Effect of fuel-to-oxidizer ratios on combustion mode and microstructure of Li2TiO3 nanoscale powders

Available online at www.sciencedirect.com

ScienceDirect Journal of the European Ceramic Society 34 (2014) 801–807

Effect of fuel-to-oxidizer ratios on combustion mode and microstructure of Li2TiO3 nanoscale powders Qilai Zhou, Yang Mou, Xiao Ma, Lihong Xue ∗ , Youwei Yan State Key Laboratory of Materials Processing and Die and Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, PR China Received 16 August 2013; received in revised form 26 September 2013; accepted 2 October 2013 Available online 29 October 2013

Abstract A single phase Li2 TiO3 powder has been fabricated through a facile solution combustion process, using citric acid as the fuel and corresponding nitrates as oxidants. The effect of fuel-to-oxidizer ratio (0.5–1.5) on the combustion process, the phase and microstructure of the products was investigated. By using different combinations of citric acid fuel and metal nitrates, the combustion mode could be controlled. When the fuel-to-oxidizer ratio is 0.75, an eruption combustion mode is realized. Thermodynamic analysis of the combustion reaction shows that as the fuelto-oxidizer ratio increases, the adiabatic flame temperature during combustion also increases, but the measured maximum temperature decreases. The crystallite size of Li2 TiO3 powders was calculated to be 18–36 nm at different combustion modes. The as-prepared Li2 TiO3 powders exhibit excellent sinterability and can be sintered to 90.7% of the theoretical density at 800 ◦ C. The grain size of the Li2 TiO3 ceramics is around 800 nm. © 2013 Elsevier Ltd. All rights reserved. Keywords: Li2 TiO3 ; Eruption; Solution combustion; Sintering; Citric acid

1. Introduction Lithium containing ceramics such as Li2 O, Li4 SiO4 , LiAlO2 , Li2 ZrO3 and Li2 TiO3 have been considered as candidate solid blanket materials for tritium breeding in nuclear fusion reactor.1–3 Among them, Li2 TiO3 is a promising tritium breeding material because of its reasonable lithium atom density, low activation, excellent tritium release performance and chemical stability.4,5 During the last years, many efforts have been aimed at the fabrication of Li2 TiO3 powders, such as solid state reaction,6–8 sol–gel,9,10 hydrothermal11 and solution combustion synthesis (SCS). SCS is an attractive technique to produce highly pure and well-crystallized oxide powders. This method has the advantages of a low initial temperature, a rapid heating rate, short reaction time, simple equipment and inexpensive raw materials.12–14 Jung et al.15,16 reported the SCS of Li2 TiO3 using TiCl4 and LiNO3 as the raw materials and glycine as the fuel. Lee et al.17,18 tried a polymer solution combustion technique using ethyleneglycol as cations carrier and a pure Li2 TiO3 phase

∗

Corresponding author. Tel.: +86 27 87543876; fax: +86 27 87541922. E-mail address: [email protected] (L. Xue).

0955-2219/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jeurceramsoc.2013.10.004

with 70 nm crystallite size was obtained by heat-treating ascombusted powders at 300 ◦ C. Sinha et al.19 developed a novel solid-liquid combustion synthesis to produce Li2 TiO3 starting from hydrated titannia powders and LiNO3 , but some peaks of TiO2 were observed in the XRD patterns. In SCS, the phase and morphology of products can be controlled by fuel types, fuel-to-oxidizer ratio (ϕ) and combustion mode. The combustion modes are usually classified into volume combustion synthesis (VCS) and self-propagating hightemperature synthesis (SHS).20–22 Smolder-SHS, as one type of SHS, is characterized by slow reaction rate and essentially flame-less reaction with the lowest maximum temperature.20 Recently, an eruption combustion mode (referred as eruptionSHS below) for the synthesis of Ni/NiO nanoparticles was reported by Wen and Wu.23 They found during SCS, lots of powders were lifted off and naturally fell down around the container, leaving highly fluffy products with a quite big volume. This phenomenon is very similar to the volcano eruption nature. Powders produced by this strategy enjoy small particle size, good dispersity and high specific surface area. However, the realization of eruption-SHS needs some additives like NaF. Citric acid is a promising fuel in SCS to synthesize nanoscale oxide powders. Moreover, it also acts as a preferable complexant for metal ions in producing a transparent precursor avoiding

