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Thermal and mechanical behavior of ZrTiO4-TiO2 porous ceramics by direct foaming
Author’s Accepted Manuscript Thermal and Mechanical Behaviour of ZrTiO4TiO2 Porous Ceramics by Direct Foaming Subhasree Bhaskar, Jung Gyu Park, Kee Sung Lee, Suk Young Kim, Ik Jin Kim www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(16)30850-1 http://dx.doi.org/10.1016/j.ceramint.2016.06.019 CERI13035
To appear in: Ceramics International Received date: 12 April 2016 Revised date: 4 June 2016 Accepted date: 4 June 2016 Cite this article as: Subhasree Bhaskar, Jung Gyu Park, Kee Sung Lee, Suk Young Kim and Ik Jin Kim, Thermal and Mechanical Behaviour of ZrTiO 4-TiO Porous Ceramics by Direct Foaming, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.06.019 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 galley proof before it is published in its final citable 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.
Thermal and Mechanical Behaviour of ZrTiO4-TiO2 Porous Ceramics by Direct Foaming
2
Subhasree Bhaskar1, Jung Gyu Park1, Kee Sung Lee , Suk Young. Kim3, and Ik Jin Kim1* 1
Institute of Processing and Application of Inorganic Materials (PAIM), Department of Materials Science and Engineering, Hanseo University, 46, Hanseo 1-ro, Haemi-myeon, Seosan, Chungcheongnam-do, 356-706, Republic of Korea. 2
3
School of Mechanical Systems Engineering, Kookmin University, 77, Jeongneung-ro, Seongbuk-gu, Seoul 02707, Republic of Korea.
School of Materials Science & Engineering, Yeungnam University, Gyeongsan, Gyeongbuk, Republic of Korea. Email: [email protected]
Abstract This paper reports the production of micro porous ceramics consisting of TiO2 and ZrO2 by direct foaming. ZrO2 particles in a colloidal suspension were partially hydrophobized using propyl gallate as an amphiphile at a suitable pH range of around 3.5 ~ 4.5. A TiO2 suspension with different mole ratios was added to the surface modified ZrO2 suspension to obtain ZrTiO4-TiO2 porous ceramics in the sintered sample. The influence of the TiO2 content and calcination temperature on the phase transformation, microstructure, and thermal properties of the materials was determined by thermal analysis, X-ray diffraction, field emission scanning electron microscopy, and dilatometry. The crystallization of ZrTiO4 (orthorhombic) was observed at 1100°C on the thermal hysteresis curve due to anisotropic thermal expansion. The compressive load and displacement of the sintered porous ceramics samples were calculated using the Hertzian indentation method.
Keywords ZrTiO4-TiO2 porous ceramics, Direct foaming, Hertzian indentations, Quasi-plastic behavior, Thermal hysteresis. 1
1.
Introduction Porous ceramics involve in a large number of structural applications with gradients and
temporal variations of high temperature and strain. The stresses developed in the ceramic parts subjected to these conditions are determined by the elastic properties of the constituent materials, being higher for stiffer materials. Therefore, the elastic characterization of ceramics at high temperatures is fundamental to evaluating their expected behavior in use [1]. Porous ceramics are used in numerous fields, such as filtration of molten metals or hot gases, photo catalysis for solar energy conversion water and air purification, humidity sensors, acoustic absorbers, refractory and insulation furnaces, and hard tissue repair engineering [24]. ZrTiO4-TiO2-based ceramics have been prepared traditionally with ZrO2 and TiO2 powders at high temperatures (above 1400°C) using solid state reactions [5]. Pure ZrO2 and TiO2 have a very low specific surface area and high hardness, and ZrO2 has excellent thermal and chemical stability. The homogeneous incorporation of ZrO2 and TiO2 is important for obtaining materials that exhibit chemical, thermal and mechanical stability [6]. The use of TiO2 as an additive has attracted interest because if it’s excellent photo-catalytic activity, high oxidation resistance, long-term stability, and low toxicity [7]. Controlling the morphology of materials during synthesis is of great importance because the structural characteristics strongly influence their performance and purpose [8]. Porous ceramics can be fabricated using a variety of methods, including the replica technique, the sacrificial template method, and the direct foaming technique [9]. The processing route determines the microstructure of the final macro porous ceramic. Therefore, the selection of a given processing method depends strongly on the microstructures required in the end application [10]. The direct foaming technique is suitable for preparing open and closed porous structures with various porosities. This technique was used in the present study because of its inherent features, such as versatility, simplicity, and production cost. In this method, the air is incorporated directly into the suspension or liquid media, which is then set to maintain the structure of air bubbles. The mechanical behavior of porous ceramics is influenced greatly by their pore structure. The introduced porosity affects and alters the mechanical properties, making it different from 2
that of dense ceramics. Therefore, mechanical evaluation techniques commonly applied for dense ceramics might not be suitable for porous ceramics. Such porous ceramics ordinarily appear completely brittle in traditional strength tests. On the other hand, Hertzian indentation mechanics have been used extensively for the analysis and characterization of the fracture and deformation properties of brittle ceramics, including porous ceramics [12, 13]. Hertzian fracture is commonly associated with energy dissipation by internal friction at sliding grains, platelet or whiskers, or other microstructural elements that bridge the crack wake [14, 15]. This paper reports the evolution of the thermal and mechanical properties of ZrTiO4TiO2 (ZTT) porous ceramics with various volume percentages of TiO2, prepared by a direct foaming process. The thermal expansion curves of the porous ceramic samples of the ZrTiO4TiO2 including differential thermal analysis (DTA) and thermo-gravimetric analysis (TGA), were measured. Hertzian indentation was performed to evaluate the damage behavior under constrained loading conditions. The mechanical behavior from the indentation loaddisplacement curves was investigated. The increase in TiO2 content produced an interconnected neck-foam with the exaggerated grain growth of the ZrTiO4 phase, thereby controlling
2. 2.1.
thermal
expansion
due
to
the
phase
transformation
to
ZrTiO4.
Experimental procedure Suspension preparation The ZrO2 suspension was prepared by the stepwise addition of ZrO2 powder (baddeleyite
polymorph, d50 ~ 14.77 µm, > 99.0 % purity, density ~ 5.68 g/cm3, Showa Chemicals Co. Ltd., Japan) into an aqueous mixture of 1 ~ 2 ml of 0.01 (M) of propyl gallate (Fluka, China) used as an amphiphile. The solid loadings and pH of the suspensions were fixed initially at 50 vol.% at a pH range of 3.5 ~ 4.0, respectively. A stable ZrO2 suspension was prepared by adjusting the pH of the suspension using 1 N NaOH (Yakuri Pure Chemicals, Kyoto, Japan) or 0.1 M HCl (35%, Yakuri Pure Chemicals, Osaka, Japan). Fig. 1 shows the zeta potential of the ZrO2 suspension. Homogenization and de-agglomeration were performed using zirconia balls (10 mm diameter with a 2 : 1 ratio of balls to powder). Ball milling (Digital ball mill, DHSL, Korea) proceeded for 24 ~ 48 h, and the solid concentration of the suspension was reduced to 30 3
vol.% to maintain the stability of the airflow by decreasing the viscosity. The propyl gallate (2 wt. % to ZrO2) was adjusted to the required concentration in the final ZrO2 suspension. Ball-milling of the TiO2 (rutile polymorph, d50 ~ 2.05 µm, > 99.0 % purity, density ~ 4.23 g/cm3, Junsei Chemicals, Japan) suspension proceeded for 24 ~ 48 h, and the solid concentration of the suspension was reduced initially from 50 vol.% to 15 vol.%. TiO2 suspensions at different mole ratios were added to the ZrO2 suspension, and the mixtures were stirred uniformly for 10 ~ 15 min to produce the ZrTiO4-TiO2 wet foams of the final suspension with maximum stability under constant atmospheric conditions, as shown in Fig. 2. 2.2.
