- Email: [email protected]
Synthesis, characterization and use of synthesized fine zirconium diboride as an additive for densification of commercial zirconium diboride powder
Ceramics International 42 (2016) 9565–9570
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Synthesis, characterization and use of synthesized fine zirconium diboride as an additive for densification of commercial zirconium diboride powder Niranjan Patra n, Nasrin Al Nasiri, Daniel Doni Jayaseelan, William Edward Lee Centre for Advanced Structural Ceramics and Department of Materials, Imperial College London, London SW7 2AZ, UK
art ic l e i nf o
a b s t r a c t
Article history: Received 13 February 2016 Received in revised form 3 March 2016 Accepted 4 March 2016 Available online 5 March 2016
Zirconium diboride (ZrB2) was synthesized by a solution-based technique using zirconyl chloride (ZrOCl2 8H2O, ZOO), boric acid (H3BO3, BA) and gum karaya (GK) as the sources of zirconium, boron and carbon, respectively. The initial formation temperature of ZrB2 was 1200 °C and complete conversion was achieved by 1400 °C. Preceramic precursors and as-synthesized ZrB2 powders were characterized by XRD, TG-DTA, SEM, TEM, EDX and compared with commercial ZrB2 powder made by carbothermic reduction. FT-IR of as-synthesized dried preceramic precursor revealed the formation of Zr–O–C and Zr–O– B whereas SEM showed agglomerated spherical particles with mean diameter of o 1 mm. Commercial ZrB2 and as-synthesized fine ZrB2 powder were spark plasma sintered (SPS) at 1900 °C for 10 min. Addition of 10 wt% of synthesized fine powder improved the fired density from 87% to 93% of theoretical. A significant cost benefit arises for the utilization of cheap synthesized fine powder as an additive for the densification of the more expensive commercial powder. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: A. Sintering UHTCs Zirconium diboride Additive SPS
1. Introduction Ultra-high-temperature ceramics (UHTCs) are materials (usually carbides, borides and nitrides) having extreme melting temperatures ( 43000 °C) along with excellent physical and chemical stability making them suitable for hypersonic flight leading edges, atmospheric re-entry and many aerospace and nuclear structural applications [1]. Transition metal diborides such as zirconium diboride (ZrB2) show relatively good oxidation resistance leading to significant interest for the scientific community. Methods used for the synthesis of metal diborides include i.e. reaction of a metal oxide with boron, borothermal reduction [2], carbothermic reduction of a metal oxide and boron oxide [3], mechanochemical synthesis in which the metal and boron are mixed by high energy milling [4] and wet chemical in which all components such as carbon, boron and the metal-containing sources are dissolved in a liquid medium and subject to condensation and hydrolysis to form a colloidal solution-based precursor [5]. Densification of diborides requires high temperatures and n Correspondence to: Department of Materials, Imperial College London, London SW7 2AZ, UK. E-mail addresses: [email protected], [email protected] (N. Patra).
http://dx.doi.org/10.1016/j.ceramint.2016.03.037 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
external pressure because of the strong covalent bonding and low self-diffusion [6]. Methods of enhancing densification of borides include reactive hot pressing and spark plasma sintering (SPS) [7– 9]. Sintering temperature can also be reduced by adding metal additives such as nickel [10]. However, metal additives lead to formation of nickel-rich phases at grain boundaries and triple junctions deteriorating the mechanical properties [10–11]. Preparation of highly pure and dense borides without any additives is desirable. Additionally, commercially available diboride powders made by carbothermic reduction of zirconia with boria have mostly coarse particles and poor sinterability [12]. SPS is commonly used to fabricate dense UHTCs mainly by concurrent application of high temperature, axial pressure and field-assisted sintering [13]. The field component is associated with an electric current which passes through the powder. SPS has advantages over conventional sintering or hot pressing, such as denser final materials, higher heating rates, shorter holding times, fine product grain size and cleaner grain boundaries [13–17]. The objective of this work was to synthesize ultrafine ZrB2 particles by a solution-based polymer pyrolysis technique which gives the advantages of intimate mixing of the reactants at the atomic level and then to validate the applicability of using the assynthesized submicron powder as an additive for densification of commercial ZrB2 powder.