802

Q. Zhou et al. / Journal of the European Ceramic Society 34 (2014) 801–807

precipitation. Jung et al.15 once chose citric acid as one of various fuels to synthesize Li2 TiO3 . However, the phase of synthesized products is amorphous and there is no systematic study of the combustion behavior. In the present study, citric acid was selected as a fuel. It is found that the eruption-SHS could be realized by merely adjusting ϕ without any other additive. Fine Li2 TiO3 powders can be obtained by this eruption-SHS technology. 2. Experimental 2.1. Fabrication process All reagents were of analytical grade and used as received without further purifications. Deionized water was used throughout the experiments. Tetrabutyltitanate (Ti(OC4 H9 )4 , 98%), lithium nitrate (LiNO3 , AR), and concentrated nitric acid (65% HNO3 , AR) were used as raw materials. Citric acid (C6 H8 O7 , 99%) was added as the fuel. In a typical synthesis, 20 mmol Ti(OC4 H9 )4 was added into deionized water to obtain a white precipitation. Then the precipitation was washed with deionized water for several times and dissolved in concentrated nitric acid to get a clear solution (150 ml). Then 40 mmol LiNO3 with citric acid were added to the solution to form precursor solution. The transparent solution was transferred to a preheated furnace. A thermocouple was suitably placed to measure the temperature of the combustion reaction. Within a few minutes, the solution boiled and was ignited to produce a self-propagating combustion, yielding fluffy powders. The stoichiometric synthesis equation is expressed as: 2LiNO3 + TiO(NO3 )2 + 1.11ϕC6 H8 O7 + (5ϕ − 5)O2 → Li2 TiO3 + 6.67ϕCO2 + 2N2 + 4.44ϕH2 O

Fig. 1. Eruption-SHS (a) and smolder-SHS (b) combustion processes and photographs of the corresponding products for ϕ = 0.75 (c) and ϕ = 1.5 (d), respectively.

pellet method. The morphologies of the powders and the sintered samples were observed using a field emission scanning electron microscopy (Nova NanoSEM 450 microscope, FEI, Netherlands). The densities of the sintered specimens were measured by the Archimedes’ method. 3. Results and discussion

(1)

In the above reaction, ϕ = 1 means that the initial mixture solution does not require atmospheric oxygen for a complete oxidation of fuel, while ϕ > 1 (<1) implies fuel-rich (lean) condition. The excess fuel in the fuel-rich (ϕ > 1) system is oxidized by the atmospheric oxygen. In our experiment, four fuel-to-oxidizer ratios, ϕ = 0.5, 0.75, 1.0, 1.5, were selected to investigate the effect of ϕ on the phase and SCS process. In order to study the sinterability of the as-synthesized Li2 TiO3 powders, pellets with 14 mm in diameter and 2 mm in thickness were fabricated by pressing the as-obtained Li2 TiO3 powders under 180 MPa pressure and sintered in an air atmosphere at temperatures from 700 to 1000 ◦ C for 2 h. 2.2. Characterization The crystalline structure of the as-prepared powders was identified by X-ray diffraction (XRD), where the X-ray source was Cu K␣ radiation (γ = 0.15406 nm), powered at 40 mA, 40 KV. Scanning was performed over 2θ angles ranging from 10◦ to 80◦ . The infrared (IR) spectra of the samples were recorded in the range of 500–4000 cm−1 on a Fourier transform infrared (FT-IR) spectrometer (Thermo-Nicolet Avatar 370) using KBr

3.1. Combustion modes The fuel-to-oxidizer ratio, ϕ, is one of the key parameters that influence the combustion mode as well as phase composition of the product. Table 1 shows the combustion process at various ϕ. It is noted that the nature of reaction is quite different under different ϕ. Fig. 1 shows some typical combustion processes and the appearance of the corresponding products. When ϕ = 0.5, the reaction is violent and the combustion mode belongs to the VCS mode. A white mixture of Li2 TiO3 and LiNO3 powders was obtained. When ϕ = 0.75, the combustion process exhibits an eruption-SHS mode (Fig. 1a). The flame existed all through the combustion process along with large quantities of gas generation. In addition, a lot of the yields were lifted off and naturally fell down around the container, leaving highly fluffy products. Thus, the yields in the eruption solution combustion spread the entire vessel wall and bottom (Fig. 1c). When ϕ increases to above 1.0, the combustion pattern changes into smolder-SHS mode (Fig. 1b). The reaction rate became rather slow and no flame was observed. The appearance of the products is just like a whole gray coral (Fig. 1d). The gray color of the products indicates the powders are carbon contaminated. In terms of phase composition, a pure Li2 TiO3 phase was obtained with ϕ ≥0.75, but the color changes from white to gray with the increase of ϕ.

