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Fabrication of high-performance Y2O3 stabilized hafnium dioxide refractories
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CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 5232–5238 www.elsevier.com/locate/ceramint
Review paper
Fabrication of high-performance Y2O3 stabilized hafnium dioxide refractories Jiaxing Zhaoa,b, Yujun Zhanga,b,n, Hongyu Gonga,b, Yubai Zhanga,b, Xianli Wanga,b, Xue Guoa,b, Yujun Zhaoa,b a
Key Laboratory for Liquid–Solid Structural Evolution & Processing of Materials of Ministry of Education, Shandong University, Jinan 250061, PR China b Key Laboratory of Special Functional Aggregated Materials, Ministry of Education, Shandong University, Jinan 250061, PR China Received 3 December 2014; received in revised form 23 December 2014; accepted 9 January 2015 Available online 14 January 2015
Abstract Y2O3 stabilized HfO2 ceramics with 5 mol% and 8 mol% Y2O3, respectively, were successfully fabricated by pressureless sintering at 1600 1C for 1 h. The phase distribution and transformation, microstructure, and relative density were investigated for Y2O3 stabilized HfO2 ceramics. Cubic phase was observed in the Y2O3 stabilized hafnium dioxide while pure HfO2 showed monoclinical phase only. The SEM images of fractured surface indicated two kinds of structures existed in modified HfO2, the solid solution region and uniform polygon grains, and some holes caused by Kirkendall effect. Refractoriness test showed that high temperature volumetric stability of the material can be effectively enhanced by adding Y2O3 into HfO2. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Hafnium dioxide; Y2O3 stabilized; High-performance refractory
Contents 1. 2.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Materials processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Materials characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Sintering densification behavior of pure hafnium oxide. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Phase composition and distribution of Y2O3 stabilized hafnium dioxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Calculation of phase distribution and density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Refractoriness test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
n
Corresponding author at: Key Laboratory for Liquid–Solid Structural Evolution and Processing of Materials of Ministry of Education, Shandong University, Jinan 250061, PR China. Tel./fax: þ 86 531 88399760. E-mail address: [email protected] (Y. Zhang). http://dx.doi.org/10.1016/j.ceramint.2015.01.047 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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1. Introduction Hafnium dioxide (HfO2) is technologically important because of its low Young’s modulus, high melting point, high chemical
J. Zhao et al. / Ceramics International 41 (2015) 5232–5238
stability under heat as well as its high neutron absorption cross section. HfO2 resembles its twin oxide, zirconia (ZrO2) in many physical and chemical properties [1]. Compared with ZrO2, HfO2 shows higher melting point (2810 1C), lower volumetric change (3.4%), lower Young’s modulus (0.57 GPa, room temperature) and lower thermal conductivity (3 W/m/1C, beyond 1200 1C). Besides, monoclinic hafnium oxide is stable to 1700 1C, which is 600 1C higher than the corresponding inversion temperature of zirconia [2]. Thus, HfO2 ceramics can serve as high-performance refractories for furnace lining [3], a dilatometer or a protecting tube [4] and high-temperature thermal analysis devices such as thermo-gravimetric analysis (TGA) and differential thermal analysis (DTA). However, during the process of thermal cycling, the allotropic transformation of HfO2 from monoclinic phase to tetragonal phase is accompanied by a 3.4% shrinkage (or a volume expansion during the opposite transformation) between 1500 1C and 1800 1C [3]. To avoid cracking caused by large volume change, HfO2 may be partly stabilized to tetragonal or cubic phase. The incorporation of the metastable tetragonal ZrO2 has been accomplished, which indicates that material toughness can be enhanced via arresting crack growth [5]. HfO2 might be similarly modified due to yttria-doped hafnia has similar mechanical properties to yttria doped zirconia [6]. The HfO2–Y2O3 phase diagram [7] suggested the possibility of cubic or tetragonal phase formation at relatively lower temperatures by adding a slight amount of Y2O3 to HfO2. In the previous study, Scheidecker et al. [8] investigated the effect of metastable tetragonal HfO2 on the elastic properties of partially stabilized HfO2. Yang et al. [9] reported the growth of nanometer thick Y2O3 doped HfO2 films on GaAs(0 0 1) by molecular beam epitaxy (MBE). Kita et al. [10] studied the increasing permittivity of the yttrium-doped HfO2 films. Gupta et al. [11] reported addition of yttrium in HfO2 thin films prepared on silicon by metal organic chemical vapor deposition. However, the fabrication of metastable rare-earth-doped HfO2 as high-temperature resistance refractory is seldom reported. In this paper, Y2O3 stabilized HfO2 with different amounts of Y2O3 were studied. In addition, microstructural development and phase evolution of sintered samples were investigated. Also, fire resistance performance was tested. 2. Experimental procedure 2.1. Materials processing HfO2 (mean grain size 300 nm, 99.9%, Beijing Founde Star Science & technology Co. Ltd) and Y2O3 (mean grain size 5 μm, 99.9%, Beijing Founde Star Science & technology Co. Ltd) were used as raw materials. Samples were prepared by adding Y2O3 as stabilizing agent which represented 5 mol% and 8 mol% relative to the total moles of HfO2, respectively. After mill mixing in alcohol for more than 6 h in a horizontal ball miller, the mixture was dried at 80 1C for 3 h and cylindrical shaped specimens of about 30 mm in diameter and 5 mm in height were formed by dry pressing (20 MPa),