Foam characterization The pendant-drop-method (KSV Instruments Ltd, Helsinki, Finland) was used to
measure the surface tension (γ) of the ZrO2-TiO2 suspension, whereas the sessile-dropmethod was used to measure the contact angle ( ). The variation in the stability of the wet foam at the particle–stabilized interfaces was attributed to the adsorption free energy
G)
required to remove an adsorbed particle of radius r from the interface of surface tension ϒαβ, which is expressed as [17] )
ϒ
)
The foaming of 100 ml of each suspension was accomplished using a household hand mixer (150 W, Super Mix, France) at full power for 15 to 20 min. Mechanical frothing facilitates the air incorporation throughout the entire suspension. The volume of air or voids in the suspension in aggregate particles is called the air content (%) of the suspension, and is usually expressed as an increased percentage of the total volume of air in the mixture before and after foaming, as follows:
Air content (%) =
(
)
x
(2)
where Vwet foam indicates the wet foam volume after foaming and Vsuspension indicates the volume of the suspension before foaming.
4
Furthermore, a broadening of the bubble size distribution occurs due to the steady diffusion of gas molecules from smaller to larger bubbles over time. The average bubble size of the foam was evaluated by optical microscopy in transmission mode (Somtech Vision, South Korea) with a connected digital camera, and was measured using the software Linear Intercept (TU Darmstadt, Germany). The size was determined by an analysis of 100 bubbles for each composition. The difference in the Laplace pressure between the bubbles of distinct sizes (R) leads to bubble disproportionation and Ostwald ripening, and can be expressed as (
)
(For spherical bubbles)
(3)
where ∆P = Laplace pressure (mPa), which is the pressure difference between the inner and outer surfaces of a bubble or droplet. This effect is caused by the surface tension
(mN/m) at
the interface between the liquid and gas. R1 and R2 are the radii of the curvature for an ellipse. For spherical bubbles, however, R1 and R2 are equal, so the second formula was used to calculate the Laplace pressure [17]. The wet foam stability (%) can be defined by the reduction of the volume of the foams after being dried at room temperature (20 ~ 25 °C), and can be represented by Eq. 4. )
(
)
(4)
where Vfinal indicates the volume of wet foam after drying at room temperature (20 ~ 25 °C), and Vinitial indicates the volume of the wet foam immediately after foaming. 2.3.
Drying and sintering The wet samples were dried at 100 °C for 1 ~ 2 h. in a convection oven (Seon Jin Co.
SD-501). The dried foams were sintered in a super kantal furnace at 1600 °C for 1 h. with heating and cooling rates of 1 °C/min and 3 °C/min, respectively. The true and apparent densities of the ZrTiO4-TiO2 porous ceramics samples were calculated using an electronic densimeter (MD 300S, Alpha Mirage). The resulting porosity was calculated from the true and apparent density values using the following equation: )
(
)
5
(5)
To analyze the phase change, a set of ZTT samples were sintered separately using a muffle furnace at three different temperatures, 600°C, 800°C, and 1000°C, for 1 h at a heating rate of 5°C/min. followed by quenching (cooled suddenly by plunging into a liquid) in water [18, 19]. The microstructures of the sintered foams were observed by field emission scanning electron microscopy (SEM & FESEM, JEOL, Japan). The phase compositions of the samples were characterized by X-ray diffraction (XRD, Rigaku D/Max 2500, Japan).
2.4 Thermal & mechanical characterization DTA TGA of the dried samples was carried out using a thermal analyzer (Shimadzu, Japan) in static air up to 1300C at a heating rate of 10C/min. A dilatometry study was carried out up to 1000C at a heating and cooling rate of 10 C/min using a thermomechanical analyzer (TMA-60 H, Shimadzu, Japan). The mechanical behavior of the load-displacements curves of the sintered ZrTiO4-TiO2 porous ceramics was examined using Hertzian indentations tests. Cylindrical specimens, 1 inch in diameter, were cut from the asreceived materials and polished. Hertzian indentation tests were performed in air using a universal testing machine (Model 5567, Instron Corp., Canton, MA) at a constant cross-head speed of 0.2 mm/min over a load range, P = 5 ~ 200 N, using tungsten carbide spheres of radius r = 6.35 mm. The load-displacement curves were plotted during loading and unloading under normal atmospheric conditions. The displacements were converted through an amplifier, converters, and the digital signal processor consecutively after measuring by an extensometer [20, 21].
3. 3.1.
Results & Discussion Physical Properties
Table 1 lists colloidal suspension and wet foam properties of the ZrTiO4-TiO2 (ZTT) porous ceramics at different stages of the direct foaming process. To avoid foam collapse in the wet state, the air-water interface of the foams needs to be stabilized. Variations in the different parameters, such as surface tension and contact angle, were observed in all the samples evaluated upon increasing the vol. % of the TiO2 particle suspension added. Typically, the surface tension of the gas-liquid interface is reduced to reduce the Laplace pressure and prolong the foam life span of the newly formed bubbles. This can also be explained by an 6
increase in the surface hydrophobicity of the particles with increasing particle concentration [18,19]. High volume foams with air contents up to 75% form upon mechanical frothing, which strongly indicates the stabilization of air bubbles, due to the attachment of particles to the air-water interface. The foam stability was measured, and ZT, ZTT1, ZTT2, ZTT3, and ZTT4 showed enhanced foam stability, which might be explained by the optimal surface hydrophobicity being achieved, due to the surface modification of the ZrO2 particles. Variations in the average bubble size with increasing particle concentration and particle hydrophobicity were observed. This reduces the resistance of air bubbles against rupture, which leads to the production of foams with variable pore sizes and various porosities of the samples sintered at 1600°C for 1 h. Table 2 summarizes physical properties of ZTT porous ceramics sintered at 1400, 1500, and 1600 C for 1 h. The true densities of ZTT ceramics was determined by the Archimedes method. These density of ZTT ceramics including the orthorhombic ZrTiO4 phase with a theoretical density of 5.06g/cm3 decreased with increasing TiO2 content because the densities of the starting oxides ZrO2 and TiO2 (rutile) are 5.83 and 4.25 g/cm3, respectively [18]. A higher sintering temperature resulted in densification of ZTT ceramics. A smaller pore size of 54.74μm (ZTT4, ZrO2 : TiO2 = 1:2) and a moderately porosity of ZTT ceramics in the range of 76.03% to 57.55% were found in sample sintered at 1600 C, respectively.
3.2.
Microstructural Characteristics Figure 3 presents the microstructures of ZrTiO4-TiO2 porous ceramics sintered at
1600 °C for 1 h. Fine interconnected neck-foam produces a highly porous stoichiometric ZrTiO4 phase (ZTT) with a mean pore size of 78.51 μm. With increasing TiO2 content (ZTT2), the mean pore size increases gradually to 89.21 μm (see Table 1), whereas a drastic decrease in the mean pore size was observed in ZTT4, 54.74 μm. In contrast to the pore size, the grain size of the sintered samples varied with the TiO2 content. An absence of microcracks can be observed in all figures. A dense and fine-grained microstructure with a uniform size distribution of the grains was observed for the composites with increasing TiO2 content. The lower mechanical properties of ZTT2 can be explained by the roughly packed grain size distribution and larger pore size of the ZTT materials (see Fig. 3b’). Therefore, the mechanical properties of the samples decrease with increasing pore size. 7
3.3.
