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A quantitative study of the calcination and sintering of nanocrystalline titanium dioxide and its flexural strength properties
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Materials Chemistry and Physics 109 (2008) 392–398
A quantitative study of the calcination and sintering of nanocrystalline titanium dioxide and its flexural strength properties Samar J. Kalita ∗ , Shipeng Qiu, Saurabh Verma Department of Mechanical, Materials and Aerospace Engineering, University of Central Florida, Orlando, FL 32816-2450, United States Received 15 August 2007; received in revised form 18 November 2007; accepted 1 December 2007
Abstract We performed a quantitative study of the calcination and sintering of nanocrystalline titanium dioxide (TiO2 ) using the Rietveld refinement technique. Previously, we developed a sol–gel technique to synthesize 5–15 nm anatase powder. Here, we performed a quantitative analysis to study the phase evolution during calcination (400 and 800 ◦ C) and the crystal structure after sintering (1400 and 1500 ◦ C). TiO2 nanopowder was obtained by hydrolyzing titanium tetraisopropoxide in a mixture of isopropanol and deionized water, after calcination at 400 ◦ C. Additionally, the powder was cold die compacted and sintered at 1200–1600 ◦ C, to study the biaxial flexural strength and microstructure as a function of sintering temperature. Powder X-ray diffraction technique was used for phase analysis. Scanning electron microscopy was used for microstructural analysis. Rietveld refinement, performed using GSAS software, provided accurate quantitative analysis. Grain size and density increased with increasing sintering temperature. Biaxial flexural tests, performed as per ASTM F-394 with some adaptation, demonstrated a maximum strength of 127.6 MPa in specimens sintered at 1500 ◦ C. © 2008 Elsevier B.V. All rights reserved. Keywords: Titanium dioxide; Nanomaterials; Nanoceramics; Biaxial flexural strength; Pressure-less sintering
1. Introduction Nanocrystalline titanium dioxide (TiO2 ) has gained significant attention during recent years due to its unique and interesting optical, dielectric and catalytic properties [1,2]. Nanophase TiO2 has also been shown to exhibit increased osteoblast adhesion and is being researched as a promising biomaterial for orthopedic applications [3]. In all applications, the properties and performance of the nanostructured TiO2 products depend on size, morphology, crystallinity and phase purity of the starting nanopowder and, sintering/processing conditions used. Particle-size reduction and understanding the associated phase evolution/transformation during calcination/sintering in the nanostructured TiO2 system is of importance. Similarly, flexural strength of TiO2 ceramic has direct relevance to their performance in service. Nano-TiO2 has significant potential to ∗
Corresponding author at: Department of Mechanical, Materials and Aerospace Engineering, P.O. Box 162450, University of Central Florida, Orlando, FL 32816-2450, United States. Tel.: +1 407 823 3159; fax: +1 407 823 0208. E-mail address: [email protected] (S.J. Kalita). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.12.031
offer remarkable physical, mechanical, optical, biological and electrical properties. TiO2 exists in three polymorphs viz., anatase, rutile and brookite (other structures exist as well, for example, cotunnite TiO2 has been synthesized at high pressures and is one of the hardest polycrystalline materials known [4]). Both anatase and rutile have tetragonal symmetry (anatase has a body centered tetragonal structure whereas rutile is simple tetragonal), while 14 -P4 /mnm brookite is rhombohedral. Rutile belongs to the D4h 2 19 space group, while anatase belongs to the D4h -I41 /amd space group [5]. In both structures, slightly distorted octahedra are the basic building blocks, which consist of a titanium atom surrounded by six oxygen atoms in a more or less distorted octahedral configuration. During recent years, many processes have been developed to synthesize nanoscale TiO2 powder which include chemical vapor deposition (CVD) [6], solvothermal [7], hydrothermal [8], oxidation of titanium tetrachloride [9], mechanical alloying/milling [10], mechanochemical [11], RF thermal plasma [12] thermal decomposition and the versatile sol–gel technique [13,14]. Among them, the sol–gel process offers unique advantages such as better control over stoichiometric composition,
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ease of synthesis, better homogeneity and production of highpurity powder. Processing conditions, such as chemical concentration, pH, peptization time, calcinations time and temperature influence the particle size and phase purity of the final powder. Although the number of current and potential applications of nanostructured TiO2 ceramic is enormous, not much research has been done to understand and evaluate its mechanical properties and quantitative phase transformation characteristics as a function of sintering/processing parameters. The current literature on nano-TiO2 also lacks information on correlation of mechanical properties with microstructure and phase composition. With an increased interest in mechanical behavior of TiO2 coatings and films, there is a need for research to investigate and enhance their mechanical performance for relevant applications in gas sensors, as wear-resistant materials or, as bioceramics for possible bone grafts in hard tissue engineering. The mechanical strength of ceramic structures is assessed by different experimental techniques including three-point bending [15], four-point bending [16], compression testing [17] nondestructive testing [18] and the biaxial flexural test [19]. The biaxial flexure testing is becoming widely recognized as a reliable technique for studying brittle materials, since the maximum tensile stress occurs within the central loading area and premature failure due to edge defects is eliminated. A thin circular test specimen rests on three symmetrically spaced peripheral points, which is being bent by a force at the center of the disk. The breaking load, the specimen dimensions and elastic constants and the radii of the support and load are used to compute the maximum tensile stress which is at the center of the tension (convex) surface. This is typically the point of origin of the fracture. The biaxial stress state is possibly more severe than the uniaxial stress state and thus a better approach for conservative design. In our previous work on nanocrystalline TiO2 , we developed a simple and easily reproducible sol–gel-based technique to synthesize nanopowder (anatase) having average particle size 5–15 nm. The work included characterization of the synthesized nanopowder [14]. In this research, we used the same technique to synthesize TiO2 nanopowder and performed a quantitative study using Rietveld analysis to understand the phase evolution during calcination (400 and 800 ◦ C) and phase transformation during sintering at 1400 and 1500 ◦ C. In addition, we fabricated dense structures of nanocrystalline TiO2 and have studied their biaxial flexural strength (Modulus of Rupture) and microstructure as a function of sintering temperature. The objective of this research was to investigate the influence of nanopowder and sintering temperature on flexural strength and to develop a correlation between flexural strength, microstructure and phase composition as analyzed through Rietveld refinement technique. 2. Materials and methods 2.1. Synthesis of nanocrystalline powder Nanocrystalline titanium dioxide powder with an average crystallite size of 5–15 nm was synthesized using a simple sol–gel technique. By hydrolyzing titanium tetraisopropoxide in a mixture of isopropanol and deionized water, TiO2 nanopowder was obtained after calcination at 400 ◦ C for 3 h in a table-
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top furnace. Details on TiO2 nanopowder synthesis and characterization are provided in Ref. [14]. Amorphous gel was also calcined at 800 ◦ C for 3 h to study quantitative phase evolution characteristics as a result of high-temperature calcination.