9566
N. Patra et al. / Ceramics International 42 (2016) 9565–9570
2. Experimental The starting materials were gum karaya (GK), zirconyl oxychloride octahydrate (ZOO) and boric acid (BO) obtained from Sigma-Aldrich Co (Dorset, UK). A flow chart for the synthesis of ZrB2 is shown in Fig. 1. GK (3 M) was dissolved in distilled water with continuous stirring by a magnetic stirrer at 60 °C. Then, BO (3 M) and ZOO (1 M) were mixed and refluxed under continuous stirring with a magnetic stirrer for 2 h followed by drying at 110 °C for 24 h in an oven and crushed to powder in a mortar with a pestle to get ZrB2 precursor powder. Finally, pyrolysis and subsequent heat treatment were carried out in a temperature range of 1000–1400 °C, dwelling for 1 h in a horizontal alumina tube furnace (Lenton furnaces and ovens, Derbyshire, UK) under controlled flow of argon (99.99% purity, 100 mL min 1) to avoid oxidation. Commercial ZrB2 powder (Grade B) prepared by carbothermic reduction with an average particle size of 2.4 mm (H.C. Starck GmbH, Goslar, Germany) was used as a comparison with the laboratory synthesized ZrB2 powder of this work. Sintering of commercial ZrB2 alone and with laboratory synthesized ZrB2 as an additive was carried out using SPS (FCT HPD 25; FCT Systeme GmbH, Rauenstein, Germany) under vacuum (5 Pa). For compositions with laboratory synthesized ZrB2 addition, commercial ZrB2 (90 wt%) with 10 wt% lab synthesized ZrB2 were mixed in acetone followed by ultrasonication for 30 min. A rotary evaporator (Rotavapour R-124, Bucchi, Flawil, Germany) was used to remove the acetone and the powder was then grind in a glass mortar and sieved. For the sintering, 6 g of ZrB2 powder was poured into a 20 mm diameter graphite die containing a graphite paper used on the inner wall of the mold to prevent sticking. The sample was pressed between two graphite punches of 3 cm diameter. The temperature was probed by the top pyrometer focused on the inner graphite wall of the pressing punch at a distance of 4 mm from the sample. Graphite felt was wrapped around the sintering mold during sintering to prevent heat loss by radiation from the sample. For comparison, the commercial samples with and without additive were sintered at 1900 °C under a uniaxial pressure of 50 MPa with a dwell time of 10 min. The heating and cooling rates were 100 °C and 50 °C min 1, respectively. X-ray diffraction (XRD) (PANAlytical Xpert3 diffractometer, Cambridge, UK) with Cu Kα1 radiation (λ ¼ 1.540598 Å) and a
Fig. 2. XRD of synthesized ZrB2 powders heat treated 1 h at 1000, 1100, 1200, 1300 and 1400 °C under Ar.
Fig. 3. FT-IR of (a) starting materials (b) as-synthesized sample dried at 110 °C.
Fig. 1. Flow chart for the synthesis of ZrB2 from solution based process.
Fig. 4. TG (M represents mass) and DTA (Q represents heatflow) of as-synthesized hybrid precursor in argon.
N. Patra et al. / Ceramics International 42 (2016) 9565–9570
Fig. 5. TG-DTA plot of commercial and as-synthesized ZrB2 in air obtained at 1400 °C.
secondary monochromator scanned from 20° to 90° 2θ was used to identify the phase composition of the synthesized powders. The crystallite size was calculated from the XRD diffractogram based on the Scherrer equation. The morphology of the phases in the composite powders and sintered samples was investigated using secondary electron imaging (SEI) in a scanning electron microscope (SEM, JEOL JSM 6010 LA Tokyo, Japan) for fractography and back scattered electron imaging (BSI) for grain orientation sharp contrast using Carl Zeiss AURIGA crossbeam (FIB-SEM) workstation. Transmission electron microscopy (TEM, using a JEOL JEM 2000 FX, Tokyo, Japan) with an acceleration voltage up to 200 kV was used to analyze the morphology of fine particles using brightfield imaging. Selected area diffraction patterns (SADP) were solved using the reciprocal distance and angles between the
9567
reflections and then comparing the theoretical calculated dhkl values and in this study FT-IR spectra of as-synthesized composite precursors were acquired using a Nicolet iS10 spectrometer (Thermo Scientific Company Ltd, Waltham, USA) in the range of 4000–400 cm 1 to identify structural changes and interactions of organic ligands with the metal salt ions. FT-IR spectroscopy has been used to investigate the structural changes in materials by various investigators [18–20]. All spectra were baseline corrected and normalized thereafter to the highest peak. Thermal analysis (thermogravimetry-differential thermal analysis, TG-DTA) of the as-synthesized powder dried at 110 °C was performed at a heating rate of 10 °C min 1 in a range of 30–1550 °C under flowing argon (60 mL min 1) using NETZSCH STA 449 F1 Jupiter, Selb, Germany. TG oxidation of the synthesized and commercial ZrB2 powder was also performed in air at a heating rate of 10 °C min 1 in a range of 30–1500 °C. The density was measured using Archimedes principle according to ASTM C373-88 with water as the immersion medium using theoretical density of ZrB2 of 6.119 g cm 3.