Air content and porosity The porosity and air content of the samples are directly proportional to each other
according to Gonzenbach et al. The results in Fig. 4 tend to match the theory explained before. This is due to an increase in the concentration of unmodified particles in the suspension, resulting in an increase in the amount of free radicals, which form a thicker lamella after sintering, as shown in Fig. 3 (c’) [23]. Moreover, the air content of the wet foam is related directly to the porosity of the sintered porous ceramics samples at 1600 °C for 1 h. The air content reaches its highest point of approximately 93% for ZTT1 with the highest porosity of 78.33 %. This indicates the fabrication of the pore structures in finer struts for producing highly porous ZrTiO4-TiO2 ceramics.
3.4.
Differential Thermal Analysis The DTA curve provides data on the transformations that have occurred, such as
crystallization, melting, formation, glass transition, and sublimation. The area under a DTA peak is the enthalpy change and is unaffected by the heat capacity of the sample [24]. In Fig. 5, the exothermic peak at 83.79 °C was assigned to the removal of the additives [2]. The exothermic peak at 345.25 °C, 360.17 °C, and 374.02 °C for different mole ratios of ZrO2TiO2 are related to the removal of organic compounds with a mass loss of 3.4 % for ZT, 4.4 % for ZTT2, and 7.6 % for ZTT4 (see Fig. 6). Upon continuous heating of the ZTT materials up to 1100°C, gradual endothermic curves appear without weight loss in Fig 6. A small exothermic increase in the samples was observed at 1192.18 °C. This can be attributed to the crystallization of the orthorhombic phase of ZrTiO4 from ZrO2 and TiO2 [25]. This ZrTiO4 phase has the α-PbO structure (space group Pcnb) with the disordering of Ti4+ and Zr4+ cations occupying the available octahedral site [13, 14].
3.5.
Thermogravimetric Analysis TGA is commonly used to determine the selected characteristics of materials that
exhibit mass loss or gain due to decomposition, oxidation, or loss of volatiles. In Fig. 6, the 8
TGA curve for two different mole ratios of ZrO2 – TiO2 were measured. A considerable weight loss of above 4.69 % was observed in the range, 180 °C to 390 °C, due to dehydration of the compounds. A steady state curve was observed, which corresponds to the continuous state of the reaction. The weight loss from 390 °C to 394 °C increased to a maximum 7.54 % due to the oxidation process and reconstructive transformation of the ZrTiO4 phase in the different mole ratios of ZrO2-TiO2 [26]. This phase is meta-stable at 400 °C to 1100 °C because of the ordering process without any weight loss. The stoichiometric ZrTiO4 is stable at high temperatures (> 1150°C), whereas for low temperatures (< 1100 °C), the composition of ZrTiO4 shifts towards a higher TiO2 content with a mass loss of 6.55 %, 7.08 %, and 9.84 % from 1100°C to 1300°C, respectively. In addition, the weight loss of all the samples began at 1150°C, which according to the DTA and XRD results, corresponds to the crystallization of ZrTiO4.
3.6.
Thermal Expansion Curves Figure 7 presents the thermal expansion curves in the heating and cooling period of
porous ceramic samples at different ZrO2-TiO2 mole ratios. The ZT and ZTT4 samples sintered at 1600 °C for 1 h showed a similar thermal expansion coefficient in the range, 201000°C, with a smaller hysteresis area [21]. The heating curves of ZT and ZTT4 showed a linear increase until 1000°C, except that ZTT2 showed a gradual decrease at 670°C. Similarly, the cooling curves of ZT and ZTT4 showed the contraction of 2 x 10-3 in the temperature range, 670°C to 540°C, whereas there was a variation in ZTT2. This was attributed to thermal anisotropic expansion of the ZrTiO4 phase (αa298-1073k = 8.0 ⅹ 10-6K-1, αb298-1073k = 10.0 ⅹ 106
K-1, αc298-1073k = 6.2 ⅹ 10-6K-1, space group Pbcn), which leads to a sudden contraction of
the porous ceramics samples [2, 24]. The difference in the starting and ending positions in the heating and cooling curves was attributed to an order-disorder transformation into the ZrTiO4 phase in ZTT materials. This is unusual because ordinarily, the lattice parameter b decreases when a disordered high-temperature form changes into more ordered low-temperature form between 680°C and 1000°C during cooling [25,26]. None of the ZrO2 - TiO2 composites showed micro cracks after cooling the porous ceramics samples, as shown in Fig. 3 [27, 28].
9
3.7.
XRD Figure 8 shows XRD patterns of the porous ZrTiO4 - TiO2 samples at different
sintering temperatures, 600°C, 800°C, 1000°C, and 1600°C prepared by quenching [29]. The patterns indicate the high crystallinity of ZrTiO4 - TiO2 porous ceramics sintered at 1600 °C for 1 h with ZrTiO4, ZrO2, and TiO2 phases. The observed Bragg peaks confirmed the lack of a reaction between ZrO2 and TiO2 when the porous ceramics samples were quenched at temperatures below 1000°C (in accordance with Figs. 5 and 6). The main intensity peak of the different composites were identified with the peak at 30.595° 2θ (hkl = 111) for the orthorhombic ZrTiO4 phase, 36° 2θ (hkl = 200) for the rutile TiO2 phase, and 28° 2θ (hkl = 11) for the m-ZrO2 (major phase) phase [30, 31]. ZrTiO4 phase showed a higher peak intensity compared to the other components, which shows the larger impact of the reaction at 1600 °C. The amount of unreacted ZrO2 and TiO2 kept decreasing. This proves that the formation of ZrO2-TiO2 to ZrTiO4 (orthorhombic) with peak at 25° 2θ (hkl = 110) can begin at temperatures above 1190°C, as shown in Figs. 5 and 6, which as a result, explains the second mass loss of ZTT, ZTT2, and ZTT4 porous ceramics with an exothermic peak above 1100°C for the formation of ZrTiO4.
3.8.
Mechanical Properties Figure 9 presents the indentation load-displacement curves for the ZrO2-TiO2 porous
ceramics with different mole ratios sintered at 1600°C for 1 h, which is a plot of indentation load P as a function of the displacement () [32] using a WC ball with a radius of r = 6.35 mm. The curve for the ZrO2-TiO2 with the (a) ZT and (b) ZTT1 samples were considerably lower and flatter than those for the higher mole ratios, indicating a quasi-plastic material. This suggests a “high damage tolerance” of the specimens under constrained spherical indentation. In contrast, the ZTT2 sample has the highest pore size of approximately 89.21 μm (as shown in Table 1), indicating the lowest compressive load with respect to the displacement. Similarly, the smallest pore size was observed for the ZTT4 samples, which showed a higher loading support with a smaller displacement. The right side figure shows optical micrographs of the surface views of the Hertzian contact damage in Fig. 9 (a’) ZT, (b’) ZTT2, and (c’) ZTT4. It is clearly seen that the quasi-plastic damage is more pronounced 10
in the porous ceramics samples with the increase in the TiO2 content and densified regions are formed during indentation, indicating pore effect, which strongly contributes to the damage tolerances.
4.
Conclusions: The thermal and mechanical properties of highly porous ZrO2-TiO2 composites prepared
by the direct foaming method were investigated. All the composites showed a very fine microstructure after sintering at 1600°C for 1 h. The grain size of the ceramics was finer as the TiO2 content increased, but coarser as the sintering temperature increased. The indentation load was relatively constant over the entire displacement range in the ZTT sample, which was investigated by the Hertzian indentation method from the compressive load vs. displacement curves. Thermal analysis of the porous ceramic samples with pure ZrO2-TiO2 ceramics was conducted. The ZrTiO4 (orthorhombic) porous ceramics with a peak at 25° 2θ can begin at temperatures above 1192.18°C, which as a result, leads to the good mechanical and thermal hysteresis properties of the sintered porous ceramics.