2.2. Processing of dense TiO2 structures The synthesized TiO2 nanopowder was compacted by a traditional cold die compaction method using a steel mold having an internal diameter of 12.7 mm at a pressure of 19.4 MPa. During the cold die compaction, the powder was densified by powder rearrangement, including sliding and rolling, so as to decrease the porosity. Since nanoceramic powders have a greater tendency to form agglomerates, it is sometimes necessary to grind the powders first to break the agglomeration. The green samples were prepared in a uniaxial single action manual hydraulic press (Model 3851-0, Carver Inc., Wabash, IN). A dry P.T.F.E. film (made with Dupont Krytox) was sprayed on the inside surface of the mold and punch to reduce the friction between the fine powder and metal surfaces. All green samples were then sintered in a high-temperature programmable muffle furnace (Model 46100, Barnstead International Co., Dubuque, IA), in air, at a temperature of 1200, 1300, 1400, 1500 and 1600 ◦ C for 3 h, separately. A sintering cycle suitable for TiO2 ceramics was developed to attain improved densification and to avoid cracks in the sintered specimens by introducing several soaking temperatures. The consequent sintering cycle used several steps: first, holding at 150 ◦ C to stabilize the furnace; second, holding at 400 ◦ C to remove residual stresses from the green structures; and the final holding at the desired sintering temperature for densification. A heating rate of 4 ◦ C min−1 and a cooling rate of 10 ◦ C min−1 were used to minimize thermal stress-induced cracking during sintering and to help in densification.
2.3. Phase and microstructural analyses of the sintered specimens A powder X-ray diffraction (XRD) technique was used to study the phase evolution/transformation in TiO2 structures sintered at 1200, 1300, 1400, 1500 and 1600 ◦ C. For this, TiO2 structures sintered at different temperatures were ground into a fine powder, separately, using a mortar and pestle. The fine powder obtained was then used for XRD analysis in a Rigaku diffractometer (Model D/MAX-B, Rigaku Co., Tokyo, Japan) equipped with Ni-filtered Cu K␣ radiation (λ = 0.15418 nm) at 35 kV and 30 mA settings. The 2θ step size was 0.04◦ , and a scanning rate of 1.5◦ min−1 was used. Scanning electron microscopy (SEM) was used to observe and analyze the microstructure of the sintered TiO2 ceramics. The TiO2 specimens sintered at 1300, 1400 and 1600 ◦ C were gold-coated for 1 min using a magnetron sputter coater from Emitech Inc. The gold-coated specimens were observed in a JOEL SEM (Model 6400F, JEOL, Tokyo, Japan).
2.4. Compositional analysis using Rietveld refinement The Rietveld refinement technique [20], which was originally introduced for the analysis of constant wavelength neutron diffraction data [21], is being broadly used for the analysis of neutron, X-ray and synchrotron diffraction data nowadays. This technique, implemented in the LANL code General Structure Analysis System (GSAS) [22], was used to analyze X-ray diffraction spectra for quantitative phase analysis in this research. In the Rietveld method, the intensity at every point in the spectrum is determined by adding the calculated background and Bragg scattering intensities corresponding to diffraction peaks. The refinement procedure varies selected parameters (e.g., phase volume fractions, lattice parameters, and phase texture, etc.), and constructs linear constraints between parameters (e.g., atomic fraction of A + atomic fraction of B = 1), until the calculated and measured spectra match in a least-squares fit. Errors are quantified and are associated with the statistics of the fit. Furthermore, Rietveld refinement can account for variations in intensity due to changes in phase volume fractions (in multiphase materials) or to preferred orientation (texture). A generalized spherical harmonic description [23] was used to account for the evolving texture in the existing phases. In this work, Reitveld refinement technique was used for quantitative phase characterization in as-synthesized powder calcined at 400 and 800 ◦ C separately, and TiO2 structures sintered at 1400 and 1500 ◦ C. The analy-
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sis provided information on lattice constants and crystal structures of the nano-TiO2 .
2.5. Biaxial flexural testing of nanostructured TiO2 Biaxial flexural strength tests were performed in accordance with ASTM F-394 standard with some alteration in fixture dimension to accommodate the specimen size. Flexural strength properties of nanostructured TiO2 ceramics were studied in specimens sintered at two different temperatures viz., 1400 and 1500 ◦ C for 3 h, separately. The tests were performed in a tensile tester (Model 3369, Instron Co., USA) at a constant crosshead speed of 0.05 mm min−1 . ASTM F394-7818 supports the use of the piston on three-ball test. This test concerns the supporting of a disc specimen by three ball bearings near its periphery but equi-distant from a load piston. The test fixture design compensated for the lack of perfectly plane and parallel surfaces and allowed for the testing of slightly warped specimens. Uniform loading may be assisted by a layer of non-ridged material such as polythene between the piston and test specimens. A flat ended ball piston (0.75 mm in diameter), pressing against a three-ball (1.98 mm in diameter) test jig was developed and used for the biaxial flexural test of circular disc-shaped sintered specimens. To closely mimic the ASTM F 394-78 standard the three-ball test jig was fabricated using hardened steel balls but the dimensions of the ball and test specimens were smaller than that specified in ASTM standard. This was done to accommodate the smaller dimensions of the specimens used in this work. Sintered disc specimens were centered and supported on three hardened steel balls positioned 120◦ apart on a circle, 7.5 mm in diameter. A thin plastic-coated thick paper sheet was placed between the specimen and flattened ball piston to distribute the load evenly, as specified in the ASTM standard. The specimens were tested on a fully automated tensile testing machine from Instron Inc. (Model 3369) with a crosshead speed of 0.05 mm min−1 . The recorded fracture load was used to calculate the biaxial fracture strength using the following equation: S=
−0.2387P(X − Y ) d2
(1)
where S is maximum center tensile stress in MPa and P is the total load causing fracture in N.
