmittances of the Dy155 and Dy165 are 59% and 57.5%, respectively, about ~12% higher than that of Dy180. It is well known that some characteristics of transparent polycrystalline ceramics are as follows: (1) High bulk density, as the pores in polycrystalline ceramics would decrease the transmittance greatly, because the difference in the refractive index between air and polycrystalline ceramic body would lead to increase of loss of light scattering. (2) No second crystalline phases, the refractive index of the second crystalline phase differs from the main crystalline phase would also lead to the loss of light scattering. (3) Microstructure, larger grain sizes, less grain boundary phase and homogenous distribution of grains are helpful to improve the optical transmittance. (4) The finish of surface, churlish surface will increase the reflection of light. All specimens in the present work have the same composition and are composed of single-crystalline aV phase; thus, the difference in optical transmittance among these specimens is attributed to their variation in bulk density, microstructures and the amount of grain boundary glass. Dy155 and Dy165 have slightly higher density than Dy180, although the difference in density between Dy155 and Dy180 is only 0.1. Both Dy155 and Dy165 consist of mostly equi-axed aVgrains with a homogenous distribution, while Dy180 has both equi-axed and elongated grains. It is difficult to have quantitative values of grain boundary phase present in these samples. However, considering the same starting composition and the same phase assemblage of aVas well as the similar cell dimensions of aVphase in Dy155, Dy165 and Dy180, in spite of little larger lattice parameters of aVin Dy165, we thought that the components and amount of grain boundary glass phase in these samples should be similar, According to the slightly higher bulk density of Dy165 than that of Dy155, Dy165 should have a higher optical transmittance. Larger grain sizes of aVphase and more homogeneous distribution of aVgrains with equiaxed morphology in Dy155 compared to that of Dy165 are positive factors that could improve the optical transmittance, however. All these factors contribute to the higher optical transmittance for Dy155 and Dy165 than for Dy180. It is therefore thought that the optical transmittance of Dy doped aV by could be related to the material’s sintering process. Table 1 also lists the hardness and fracture toughness of the specimens prepared. The hardness of the three specimens is very close, but Dy180 has slightly higher fracture
3343
toughness than Dy155 and Dy165. The minor difference in the mechanical properties of the three specimens is associated with their microstructures. The cell dimensions of the aV phases in all three specimens are very similar, suggesting the aVcompositions and the amounts of glassy grain boundary phase in these samples are comparable. This can explain the similar hardness observed in the three specimens. Moreover, the elongated aV grain morphology, although having a relatively small aspect ratio, should be responsible for the improvement of fracture toughness in Dy180 [15].
4. Summary Compared with the one-step hot pressing, an intermediate dwelling at 1550 or 1650 8C during hot pressing of DyaV compositions is beneficial to grain growth and homogenous distribution of aV with equi-axed morphology and improvements in densification. The two-step sintered Dy-aV ceramics also show higher optical transmittance than the one-step sintered sample. The maximum infrared transmission for both the two-step sintered specimens (1 mm in thickness) reaches ~60% in the range 2600–3000 cm 1, about 12% higher than that for the one-step sintered sample. This difference is attributed to the higher density and the homogeneous equi-axed grain microstructure. Combining the improved transmittance in infrared range with its good mechanical and thermal as well as chemical stability properties, it is expect that the new produced aV ceramics could be used as window material under harsh conditions.
Acknowledgements This work was supported by the Outstanding Overseas Chinese Scholars Fund of Chinese Academy of Sciences and the National Natural Sciences Foundation of China.
References [1] T. Ekstrfm, M. Nygren, J. Am. Ceram. Soc. 75 (1992) 259 – 276. [2] V.A. Izhevskiy, L.A. Genova, J.C. Bressiani, F. Aldinger, J. Eur. Ceram. Soc. 20 (2000) 2275 – 2295. [3] P.L. Wang, W.Y. Sun, T.S. Yen, J. Solid State Inorg. Chem. 31 (1994) 93 – 104. [4] M. Mitomo, Y. Moriyoshi, T. Sakai, T. Ohsaka, M. Kobayashi, J. Mater. Sci. Lett. 1 (1982) 25 – 26. [5] H. Mandal, J. Eur. Ceram. Soc. 19 (1999) 2349 – 2357. [6] B.S.B. Karunaratne, R.J. Lumby, M.H. Lewis, J. Mater. Res. 31 (1996) 2790 – 2794. [7] M.I. Jones, H. Hyuga, K. Hirao, J. Am. Ceram. Soc. 86 (2003) 520 – 522. [8] I.-J., Bae, S. Baik, J. Am. Ceram. Soc. 80 (1997) 1149 – 1156. [9] C. Greskovich, K.N. Woods, 52 (1973) 473–478.
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