Acknowledgements This study was financially and technically supported by Hanseo University, Kookmin University, and Yeungnam University in Republic of Korea
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16. S. Bhaskar, B. H. Kang, J. G. Park, T. Y. Lim and I. J. Kim, “ZrTiO4 porous ceramics from particle stabilized wet foam by direct foaming,” J. Kor. Phy. Soc. 68 (1) (2016) 77. 17. S. Bhaskar, J. G. Park, G. H. Cho, S. Y. Kim, and I. J. Kim, “Wet foam stability and tailoring the microstructure of porous ceramics using polymer beads,” Adv. Appl. Ceram. 114(6) (2015) 333. 18. I. J. Kim and G. Cao, “Low thermal expansion behaviour and thermal durability of ZrTiO4-Al2TiO5-Fe2O3 ceramics between 750 and 1400 °C,” J. Eur. Ceram. Soc. 22 (2002) 2627. 19. E. López-López, I. Santacruz, L. Leon-Reina, M. A. G. Aranda, R. Moreno, and C. Baudín, “Reaction sintered zirconium titanate – zirconia bulk materials form 3Y2O3 – stabilized zirconia and TiO2,” J. Eur. Ceram. Soc. 32 (2012) 1205. 20. K. S. Lee, Y. G. Jung, I. M. Peterson, B. R. Lawn, “Model for cyclic fatigue of quasiplastic ceramics in contact with spheres,” J. Am. Ceram. Soc. 83 (9) (2000) 2255. 21. J. G. Yeo, K. S. Lee, and B. R. Lawn, “Role of microstructure in dynamic fatigue of glass-ceramics after contact with spheres,” J. Am. Ceram. Soc. 83 (6) (2000) 1545. 22. S. Bhaskar, J. G. Park, M. J. Lee, T. Y. Lim, I. S. Han, and I. J. Kim, “ZrO2-TiO2 porous ceramics from particle stabilized wet foam by colloidal processing,” J. Ceram. Soc. Japan. 124 (1) (2016) 106. 23. U. T. Gonzenabach, A. R Studart, E. Tervoort and L. J. Gauckler, Langmuir, “Stabilization of foams with inorganic colloidal particles,” 10 (2006) 983. 24. H. K. D. H. Bhadeshia, “Thermal analyses techniques. differential thermal analysis,” Mat. Sci. Metgy. Phase Trans (2002). 25. V. dos Santos, M. Zeni, J.M. Hohemberger and C.P. Bergmann, “Preparation of crystalline ZrTiO4 at low thermal treatment temperatures,” Rev. Adv. Mat. Sci. 24 (2010) 44. 26. J. Macan, A. Gajovic, D. Dekanic, H. Ivankovic, “Zirconium titanate ceramics for humidity sensors synthesized by sol-gel process,” Proc. 10th ECerS Conf. (2007) 866. 27. E. López-López, M. L. Sanjuán, R. Moreno, and C. Baudín, “Phase evolution in reaction sintered zirconium titanate based materials,” J. Eur. Ceram. Soc. 30 (2010) 981. 13
28. E. López-López, C. Baudín, R. Moreno, I. Santacruz, L. Leon-Reina, and M.A.G. Aranda, “Structural characterization of bulk ZrTiO4 and its potential for thermal shock applications,” J. Eur. Ceram. Soc. 32 (2012) 299. 29. M. E. Manriquez, M. Picquart, X. Bokhimi, T. Lopez, P. Quintana, and J. M. Coronado, “X-ray diffraction , and Raman scattering study of nanostructured ZrO2TiO2 oxides by sol-gel,” J. Nanosc. Nanotech. 8 (12) (2008) 6623. 30. N. Vittayakorn, “Synthesis and a crystal structural study of microwave dielectric zirconium titanate (ZrTiO4) powders via a mixed oxide synthesis route,” J. Ceram. Pro. Res. 7 (4) (2006) 288. 31. I. J. Kim and L. G. Gauckler, “Formation, Decomposition, and Thermal Stability of Al2TiO5 Ceramics”, J. Ceram. Sci. Eng., 3 (2), (2012) 49 32. E. López-López, R. Moreno, and C. Baudín, “Fracture strength and fracture toughness of zirconium titanate – zirconia bulk composite materials,” J. Eur. Ceram. Soc. 35 (2015) 277
14
Fig. 1. Zeta potential of ZrO2 with respect to the pH of the ZrO2 suspension [16].
15
Fig. 2. Schematic diagram of the direct foaming technique to prepare the ZrTiO4-TiO2 porous ceramics.
16
Fig. 3. Microstructure of ZrTiO4-TiO2 porous ceramics sintered at the different mole ratios of ZrO2 and TiO2 at 1600 °C for 1 h – (a) and (a’) ZT, (b) and (b’) ZTT2, and (c) and (c’) ZTT4 respectively.
17
Fig. 4. Air content and porosity of the ZrTiO4-TiO2 porous ceramics with respect to the mole ratio of TiO2 in the colloidal suspension sintered at 1600 °C for 1 h.
18
Fig. 5. DTA curves of various ZrTiO4-TiO2 porous ceramics: (a) ZT, (b) ZTT2, and (c) ZTT4.
19
Fig. 6. TGA curves of various ZrTiO4-TiO2 porous ceramics: (a) ZT, (b) ZTT2, and (c) ZTT4.
20
Fig. 7. Thermal hysteresis curves of various ZTT porous ceramics sintered at 1600°C for 1 h: (a) ZT, (b) ZTT2, and (c) ZTT4.
21
Fig. 8. XRD patterns of the ZTT porous ceramics sintered at (a) 600 °C, (b) 800 °C, (c) 1000 °C, and (d) 1600 °C for 1 h.
22
Fig. 9. [Left] Compressive load vs. displacement curve for the ZrTiO4-TiO2 porous ceramics sintered at 1600 °C. [Right] Optical micrographs of the indented sites with WC spheres, comparing the damage in various ZrTiO4-TiO2 porous ceramics; (a’) ZT, (b’) ZTT2 and (c’) ZTT4.
23
Table 1 Colloidal suspension and wet foam characterization of the ZrTiO4-TiO2 porous ceramics in different stages of direct foaming process [22].