B 2
X = (1 + ν) ln
+
(1 − ν) B 2
C 2 2 A
Y = (1 + ν) 1 + ln
C
C
A 2
+ (1 − ν)
C
Fig. 1. Rietveld refinement results of synthesized anatase nanopowder (calcined at 400 ◦ C). The observed diffraction intensities are displayed as crosses in red, with the calculated values drawn as a curve in green. The reflection positions are marked and the difference curve (Io − Ic ) in purple is displayed near the bottom of the graph. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
The reduced χ2 -value was found to be 1.380 and the convergence was achieved, which meant the quality of the fit was good. The calculated unit cell formula weight found from the Rietveld refinement was 319.592. The density was calculated to be 3.951 g cm−3 . The lattice parameters for the as-synthesized anatase nano-TiO2 powder, calcined at 400 ◦ C, were calculated as a = b = 0.3793 nm, c = 0.9502 nm, α = β = γ = 90◦ , which were quite reasonable when comparing with the values for anatase phase reported in Ref. [5]. The volume of the unit cell was ˚ 3. computed to be 134.304 (±0.344) A Fig. 2 shows the results of Rietveld refinement of the X-ray diffraction data for the synthesized TiO2 nanopowder calcined at 800 ◦ C for 3 h. As calcination temperature was raised to 800 ◦ C, the rutile phase was 100%, and the anatase to rutile phase
where ν is Poisson’s ratio (taken as 0.27), A is the radius of support circle in mm, B is the radius of loaded area or ram tip in mm, C is the radius of specimen in mm, and d is the specimen thickness at fracture origin in mm. Earlier [14], we reported hardness and compression strength in TiO2 structures sintered at 1300–1600 ◦ C. The specimens sintered at 1500 ◦ C exhibited the highest compressive strength, while the specimens sintered at 1600 ◦ C exhibited the highest hardness. It is reported that Vickers hardness is a convenient method for estimating the yield strength (σ y ) of ceramics as HV ≈ 3σ y [24]. Accordingly, 1400 and 1500 ◦ C were selected as sintering temperatures of choice in this study for assessing the flexural strength. Seven specimens sintered at 1500 ◦ C and six specimens sintered at 1400 ◦ C were tested for the accuracy of results.
3. Results 3.1. Compositional analysis of the calcined nano-TiO2 using Rietveld technique Fig. 1 shows the results of Rietveld refinement of the X-ray diffraction data for the synthesized TiO2 nanopowder calcined at 400 ◦ C for 3 h. Rietveld refinement confirmed that the powder was pure anatase. The crystallite size corresponding to the ˚ β = 0.774◦ ) was computed peak at 2θ = 24.92◦ (λ = 1.5418 A, using the Scherrer’s equation and was found to be 10.5 nm.
Fig. 2. Rietveld refinement results of synthesized TiO2 (rutile phase) nanopowder, obtained through calcination of the gel at 800 ◦ C. The observed diffraction intensities are displayed as crosses in red, with the calculated values drawn as a curve in green. The reflection positions are marked and the difference curve (Io − Ic ) in purple is displayed near the bottom of the graph. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
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Table 1 The lattice parameters of TiO2 (calcined/sintered) and their corresponding unit cell volume from Rietveld analysis Thermal treatment/temperature/phase
Calcination/400 ◦ C/anatase Calcination/800 ◦ C/rutile Sintering/1400 ◦ C/rutile Sintering/1500 ◦ C/rutile
˚ Lattice parameters (A) a
b
c
3.772105 (±0.003482) 4.592226 (±0.000181) 4.599384 (±0.000342) 4.598760 (±0.000507)
– – – –
9.438909 (±0.009544) 2.959896 (±0.000208) 2.961549 (±0.000262) 2.960840 (±0.000346)
transformation had already been completed. Based on Rietveld refinement, the reduced χ2 -value was found to be 1.364 and the convergence was achieved, which meant quality of the fit was good. The crystallite size, computed using Scherrer’s equation corresponding to a rutile (1 1 0) reflection, was 46.2 nm. Calculated unit cell formula weight found from Rietveld refinement was 159.796. The density was calculated to be 4.251 g cm−3 . The lattice parameters of rutile TiO2 nanocrystallite and computed volume of the unit cell are presented in Table 1. The values obtained in our analysis are in line with the values for rutile phase reported in Ref. [5]. 3.2. Phase and microstructural analyses X-ray powder diffraction analysis was conducted to analyze phase transformation in TiO2 structures sintered at different temperatures viz., 1200, 1300, 1400, 1500 and 1600 ◦ C, separately. The results of phase analysis are presented in Fig. 3. Peaks in each X-ray diffraction pattern were recorded and verified using standard JCPDS standard files #21-1276, for rutile TiO2 . Identical patterns were recorded in all the specimens con-
c/a
˚ 3) Unit cell volume (A
2.5023 0.6445 0.6439 0.6438
134.304 (±0.344) 62.42 (±0.005) 62.650 (±0.010) 62.618 (±0.018)
firming no further phase change, in the temperature range of 1300–1600 ◦ C. However, the intensity and sharpness of the rutile peaks grew with the increase in sintering temperature indicating an increase in the crystallite size in the sintered structures, as the sintering temperature was raised from 1200 to 1600 ◦ C. A typical green structure formed after uniaxial compaction and a sintered TiO2 structure are shown in Fig. 4. Sintering at 1200 ◦ C showed poor density and relatively low crystallinity (see Fig. 3) and therefore was not considered for SEM characterization. The effects of sintering on the microstructure of the sintered TiO2 ceramics were studied in a JOEL SEM and the SEM micrographs obtained are presented in Fig. 5. It is clear from these SEM micrographs that grain growth continued with the increase in the sintering temperature. Grain boundaries are evident in the micrographs, confirming crystallinity and good sintering. This result is consistent with our X-ray diffraction phase analysis, where peak intensity of rutile phase increased with the sintering temperature. The pore sizes were reduced with the increase in sintering temperature from 1300 to 1600 ◦ C. 3.3. Compositional analysis of sintered structures using Rietveld refinement Fig. 6a and b presents the Rietveld refinement results of the X-ray diffraction data for TiO2 structures sintered at 1400 and
Fig. 3. Phase analysis of nanostructured TiO2 ceramics as a function of sintering temperature. Specimens were sintered in air at 1200–1600 ◦ C for 3 h, separately, in a muffle furnace.
Fig. 4. Effect of sintering in sol–gel processed nanocrystalline (anatase powder, 5–15 nm) TiO2 ceramics.
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Fig. 5. Scanning electron micrographs of TiO2 ceramics sintered at (a) 1300 ◦ C, (b) 1400 ◦ C and (c) 1600 ◦ C for 3 h; (d) shows a high magnification SEM of TiO2 sintered at 1400 ◦ C.
1500 ◦ C for 3 h, separately. The phase composition is pure rutile as revealed by XRD results (Fig. 3). Based on Rietveld refinement, the reduced χ2 -value was found to be 1.890 and 1.548 for sintering at 1400 and 1500 ◦ C, respectively. This means the convergence was achieved, which meant the quality of the fit
was good in both cases. The calculated unit cell formula weight found from Rietveld refinement was 159.796. The density was calculated to be 4.235 and 4.238 g cm−3 sintering at 1400 and 1500 ◦ C, respectively. The lattice parameters of rutile TiO2 and computed volume of the unit cell are presented in Table 1.