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then isostatic pressing (160 MPa) is carried out. Pure HfO2 specimens designed as control samples were first sintered in air at 1200 1C, 1400 1C, 1550 1C, 1600 1C, 1650 1C, respectively, for 1 h in Al2O3 tube furnace (NBD-T). The sintering temperature of Y2O3-doped HfO2 was set as 1600 1C. The soaking time was 1 h and the heating rate was 10 1C/min. 2.2. Materials characterization Bulk density and apparent porosity are measured using the Archemedes’s method by boiling with water. Phases were identified by X-ray diffraction (XRD, D/Max-cA, Japan). Data were collected by step scanning over a 2θ range from 101 to 701 with a step size of 0.081 and 5 s counting time at each step. The morphology of fracture surfaces was characterized by scanning electron microscopy (SEM, model no: EVO-18, CARL ZEISS SMT LTD), with an acceleration voltage of 20 kV. Elementary compositions of Y2O3 stabilized HfO2 were determined by Energy Dispersive Spectrometer (EDS). At last, the ceramics obtained by the reference process were heated above more than 1850 1C for 1 h and then allowed to cool down at the natural cooling rate of the furnace. In order to study the fire resistance performance in the temperature range where the volumetric expansion occurs, the thermocycling process is implemented for several times. 3. Results and discussion 3.1. Sintering densification behavior of pure hafnium oxide Sintering densification behavior of pure HfO2 specimens are studied to determine the optimal sintering temperature. Fig. 1 showed that with the increase of the sintering temperature, the relative density of pure HfO2 have increased obviously. Fig. 2 displays the effect of sintering temperature on microstructures of fractures in pure HfO2 ceramics. The grain size was about 300–500 nm and some pores between grains can be seen when HfO2 was sintered at 1400 1C (as shown in Fig. 2(a)). With the increasing of sintering temperature, polycrystalline grains grew and filled up the pores between them. HfO2 was densed and grain size reached 1–1.5 μm when sintering temperature reached 1600 1C (Fig. 2(b)). At 1650 1C, however, abnormal grain growth occurred and mean grain size was 2–4 μm but the largest grain size reached up to 5 μm (Fig. 2(c)). Excessive grain growth and broad grain size distribution would be negative to mechanical properties. Comprehensively considering relative density and grain size, 1600 1C would be the optimal sintering temperature. 3.2. Phase composition and distribution of Y2O3 stabilized hafnium dioxide Pure HfO2 shows a low-temperature structure which is monoclinic and and transforms into tetragonal phase at a higher temperature [8,12]. These transformations took place at the atmospheric pressure as follows [13,14]: 1700 1C
2600 1C
2800 1C
monoclinic ⟷ tetragonal ⟷ cubic ⟷ liquid
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Fig. 3 shows XRD patterns of pure HfO2 and Y2O3 stabilized HfO2 sintered at 1600 1C. For pure HfO2 (Fig. 3(a)), only monoclinic HfO2 could be observed while the main crystal phases of HfO2 ceramics containing 5 mol% and 8 mol% of Y2O3 are monoclinic and cubic HfO2, which means that Y2O3 contributed to the transformation from monoclinic phase to cubic phase at a much lower temperature than that on theoretical level (Fig. 3(b) and (c)). With the increase of
yttrium oxide content, peak intensity of cubic HfO2 increased obviously. Therefore, as a dopant, yttrium serves effectively to induce phase transformation of HfO2 from monoclinic to cubic phase. Y2O3 phase cannot be detected by XRD. SEM images in Fig. 4 reveal that modified HfO2 is a composite, that is, comprising hafnium dioxide in two distinct crystalline phases which are homogeneously distributed, cubic
Fig. 1. Relationship between sintering temperature and relative density of pure HfO2.
Fig. 3. X-rays diffraction diagrams of pure HfO2 and Y2O3 stabilized HfO2 which were sintering at 1600 1C.
Fig. 2. SEM micrographs of pure HfO2 ceramics sinering at (a) 1400 1C, (b) 1600 1C, (c) 1650 1C. (a) SEM micrographs of pure HfO2 ceramics sinering at 1400 1C. (b) SEM micrographs of pure HfO2 ceramics sinering at 1600 1C. (c) SEM micrographs of pure HfO2 ceramics sinering at 1650 1C.
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Fig. 4. SEM micrographs of Y2O3 stabilized HfO2 at 1600 1C: (a) SEM micrographs of 5 mol% Y2O3 stabilized HfO2 at 1600 1C, (b) SEM micrographs of 8 mol% Y2O3 stabilized HfO2 at 1600 1C.