Q. Zhou et al. / Journal of the European Ceramic Society 34 (2014) 801–807

803

Table 1 Different combustion mode and nature of the products at various ϕ. Fuel-to-oxidizer ratio, ϕ

0.5 0.75 1.0 1.5

Combustion mode

Combustion process

VCS Eruption-SHS Smolder-SHS Smolder-SHS

Products characteristics

Duration of combustion (s)

Measured maximum temperature (K)

Products color

Phase

120 200 420 460

751 700 605 583

White White Gray Gray

Li2 TiO3 and LiNO3 Li2 TiO3 Li2 TiO3 Li2 TiO3

Table 2 Thermodynamic data of materials for calculating adiabatic flame temperature. Hf0 (kcal mol−1 )

Compounda LiNO3 (c) TiO(NO3 )2 (c) C6 H8 O7 (c) O2 (g) Li2 TiO3 (c) CO2 (g) N2 (g) H2 O(g) a b c

Cp (cal mol−1 K−1 )

–115.4 –235.6b –245.6 0 –339.1 –94.05 0 –57.79

– – – 5.92 + 0.00367Tc 26.3 35.36 6.5 + 0.00100T 7.2 + 0.0036T

(c) = Crystallite, (g) = gas. Estimated from bond energies and Ref. 26 approximately. Absolute temperature.

3.2. Thermodynamic analysis To calculate the theoretical adiabatic flame temperature as well as the gas volume generated during SCS, the combustion was assumed to be complete, and the combustible excess was burned by atmospheric oxygen. Heat loss by radiation or conduction was ignored. The adiabatic flame temperature of each fuel-to-oxidizer system was approximately calculated by the following equations24 : Tad   0 nCp Q = −H = − (2) dT products

T0

where Q is the heat absorbed by products under adiabatic condition, Cp is the heat capacity of the products at constant pressure. For adiabatic calculations, room temperature is considered as T0 = 298 K. The change in enthalpy of reaction (H0 ) can be obtained as:   ΔH 0 = n(Hf0 ) − n(Hf0 ) (3) products

reactants

Fig. 2. The dependence of calculated adiabatic temperatures (Tf ) and measured maximum temperatures (Tc ) on ϕ.

substantially with ϕ increasing. However, the measured maximum temperatures (Tc ) of each system during combustion are quite different from the calculated adiabatic temperature (Fig. 2). Firstly, the temperatures measured are lower than the calculated ones due to the heat conduction as well as gas generation, which cause the dissipation of the combustion heat, decreasing the temperature in the reagents mass. Secondly, the dependence of the measured maximum temperatures on ϕ is just the opposite of that of the calculated ones. This tendency is consistent with the change of combustion mode by observation and can be explained as follows: For the fuel-lean, stoichiometric and fuel-rich systems, the redox reactions could be written as: 2LiNO3 + TiO(NO3 )2 + 0.555C6 H8 O7 → Li2 TiO3 + 3.335CO2 + 2N2 + 2.22H2 O + 2.5O2 (ϕ = 0.5)

(5)

Hf0

is the enthalpy of forHere, n is the number of moles and mation. Based on Eq. (2), the following equation can be used to approximate the adiabatic flame temperature (Tf ) for combustion reaction: H 0 Tf = T0 −  nCp

2LiNO3 + TiO(NO3 )2 + 0.833C6 H8 O7 → Li2 TiO3 + 5.003CO2 + 2N2 + 3.33H2 O + 1.25O2 (ϕ = 0.75)

(6)

(4)

Related thermodynamic data are listed in Table 2.25 The dependence of calculated adiabatic flame temperature on the fuel-to-oxidizer ratio is presented in Fig. 2. The values increase

2LiNO3 + TiO(NO3 )2 + 1.11C6 H8 O7 → Li2 TiO3 + 6.67CO2 + 2N2 + 4.44H2 O(ϕ = 1.0)

(7)

804

Q. Zhou et al. / Journal of the European Ceramic Society 34 (2014) 801–807

Fig. 3. Calculated volume of gases produced at room temperature and pressure at different ϕ.