SAMPLE NAME
ZT (1:1.00) ZTT1 (1:1.25) ZTT2 (1:1.50) ZTT3 (1:1.75) ZTT4 (1:2.00)
SUSPENSION CHARACTERIZATION
WET FOAM CHRACTERIZATION
Contact angle
Surface Tension
Air content
Wet foam
Adsorption free
Laplace pressure
Average bubble size
[θ]
[mN/m]
[%]
stability [%]
energy [J]
[mPa]
[μm]
71.35
55.61
68.75
76.30
1.48
83.32
75.47
52.65
75.00
80.00
1.20
99.18
56.20
62.69
56.52
74.00
1.32
127.36
58.25
54.67
60.00
74.80
1.18
107.56
54.73
42.15
44.44
72.00
1.16
80.74
24
Table 2 Physical properties of the ZrTiO4-TiO2 porous ceramics in different sintering temperature SAMPLE NAME
ZT
ZTT1
ZTT2
ZTT3
ZTT4
SINTERED FOAM CHARACTERIZATION
TEMPERATURE [℃]
Average pore size [μm]
True density[g/cm3]
Porosity [%]
Volume shrinkage [%]
1400
99.44
5.00
76.24
22.61
1500
101.30
5.28
72.36
19.19
1600
78.51
5.50
76.03
43.66
1400
97.15
4.65
77.99
19.82
1500
98.17
4.97
71.33
18.89
1600
86.97
5.26
78.33
30.30
1400
93.92
4.35
74.02
18.00
1500
94.77
4.66
68.66
17.10
1600
89.21
5.10
61.77
23.00
1400
66.30
4.07
66.76
17.31
1500
66.88
4.42
65.73
16.44
1600
65.35
4.90
67.32
18.74
1400
53.77
3.94
62.41
17.06
1500
55.04
4.33
63.92
16.32
1600
54.74
4.80
57.55
16.88
25
PII: DOI: Reference:
S0272-8842(16)30850-1 http://dx.doi.org/10.1016/j.ceramint.2016.06.019 CERI13035
To appear in: Ceramics International Received date: 12 April 2016 Revised date: 4 June 2016 Accepted date: 4 June 2016 Cite this article as: Subhasree Bhaskar, Jung Gyu Park, Kee Sung Lee, Suk Young Kim and Ik Jin Kim, Thermal and Mechanical Behaviour of ZrTiO 4-TiO Porous Ceramics by Direct Foaming, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.06.019 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 galley proof before it is published in its final citable 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.
Thermal and Mechanical Behaviour of ZrTiO4-TiO2 Porous Ceramics by Direct Foaming
2
Subhasree Bhaskar1, Jung Gyu Park1, Kee Sung Lee , Suk Young. Kim3, and Ik Jin Kim1* 1
Institute of Processing and Application of Inorganic Materials (PAIM), Department of Materials Science and Engineering, Hanseo University, 46, Hanseo 1-ro, Haemi-myeon, Seosan, Chungcheongnam-do, 356-706, Republic of Korea. 2
3
School of Mechanical Systems Engineering, Kookmin University, 77, Jeongneung-ro, Seongbuk-gu, Seoul 02707, Republic of Korea.
School of Materials Science & Engineering, Yeungnam University, Gyeongsan, Gyeongbuk, Republic of Korea. Email: [email protected]
Abstract This paper reports the production of micro porous ceramics consisting of TiO2 and ZrO2 by direct foaming. ZrO2 particles in a colloidal suspension were partially hydrophobized using propyl gallate as an amphiphile at a suitable pH range of around 3.5 ~ 4.5. A TiO2 suspension with different mole ratios was added to the surface modified ZrO2 suspension to obtain ZrTiO4-TiO2 porous ceramics in the sintered sample. The influence of the TiO2 content and calcination temperature on the phase transformation, microstructure, and thermal properties of the materials was determined by thermal analysis, X-ray diffraction, field emission scanning electron microscopy, and dilatometry. The crystallization of ZrTiO4 (orthorhombic) was observed at 1100°C on the thermal hysteresis curve due to anisotropic thermal expansion. The compressive load and displacement of the sintered porous ceramics samples were calculated using the Hertzian indentation method.
Keywords ZrTiO4-TiO2 porous ceramics, Direct foaming, Hertzian indentations, Quasi-plastic behavior, Thermal hysteresis. 1
1.
Introduction Porous ceramics involve in a large number of structural applications with gradients and
temporal variations of high temperature and strain. The stresses developed in the ceramic parts subjected to these conditions are determined by the elastic properties of the constituent materials, being higher for stiffer materials. Therefore, the elastic characterization of ceramics at high temperatures is fundamental to evaluating their expected behavior in use [1]. Porous ceramics are used in numerous fields, such as filtration of molten metals or hot gases, photo catalysis for solar energy conversion water and air purification, humidity sensors, acoustic absorbers, refractory and insulation furnaces, and hard tissue repair engineering [24]. ZrTiO4-TiO2-based ceramics have been prepared traditionally with ZrO2 and TiO2 powders at high temperatures (above 1400°C) using solid state reactions [5]. Pure ZrO2 and TiO2 have a very low specific surface area and high hardness, and ZrO2 has excellent thermal and chemical stability. The homogeneous incorporation of ZrO2 and TiO2 is important for obtaining materials that exhibit chemical, thermal and mechanical stability [6]. The use of TiO2 as an additive has attracted interest because if it’s excellent photo-catalytic activity, high oxidation resistance, long-term stability, and low toxicity [7]. Controlling the morphology of materials during synthesis is of great importance because the structural characteristics strongly influence their performance and purpose [8]. Porous ceramics can be fabricated using a variety of methods, including the replica technique, the sacrificial template method, and the direct foaming technique [9]. The processing route determines the microstructure of the final macro porous ceramic. Therefore, the selection of a given processing method depends strongly on the microstructures required in the end application [10]. The direct foaming technique is suitable for preparing open and closed porous structures with various porosities. This technique was used in the present study because of its inherent features, such as versatility, simplicity, and production cost. In this method, the air is incorporated directly into the suspension or liquid media, which is then set to maintain the structure of air bubbles. The mechanical behavior of porous ceramics is influenced greatly by their pore structure. The introduced porosity affects and alters the mechanical properties, making it different from 2
that of dense ceramics. Therefore, mechanical evaluation techniques commonly applied for dense ceramics might not be suitable for porous ceramics. Such porous ceramics ordinarily appear completely brittle in traditional strength tests. On the other hand, Hertzian indentation mechanics have been used extensively for the analysis and characterization of the fracture and deformation properties of brittle ceramics, including porous ceramics [12, 13]. Hertzian fracture is commonly associated with energy dissipation by internal friction at sliding grains, platelet or whiskers, or other microstructural elements that bridge the crack wake [14, 15]. This paper reports the evolution of the thermal and mechanical properties of ZrTiO4TiO2 (ZTT) porous ceramics with various volume percentages of TiO2, prepared by a direct foaming process. The thermal expansion curves of the porous ceramic samples of the ZrTiO4TiO2 including differential thermal analysis (DTA) and thermo-gravimetric analysis (TGA), were measured. Hertzian indentation was performed to evaluate the damage behavior under constrained loading conditions. The mechanical behavior from the indentation loaddisplacement curves was investigated. The increase in TiO2 content produced an interconnected neck-foam with the exaggerated grain growth of the ZrTiO4 phase, thereby controlling
2. 2.1.
thermal
expansion
due
to
the
phase
transformation
to
ZrTiO4.
Experimental procedure Suspension preparation The ZrO2 suspension was prepared by the stepwise addition of ZrO2 powder (baddeleyite
polymorph, d50 ~ 14.77 µm, > 99.0 % purity, density ~ 5.68 g/cm3, Showa Chemicals Co. Ltd., Japan) into an aqueous mixture of 1 ~ 2 ml of 0.01 (M) of propyl gallate (Fluka, China) used as an amphiphile. The solid loadings and pH of the suspensions were fixed initially at 50 vol.% at a pH range of 3.5 ~ 4.0, respectively. A stable ZrO2 suspension was prepared by adjusting the pH of the suspension using 1 N NaOH (Yakuri Pure Chemicals, Kyoto, Japan) or 0.1 M HCl (35%, Yakuri Pure Chemicals, Osaka, Japan). Fig. 1 shows the zeta potential of the ZrO2 suspension. Homogenization and de-agglomeration were performed using zirconia balls (10 mm diameter with a 2 : 1 ratio of balls to powder). Ball milling (Digital ball mill, DHSL, Korea) proceeded for 24 ~ 48 h, and the solid concentration of the suspension was reduced to 30 3
vol.% to maintain the stability of the airflow by decreasing the viscosity. The propyl gallate (2 wt. % to ZrO2) was adjusted to the required concentration in the final ZrO2 suspension. Ball-milling of the TiO2 (rutile polymorph, d50 ~ 2.05 µm, > 99.0 % purity, density ~ 4.23 g/cm3, Junsei Chemicals, Japan) suspension proceeded for 24 ~ 48 h, and the solid concentration of the suspension was reduced initially from 50 vol.% to 15 vol.%. TiO2 suspensions at different mole ratios were added to the ZrO2 suspension, and the mixtures were stirred uniformly for 10 ~ 15 min to produce the ZrTiO4-TiO2 wet foams of the final suspension with maximum stability under constant atmospheric conditions, as shown in Fig. 2. 2.2.