Fig. 5. EDS of hafnium oxide stabilized by 5 mol% of Y2O3.
and monoclinic structure, based on XRD results. Some holes homogeneously distributed inside. The grey area around the holes is the solid solution of hafnium oxide and yttrium oxide, Hf(1 2x)Y2xO(2 x), which is cubic phase. The presence of the solid solution around the holes might be explained by Kirkendall effect during the sintering process. Specifically, yttrium diffuses more quickly than hafnium in the solid solution, therefore the position previously occupied by Y2O3 would be vacant and pores developed after the sintering. The hole is about 1–5 μm, which is consistent with the size of the yttrium oxide agglomerates. The rest of the material is hafnium oxide of monoclinic phase. Due to the composite nature, the material is only partially stabilized by the formation of a solid solution of cubic structure with the advantages of both (i) avoiding the above mentioned volume expansion and (ii) preserving a solidus temperature as high as possible, and as close as possible to the melting temperature of pure HfO2. Interestingly, the grains of monoclinic hafnium oxide in the stabilized HfO2 (about 500 nm, Fig. 4) were markedly smaller, more compact and distributed more uniformly than that in pure HfO2 (about 3–5 μm, Fig. 2b). In order to confirm the composition of each phase, Energy Diffraction Spectrum (EDS) was applied for hafnium oxide
which was stabilized by 5 mol% of Y2O3. The element contents of two test points are shown in Fig. 5. O, Hf, Y were detected at test point 1 while only O and Hf were detected at test point 2, which further illustrated the formation of a solid solution of Y2O3 and HfO2. The composition of the solid solution was obtained based on the atomic ratio of Y and Hf: Hf0.83Y0.17O1.91. This accurate expression is taken to estimate the theoretical density of the sample later.
3.3. Calculation of phase distribution and density By taking account of the factor of multiplicity and the linear absorption coefficient, the volumetric fraction of the monoclinic phase in a zirconia stabilized by magnesia could be obtained by formula (1): [13] 1:6031I m 1 1 1 Vm ¼ ð1Þ 1:6031I m 1 1 1 þ I c 1 1 1 (Im and Ic is intensity of monoclinic phase and cubic phase, respectively.) ρ ¼ ρm V m þ ρs V s
ð2Þ
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Table 1 Densities and phase compositions of Y2O3 modified HfO2 ceramics. composition
HfO2 þ 5 mol% Y2O3 HfO2 þ 8 mol% Y2O3
Density Theoretical (g/cm3) density (g/cm3)
Relative density (%)
Water absorption (%)
Phase distribution Monoclinic phase (%)
Cubic solid solution phase (%)
8.64
9.76
88.52
0.42
3.67
46.98
53.02
8.33
9.63
86.50
0.01
0.03
25.91
74.09
(ρm: 10.1 g/cm3, theoretical density of monoclinic phase; ρs: 9.47 g/cm3, theoretical density of solid solution; Vm (%): phase distribution of monoclinic phase; Vs (%): phase distribution of cubic solid solution phase.) It is possible to estimate the percentages of monoclinic and cubic phases of Y2O3 stabilized HfO2 by using Formula (1) since phase evolution of HfO2 is similar to ZrO2. The main phase of Y2O3 stabilized HfO2 ceramics is cubic, of which the content ranged from 53.02% (5 mol% of Y2O3) to 74.09% (8 mol% of Y2O3) after sintering at 1600 1C. The unit cell parameters of Y2O3 stabilized HfO2 ceramics vary in a rather restricted range. The analysis found that the diffraction peak corresponding to lattice plane {1 1 1} of cubic HfO2 evidently moved left. Specifically, the peak of cubic solid solution HfO2 was at 30.071 while that of pure HfO2 was at 30.361, which suggested a change in lattice constant. Interplanar spacing (d value) of {1 1 1} lattice plane (2.97 Å) of solid solution is also larger than standard cubic HfO2 (2.94 Å) due to the replacing of Hf position by yttrium in the cubic phase and introduction of O vacancy, however, it is rarely affected by the different amounts of Y2O3 (2.9679 Å and 2.9702 Å corresponding to 5 mol% and 8 mol% addition of Y2O3, respectively). The theoretical density of the solid solution is estimated to be 9.47 g/cm3 and for stabilized HfO2, theoretical density could be calculated by formula (2), knowing the phase distribution. These calculated results highlight that the theoretical density of ceramic was decreased with the increase of Y2O3 content. All the calculated data is given in Table 1. The addition of Y2O3 promotes the densification of the refractory. Both water absorption and open porosity are close to 0 when yttrium oxide was added 8 mol%. The lower relative density of 8 mol% Y2O3 stabilized HfO2 is attributed to a large number of closed pores caused by Kirkendall effect. atom ratioHf=Y ¼
Open porosity (%)
V m þ 0:83V s 0:17V s
ð3Þ
We can use the accurate expression of solid solution Hf0.83Y0.17O1.91 and phase distribution to calculate the hafnium yttrium atomic ratio by formula (3). Comparing the calculated results with the actual content in a sample (listed in Table 2), it can be easily found that two lists of data are very similar, which testified that formula (1) is suitable for the hafnium oxide phase calculation.