Fig. 6. Rietveld refinement of X-ray diffraction patterns of nanostructured TiO2 ceramics sintered at: (a) 1400 ◦ C and (b) 1500 ◦ C. The observed diffraction intensities are displayed as crosses in red, with the calculated values drawn as a curve in green. The reflection positions are marked and the difference curve (Io − Ic ) in purple is displayed near the bottom of the graph. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
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though the average sintered density in both sets are different, this difference is not statistically significant. 4. Discussion
Fig. 7. Flexural strength and geometric bulk density (ρg ) of sintered TiO2 structures.
These values are comparable with the values that are reported in literature [5]. 3.4. Assessment of biaxial flexural strength In this work, we determined flexural strength properties of sintered TiO2 structures at 1400 and 1500 ◦ C, separately, prepared using 5–15 nm starting powder. In our previous work [14], we performed Vickers hardness testing and compression testing on similar structures sintered at 1300–1600 ◦ C. Results of biaxial flexural strength test, as shown in Fig. 7, revealed that using fine nanopowder, flexural strength of TiO2 sintered structures can be enhanced. The biaxial flexural strength was measured on seven specimens sintered at 1500 ◦ C and six specimens sintered at 1400 ◦ C. The average flexural strength computed to be 127.60 (±7.01) MPa and 111.74 (±6.26) MPa, respectively for specimens sintered at 1500 and 1400 ◦ C. The geometric bulk density (ρg ) of the sintered specimens was evaluated from the measurements of the mass of specimen and its volume (determined by dimensional measurements) using the following equation: geometric bulk density (ρg ) =
mass (m) volume (V )
(2)
The average sintered density (ρg ) computed for the same sets of specimens was found to be 3.82 (±0.06) and 3.78 (±0.11) g cm−3 , respectively for sintering temperatures of 1500 and 1400 ◦ C. To determine and compare statistical significance of our flexural strength test results, Student’s t-test was performed to show the significance in data for samples sintered at two different temperatures viz., 1400 and 1500 ◦ C. Probability associated with a Student’s paired t-test (two samples assuming equal variance, i.e. homoscedastic Student’s t-test), with a two-tailed distribution, was computed and reported. Our calculated p and t values were 0.0225 and 2.65, respectively. The calculated t value exceeds the tabulated value corresponding to p = 0.05, consequently the means are significantly different. The difference between the means is at the 95% level and therefore the difference is statistically “significant”. Similarly, Student’s paired t-test was performed to determine statistical significance of sintered density data and the t value was found to be 1.64, which means
In our previous work [14], we discussed the synthesis and characterization of nanocrystalline TiO2 powder. Using the same technique, we processed TiO2 nanopowder of size 5–15 nm after calcination at 400 ◦ C in pure anatase phase. The calcination temperature was high enough to achieve crystallization, and at the same time, helped minimize the thermal growth of the crystallites and maintain nanoscale features in the calcined powder in the lower end of nano-range. The Rietveld refinement results, as presented in Table 1, were in good agreement with the available literature. Rutile belongs 14 -P4 /mnm space group (lattice constant a = 0.4584 nm, to D4h 2 19 c = 0.2953 nm, c/a = 0.644), anatase belongs to D4h -I41 /amd space group (lattice constant a = 0.3733 nm, c = 0.937 nm, 15 -Pbca space group c/a = 2.51), while brookite belongs to D2h (lattice constant a = 0.5436 nm, c = 0.9166 nm, c/a = 0.944) [5]. The lattice parameters of our nano-TiO2 powder calcined at 400 ◦ C, were calculated to be a = 0.3772 nm, c = 0.9439 nm, c/a = 2.50, α = β = γ = 90◦ through Rietveld refinement, which were comparable with the literature. Similar comparable results were obtained for rutile phase at different temperature (Table 1). The three-point flexural strength test is the standard for advanced ceramics. However, inherent sensitivity to flaws and defects near specimen edges is a major drawback of this test [25]. This problem is not seen in the biaxial flexural test because of the difference in loading conditions, specimen geometry, and test set-up [25]. The strength measurement with the biaxial test data does, however, require material information such as the Poisson’s ratio. Specimen flaws regardless of orientation are also taken into account in the biaxial flexural testing, which has made it a widely recognized as a reliable technique for studying the strength of brittle materials since the maximum tensile stress occurs within the central loading area and failures at the edge of the specimens, a serious concern in three-point bend test, are avoided. We observed that compression and flexural strength of nanostructured TiO2 increases with the increase in sintering temperature, which can be explained based on SEM analysis of the microstructure. Evidently (Fig. 5), in the case of the 1300 ◦ C sintering temperature materials, the porosity was high and most pores were interconnected. For the specimens sintered at 1400 and 1600 ◦ C, continuous grain boundary networks were formed. Most of the pores were present at triple junctions and the grain boundaries. In light of the obtained microstructural information, the decrease in porosity at 1400 ◦ C and higher temperatures is possibly due to the bridging of fine crystallites and formation of closed pores. There is no apparent secondary phase/s present within the grain or along the grain boundaries. It is also clear from our XRD analysis that only the rutile phase is present within this range of sintering temperature. Below 1500 ◦ C, the removal of pores during the densification process played a significant role in increasing the flexural and compressive strength, since
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specimens had smaller grain-sizes and the distribution of grain size was relatively homogenous and uniform compared to those sintered at 1600 ◦ C. Again, with the increase in the sintering temperature, it was expected that the material would achieve better densification at the expense of grain growth. This effect was especially significant at 1600 ◦ C where large and irregularshaped grains were observed. 5. Conclusions Synthesis of nanocrystalline TiO2 (anatase) powder using sol–gel technique is simple and easily reproducible. Rietveld refinement results showed reduced χ2 -values, confirming that the obtained results were accurate. Calculated unit cell densities were found to be 3.951 g cm−3 (anatase at 400 ◦ C), 4.251 g cm−3 (rutile at 800 ◦ C), 4.235 g cm−3 (rutile at 1400 ◦ C) and 4.238 g cm−3 (rutile at 1500 ◦ C). Both sintered density and grain-size increased with the sintering temperature. The highest sintered density of 3.82 g cm−3 was achieved at 1500 ◦ C. Maximum biaxial flexural strength of 127.60 (±7.01) MPa was observed in structures sintered at 1500 ◦ C. The biaxial flexural strength results were in agreement with hardness and compression strength results reported earlier, i.e. strength and hardness increased with increase in sintering temperature from 1400 to 1500 ◦ C. We conclude from our research that sintering at 1500 ◦ C gives combination of high compression strength, high flexural strength and high hardness. Acknowledgements We are grateful for the editorial help by Chayanika D. Kalita in preparing this manuscript. Assistance from Materials Characterization Facility of the University of Central Florida is thankfully recognized, as well.