2LiNO3 + TiO(NO3 )2 + 1.665C6 H8 O7 + 2.5O2 → Li2 TiO3 + 10.005CO2 + 2N2 + 6.66H2 O(ϕ = 1.5)

(8)

From the redox reactions mentioned above, atmospheric oxygen is not necessary in the fuel-lean or stoichiometric system (Eqs. (5)–(7)). Thus, the reactants are able to react completely as assumed even in the oxygen deficient condition. However, for the fuel-rich system, when calculating the adiabatic temperatures, it was assumed that the excess citric acid was combusted by the atmospheric oxygen and released heat at the same time (Eq. (8)). In fact, the excess citric acid did not fully react with atmospheric oxygen because of oxygen slow diffusion into the reagents mass.26 Instead, the excess citric acid decomposed as follows: C6 H8 O7 → 6C + 4H2 O + 1.5O2

(9)

This reaction is endothermic and yields carbon (Fig. 1b), which just explains why the color of the product in the fuelrich system became gray. The carbonization of citric acid would further decrease the combustion temperature. On the other hand, the gases generated in the combustion process affect the temperature to a large extent, because lots of heat would be taken away by the gas flow. As shown in Fig. 3, the amounts of gases increase with the ϕ rising. In the fuel-rich system, the amount of gases generation is two times more than that in the fuel-lean one. According to the above reasons, the measured maximum temperature of the fuel-lean system is much higher than that of the fuel-rich system. 3.3. Powder characteristics Fig. 4 presents the SEM images of the resultant powders obtained at different ϕ. It is observed that the aggregates are well-distributed when ϕ ≤ 1.0 (Fig. 4a, c, and e). The volume of aggregates increases remarkably at ϕ = 1.5 (Fig. 4g). In the images at higher magnification, no obvious gas tube can be observed in the product at fuel-lean system (Fig. 4b and d). The gas tubes emerge under stoichiometric system (Fig. 4f). In the

Fig. 4. SEM images of as-synthesized powders with different ϕ, (a) and (b) ϕ = 0.5, (c) and (d) ϕ = 0.75, (e) and (f) ϕ = 1.0, (g) and (h) ϕ = 1.5.

fuel-rich system, the number of the gas tubes increase obviously and the dimensions of the gas tubes are ten times bigger than that obtained in the stoichiometric system (Fig. 4h). Fig. 5 shows the XRD patterns of as-synthesized powders at different ϕ. When ϕ = 0.5, LiNO3 and TiO2 as well as some unidentified reflection peaks exist. With the ϕ increased to 0.75, the impurity phase disappears and the reflection peaks of Li2 TiO3 become sharper. All the diffraction peaks are in good agreement with the monoclinic crystal Li2 TiO3 registered in the Joint Committee on the Powder Diffraction Strands Card (JCPDS Card No. 33-0831). However, with further increasing ϕ, the characteristic peaks reduce significantly, along with the broadening of the full-width at half-maximum (FWHM) of the peaks. The crystallite size was also calculated by the Scherrer equation27 based on the XRD patterns (Fig. 6). For ϕ = 1.0 and

Q. Zhou et al. / Journal of the European Ceramic Society 34 (2014) 801–807

805

Fig. 5. XRD patterns of the as-synthesized powders at various ϕ. Fig. 7. FT-IR spectra of the as-synthesized powders with ϕ = 0.5, 0.75, 1.0, 1.5, respectively.

Fig. 6. Crystallite size calculated based on the XRD patterns.

1.5, the crystallite sizes are 17.9 and 21.4 nm respectively, which are smaller than that of the other two products. The smaller crystallite sizes can be ascribed to the lower combustion temperature under the smolder-SHS mode. For ϕ = 0.75, the crystallite size is 35.8 nm, which is larger than that of powders obtained at ϕ = 0.5 (30.7 nm). This is due to the relatively longer duration of combustion under eruption-SHS compared with VCS mode. Fig. 7 depicts the FT-IR spectra of the as-synthesized powders. The typical absorption bands appear at 1383, 1439, 1510 cm−1 for the powders obtained at ϕ = 0.5. The bands at 1439 and 1510 cm−1 can be assigned to the antisymmetric stretching of CO3 2− anions due to the absorption of CO2 in the air.28 The bands at 1383 cm−1 correspond to the NO3 − ions.29 The appearance of NO3 − is due to the relatively more oxidizers than fuel in fuel-lean system and rapid reaction rate under VCS mode. For ϕ = 0.75, the absorption bands of NO3 − ions greatly reduce. The bands appear around 600 cm−1 could be attributed to the characteristics vibration of Ti O bond30 which indicates the formation of Li2 TiO3 . Besides, there is no obvious band corresponding to the carboxyl group and hydroxyl group for all the resultant powders.