Table 2 Atomic ratio of Hf/Y. Composition
HfO2 þ 5 mol%Y2O3 HfO2 þ 8 mol%Y2O3
Atomic ratio (Hf/Y) Calculated result
Actual content
9.93 6.82
10 6.25
3.4. Refractoriness test Dense HfO2 ceramics which were stabilized by 5 mol% and 8 mol% of Y2O3 respectively were heated above 1800 1C, maintained at the temperature for 1 h and were cooled down in the furnace at the natural cooling rate to qualify the temperature resistance. As the control group, tests were also carried out on pure HfO2 ceramic. The test was performed in a graphite furnace. All the samples were buried in boron nitride powders in case of reduction by carbon atmosphere due to direct contact of HfO2 and the graphite heater. The thermocycling was proceeded for seven times to study the behavior in the temperature range where the volumetric expansion occurred. After high-temperature treatment, all grain boundaries were gradually blurred and became out of sight for pure and Y2O3 modified HfO2 (Fig. 6). Fractured surface of stabilized specimens are flat and smooth (Fig. 6(a) and (b)) and all of the residual monoclinic phase is transformed into cubic phase due to yttrium diffusion above 1850 1C characterized by the X-rays diffraction (Fig. 8(b) and (c)) and EDS (Fig. 7). By contrast, fractured surface of pure HfO2 is rough and full of cracks caused by vast volumetric expansion of phase transformation (Fig. 6(c)). Principal phases of pure HfO2 maintained to be monoclinic phase accompanied by a small amount of tetragonal HfO2 (Fig. 8(a)). Cubic HfC is detected because diffusion in solid state is very rapid at such a high temperature and reactions occurred between HfO2 and C, forming HfC. The macroscopic results showed that pure HfO2 underwent significant cracking in the first thermocycling. The volume change associated with this transformation would cause localized cracking from microcrack and crack propagation with the increase of thermal cycles, while the stabilized pieces were intact without any crack. We can draw a conclusion that the high-temperature volumetric stability of the material can be effectively enhanced by adding Y2O3 into HfO2.
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Fig. 6. Fractured surface SEM micrographs of HfO2 ceramic after refractoriness test. (a) Fractured surface SEM micrographs of HfO2 ceramic adding 5 mol% of Y2O3 after refractoriness test. (b) Fractured surface SEM micrographs of HfO2 ceramic adding 8 mol% of Y2O3 after refractoriness test. (c) Fractured surface SEM micrographs of pure HfO2 ceramic after refractoriness test.
Fig. 7. EDS of HfO2 ceramic after refractoriness test which was stabilized by 5 mol% Y2O3 (a) and 8 mol% Y2O3 (b). (a) EDS of HfO2 ceramic after refractoriness test which was stabilized by 5 mol% Y2O3. (b) EDS of HfO2 ceramic after refractoriness test which was stabilized by 8 mol% Y2O3.
4. Conclusion Y2O3 stabilized HfO2 ceramics with 5 mol% and 8 mol% Y2O3, respectively, were successfully fabricated by pressureless sintering at the optimal sintering temperature 1600 1C for 1 h. The main phase of Y2O3 stabilized HfO2 ceramics was cubic, of which the content ranged from 53.02% (5 mol% of Y2O3) to 74.09% (8 mol% of Y2O3) at 1600 1C and the content rising to nearly 100% after firing at above 1800 1C. It indicated that most of
monoclinic HfO2 was transformed into a more stable phase, cubic HfO2, due to Y2O3 addition while XRD pattern of pure HfO2 showed only monoclinic phase. The SEM images of fractured surface further confirmed the presence of two crystal phases accompanied by some holes caused by Kirkendall effect. Accurate expression of solid solution was Hf0.83Y0.17O1.91. Phase distribution and theoretical density was calculated. Refractoriness test showed that the high-temperature stability of the material can be effectively improved by Y2O3 stabilized HfO2 pieces.
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Fig. 8. X-rays diffraction diagrams of HfO2 ceramic after refractoriness test.
Acknowledgements This work was financially supported by the National Nature Science Foundation of China (no. 51472146). References [1] X. Zhao, D. Vanderbilt, First-principles study of structural, vibrational, and lattice dielectric properties of hafnium oxide, Phys. Rev. B 65 (23) (2002) 233106. [2] C.E. Curtis, L.M. Doney, J.R. Johnson, Some properties of hafnium oxide, hafnium silicate, calcium hafnate, and hafnium carbide, J. Am. Ceram. Soc. 37 (10) (1954) 458–465.