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Materials Chemistry and Physics 109 (2008) 392–398
A quantitative study of the calcination and sintering of nanocrystalline titanium dioxide and its flexural strength properties Samar J. Kalita ∗ , Shipeng Qiu, Saurabh Verma Department of Mechanical, Materials and Aerospace Engineering, University of Central Florida, Orlando, FL 32816-2450, United States Received 15 August 2007; received in revised form 18 November 2007; accepted 1 December 2007
Abstract We performed a quantitative study of the calcination and sintering of nanocrystalline titanium dioxide (TiO2 ) using the Rietveld refinement technique. Previously, we developed a sol–gel technique to synthesize 5–15 nm anatase powder. Here, we performed a quantitative analysis to study the phase evolution during calcination (400 and 800 ◦ C) and the crystal structure after sintering (1400 and 1500 ◦ C). TiO2 nanopowder was obtained by hydrolyzing titanium tetraisopropoxide in a mixture of isopropanol and deionized water, after calcination at 400 ◦ C. Additionally, the powder was cold die compacted and sintered at 1200–1600 ◦ C, to study the biaxial flexural strength and microstructure as a function of sintering temperature. Powder X-ray diffraction technique was used for phase analysis. Scanning electron microscopy was used for microstructural analysis. Rietveld refinement, performed using GSAS software, provided accurate quantitative analysis. Grain size and density increased with increasing sintering temperature. Biaxial flexural tests, performed as per ASTM F-394 with some adaptation, demonstrated a maximum strength of 127.6 MPa in specimens sintered at 1500 ◦ C. © 2008 Elsevier B.V. All rights reserved. Keywords: Titanium dioxide; Nanomaterials; Nanoceramics; Biaxial flexural strength; Pressure-less sintering
1. Introduction Nanocrystalline titanium dioxide (TiO2 ) has gained significant attention during recent years due to its unique and interesting optical, dielectric and catalytic properties [1,2]. Nanophase TiO2 has also been shown to exhibit increased osteoblast adhesion and is being researched as a promising biomaterial for orthopedic applications [3]. In all applications, the properties and performance of the nanostructured TiO2 products depend on size, morphology, crystallinity and phase purity of the starting nanopowder and, sintering/processing conditions used. Particle-size reduction and understanding the associated phase evolution/transformation during calcination/sintering in the nanostructured TiO2 system is of importance. Similarly, flexural strength of TiO2 ceramic has direct relevance to their performance in service. Nano-TiO2 has significant potential to ∗
Corresponding author at: Department of Mechanical, Materials and Aerospace Engineering, P.O. Box 162450, University of Central Florida, Orlando, FL 32816-2450, United States. Tel.: +1 407 823 3159; fax: +1 407 823 0208. E-mail address: [email protected] (S.J. Kalita). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.12.031
offer remarkable physical, mechanical, optical, biological and electrical properties. TiO2 exists in three polymorphs viz., anatase, rutile and brookite (other structures exist as well, for example, cotunnite TiO2 has been synthesized at high pressures and is one of the hardest polycrystalline materials known [4]). Both anatase and rutile have tetragonal symmetry (anatase has a body centered tetragonal structure whereas rutile is simple tetragonal), while 14 -P4 /mnm brookite is rhombohedral. Rutile belongs to the D4h 2 19 space group, while anatase belongs to the D4h -I41 /amd space group [5]. In both structures, slightly distorted octahedra are the basic building blocks, which consist of a titanium atom surrounded by six oxygen atoms in a more or less distorted octahedral configuration. During recent years, many processes have been developed to synthesize nanoscale TiO2 powder which include chemical vapor deposition (CVD) [6], solvothermal [7], hydrothermal [8], oxidation of titanium tetrachloride [9], mechanical alloying/milling [10], mechanochemical [11], RF thermal plasma [12] thermal decomposition and the versatile sol–gel technique [13,14]. Among them, the sol–gel process offers unique advantages such as better control over stoichiometric composition,
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ease of synthesis, better homogeneity and production of highpurity powder. Processing conditions, such as chemical concentration, pH, peptization time, calcinations time and temperature influence the particle size and phase purity of the final powder. Although the number of current and potential applications of nanostructured TiO2 ceramic is enormous, not much research has been done to understand and evaluate its mechanical properties and quantitative phase transformation characteristics as a function of sintering/processing parameters. The current literature on nano-TiO2 also lacks information on correlation of mechanical properties with microstructure and phase composition. With an increased interest in mechanical behavior of TiO2 coatings and films, there is a need for research to investigate and enhance their mechanical performance for relevant applications in gas sensors, as wear-resistant materials or, as bioceramics for possible bone grafts in hard tissue engineering. The mechanical strength of ceramic structures is assessed by different experimental techniques including three-point bending [15], four-point bending [16], compression testing [17] nondestructive testing [18] and the biaxial flexural test [19]. The biaxial flexure testing is becoming widely recognized as a reliable technique for studying brittle materials, since the maximum tensile stress occurs within the central loading area and premature failure due to edge defects is eliminated. A thin circular test specimen rests on three symmetrically spaced peripheral points, which is being bent by a force at the center of the disk. The breaking load, the specimen dimensions and elastic constants and the radii of the support and load are used to compute the maximum tensile stress which is at the center of the tension (convex) surface. This is typically the point of origin of the fracture. The biaxial stress state is possibly more severe than the uniaxial stress state and thus a better approach for conservative design. In our previous work on nanocrystalline TiO2 , we developed a simple and easily reproducible sol–gel-based technique to synthesize nanopowder (anatase) having average particle size 5–15 nm. The work included characterization of the synthesized nanopowder [14]. In this research, we used the same technique to synthesize TiO2 nanopowder and performed a quantitative study using Rietveld analysis to understand the phase evolution during calcination (400 and 800 ◦ C) and phase transformation during sintering at 1400 and 1500 ◦ C. In addition, we fabricated dense structures of nanocrystalline TiO2 and have studied their biaxial flexural strength (Modulus of Rupture) and microstructure as a function of sintering temperature. The objective of this research was to investigate the influence of nanopowder and sintering temperature on flexural strength and to develop a correlation between flexural strength, microstructure and phase composition as analyzed through Rietveld refinement technique. 2. Materials and methods 2.1. Synthesis of nanocrystalline powder Nanocrystalline titanium dioxide powder with an average crystallite size of 5–15 nm was synthesized using a simple sol–gel technique. By hydrolyzing titanium tetraisopropoxide in a mixture of isopropanol and deionized water, TiO2 nanopowder was obtained after calcination at 400 ◦ C for 3 h in a table-
393
top furnace. Details on TiO2 nanopowder synthesis and characterization are provided in Ref. [14]. Amorphous gel was also calcined at 800 ◦ C for 3 h to study quantitative phase evolution characteristics as a result of high-temperature calcination.