Fig. 8. Curve of relative density of Li2 TiO3 ceramic against sintering temperatures.

3.4. Sinterability of Li2 TiO3 powders To study the sinterability of the powders, Li2 TiO3 powders prepared by the eruption-SHS mode were pressed and sintered at different temperatures. Fig. 8 depicts the relative density of Li2 TiO3 ceramic sintered at various temperatures. As presented in the plot, the relative density of the Li2 TiO3 ceramic increases marginally between 700 and 800 ◦ C. The density of the Li2 TiO3 ceramic obtained at 800 ◦ C reaches 90.7% of theoretical density, which shows the high sinterability of the as-synthesized powders at fairly low temperature. The Li2 TiO3 powders prepared by other conventional routes require higher temperature (>900 ◦ C) to achieve the same densification.11,19,31 With the sintering temperature further going up to 900 ◦ C, the grain size increases considerably (Fig. 9d). However, the relative density remained nearly unchanged.

806

Q. Zhou et al. / Journal of the European Ceramic Society 34 (2014) 801–807

References

Fig. 9. SEM images of the surface of the Li2 TiO3 ceramic sintered at (a) 700 ◦ C, (b) 750 ◦ C, (c) 800 ◦ C, (d) 900 ◦ C, respectively.

The surface microstructures of the Li2 TiO3 ceramic sintered at different temperatures are shown in Fig. 9. The Li2 TiO3 ceramic sintered at 700 ◦ C is loose and porous. With an increase in sintering temperature, the pores become less and the grains grow. When the temperature reaches 800 ◦ C, the Li2 TiO3 ceramic becomes dense and the grains have regular shape with the average grain size of 800 nm, which is much smaller than that of the Li2 TiO3 ceramic fabricated by other techniques.10,11,19 4. Conclusion Well crystalline and nano-size Li2 TiO3 powders could be synthesized in a single step via solution combustion using citric acid as the fuel. It is found that the fuel-to-oxidizer ratio (ϕ) has significantly influenced the combustion mode and microstructure of the synthesized powders. Thermodynamic analysis of the combustion reaction shows that as the fuel-to-oxidizer ratio increases, the amount of gases produced and adiabatic flame temperatures during combustion also increase. Their synergistic effect causes the measured maximum temperature decrease and it also changes the combustion modes from VCS to eruptionSHS and smolder-SHS. The crystallite size of Li2 TiO3 powders was calculated to be in the range of 18–36 nm. The powders can be sintered at a temperature of 800 ◦ C with a density more than 90% of the theoretical density. The as-sintered Li2 TiO3 ceramic shows an average grain size of 800 nm. This new solution combustion method provides an efficient and convenient alternative to produce Li2 TiO3 powders with high sinterability. Acknowledgements This work is supported by Chinese National Fusion Project for ITER (2013GB110000) and the National Natural Scheme Foundation of China (51272079, 51005054).