[3] P. Piluso, M. Ferrier, C. Chaput, et al., Hafnium dioxide for porous and dense high-temperature refractories (2600 1C), J. Eur. Ceram. Soc. 29 (5) (2009) 961–968. [4] J.Y. Tewg, Y. Kuo, J. Lu, Zirconium-doped tantalum oxide gate dielectric films integrated with molybdenum, molybdenum nitride, and tungsten nitride gate electrodes, J. Electrochem. Soc. 152 (8) (2005) G643–G650. [5] E.R. Andrievskaya, L.M. Lopato, Influence of composition on the T-M transformation in the systems ZrO2–Ln2O3 (Ln¼La, Nd, Sm, Eu), J. Mater. Sci. 30 (10) (1995) 2591–2596. [6] M. Yashima, H. Takahashi, K. Ohtake, et al., Formation of metastable forms by quenching of the HfO2RO1.5 melts (R¼ Gd, Y and Yb), J. Phys. Chem. Solids 57 (3) (1996) 289–295. [7] E.M. Levin, Phase diagrams for ceramists, Am. Ceram. Soc. (1964). [8] R.W. Scheidecker, O. Hunter Jr, F.W. Calderwood, Elastic properties of partially-stabilized HfO2 compositions, J. Mater. Sci. 14 (10) (1979) 2284–2288. [9] Z.K. Yang, W.C. Lee, Y.J. Lee, et al., Cubic HfO2 doped with Y2O3 epitaxial films on GaAs (0 0 1) of enhanced dielectric constant, Appl. Phys. Lett. 90 (15) (2007) 152908. [10] K. Kita, K. Kyuno, A. Toriumi, Permittivity increase of yttrium-doped HfO2 through structural phase transformation, Appl. Phys. Lett. 86 (10) (2005) 102906. [11] E. Rauwel, C. Dubourdieu, B. Hollander, et al., Stabilization of the cubic phase of HfO2 by Y addition in films grown by metal organic chemical vapor deposition, Appl. Phys. Lett. 89 (1) (2006) 012902–012902-3. [12] C.T. LYNCH, High Temperature Oxides, Part ΙΙ: Oxides of Rare Earths, Titanium, Zirconium, Hafnium, Niobium and Tantalum, in: A.M. Alper (Ed.), Academic Press, New York, NY, 1970, pp. 193–216. [13] D.L. Porter, A.H. Heuer, Microstructural development in MgO—partially stabilized zirconia (Mg—PSZ), J. Am. Ceram. Soc. 62 (5‐6) (1979) 298–305. [14] D.W. Stacy, D.R. Wilder, The yttria–hafnia system, J. Am. Ceram. Soc. 58 (7‐8) (1975) 285–288.
CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 5232–5238 www.elsevier.com/locate/ceramint
Review paper
Fabrication of high-performance Y2O3 stabilized hafnium dioxide refractories Jiaxing Zhaoa,b, Yujun Zhanga,b,n, Hongyu Gonga,b, Yubai Zhanga,b, Xianli Wanga,b, Xue Guoa,b, Yujun Zhaoa,b a
Key Laboratory for Liquid–Solid Structural Evolution & Processing of Materials of Ministry of Education, Shandong University, Jinan 250061, PR China b Key Laboratory of Special Functional Aggregated Materials, Ministry of Education, Shandong University, Jinan 250061, PR China Received 3 December 2014; received in revised form 23 December 2014; accepted 9 January 2015 Available online 14 January 2015
Abstract Y2O3 stabilized HfO2 ceramics with 5 mol% and 8 mol% Y2O3, respectively, were successfully fabricated by pressureless sintering at 1600 1C for 1 h. The phase distribution and transformation, microstructure, and relative density were investigated for Y2O3 stabilized HfO2 ceramics. Cubic phase was observed in the Y2O3 stabilized hafnium dioxide while pure HfO2 showed monoclinical phase only. The SEM images of fractured surface indicated two kinds of structures existed in modified HfO2, the solid solution region and uniform polygon grains, and some holes caused by Kirkendall effect. Refractoriness test showed that high temperature volumetric stability of the material can be effectively enhanced by adding Y2O3 into HfO2. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Hafnium dioxide; Y2O3 stabilized; High-performance refractory
Contents 1. 2.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Materials processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Materials characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Sintering densification behavior of pure hafnium oxide. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Phase composition and distribution of Y2O3 stabilized hafnium dioxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Calculation of phase distribution and density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Refractoriness test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Corresponding author at: Key Laboratory for Liquid–Solid Structural Evolution and Processing of Materials of Ministry of Education, Shandong University, Jinan 250061, PR China. Tel./fax: þ 86 531 88399760. E-mail address: [email protected] (Y. Zhang). http://dx.doi.org/10.1016/j.ceramint.2015.01.047 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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1. Introduction Hafnium dioxide (HfO2) is technologically important because of its low Young’s modulus, high melting point, high chemical
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stability under heat as well as its high neutron absorption cross section. HfO2 resembles its twin oxide, zirconia (ZrO2) in many physical and chemical properties [1]. Compared with ZrO2, HfO2 shows higher melting point (2810 1C), lower volumetric change (3.4%), lower Young’s modulus (0.57 GPa, room temperature) and lower thermal conductivity (3 W/m/1C, beyond 1200 1C). Besides, monoclinic hafnium oxide is stable to 1700 1C, which is 600 1C higher than the corresponding inversion temperature of zirconia [2]. Thus, HfO2 ceramics can serve as high-performance refractories for furnace lining [3], a dilatometer or a protecting tube [4] and high-temperature thermal analysis devices such as thermo-gravimetric analysis (TGA) and differential thermal analysis (DTA). However, during the process of thermal cycling, the allotropic transformation of HfO2 from monoclinic phase to tetragonal phase is accompanied by a 3.4% shrinkage (or a volume expansion during the opposite transformation) between 1500 1C and 1800 1C [3]. To avoid cracking caused by large volume change, HfO2 may be partly stabilized to tetragonal