2.2. Processing of dense TiO2 structures The synthesized TiO2 nanopowder was compacted by a traditional cold die compaction method using a steel mold having an internal diameter of 12.7 mm at a pressure of 19.4 MPa. During the cold die compaction, the powder was densified by powder rearrangement, including sliding and rolling, so as to decrease the porosity. Since nanoceramic powders have a greater tendency to form agglomerates, it is sometimes necessary to grind the powders first to break the agglomeration. The green samples were prepared in a uniaxial single action manual hydraulic press (Model 3851-0, Carver Inc., Wabash, IN). A dry P.T.F.E. film (made with Dupont Krytox) was sprayed on the inside surface of the mold and punch to reduce the friction between the fine powder and metal surfaces. All green samples were then sintered in a high-temperature programmable muffle furnace (Model 46100, Barnstead International Co., Dubuque, IA), in air, at a temperature of 1200, 1300, 1400, 1500 and 1600 ◦ C for 3 h, separately. A sintering cycle suitable for TiO2 ceramics was developed to attain improved densification and to avoid cracks in the sintered specimens by introducing several soaking temperatures. The consequent sintering cycle used several steps: first, holding at 150 ◦ C to stabilize the furnace; second, holding at 400 ◦ C to remove residual stresses from the green structures; and the final holding at the desired sintering temperature for densification. A heating rate of 4 ◦ C min−1 and a cooling rate of 10 ◦ C min−1 were used to minimize thermal stress-induced cracking during sintering and to help in densification.
2.3. Phase and microstructural analyses of the sintered specimens A powder X-ray diffraction (XRD) technique was used to study the phase evolution/transformation in TiO2 structures sintered at 1200, 1300, 1400, 1500 and 1600 ◦ C. For this, TiO2 structures sintered at different temperatures were ground into a fine powder, separately, using a mortar and pestle. The fine powder obtained was then used for XRD analysis in a Rigaku diffractometer (Model D/MAX-B, Rigaku Co., Tokyo, Japan) equipped with Ni-filtered Cu K␣ radiation (λ = 0.15418 nm) at 35 kV and 30 mA settings. The 2θ step size was 0.04◦ , and a scanning rate of 1.5◦ min−1 was used. Scanning electron microscopy (SEM) was used to observe and analyze the microstructure of the sintered TiO2 ceramics. The TiO2 specimens sintered at 1300, 1400 and 1600 ◦ C were gold-coated for 1 min using a magnetron sputter coater from Emitech Inc. The gold-coated specimens were observed in a JOEL SEM (Model 6400F, JEOL, Tokyo, Japan).
2.4. Compositional analysis using Rietveld refinement The Rietveld refinement technique [20], which was originally introduced for the analysis of constant wavelength neutron diffraction data [21], is being broadly used for the analysis of neutron, X-ray and synchrotron diffraction data nowadays. This technique, implemented in the LANL code General Structure Analysis System (GSAS) [22], was used to analyze X-ray diffraction spectra for quantitative phase analysis in this research. In the Rietveld method, the intensity at every point in the spectrum is determined by adding the calculated background and Bragg scattering intensities corresponding to diffraction peaks. The refinement procedure varies selected parameters (e.g., phase volume fractions, lattice parameters, and phase texture, etc.), and constructs linear constraints between parameters (e.g., atomic fraction of A + atomic fraction of B = 1), until the calculated and measured spectra match in a least-squares fit. Errors are quantified and are associated with the statistics of the fit. Furthermore, Rietveld refinement can account for variations in intensity due to changes in phase volume fractions (in multiphase materials) or to preferred orientation (texture). A generalized spherical harmonic description [23] was used to account for the evolving texture in the existing phases. In this work, Reitveld refinement technique was used for quantitative phase characterization in as-synthesized powder calcined at 400 and 800 ◦ C separately, and TiO2 structures sintered at 1400 and 1500 ◦ C. The analy-
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sis provided information on lattice constants and crystal structures of the nano-TiO2 .
2.5. Biaxial flexural testing of nanostructured TiO2 Biaxial flexural strength tests were performed in accordance with ASTM F-394 standard with some alteration in fixture dimension to accommodate the specimen size. Flexural strength properties of nanostructured TiO2 ceramics were studied in specimens sintered at two different temperatures viz., 1400 and 1500 ◦ C for 3 h, separately. The tests were performed in a tensile tester (Model 3369, Instron Co., USA) at a constant crosshead speed of 0.05 mm min−1 . ASTM F394-7818 supports the use of the piston on three-ball test. This test concerns the supporting of a disc specimen by three ball bearings near its periphery but equi-distant from a load piston. The test fixture design compensated for the lack of perfectly plane and parallel surfaces and allowed for the testing of slightly warped specimens. Uniform loading may be assisted by a layer of non-ridged material such as polythene between the piston and test specimens. A flat ended ball piston (0.75 mm in diameter), pressing against a three-ball (1.98 mm in diameter) test jig was developed and used for the biaxial flexural test of circular disc-shaped sintered specimens. To closely mimic the ASTM F 394-78 standard the three-ball test jig was fabricated using hardened steel balls but the dimensions of the ball and test specimens were smaller than that specified in ASTM standard. This was done to accommodate the smaller dimensions of the specimens used in this work. Sintered disc specimens were centered and supported on three hardened steel balls positioned 120◦ apart on a circle, 7.5 mm in diameter. A thin plastic-coated thick paper sheet was placed between the specimen and flattened ball piston to distribute the load evenly, as specified in the ASTM standard. The specimens were tested on a fully automated tensile testing machine from Instron Inc. (Model 3369) with a crosshead speed of 0.05 mm min−1 . The recorded fracture load was used to calculate the biaxial fracture strength using the following equation: S=
−0.2387P(X − Y ) d2
(1)
where S is maximum center tensile stress in MPa and P is the total load causing fracture in N.