Foam characterization The pendant-drop-method (KSV Instruments Ltd, Helsinki, Finland) was used to
measure the surface tension (γ) of the ZrO2-TiO2 suspension, whereas the sessile-dropmethod was used to measure the contact angle ( ). The variation in the stability of the wet foam at the particle–stabilized interfaces was attributed to the adsorption free energy
G)
required to remove an adsorbed particle of radius r from the interface of surface tension ϒαβ, which is expressed as [17] )
ϒ
)
The foaming of 100 ml of each suspension was accomplished using a household hand mixer (150 W, Super Mix, France) at full power for 15 to 20 min. Mechanical frothing facilitates the air incorporation throughout the entire suspension. The volume of air or voids in the suspension in aggregate particles is called the air content (%) of the suspension, and is usually expressed as an increased percentage of the total volume of air in the mixture before and after foaming, as follows:
Air content (%) =
(
)
x
(2)
where Vwet foam indicates the wet foam volume after foaming and Vsuspension indicates the volume of the suspension before foaming.
4
Furthermore, a broadening of the bubble size distribution occurs due to the steady diffusion of gas molecules from smaller to larger bubbles over time. The average bubble size of the foam was evaluated by optical microscopy in transmission mode (Somtech Vision, South Korea) with a connected digital camera, and was measured using the software Linear Intercept (TU Darmstadt, Germany). The size was determined by an analysis of 100 bubbles for each composition. The difference in the Laplace pressure between the bubbles of distinct sizes (R) leads to bubble disproportionation and Ostwald ripening, and can be expressed as (
)
(For spherical bubbles)
(3)
where ∆P = Laplace pressure (mPa), which is the pressure difference between the inner and outer surfaces of a bubble or droplet. This effect is caused by the surface tension
(mN/m) at
the interface between the liquid and gas. R1 and R2 are the radii of the curvature for an ellipse. For spherical bubbles, however, R1 and R2 are equal, so the second formula was used to calculate the Laplace pressure [17]. The wet foam stability (%) can be defined by the reduction of the volume of the foams after being dried at room temperature (20 ~ 25 °C), and can be represented by Eq. 4. )
(
)
(4)
where Vfinal indicates the volume of wet foam after drying at room temperature (20 ~ 25 °C), and Vinitial indicates the volume of the wet foam immediately after foaming. 2.3.
Drying and sintering The wet samples were dried at 100 °C for 1 ~ 2 h. in a convection oven (Seon Jin Co.
SD-501). The dried foams were sintered in a super kantal furnace at 1600 °C for 1 h. with heating and cooling rates of 1 °C/min and 3 °C/min, respectively. The true and apparent densities of the ZrTiO4-TiO2 porous ceramics samples were calculated using an electronic densimeter (MD 300S, Alpha Mirage). The resulting porosity was calculated from the true and apparent density values using the following equation: )
(
)
5
(5)
To analyze the phase change, a set of ZTT samples were sintered separately using a muffle furnace at three different temperatures, 600°C, 800°C, and 1000°C, for 1 h at a heating rate of 5°C/min. followed by quenching (cooled suddenly by plunging into a liquid) in water [18, 19]. The microstructures of the sintered foams were observed by field emission scanning electron microscopy (SEM & FESEM, JEOL, Japan). The phase compositions of the samples were characterized by X-ray diffraction (XRD, Rigaku D/Max 2500, Japan).
2.4 Thermal & mechanical characterization DTA TGA of the dried samples was carried out using a thermal analyzer (Shimadzu, Japan) in static air up to 1300C at a heating rate of 10C/min. A dilatometry study was carried out up to 1000C at a heating and cooling rate of 10 C/min using a thermomechanical analyzer (TMA-60 H, Shimadzu, Japan). The mechanical behavior of the load-displacements curves of the sintered ZrTiO4-TiO2 porous ceramics was examined using Hertzian indentations tests. Cylindrical specimens, 1 inch in diameter, were cut from the asreceived materials and polished. Hertzian indentation tests were performed in air using a universal testing machine (Model 5567, Instron Corp., Canton, MA) at a constant cross-head speed of 0.2 mm/min over a load range, P = 5 ~ 200 N, using tungsten carbide spheres of radius r = 6.35 mm. The load-displacement curves were plotted during loading and unloading under normal atmospheric conditions. The displacements were converted through an amplifier, converters, and the digital signal processor consecutively after measuring by an extensometer [20, 21].
3. 3.1.
Results & Discussion Physical Properties
Table 1 lists colloidal suspension and wet foam properties of the ZrTiO4-TiO2 (ZTT) porous ceramics at different stages of the direct foaming process. To avoid foam collapse in the wet state, the air-water interface of the foams needs to be stabilized. Variations in the different parameters, such as surface tension and contact angle, were observed in all the samples evaluated upon increasing the vol. % of the TiO2 particle suspension added. Typically, the surface tension of the gas-liquid interface is reduced to reduce the Laplace pressure and prolong the foam life span of the newly formed bubbles. This can also be explained by an 6
increase in the surface hydrophobicity of the particles with increasing particle concentration [18,19]. High volume foams with air contents up to 75% form upon mechanical frothing, which strongly indicates the stabilization of air bubbles, due to the attachment of particles to the air-water interface. The foam stability was measured, and ZT, ZTT1, ZTT2, ZTT3, and ZTT4 showed enhanced foam stability, which might be explained by the optimal surface hydrophobicity being achieved, due to the surface modification of the ZrO2 particles. Variations in the average bubble size with increasing particle concentration and particle hydrophobicity were observed. This reduces the resistance of air bubbles against rupture, which leads to the production of foams with variable pore sizes and various porosities of the samples sintered at 1600°C for 1 h. Table 2 summarizes physical properties of ZTT porous ceramics sintered at 1400, 1500, and 1600 C for 1 h. The true densities of ZTT ceramics was determined by the Archimedes method. These density of ZTT ceramics including the orthorhombic ZrTiO4 phase with a theoretical density of 5.06g/cm3 decreased with increasing TiO2 content because the densities of the starting oxides ZrO2 and TiO2 (rutile) are 5.83 and 4.25 g/cm3, respectively [18]. A higher sintering temperature resulted in densification of ZTT ceramics. A smaller pore size of 54.74μm (ZTT4, ZrO2 : TiO2 = 1:2) and a moderately porosity of ZTT ceramics in the range of 76.03% to 57.55% were found in sample sintered at 1600 C, respectively.
3.2.