or cubic phase. The incorporation of the metastable tetragonal ZrO2 has been accomplished, which indicates that material toughness can be enhanced via arresting crack growth [5]. HfO2 might be similarly modified due to yttria-doped hafnia has similar mechanical properties to yttria doped zirconia [6]. The HfO2–Y2O3 phase diagram [7] suggested the possibility of cubic or tetragonal phase formation at relatively lower temperatures by adding a slight amount of Y2O3 to HfO2. In the previous study, Scheidecker et al. [8] investigated the effect of metastable tetragonal HfO2 on the elastic properties of partially stabilized HfO2. Yang et al. [9] reported the growth of nanometer thick Y2O3 doped HfO2 films on GaAs(0 0 1) by molecular beam epitaxy (MBE). Kita et al. [10] studied the increasing permittivity of the yttrium-doped HfO2 films. Gupta et al. [11] reported addition of yttrium in HfO2 thin films prepared on silicon by metal organic chemical vapor deposition. However, the fabrication of metastable rare-earth-doped HfO2 as high-temperature resistance refractory is seldom reported. In this paper, Y2O3 stabilized HfO2 with different amounts of Y2O3 were studied. In addition, microstructural development and phase evolution of sintered samples were investigated. Also, fire resistance performance was tested. 2. Experimental procedure 2.1. Materials processing HfO2 (mean grain size 300 nm, 99.9%, Beijing Founde Star Science & technology Co. Ltd) and Y2O3 (mean grain size 5 μm, 99.9%, Beijing Founde Star Science & technology Co. Ltd) were used as raw materials. Samples were prepared by adding Y2O3 as stabilizing agent which represented 5 mol% and 8 mol% relative to the total moles of HfO2, respectively. After mill mixing in alcohol for more than 6 h in a horizontal ball miller, the mixture was dried at 80 1C for 3 h and cylindrical shaped specimens of about 30 mm in diameter and 5 mm in height were formed by dry pressing (20 MPa),
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then isostatic pressing (160 MPa) is carried out. Pure HfO2 specimens designed as control samples were first sintered in air at 1200 1C, 1400 1C, 1550 1C, 1600 1C, 1650 1C, respectively, for 1 h in Al2O3 tube furnace (NBD-T). The sintering temperature of Y2O3-doped HfO2 was set as 1600 1C. The soaking time was 1 h and the heating rate was 10 1C/min. 2.2. Materials characterization Bulk density and apparent porosity are measured using the Archemedes’s method by boiling with water. Phases were identified by X-ray diffraction (XRD, D/Max-cA, Japan). Data were collected by step scanning over a 2θ range from 101 to 701 with a step size of 0.081 and 5 s counting time at each step. The morphology of fracture surfaces was characterized by scanning electron microscopy (SEM, model no: EVO-18, CARL ZEISS SMT LTD), with an acceleration voltage of 20 kV. Elementary compositions of Y2O3 stabilized HfO2 were determined by Energy Dispersive Spectrometer (EDS). At last, the ceramics obtained by the reference process were heated above more than 1850 1C for 1 h and then allowed to cool down at the natural cooling rate of the furnace. In order to study the fire resistance performance in the temperature range where the volumetric expansion occurs, the thermocycling process is implemented for several times. 3. Results and discussion 3.1. Sintering densification behavior of pure hafnium oxide Sintering densification behavior of pure HfO2 specimens are studied to determine the optimal sintering temperature. Fig. 1 showed that with the increase of the sintering temperature, the relative density of pure HfO2 have increased obviously. Fig. 2 displays the effect of sintering temperature on microstructures of fractures in pure HfO2 ceramics. The grain size was about 300–500 nm and some pores between grains can be seen when HfO2 was sintered at 1400 1C (as shown in Fig. 2(a)). With the increasing of sintering temperature, polycrystalline grains grew and filled up the pores between them. HfO2 was densed and grain size reached 1–1.5 μm when sintering temperature reached 1600 1C (Fig. 2(b)). At 1650 1C, however, abnormal grain growth occurred and mean grain size was 2–4 μm but the largest grain size reached up to 5 μm (Fig. 2(c)). Excessive grain growth and broad grain size distribution would be negative to mechanical properties. Comprehensively considering relative density and grain size, 1600 1C would be the optimal sintering temperature. 3.2. Phase composition and distribution of Y2O3 stabilized hafnium dioxide Pure HfO2 shows a low-temperature structure which is monoclinic and and transforms into tetragonal phase at a higher temperature [8,12]. These transformations took place at the atmospheric pressure as follows [13,14]: 1700 1C
2600 1C
2800 1C
monoclinic ⟷ tetragonal ⟷ cubic ⟷ liquid
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Fig. 3 shows XRD patterns of pure HfO2 and Y2O3 stabilized HfO2 sintered at 1600 1C. For pure HfO2 (Fig. 3(a)), only monoclinic HfO2 could be observed while the main crystal phases of HfO2 ceramics containing 5 mol% and 8 mol% of Y2O3 are monoclinic and cubic HfO2, which means that Y2O3 contributed to the transformation from monoclinic phase to cubic phase at a much lower temperature than that on theoretical level (Fig. 3(b) and (c)). With the increase of
yttrium oxide content, peak intensity of cubic HfO2 increased obviously. Therefore, as a dopant, yttrium serves effectively to induce phase transformation of HfO2 from monoclinic to cubic phase. Y2O3 phase cannot be detected by XRD. SEM images in Fig. 4 reveal that modified HfO2 is a composite, that is, comprising hafnium dioxide in two distinct crystalline phases which are homogeneously distributed, cubic
Fig. 1. Relationship between sintering temperature and relative density of pure HfO2.