B 2
X = (1 + ν) ln
+
(1 − ν) B 2
C 2 2 A
Y = (1 + ν) 1 + ln
C
C
A 2
+ (1 − ν)
C
Fig. 1. Rietveld refinement results of synthesized anatase nanopowder (calcined at 400 ◦ C). The observed diffraction intensities are displayed as crosses in red, with the calculated values drawn as a curve in green. The reflection positions are marked and the difference curve (Io − Ic ) in purple is displayed near the bottom of the graph. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
The reduced χ2 -value was found to be 1.380 and the convergence was achieved, which meant the quality of the fit was good. The calculated unit cell formula weight found from the Rietveld refinement was 319.592. The density was calculated to be 3.951 g cm−3 . The lattice parameters for the as-synthesized anatase nano-TiO2 powder, calcined at 400 ◦ C, were calculated as a = b = 0.3793 nm, c = 0.9502 nm, α = β = γ = 90◦ , which were quite reasonable when comparing with the values for anatase phase reported in Ref. [5]. The volume of the unit cell was ˚ 3. computed to be 134.304 (±0.344) A Fig. 2 shows the results of Rietveld refinement of the X-ray diffraction data for the synthesized TiO2 nanopowder calcined at 800 ◦ C for 3 h. As calcination temperature was raised to 800 ◦ C, the rutile phase was 100%, and the anatase to rutile phase
where ν is Poisson’s ratio (taken as 0.27), A is the radius of support circle in mm, B is the radius of loaded area or ram tip in mm, C is the radius of specimen in mm, and d is the specimen thickness at fracture origin in mm. Earlier [14], we reported hardness and compression strength in TiO2 structures sintered at 1300–1600 ◦ C. The specimens sintered at 1500 ◦ C exhibited the highest compressive strength, while the specimens sintered at 1600 ◦ C exhibited the highest hardness. It is reported that Vickers hardness is a convenient method for estimating the yield strength (σ y ) of ceramics as HV ≈ 3σ y [24]. Accordingly, 1400 and 1500 ◦ C were selected as sintering temperatures of choice in this study for assessing the flexural strength. Seven specimens sintered at 1500 ◦ C and six specimens sintered at 1400 ◦ C were tested for the accuracy of results.
3. Results 3.1. Compositional analysis of the calcined nano-TiO2 using Rietveld technique Fig. 1 shows the results of Rietveld refinement of the X-ray diffraction data for the synthesized TiO2 nanopowder calcined at 400 ◦ C for 3 h. Rietveld refinement confirmed that the powder was pure anatase. The crystallite size corresponding to the ˚ β = 0.774◦ ) was computed peak at 2θ = 24.92◦ (λ = 1.5418 A, using the Scherrer’s equation and was found to be 10.5 nm.
Fig. 2. Rietveld refinement results of synthesized TiO2 (rutile phase) nanopowder, obtained through calcination of the gel at 800 ◦ C. The observed diffraction intensities are displayed as crosses in red, with the calculated values drawn as a curve in green. The reflection positions are marked and the difference curve (Io − Ic ) in purple is displayed near the bottom of the graph. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
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Table 1 The lattice parameters of TiO2 (calcined/sintered) and their corresponding unit cell volume from Rietveld analysis Thermal treatment/temperature/phase
Calcination/400 ◦ C/anatase Calcination/800 ◦ C/rutile Sintering/1400 ◦ C/rutile Sintering/1500 ◦ C/rutile
˚ Lattice parameters (A) a
b
c
3.772105 (±0.003482) 4.592226 (±0.000181) 4.599384 (±0.000342) 4.598760 (±0.000507)
– – – –
9.438909 (±0.009544) 2.959896 (±0.000208) 2.961549 (±0.000262) 2.960840 (±0.000346)
transformation had already been completed. Based on Rietveld refinement, the reduced χ2 -value was found to be 1.364 and the convergence was achieved, which meant quality of the fit was good. The crystallite size, computed using Scherrer’s equation corresponding to a rutile (1 1 0) reflection, was 46.2 nm. Calculated unit cell formula weight found from Rietveld refinement was 159.796. The density was calculated to be 4.251 g cm−3 . The lattice parameters of rutile TiO2 nanocrystallite and computed volume of the unit cell are presented in Table 1. The values obtained in our analysis are in line with the values for rutile phase reported in Ref. [5]. 3.2. Phase and microstructural analyses X-ray powder diffraction analysis was conducted to analyze phase transformation in TiO2 structures sintered at different temperatures viz., 1200, 1300, 1400, 1500 and 1600 ◦ C, separately. The results of phase analysis are presented in Fig. 3. Peaks in each X-ray diffraction pattern were recorded and verified using standard JCPDS standard files #21-1276, for rutile TiO2 . Identical patterns were recorded in all the specimens con-
c/a
˚ 3) Unit cell volume (A
2.5023 0.6445 0.6439 0.6438
134.304 (±0.344) 62.42 (±0.005) 62.650 (±0.010) 62.618 (±0.018)
firming no further phase change, in the temperature range of 1300–1600 ◦ C. However, the intensity and sharpness of the rutile peaks grew with the increase in sintering temperature indicating an increase in the crystallite size in the sintered structures, as the sintering temperature was raised from 1200 to 1600 ◦ C. A typical green structure formed after uniaxial compaction and a sintered TiO2 structure are shown in Fig. 4. Sintering at 1200 ◦ C showed poor density and relatively low crystallinity (see Fig. 3) and therefore was not considered for SEM characterization. The effects of sintering on the microstructure of the sintered TiO2 ceramics were studied in a JOEL SEM and the SEM micrographs obtained are presented in Fig. 5. It is clear from these SEM micrographs that grain growth continued with the increase in the sintering temperature. Grain boundaries are evident in the micrographs, confirming crystallinity and good sintering. This result is consistent with our X-ray diffraction phase analysis, where peak intensity of rutile phase increased with the sintering temperature. The pore sizes were reduced with the increase in sintering temperature from 1300 to 1600 ◦ C. 3.3. Compositional analysis of sintered structures using Rietveld refinement Fig. 6a and b presents the Rietveld refinement results of the X-ray diffraction data for TiO2 structures sintered at 1400 and
Fig. 3. Phase analysis of nanostructured TiO2 ceramics as a function of sintering temperature. Specimens were sintered in air at 1200–1600 ◦ C for 3 h, separately, in a muffle furnace.
Fig. 4. Effect of sintering in sol–gel processed nanocrystalline (anatase powder, 5–15 nm) TiO2 ceramics.
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Fig. 5. Scanning electron micrographs of TiO2 ceramics sintered at (a) 1300 ◦ C, (b) 1400 ◦ C and (c) 1600 ◦ C for 3 h; (d) shows a high magnification SEM of TiO2 sintered at 1400 ◦ C.
1500 ◦ C for 3 h, separately. The phase composition is pure rutile as revealed by XRD results (Fig. 3). Based on Rietveld refinement, the reduced χ2 -value was found to be 1.890 and 1.548 for sintering at 1400 and 1500 ◦ C, respectively. This means the convergence was achieved, which meant the quality of the fit
was good in both cases. The calculated unit cell formula weight found from Rietveld refinement was 159.796. The density was calculated to be 4.235 and 4.238 g cm−3 sintering at 1400 and 1500 ◦ C, respectively. The lattice parameters of rutile TiO2 and computed volume of the unit cell are presented in Table 1.