3. Results and discussion Fig. 2 shows XRD of synthesized powders heat treated at different temperatures to identify the formation of ZrB2 (phase compositions and crystalline state). After heat treatment at 1000 °C, t-ZrO2 and m-ZrO2 were detected. Initial formation of ZrB2 was observed after heating to 1200 °C along with some monoclinic zirconia (m-ZrO2). The intensity of m-ZrO2 decreases with ZrB2 peaks appearing at 1200 °C. Strong ZrB2 crystalline peaks appear in powder pyrolysed at 1300 °C. Well defined ZrB2 XRD peaks were observed in pyrolysed precursors after 1 h at 1400 °C, which is lower than the temperature required for conventional solid state synthesis of ZrB2 ( 2000 °C) generally employed to synthesize diborides [21]. The stable polymorph of
Fig. 6. SEM of (a) commercial ZrB2 and (b) as-synthesized ZrB2 obtained after 1 h heat treatment at 1400 in Ar.
9568
N. Patra et al. / Ceramics International 42 (2016) 9565–9570
Fig. 7. (a) EDX (b)BF-TEM and (c) SADP of the synthesized ZrB2 powder obtained after heating 1 h at 1400 °C.
Fig. 8. XRD of commercial and as-synthesized powder, sintered commercial and sintered commercial with 10 wt% of fine as-synthesized powder as an additive.
Fig. 9. SPS event profile of (a) commercial ZrB2 and (b) commercial with 10% assynthesized ZrB2.
zirconia at room temperature and atmospheric pressure is monoclinic, which transforms to tetragonal at 1170 °C on heating. Polymorphic transformation is also a function of the crystallite size [22]. The crystallite size calculated for ZrB2 was 70 nm. When the particle size reaches 1 mm, metastable tetragonal zirconia can exist at room temperature [23]. This means that at low temperature metastable tetragonal ZrO2 is formed along with stable
monoclinic phase at 1000 °C. Fig. 3a and b shows the normalized FT-IR absorption spectral band of starting materials used for synthesis of the hybrid precursor i.e. GK, ZOO, BO and the hybrid complex after drying at 110 °C to determine the structural changes and interactions of organic ligands with the metal salt ions (e.g. Zr and B). For GK (Fig. 3a), the major bands observed correspond to vibrations of characteristic groups; the broad band at 3330 cm 1 corresponds to the O–H stretching band of the hydroxyl group; the band at 1725 cm 1 is due to the C ¼O stretching vibration; that at 1595 cm 1 to the C ¼C band of CO2 asymmetric stretching from carboxylic acid salts; that of 1417 cm 1 to the CO2 symmetric stretching and 1371 cm 1 represents the C–H deformation. The band at 1244 cm 1 is due to the CO2 symmetrical stretching from carboxylic acid salts or C–O–C asymmetrical stretching. For ZOO (Fig. 3a) the band at 1623 cm 1 can be assigned to the “scissor” bending mode of coordinated water. The wide peak at 3330 cm 1 can be attributed to the stretching vibration of the –OH group. For BO (Fig. 3a) the characteristic O–H stretching band of the hydroxyl group appears at 3183 cm 1. The absorption band near 1410 cm 1 was assigned to the anti-symmetric stretching vibration of the B–O band. The intense peak at 545 cm 1 was attributed to vibration of B–O band. Comparing the FT-IR results of the starting materials with the as-synthesized dried hybrid precursor reveals the structural changes occurring. For the as-synthesized hybrid complex (Fig. 3b) the O–H stretching band appears at 3180 cm 1 which is among the O–H bands of BO and ZOO. The narrow O–H band for the as-synthesized hybrid complex (Fig. 3b) is an indication of the association of Zr, B with the organic polymer. The existence of a B–O–B linkage was indicated by the peak located at 495 cm 1. The organic ligands ν(C ¼O) chelate to zirconium which is evident from the differences in the FT-IR spectra [24]. The peaks at 1723 and 1604 cm 1 assigned to ν(C ¼O) of GK indicate it chelates