Fig. 3. X-rays diffraction diagrams of pure HfO2 and Y2O3 stabilized HfO2 which were sintering at 1600 1C.
Fig. 2. SEM micrographs of pure HfO2 ceramics sinering at (a) 1400 1C, (b) 1600 1C, (c) 1650 1C. (a) SEM micrographs of pure HfO2 ceramics sinering at 1400 1C. (b) SEM micrographs of pure HfO2 ceramics sinering at 1600 1C. (c) SEM micrographs of pure HfO2 ceramics sinering at 1650 1C.
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Fig. 4. SEM micrographs of Y2O3 stabilized HfO2 at 1600 1C: (a) SEM micrographs of 5 mol% Y2O3 stabilized HfO2 at 1600 1C, (b) SEM micrographs of 8 mol% Y2O3 stabilized HfO2 at 1600 1C.
Fig. 5. EDS of hafnium oxide stabilized by 5 mol% of Y2O3.
and monoclinic structure, based on XRD results. Some holes homogeneously distributed inside. The grey area around the holes is the solid solution of hafnium oxide and yttrium oxide, Hf(1 2x)Y2xO(2 x), which is cubic phase. The presence of the solid solution around the holes might be explained by Kirkendall effect during the sintering process. Specifically, yttrium diffuses more quickly than hafnium in the solid solution, therefore the position previously occupied by Y2O3 would be vacant and pores developed after the sintering. The hole is about 1–5 μm, which is consistent with the size of the yttrium oxide agglomerates. The rest of the material is hafnium oxide of monoclinic phase. Due to the composite nature, the material is only partially stabilized by the formation of a solid solution of cubic structure with the advantages of both (i) avoiding the above mentioned volume expansion and (ii) preserving a solidus temperature as high as possible, and as close as possible to the melting temperature of pure HfO2. Interestingly, the grains of monoclinic hafnium oxide in the stabilized HfO2 (about 500 nm, Fig. 4) were markedly smaller, more compact and distributed more uniformly than that in pure HfO2 (about 3–5 μm, Fig. 2b). In order to confirm the composition of each phase, Energy Diffraction Spectrum (EDS) was applied for hafnium oxide
which was stabilized by 5 mol% of Y2O3. The element contents of two test points are shown in Fig. 5. O, Hf, Y were detected at test point 1 while only O and Hf were detected at test point 2, which further illustrated the formation of a solid solution of Y2O3 and HfO2. The composition of the solid solution was obtained based on the atomic ratio of Y and Hf: Hf0.83Y0.17O1.91. This accurate expression is taken to estimate the theoretical density of the sample later.
3.3. Calculation of phase distribution and density By taking account of the factor of multiplicity and the linear absorption coefficient, the volumetric fraction of the monoclinic phase in a zirconia stabilized by magnesia could be obtained by formula (1): [13] 1:6031I m 1 1 1 Vm ¼ ð1Þ 1:6031I m 1 1 1 þ I c 1 1 1 (Im and Ic is intensity of monoclinic phase and cubic phase, respectively.) ρ ¼ ρm V m þ ρs V s
ð2Þ
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Table 1 Densities and phase compositions of Y2O3 modified HfO2 ceramics. composition
HfO2 þ 5 mol% Y2O3 HfO2 þ 8 mol% Y2O3
Density Theoretical (g/cm3) density (g/cm3)
Relative density (%)
Water absorption (%)
Phase distribution Monoclinic phase (%)
Cubic solid solution phase (%)
8.64
9.76
88.52
0.42
3.67
46.98
53.02
8.33
9.63
86.50
0.01
0.03
25.91
74.09
(ρm: 10.1 g/cm3, theoretical density of monoclinic phase; ρs: 9.47 g/cm3, theoretical density of solid solution; Vm (%): phase distribution of monoclinic phase; Vs (%): phase distribution of cubic solid solution phase.) It is possible to estimate the percentages of monoclinic and cubic phases of Y2O3 stabilized HfO2 by using Formula (1) since phase evolution of HfO2 is similar to ZrO2. The main phase of Y2O3 stabilized HfO2 ceramics is cubic, of which the content ranged from 53.02% (5 mol% of Y2O3) to 74.09% (8 mol% of Y2O3) after sintering at 1600 1C. The unit cell parameters of Y2O3 stabilized HfO2 ceramics vary in a rather restricted range. The analysis found that the diffraction peak corresponding to lattice plane {1 1 1} of cubic HfO2 evidently moved left. Specifically, the peak of cubic solid solution HfO2 was at 30.071 while that of pure HfO2 was at 30.361, which suggested a change in lattice constant. Interplanar spacing (d value) of {1 1 1} lattice plane (2.97 Å) of solid solution is also larger than standard cubic HfO2 (2.94 Å) due to the replacing of Hf position by yttrium in the cubic phase and introduction of O vacancy, however, it is rarely affected by the different amounts of Y2O3 (2.9679 Å and 2.9702 Å corresponding to 5 mol% and 8 mol% addition of Y2O3, respectively). The theoretical density of the solid solution is estimated to be 9.47 g/cm3 and for stabilized HfO2, theoretical density could be calculated by formula (2), knowing the phase distribution. These calculated results highlight that the theoretical density of ceramic was decreased with the increase of Y2O3 content. All the calculated data is given in Table 1. The addition of Y2O3 promotes the densification of the refractory. Both water absorption and open porosity are close to 0 when yttrium oxide was added 8 mol%. The lower relative density of 8 mol% Y2O3 stabilized HfO2 is attributed to a large number of closed pores caused by Kirkendall effect. atom ratioHf=Y ¼
Open porosity (%)
V m þ 0:83V s 0:17V s
ð3Þ
We can use the accurate expression of solid solution Hf0.83Y0.17O1.91 and phase distribution to calculate the hafnium yttrium atomic ratio by formula (3). Comparing the calculated results with the actual content in a sample (listed in Table 2), it can be easily found that two lists of data are very similar, which testified that formula (1) is suitable for the hafnium oxide phase calculation.