Fig. 6. Rietveld refinement of X-ray diffraction patterns of nanostructured TiO2 ceramics sintered at: (a) 1400 ◦ C and (b) 1500 ◦ C. The observed diffraction intensities are displayed as crosses in red, with the calculated values drawn as a curve in green. The reflection positions are marked and the difference curve (Io − Ic ) in purple is displayed near the bottom of the graph. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
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though the average sintered density in both sets are different, this difference is not statistically significant. 4. Discussion
Fig. 7. Flexural strength and geometric bulk density (ρg ) of sintered TiO2 structures.
These values are comparable with the values that are reported in literature [5]. 3.4. Assessment of biaxial flexural strength In this work, we determined flexural strength properties of sintered TiO2 structures at 1400 and 1500 ◦ C, separately, prepared using 5–15 nm starting powder. In our previous work [14], we performed Vickers hardness testing and compression testing on similar structures sintered at 1300–1600 ◦ C. Results of biaxial flexural strength test, as shown in Fig. 7, revealed that using fine nanopowder, flexural strength of TiO2 sintered structures can be enhanced. The biaxial flexural strength was measured on seven specimens sintered at 1500 ◦ C and six specimens sintered at 1400 ◦ C. The average flexural strength computed to be 127.60 (±7.01) MPa and 111.74 (±6.26) MPa, respectively for specimens sintered at 1500 and 1400 ◦ C. The geometric bulk density (ρg ) of the sintered specimens was evaluated from the measurements of the mass of specimen and its volume (determined by dimensional measurements) using the following equation: geometric bulk density (ρg ) =
mass (m) volume (V )
(2)
The average sintered density (ρg ) computed for the same sets of specimens was found to be 3.82 (±0.06) and 3.78 (±0.11) g cm−3 , respectively for sintering temperatures of 1500 and 1400 ◦ C. To determine and compare statistical significance of our flexural strength test results, Student’s t-test was performed to show the significance in data for samples sintered at two different temperatures viz., 1400 and 1500 ◦ C. Probability associated with a Student’s paired t-test (two samples assuming equal variance, i.e. homoscedastic Student’s t-test), with a two-tailed distribution, was computed and reported. Our calculated p and t values were 0.0225 and 2.65, respectively. The calculated t value exceeds the tabulated value corresponding to p = 0.05, consequently the means are significantly different. The difference between the means is at the 95% level and therefore the difference is statistically “significant”. Similarly, Student’s paired t-test was performed to determine statistical significance of sintered density data and the t value was found to be 1.64, which means
In our previous work [14], we discussed the synthesis and characterization of nanocrystalline TiO2 powder. Using the same technique, we processed TiO2 nanopowder of size 5–15 nm after calcination at 400 ◦ C in pure anatase phase. The calcination temperature was high enough to achieve crystallization, and at the same time, helped minimize the thermal growth of the crystallites and maintain nanoscale features in the calcined powder in the lower end of nano-range. The Rietveld refinement results, as presented in Table 1, were in good agreement with the available literature. Rutile belongs 14 -P4 /mnm space group (lattice constant a = 0.4584 nm, to D4h 2 19 c = 0.2953 nm, c/a = 0.644), anatase belongs to D4h -I41 /amd space group (lattice constant a = 0.3733 nm, c = 0.937 nm, 15 -Pbca space group c/a = 2.51), while brookite belongs to D2h (lattice constant a = 0.5436 nm, c = 0.9166 nm, c/a = 0.944) [5]. The lattice parameters of our nano-TiO2 powder calcined at 400 ◦ C, were calculated to be a = 0.3772 nm, c = 0.9439 nm, c/a = 2.50, α = β = γ = 90◦ through Rietveld refinement, which were comparable with the literature. Similar comparable results were obtained for rutile phase at different temperature (Table 1). The three-point flexural strength test is the standard for advanced ceramics. However, inherent sensitivity to flaws and defects near specimen edges is a major drawback of this test [25]. This problem is not seen in the biaxial flexural test because of the difference in loading conditions, specimen geometry, and test set-up [25]. The strength measurement with the biaxial test data does, however, require material information such as the Poisson’s ratio. Specimen flaws regardless of orientation are also taken into account in the biaxial flexural testing, which has made it a widely recognized as a reliable technique for studying the strength of brittle materials since the maximum tensile stress occurs within the central loading area and failures at the edge of the specimens, a serious concern in three-point bend test, are avoided. We observed that compression and flexural strength of nanostructured TiO2 increases with the increase in sintering temperature, which can be explained based on SEM analysis of the microstructure. Evidently (Fig. 5), in the case of the 1300 ◦ C sintering temperature materials, the porosity was high and most pores were interconnected. For the specimens sintered at 1400 and 1600 ◦ C, continuous grain boundary networks were formed. Most of the pores were present at triple junctions and the grain boundaries. In light of the obtained microstructural information, the decrease in porosity at 1400 ◦ C and higher temperatures is possibly due to the bridging of fine crystallites and formation of closed pores. There is no apparent secondary phase/s present within the grain or along the grain boundaries. It is also clear from our XRD analysis that only the rutile phase is present within this range of sintering temperature. Below 1500 ◦ C, the removal of pores during the densification process played a significant role in increasing the flexural and compressive strength, since
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specimens had smaller grain-sizes and the distribution of grain size was relatively homogenous and uniform compared to those sintered at 1600 ◦ C. Again, with the increase in the sintering temperature, it was expected that the material would achieve better densification at the expense of grain growth. This effect was especially significant at 1600 ◦ C where large and irregularshaped grains were observed. 5. Conclusions Synthesis of nanocrystalline TiO2 (anatase) powder using sol–gel technique is simple and easily reproducible. Rietveld refinement results showed reduced χ2 -values, confirming that the obtained results were accurate. Calculated unit cell densities were found to be 3.951 g cm−3 (anatase at 400 ◦ C), 4.251 g cm−3 (rutile at 800 ◦ C), 4.235 g cm−3 (rutile at 1400 ◦ C) and 4.238 g cm−3 (rutile at 1500 ◦ C). Both sintered density and grain-size increased with the sintering temperature. The highest sintered density of 3.82 g cm−3 was achieved at 1500 ◦ C. Maximum biaxial flexural strength of 127.60 (±7.01) MPa was observed in structures sintered at 1500 ◦ C. The biaxial flexural strength results were in agreement with hardness and compression strength results reported earlier, i.e. strength and hardness increased with increase in sintering temperature from 1400 to 1500 ◦ C. We conclude from our research that sintering at 1500 ◦ C gives combination of high compression strength, high flexural strength and high hardness. Acknowledgements We are grateful for the editorial help by Chayanika D. Kalita in preparing this manuscript. Assistance from Materials Characterization Facility of the University of Central Florida is thankfully recognized, as well.
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