to the zirconium ion which is shifted to lower wave number of 1711 cm 1 forming Zr–O–C. The B–O bond of boric acid chelates to the Zr–O–C which induced condensation so forming a Zr–O–C–B complex network evident from the band shifting to lower wave numbers (1385 and 1338 cm 1). Thermogravimetric analysis has been used to investigate the decomposition, stability, purity and yield by various investigators [25–28]. Fig. 4 shows representative TG and DTA curves of dried samples as synthesized under argon atmosphere. TG reveals the mass loss for the hybrid occurs in three steps with respect to temperature. The first major mass loss (30%) occurs from 30 to 400 °C, the second (of 12%) from 400 to 1200 °C and the third mass
N. Patra et al. / Ceramics International 42 (2016) 9565–9570
9569
Fig. 10. SEM BSI of the polished surface of (a) commercial ZrB2 and (b) commercial with 10 wt% as-synthesized ZrB2 powder spark plasma sintered at 1900 °C for 10 min under 50 MPa pressure.
loss (30%) occurs from 1200 to 1550 °C. The first major mass loss is due to the decomposition of organic polymer forming carbon along with evaporation of physisorbed water and dehydration of boric acid [29]. The second mass loss of 12% is due to the melting of amorphous boron oxide and evaporation of some impurities during polymorphic changes of boron and zirconium oxides. The third major mass loss of 30% is due to the carbothermic reaction started following evaporation of CO gas. DTA in argon of the hybrid complex shows five major reactions. The first endothermic peak at 180 °C is due to the dehydration of boric acid and physisorbed water. The second exothermic peak was due to the decomposition of organic polymer to form carbon. The third endothermic peak at 550 °C was due to the melting of amorphous boron trioxide [29]. The fourth peak at 1025 °C is due to evaporation of boron oxide and the last endothermic peak is due to the carbothermic reaction to form ZrB2 powder. To complement the structural and compositional information, the oxidation behavior of the commercial ZrB2 was compared with our laboratory synthesized ZrB2 powder by TG-DTA in air atmosphere. The changes in mass as well as the reaction profile as a function of temperature are shown in Fig. 5. From the TG curve, there is no change in mass up to 550 °C for both as-synthesized and as-received commercial powder. The onsets of oxidation starts at about 560 and 680 °C with gain in mass for the commercial and laboratory synthesized ZrB2 powder which corresponds to the temperature at which ZrB2 started to oxidize [30] while the commercial ZrB2 powder shows a sharp increase in mass at 650 °C as can be clearly seen from the onset of the corresponding DTA peak, the laboratory-synthesized powder shows a broad oxidation range from 680 to 1390 °C. Both powders show the same amount of gain in mass of 57%. The laboratory synthesized powder shows a plateau after 1390 °C whereas the commercial powder still shows slight increment in mass. From the DTA curve, the lower
temperature exothermic peak is due to the oxidation of Zr and B of ZrB2 to form ZrO2 and B2O3. The SEM micrographs of commercial and synthesized ZrB2 powder at 1400 °C for 1 h are shown in Fig. 6a and b. Fig. 6a and b shows that both ZrB2 particles having similar grain sizes whereas synthesized ZrB2 particles (Fig. 7b have rounded morphology with mean particle (crystallites) of about 200 nm. Fig. 7 shows EDX, and a BF-TEM image and the corresponding diffraction pattern of the ZrB2 powder prepared by solution technique. TEM imaging revealed rounded nanosized particle morphology. TEM investigation of the powders revealed no evidence of the presence of amorphous carbon consistent with the results obtained from the XRD. SADP reveals mainly ring type pattern indicating nanocrystallinity