1. Plichta E, Salomon M, Slane S, Uchiyama M, Chua D, Ebner WB, et al. A rechargeable Li/Lix CoO2 cell. J Power Sources 1987;21(1):25–31. 2. Alvani C, Carconi PL, Casadio S. Lithium titanate pebbles reprocessing by wet chemistry. J Nucl Mater 2001;289(3):303–7. 3. Raffray AR, Akiba M, Chuyanov V, Giancarli L, Malang S. Breeding blanket concepts for fusion and materials requirements. J Nucl Mater 2002;307:21–30. 4. Johnson CE, Kummerer KR, Roth E. Ceramic breeder materials. J Nucl Mater 1988;155–157:188–207. 5. Johnson CE, Hollenberg GW, Roux N, Watanabe H. Current experimental activities for solid breeder development. Fusion Eng Des 1989;(8): 145–53. 6. Renoult O, Boilot JP, Korb JP, Boncoeur M. Lithium sol–gel ceramics for tritium breeding applications. J Nucl Mater 1995;233(2):126–34. 7. Miller JM, Hamilton HB, Sullivan JD. Testing of lithium titanate as an alternate blanker material. J Nucl Mater 1994;212–215:877. 8. Ramaraghavulu R, Buddhudu S, Kumar GB. Analysis of structural and thermal properties of Li2 TiO3 ceramic powders. Ceram Int 2011;37(4): 1245–9. 9. Deptula A, Lada W, Olczak T, Chmielewski AG, Sartowska B, Goretta K, et al. Preparation of lithium titanate by sol–gel method. Nukleonika 2001;46(3):95–100. 10. Wu X, Wen Z, Bin L, Xu X. Sol–gel synthesis and sintering of nano-size Li2 TiO3 powder. Mater Lett 2008;62(6–7):837–9. 11. Li Y, Xu C, Wang X, Li L, Kong L. Synthesis of Li2 TiO3 ceramic breeder powders by in-situ hydrolysis and its characterization. Mater Lett 2012;89(8):25–7. 12. Patil KC, Aruna ST, Minami T. Combustion synthesis: an update. Curr Opin Solid State Mater Sci 2002;6(6):507–12. 13. Aruna ST, Mukasyan AS. Combustion synthesis and nanomaterials. Curr Opin Solid State Mater Sci 2008;12(3–4):44–50. 14. Mukasyan AS, Epstein P, Dinka P. Solution combustion synthesis of nanomaterials. Proc Combust Inst 2007;31(2):1789–95. 15. Jung CH, Park JY, Oha SJ, Park HK, Kim YS, Kim DK, et al. Synthesis of Li2 TiO3 ceramic breeder powders by the combustion process. J Nucl Mater 1998;253(1–3):203–12. 16. Jung CH, Park JY, Kim WJ, Ryu WS, Lee SY. Charaterizations of Li2 TiO3 prepared by a solution combustion synthesis and fabrication of spherical particles by dry-rolling granulation process. Fusion Eng Des 2006;81(8–14):1039–44. 17. Jung CH, Lee SJ, Kriven WM, Park JY, Ryu WS. A polymer solution technique for the synthesis of nano-sized Li2 TiO3 ceramic breeder powders. J Nucl Mater 2008;373(1–3):194–8. 18. Lee SJ. Characteristics of lithium titanate fabricated by an organic–inorganic solution route. J Ceram Process Res 2008;9(1):64–7. 19. Sinha A, Nair SR, Sinha PK. Single step synthesis of Li2 TiO3 powder. J Nucl Mater 2010;399(2–3):162–6. 20. Mukasyan AS, Costello C, Sherlock KP, Lafarga D, Varma A. Perovskite membranes by aqueous combustion synthesis: synthesis and properties. Sep Purif Technol 2003;25(1–3):117–26. 21. Lesnikovich AI, Sviridov VV, Printsev GV, Ivashkevich OA, Gaponik PN. A new type of self-organization in combustion. Nature 1986;323(1986):706–7. 22. Lee IS, Lee NY, Park JN, Kim BH, Yi YW, Kim TU, et al. Ni/NiO core/shell nanoparticles for selective binding and magnetic separation of histidinetagged proteins. J Am Chem Soc 2006;128(33):10658–9. 23. Wen W, Wu J. Eruption combustion synthesis of NiO/Ni nanocomposites with enhanced properties for dye-absorption and lithium storage. ACS Appl Mater Interfaces 2011;3(10):4112–9. 24. Manoharan SS, Patil KC. Combustion synthesis of metal chromite powders. J Am Ceram Soc 1992;75(4):1012–5. 25. Dean JA. Lange’s handbook of chemistry. New York: McGraw-Hill; 2003. 26. Lima MD, Bonadimann R, Andrade MJ, Toniolo JC, Bergmann CP, Nanocrystalline. Cr2 O3 and amorphous CrO3 produced by solution combustion synthesis. J Eur Ceram Soc 2006;26(7):1213–20. 27. Klug HP, Alexander LE. X-ray diffraction procedures for poly-crystalline and amorphous materials. New York, NY: Wiley; 1997. p. 637.

Q. Zhou et al. / Journal of the European Ceramic Society 34 (2014) 801–807 28. Mali A, Ataie A. Structural characterization of nano-crystalline BaFe12 O19 powders synthesized by sol–gel combustion route. Scr Mater 2005;53(9):1065–70. 29. Predoana L, Malic B, Kosec M, Carata M, Caldararu M, Zaharescu M. Characterization of LaCoO3 powders obtained by water-based sol–gel method with citric acid. J Eur Ceram Soc 2007;27(13):4407–11.

807

30. Ivanova T, Harizanova A, Surtchev M. Formation and investigation of sol–gel TiO2 –V2 O5 system. Mater Lett 2002;55(5):327–33. 31. Rhee YW, Yang JH, Kim KS, Song KW, Kim DJ. Effect of rapid sintering on the densification and the thermal diffusivity of Li2 TiO3 pellet. Thermochim Acta 2007;455(1):86–9.