Microstructural Characteristics Figure 3 presents the microstructures of ZrTiO4-TiO2 porous ceramics sintered at
1600 °C for 1 h. Fine interconnected neck-foam produces a highly porous stoichiometric ZrTiO4 phase (ZTT) with a mean pore size of 78.51 μm. With increasing TiO2 content (ZTT2), the mean pore size increases gradually to 89.21 μm (see Table 1), whereas a drastic decrease in the mean pore size was observed in ZTT4, 54.74 μm. In contrast to the pore size, the grain size of the sintered samples varied with the TiO2 content. An absence of microcracks can be observed in all figures. A dense and fine-grained microstructure with a uniform size distribution of the grains was observed for the composites with increasing TiO2 content. The lower mechanical properties of ZTT2 can be explained by the roughly packed grain size distribution and larger pore size of the ZTT materials (see Fig. 3b’). Therefore, the mechanical properties of the samples decrease with increasing pore size. 7
3.3.
Air content and porosity The porosity and air content of the samples are directly proportional to each other
according to Gonzenbach et al. The results in Fig. 4 tend to match the theory explained before. This is due to an increase in the concentration of unmodified particles in the suspension, resulting in an increase in the amount of free radicals, which form a thicker lamella after sintering, as shown in Fig. 3 (c’) [23]. Moreover, the air content of the wet foam is related directly to the porosity of the sintered porous ceramics samples at 1600 °C for 1 h. The air content reaches its highest point of approximately 93% for ZTT1 with the highest porosity of 78.33 %. This indicates the fabrication of the pore structures in finer struts for producing highly porous ZrTiO4-TiO2 ceramics.
3.4.
Differential Thermal Analysis The DTA curve provides data on the transformations that have occurred, such as
crystallization, melting, formation, glass transition, and sublimation. The area under a DTA peak is the enthalpy change and is unaffected by the heat capacity of the sample [24]. In Fig. 5, the exothermic peak at 83.79 °C was assigned to the removal of the additives [2]. The exothermic peak at 345.25 °C, 360.17 °C, and 374.02 °C for different mole ratios of ZrO2TiO2 are related to the removal of organic compounds with a mass loss of 3.4 % for ZT, 4.4 % for ZTT2, and 7.6 % for ZTT4 (see Fig. 6). Upon continuous heating of the ZTT materials up to 1100°C, gradual endothermic curves appear without weight loss in Fig 6. A small exothermic increase in the samples was observed at 1192.18 °C. This can be attributed to the crystallization of the orthorhombic phase of ZrTiO4 from ZrO2 and TiO2 [25]. This ZrTiO4 phase has the α-PbO structure (space group Pcnb) with the disordering of Ti4+ and Zr4+ cations occupying the available octahedral site [13, 14].
3.5.
Thermogravimetric Analysis TGA is commonly used to determine the selected characteristics of materials that
exhibit mass loss or gain due to decomposition, oxidation, or loss of volatiles. In Fig. 6, the 8
TGA curve for two different mole ratios of ZrO2 – TiO2 were measured. A considerable weight loss of above 4.69 % was observed in the range, 180 °C to 390 °C, due to dehydration of the compounds. A steady state curve was observed, which corresponds to the continuous state of the reaction. The weight loss from 390 °C to 394 °C increased to a maximum 7.54 % due to the oxidation process and reconstructive transformation of the ZrTiO4 phase in the different mole ratios of ZrO2-TiO2 [26]. This phase is meta-stable at 400 °C to 1100 °C because of the ordering process without any weight loss. The stoichiometric ZrTiO4 is stable at high temperatures (> 1150°C), whereas for low temperatures (< 1100 °C), the composition of ZrTiO4 shifts towards a higher TiO2 content with a mass loss of 6.55 %, 7.08 %, and 9.84 % from 1100°C to 1300°C, respectively. In addition, the weight loss of all the samples began at 1150°C, which according to the DTA and XRD results, corresponds to the crystallization of ZrTiO4.
3.6.
Thermal Expansion Curves Figure 7 presents the thermal expansion curves in the heating and cooling period of
porous ceramic samples at different ZrO2-TiO2 mole ratios. The ZT and ZTT4 samples sintered at 1600 °C for 1 h showed a similar thermal expansion coefficient in the range, 201000°C, with a smaller hysteresis area [21]. The heating curves of ZT and ZTT4 showed a linear increase until 1000°C, except that ZTT2 showed a gradual decrease at 670°C. Similarly, the cooling curves of ZT and ZTT4 showed the contraction of 2 x 10-3 in the temperature range, 670°C to 540°C, whereas there was a variation in ZTT2. This was attributed to thermal anisotropic expansion of the ZrTiO4 phase (αa298-1073k = 8.0 ⅹ 10-6K-1, αb298-1073k = 10.0 ⅹ 106
K-1, αc298-1073k = 6.2 ⅹ 10-6K-1, space group Pbcn), which leads to a sudden contraction of
the porous ceramics samples [2, 24]. The difference in the starting and ending positions in the heating and cooling curves was attributed to an order-disorder transformation into the ZrTiO4 phase in ZTT materials. This is unusual because ordinarily, the lattice parameter b decreases when a disordered high-temperature form changes into more ordered low-temperature form between 680°C and 1000°C during cooling [25,26]. None of the ZrO2 - TiO2 composites showed micro cracks after cooling the porous ceramics samples, as shown in Fig. 3 [27, 28].
9
3.7.
XRD Figure 8 shows XRD patterns of the porous ZrTiO4 - TiO2 samples at different
sintering temperatures, 600°C, 800°C, 1000°C, and 1600°C prepared by quenching [29]. The patterns indicate the high crystallinity of ZrTiO4 - TiO2 porous ceramics sintered at 1600 °C for 1 h with ZrTiO4, ZrO2, and TiO2 phases. The observed Bragg peaks confirmed the lack of a reaction between ZrO2 and TiO2 when the porous ceramics samples were quenched at temperatures below 1000°C (in accordance with Figs. 5 and 6). The main intensity peak of the different composites were identified with the peak at 30.595° 2θ (hkl = 111) for the orthorhombic ZrTiO4 phase, 36° 2θ (hkl = 200) for the rutile TiO2 phase, and 28° 2θ (hkl = 11) for the m-ZrO2 (major phase) phase [30, 31]. ZrTiO4 phase showed a higher peak intensity compared to the other components, which shows the larger impact of the reaction at 1600 °C. The amount of unreacted ZrO2 and TiO2 kept decreasing. This proves that the formation of ZrO2-TiO2 to ZrTiO4 (orthorhombic) with peak at 25° 2θ (hkl = 110) can begin at temperatures above 1190°C, as shown in Figs. 5 and 6, which as a result, explains the second mass loss of ZTT, ZTT2, and ZTT4 porous ceramics with an exothermic peak above 1100°C for the formation of ZrTiO4.
3.8.
Mechanical Properties Figure 9 presents the indentation load-displacement curves for the ZrO2-TiO2 porous
ceramics with different mole ratios sintered at 1600°C for 1 h, which is a plot of indentation load P as a function of the displacement () [32] using a WC ball with a radius of r = 6.35 mm. The curve for the ZrO2-TiO2 with the (a) ZT and (b) ZTT1 samples were considerably lower and flatter than those for the higher mole ratios, indicating a quasi-plastic material. This suggests a “high damage tolerance” of the specimens under constrained spherical indentation. In contrast, the ZTT2 sample has the highest pore size of approximately 89.21 μm (as shown in Table 1), indicating the lowest compressive load with respect to the displacement. Similarly, the smallest pore size was observed for the ZTT4 samples, which showed a higher loading support with a smaller displacement. The right side figure shows optical micrographs of the surface views of the Hertzian contact damage in Fig. 9 (a’) ZT, (b’) ZTT2, and (c’) ZTT4. It is clearly seen that the quasi-plastic damage is more pronounced 10
in the porous ceramics samples with the increase in the TiO2 content and densified regions are formed during indentation, indicating pore effect, which strongly contributes to the damage tolerances.