of the produced material. The ZrB2 crystallites were in the range of 30–100 nm. The crystallite sizes estimated from the TEM micrographs are roughly in agreement with the XRD. XRD analysis (Fig. 8) did not show any difference in the phases present between as-synthesized powder and commercial SPS sintered samples with and without as-synthesized ZrB2 additives. However, the lattice parameter increased slightly with as-synthesized powder addition which is believed to be due to the buckling of the boron planes. The commercial ZrB2 powder and the commercial powder with 10 wt% of as-synthesized fine ZrB2 powder were sintered at 1900 °C. The times, temperature, force and displacement (shrinkage) profiles are given in Fig. 9. The pressure was kept constant at 50 MPa in both cases. The densification of both commercial and as-synthesized ZrB2 appeared to start around 900 °C. However, the densification increases when 10 wt% as-synthesized fine powder was added which is indicated by lower displacement. Fig. 10 shows BSI of the sintered commercial powders (Fig. 10a) and with 10 wt% of as-synthesized powder as an additive (Fig. 10b)
9570
N. Patra et al. / Ceramics International 42 (2016) 9565–9570
sintered at 1900 °C for 10 min. The relative density of the ceramic SPS for the commercial powder was 87%. There is a notable increase in density with fine grain size for 10 wt% fine as-synthesized powder used as an additive. The ceramics relative density increases from 87% to 93% at 1900 °C while retaining a fine grain size due to the fine grain size of the synthesized powder the shrinkage rate increases. The smallest grain size ( 2 mm) and highest density (93%) was attributed to the absence of surface oxidation or oxide contamination.
[9]
[10]
[11]
[12]
4. Conclusions [13]
ZrB2 powder was successfully synthesized by boro/carbothermal reduction of GK, ZOO and boric acid, respectively. It was found out that the formation of ZrB2 started at 1200°C and pure ZrB2 phase was obtained at 1400 °C. As-synthesized ZrB2 fine powder was used as an additive to assist the densification of commercial ZrB2 powder. Ceramic samples were produced using commercial powder with 10 wt% of fine synthesized powder and SPS'd at 1900 °C for 10 min with an applied pressure of 50 MPa. The addition of synthesized powder to the system improved the relative density from 87 to 93%. Additionally, the obtained microstructure consists of fine grains with size of 1–2 mm compared to 5–10 mm for commercial powders sintered without added synthesized powder.
[14]
[15]
[16]
[17] [18]
[19]
Acknowledgment Financial support provided by Office of Naval Research Global (ONRG), USA under contract number N62909-13-1-N055 is gratefully acknowledged. The authors would like to thank Dr Ben Milsom at Queen Mary University London for help with spark plasma sintering.
[20] [21]
[22]
[23] [24]
References [25] [1] S.R. Levine, E.J. Opila, M.C. Halbig, J.D. Kiser, M. Singh, J.A. Salem, Evaluation of ultra-high temperature ceramics for aeropropulsion use, J. Eur. Ceram. Soc. 22 (2002) 2757–2767. [2] S. Ran, O. Van der Biest, J. Vleugels, ZrB2 powders synthesis by borothermal reduction, J. Am. Ceram. Soc. 1590 (2010) 1586–1590. [3] E.Y. Jung, J.H. Kim, S.H. Jung, S.C. Choi, Synthesis of ZrB2 powders by carbothermal and borothermal reduction, J. Alloy. Compd. 538 (2012) 164–168. [4] S. Guo, C. Hu, Y. Kagawa, Mechanochemical processing of nanocrystalline zirconium diboride powder, J. Am. Ceram. Soc. 94 (2011) 3643–3647. [5] N. Patra, D.D. Jayaseelan, W.E. Lee, Synthesis of ZrB2/SiC composite powders by single-step solution process from organic–inorganic hybrid precursor, Adv. Appl. Ceram. 115 (2016) 36–42. [6] W.G. Fahrenholtz, G.E. Hilmas, I.G. Talmy, J.A. Zaykoski, Refractory diborides of zirconium and hafnium, J. Am. Ceram. Soc. 90 (2007) 1347–1364. [7] W.-W. Wu, G.-J. Zhang, Y.-M. Kan, P.-L. Wang, Reactive hot pressing of ZrB2– SiC–ZrC composites at 1600 °C, J. Am. Ceram. Soc. 91 (2008) 2501–2508. [8] W.-W. Wu, G.-J. Zhang, Y.-M. Kan, P.-L. Wang, K. Vanmeensel, J. Vleugels, et al., Synthesis and microstructural features of ZrB2–SiC-based composites by