Table 2 Atomic ratio of Hf/Y. Composition
HfO2 þ 5 mol%Y2O3 HfO2 þ 8 mol%Y2O3
Atomic ratio (Hf/Y) Calculated result
Actual content
9.93 6.82
10 6.25
3.4. Refractoriness test Dense HfO2 ceramics which were stabilized by 5 mol% and 8 mol% of Y2O3 respectively were heated above 1800 1C, maintained at the temperature for 1 h and were cooled down in the furnace at the natural cooling rate to qualify the temperature resistance. As the control group, tests were also carried out on pure HfO2 ceramic. The test was performed in a graphite furnace. All the samples were buried in boron nitride powders in case of reduction by carbon atmosphere due to direct contact of HfO2 and the graphite heater. The thermocycling was proceeded for seven times to study the behavior in the temperature range where the volumetric expansion occurred. After high-temperature treatment, all grain boundaries were gradually blurred and became out of sight for pure and Y2O3 modified HfO2 (Fig. 6). Fractured surface of stabilized specimens are flat and smooth (Fig. 6(a) and (b)) and all of the residual monoclinic phase is transformed into cubic phase due to yttrium diffusion above 1850 1C characterized by the X-rays diffraction (Fig. 8(b) and (c)) and EDS (Fig. 7). By contrast, fractured surface of pure HfO2 is rough and full of cracks caused by vast volumetric expansion of phase transformation (Fig. 6(c)). Principal phases of pure HfO2 maintained to be monoclinic phase accompanied by a small amount of tetragonal HfO2 (Fig. 8(a)). Cubic HfC is detected because diffusion in solid state is very rapid at such a high temperature and reactions occurred between HfO2 and C, forming HfC. The macroscopic results showed that pure HfO2 underwent significant cracking in the first thermocycling. The volume change associated with this transformation would cause localized cracking from microcrack and crack propagation with the increase of thermal cycles, while the stabilized pieces were intact without any crack. We can draw a conclusion that the high-temperature volumetric stability of the material can be effectively enhanced by adding Y2O3 into HfO2.
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Fig. 6. Fractured surface SEM micrographs of HfO2 ceramic after refractoriness test. (a) Fractured surface SEM micrographs of HfO2 ceramic adding 5 mol% of Y2O3 after refractoriness test. (b) Fractured surface SEM micrographs of HfO2 ceramic adding 8 mol% of Y2O3 after refractoriness test. (c) Fractured surface SEM micrographs of pure HfO2 ceramic after refractoriness test.
Fig. 7. EDS of HfO2 ceramic after refractoriness test which was stabilized by 5 mol% Y2O3 (a) and 8 mol% Y2O3 (b). (a) EDS of HfO2 ceramic after refractoriness test which was stabilized by 5 mol% Y2O3. (b) EDS of HfO2 ceramic after refractoriness test which was stabilized by 8 mol% Y2O3.
4. Conclusion Y2O3 stabilized HfO2 ceramics with 5 mol% and 8 mol% Y2O3, respectively, were successfully fabricated by pressureless sintering at the optimal sintering temperature 1600 1C for 1 h. The main phase of Y2O3 stabilized HfO2 ceramics was cubic, of which the content ranged from 53.02% (5 mol% of Y2O3) to 74.09% (8 mol% of Y2O3) at 1600 1C and the content rising to nearly 100% after firing at above 1800 1C. It indicated that most of
monoclinic HfO2 was transformed into a more stable phase, cubic HfO2, due to Y2O3 addition while XRD pattern of pure HfO2 showed only monoclinic phase. The SEM images of fractured surface further confirmed the presence of two crystal phases accompanied by some holes caused by Kirkendall effect. Accurate expression of solid solution was Hf0.83Y0.17O1.91. Phase distribution and theoretical density was calculated. Refractoriness test showed that the high-temperature stability of the material can be effectively improved by Y2O3 stabilized HfO2 pieces.
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Fig. 8. X-rays diffraction diagrams of HfO2 ceramic after refractoriness test.
Acknowledgements This work was financially supported by the National Nature Science Foundation of China (no. 51472146). References [1] X. Zhao, D. Vanderbilt, First-principles study of structural, vibrational, and lattice dielectric properties of hafnium oxide, Phys. Rev. B 65 (23) (2002) 233106. [2] C.E. Curtis, L.M. Doney, J.R. Johnson, Some properties of hafnium oxide, hafnium silicate, calcium hafnate, and hafnium carbide, J. Am. Ceram. Soc. 37 (10) (1954) 458–465.
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