4.
Conclusions: The thermal and mechanical properties of highly porous ZrO2-TiO2 composites prepared
by the direct foaming method were investigated. All the composites showed a very fine microstructure after sintering at 1600°C for 1 h. The grain size of the ceramics was finer as the TiO2 content increased, but coarser as the sintering temperature increased. The indentation load was relatively constant over the entire displacement range in the ZTT sample, which was investigated by the Hertzian indentation method from the compressive load vs. displacement curves. Thermal analysis of the porous ceramic samples with pure ZrO2-TiO2 ceramics was conducted. The ZrTiO4 (orthorhombic) porous ceramics with a peak at 25° 2θ can begin at temperatures above 1192.18°C, which as a result, leads to the good mechanical and thermal hysteresis properties of the sintered porous ceramics.
Acknowledgements This study was financially and technically supported by Hanseo University, Kookmin University, and Yeungnam University in Republic of Korea
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16. S. Bhaskar, B. H. Kang, J. G. Park, T. Y. Lim and I. J. Kim, “ZrTiO4 porous ceramics from particle stabilized wet foam by direct foaming,” J. Kor. Phy. Soc. 68 (1) (2016) 77. 17. S. Bhaskar, J. G. Park, G. H. Cho, S. Y. Kim, and I. J. Kim, “Wet foam stability and tailoring the microstructure of porous ceramics using polymer beads,” Adv. Appl. Ceram. 114(6) (2015) 333. 18. I. J. Kim and G. Cao, “Low thermal expansion behaviour and thermal durability of ZrTiO4-Al2TiO5-Fe2O3 ceramics between 750 and 1400 °C,” J. Eur. Ceram. Soc. 22 (2002) 2627. 19. E. López-López, I. Santacruz, L. Leon-Reina, M. A. G. Aranda, R. Moreno, and C. Baudín, “Reaction sintered zirconium titanate – zirconia bulk materials form 3Y2O3 – stabilized zirconia and TiO2,” J. Eur. Ceram. Soc. 32 (2012) 1205. 20. K. S. Lee, Y. G. Jung, I. M. Peterson, B. R. Lawn, “Model for cyclic fatigue of quasiplastic ceramics in contact with spheres,” J. Am. Ceram. Soc. 83 (9) (2000) 2255. 21. J. G. Yeo, K. S. Lee, and B. R. Lawn, “Role of microstructure in dynamic fatigue of glass-ceramics after contact with spheres,” J. Am. Ceram. Soc. 83 (6) (2000) 1545. 22. S. Bhaskar, J. G. Park, M. J. Lee, T. Y. Lim, I. S. Han, and I. J. Kim, “ZrO2-TiO2 porous ceramics from particle stabilized wet foam by colloidal processing,” J. Ceram. Soc. Japan. 124 (1) (2016) 106. 23. U. T. Gonzenabach, A. R Studart, E. Tervoort and L. J. Gauckler, Langmuir, “Stabilization of foams with inorganic colloidal particles,” 10 (2006) 983. 24. H. K. D. H. Bhadeshia, “Thermal analyses techniques. differential thermal analysis,” Mat. Sci. Metgy. Phase Trans (2002). 25. V. dos Santos, M. Zeni, J.M. Hohemberger and C.P. Bergmann, “Preparation of crystalline ZrTiO4 at low thermal treatment temperatures,” Rev. Adv. Mat. Sci. 24 (2010) 44. 26. J. Macan, A. Gajovic, D. Dekanic, H. Ivankovic, “Zirconium titanate ceramics for humidity sensors synthesized by sol-gel process,” Proc. 10th ECerS Conf. (2007) 866. 27. E. López-López, M. L. Sanjuán, R. Moreno, and C. Baudín, “Phase evolution in reaction sintered zirconium titanate based materials,” J. Eur. Ceram. Soc. 30 (2010) 981. 13
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14
Fig. 1. Zeta potential of ZrO2 with respect to the pH of the ZrO2 suspension [16].
15
Fig. 2. Schematic diagram of the direct foaming technique to prepare the ZrTiO4-TiO2 porous ceramics.
16
Fig. 3. Microstructure of ZrTiO4-TiO2 porous ceramics sintered at the different mole ratios of ZrO2 and TiO2 at 1600 °C for 1 h – (a) and (a’) ZT, (b) and (b’) ZTT2, and (c) and (c’) ZTT4 respectively.
17
Fig. 4. Air content and porosity of the ZrTiO4-TiO2 porous ceramics with respect to the mole ratio of TiO2 in the colloidal suspension sintered at 1600 °C for 1 h.
18
Fig. 5. DTA curves of various ZrTiO4-TiO2 porous ceramics: (a) ZT, (b) ZTT2, and (c) ZTT4.
19
Fig. 6. TGA curves of various ZrTiO4-TiO2 porous ceramics: (a) ZT, (b) ZTT2, and (c) ZTT4.
20
Fig. 7. Thermal hysteresis curves of various ZTT porous ceramics sintered at 1600°C for 1 h: (a) ZT, (b) ZTT2, and (c) ZTT4.
21
Fig. 8. XRD patterns of the ZTT porous ceramics sintered at (a) 600 °C, (b) 800 °C, (c) 1000 °C, and (d) 1600 °C for 1 h.
22
Fig. 9. [Left] Compressive load vs. displacement curve for the ZrTiO4-TiO2 porous ceramics sintered at 1600 °C. [Right] Optical micrographs of the indented sites with WC spheres, comparing the damage in various ZrTiO4-TiO2 porous ceramics; (a’) ZT, (b’) ZTT2 and (c’) ZTT4.
23
Table 1 Colloidal suspension and wet foam characterization of the ZrTiO4-TiO2 porous ceramics in different stages of direct foaming process [22].
SAMPLE NAME
ZT (1:1.00) ZTT1 (1:1.25) ZTT2 (1:1.50) ZTT3 (1:1.75) ZTT4 (1:2.00)
SUSPENSION CHARACTERIZATION
WET FOAM CHRACTERIZATION
Contact angle
Surface Tension
Air content
Wet foam
Adsorption free
Laplace pressure
Average bubble size
[θ]
[mN/m]
[%]
stability [%]
energy [J]
[mPa]
[μm]
71.35
55.61
68.75
76.30
1.48
83.32
75.47
52.65
75.00
80.00
1.20
99.18
56.20
62.69
56.52
74.00
1.32
127.36
58.25
54.67
60.00
74.80
1.18
107.56
54.73
42.15
44.44
72.00
1.16
80.74
24
Table 2 Physical properties of the ZrTiO4-TiO2 porous ceramics in different sintering temperature SAMPLE NAME
ZT
ZTT1
ZTT2
ZTT3
ZTT4
SINTERED FOAM CHARACTERIZATION
TEMPERATURE [℃]
Average pore size [μm]
True density[g/cm3]
Porosity [%]
Volume shrinkage [%]
1400
99.44
5.00
76.24
22.61
1500
101.30
5.28
72.36
19.19
1600
78.51
5.50
76.03
43.66
1400
97.15
4.65
77.99
19.82
1500
98.17
4.97
71.33
18.89
1600
86.97
5.26
78.33
30.30
1400
93.92
4.35
74.02
18.00
1500
94.77
4.66
68.66
17.10
1600
89.21
5.10
61.77
23.00
1400
66.30
4.07
66.76
17.31
1500
66.88
4.42
65.73
16.44
1600
65.35
4.90
67.32
18.74
1400
53.77
3.94
62.41
17.06
1500
55.04
4.33
63.92
16.32
1600
54.74
4.80
57.55
16.88
25