[26]
[27]
[28]
[29]
[30]
reactive spark plasma sintering and reactive hot pressing, Scr. Mater. 57 (2007) 317–320. X.-J. Zhou, G.-J. Zhang, Y.-G. Li, Y.-M. Kan, P.-L. Wang, Hot pressed ZrB2–SiC–C ultra high temperature ceramics with polycarbosilane as a precursor, Mater. Lett. 61 (2007) 960–963. J.J. Melendez-Martınez, A. Domınguez-Rodrıguez, F. Monteverde, C. Melandri, G. de Portu, Characterisation and high temperature mechanical properties of zirconium boride-based materials, J. Eur. Ceram. Soc. 22 (2002) 2543–2549. E. Wuchina, M. Opeka, S. Causey, K. Buesking, J. Spain, A. Cull, J. Routbort, F. Guitierrez-Mora, Designing for ultrahigh-temperature applications : the mechanical and thermal properties of HfB2, HfCx, HfNx and αHf (N), J. Mater. Sci 39 (2004) 5939–5949. S. Zhu, W.G. Fahrenholtz, G.E. Hilmas, Enhanced densification and mechanical properties of ZrB2–SiC processed by a preceramic polymer coating route, Scr. Mater. 59 (2008) 123–126. S. Grasso, T. Saunders, H. Porwal, O. Cedillos-Barraza, D.D. Jayaseelan, W.E. Lee, et al., Flash spark plasma sintering (FSPS) of pure ZrB2, J. Am. Ceram. Soc. 97 (2014) 2405–2408. S. Venugopal, A. Paul, B. Vaidhyanathan, J.G.P. Binner, A. Heaton, P.M. Brown, Synthesis and spark plasma sintering of sub-micron HfB2: effect of various carbon sources, J. Eur. Ceram. Soc. 34 (2014) 1471–1479. S.R. Bakshi, V. Musaramthota, D. Lahiri, V. Singh, S. Seal, A. Agarwal, Spark plasma sintered tantalum carbide: effect of pressure and nano-boron carbide addition on microstructure and mechanical properties, Mater. Sci. Eng. A 528 (2011) 1287–1295. V. Medri, F. Monteverde, A. Balbo, A. Bellosi, Comparison of ZrB2-ZrC-SiC composites fabricated by spark plasma sintering and hot-pressing, Adv. Eng. Mater. 7 (2005) 159–163. E. Khaleghi, Y.S. Lin, M.A. Meyers, E.A. Olevsky, Spark plasma sintering of tantalum carbide, Scr. Mater. 63 (2010) 577–580. S. Thorat, N. Patra, R. Ruffilli, A. Diaspro, M. Salerno, Preparation and characterization of a BisGMA-resin dental restorative composites with glass, silica and titania fillers, Dent. Mater. J. 31 (2012) 635–644. N. Patra, A.C. Barone, M. Salerno, Solvent effects on the thermal and mechanical properties of poly(methyl methacrylate) casted from concentrated solutions, Adv. Polym. Technol. 30 (2011) 12–20. N. Patra, D.D. Jayaseelan, W.E. Lee, Synthesis of biopolymer-derived zirconium carbide powder by facile one-pot reaction, J. Am. Ceram. Soc. 98 (2015) 71–77. R. Thompson, Production, fabrication, and uses of borides, in: R. Freer (Ed.), The Physics and Chemistry of Carbides, Nitrides and Borides, Kluwer Academic Publishers, Dordrecht, Dordrecht, 1990, pp. 113–120. N. Igawa, T. Nagasaki, Y. Ishii, K. Noda, H. Ohno, Y. Morii, Phase-transformation study of metastable tetragonal zirconia powder, J. Mater. Sci. 33 (1998) 4747–4758. X.-M. Liu, Z.-F. Yan, Phase transformation of nanosized zirconia, Chin. J. Struct. Chem. 25 (2006) 424–432. C. Yan, R. Liu, Y. Cao, C. Zhang, D. Zhang, Synthesis of submicrometer zirconium carbide formed from inorganic–organic hybrid precursor pyrolysis, J. Sol–Gel Sci. Technol. 64 (2012) 251–256. N. Mishra, N. Patra, S. Pandey, M. Salerno, M. Sharon, M. Sharon, Taguchi method optimization of wax production from pyrolysis of waste polypropylene, J. Therm. Anal. Calorim. 117 (2014) 885–892. N. Patra, J. Hladik, L. Martinova, Investigating the thermal properties of polyethylene plasma modified by using unconventional chemical vapors, J. Therm. Anal. Calorim. 117 (2014) 229–234. N. Patra, M. Salerno, P.D. Cozzoli, A. Athanassiou, Surfactant-induced thermomechanical and morphological changes in TiO2-polystyrene nanocomposites, J. Colloid Interface Sci. 405 (2013) 103–108. N. Patra, M. Salerno, P.D. Cozzoli, A.C. Barone, L. Ceseracciu, F. Pignatelli, et al., Thermal and mechanical characterization of poly(methyl methacrylate) nanocomposites filled with TiO2 nanorods, Compos. Part B: Eng. 43 (2012) 3114–3119. F. Sevim, F. Demir, M. Bilen, H. Okur, Kinetic analysis of thermal decomposition of boric acid from thermogravimetric data, Korean J. Chem. Eng. 23 (2006) 736–740. W.C. Tripp, H.C. Graham, Thermogravimetric study of the oxidation of ZrB2 in the temperature range of 800 °C to 1500 °C, J. Electrochem. Soc. 118 (1971) 1195–1199.
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Synthesis, characterization and use of synthesized fine zirconium diboride as an additive for densification of commercial zirconium diboride powder Niranjan Patra n, Nasrin Al Nasiri, Daniel Doni Jayaseelan, William Edward Lee Centre for Advanced Structural Ceramics and Department of Materials, Imperial College London, London SW7 2AZ, UK
art ic l e i nf o
a b s t r a c t
Article history: Received 13 February 2016 Received in revised form 3 March 2016 Accepted 4 March 2016 Available online 5 March 2016
Zirconium diboride (ZrB2) was synthesized by a solution-based technique using zirconyl chloride (ZrOCl2 8H2O, ZOO), boric acid (H3BO3, BA) and gum karaya (GK) as the sources of zirconium, boron and carbon, respectively. The initial formation temperature of ZrB2 was 1200 °C and complete conversion was achieved by 1400 °C. Preceramic precursors and as-synthesized ZrB2 powders were characterized by XRD, TG-DTA, SEM, TEM, EDX and compared with commercial ZrB2 powder made by carbothermic reduction. FT-IR of as-synthesized dried preceramic precursor revealed the formation of Zr–O–C and Zr–O– B whereas SEM showed agglomerated spherical particles with mean diameter of o 1 mm. Commercial ZrB2 and as-synthesized fine ZrB2 powder were spark plasma sintered (SPS) at 1900 °C for 10 min. Addition of 10 wt% of synthesized fine powder improved the fired density from 87% to 93% of theoretical. A significant cost benefit arises for the utilization of cheap synthesized fine powder as an additive for the densification of the more expensive commercial powder. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: A. Sintering UHTCs Zirconium diboride Additive SPS
1. Introduction Ultra-high-temperature ceramics (UHTCs) are materials (usually carbides, borides and nitrides) having extreme melting temperatures ( 43000 °C) along with excellent physical and chemical stability making them suitable for hypersonic flight leading edges, atmospheric re-entry and many aerospace and nuclear structural applications [1]. Transition metal diborides such as zirconium diboride (ZrB2) show relatively good oxidation resistance leading to significant interest for the scientific community. Methods used for the synthesis of metal diborides include i.e. reaction of a metal oxide with boron, borothermal reduction [2], carbothermic reduction of a metal oxide and boron oxide [3], mechanochemical synthesis in which the metal and boron are mixed by high energy milling [4] and wet chemical in which all components such as carbon, boron and the metal-containing sources are dissolved in a liquid medium and subject to condensation and hydrolysis to form a colloidal solution-based precursor [5]. Densification of diborides requires high temperatures and n Correspondence to: Department of Materials, Imperial College London, London SW7 2AZ, UK. E-mail addresses: [email protected], [email protected] (N. Patra).
http://dx.doi.org/10.1016/j.ceramint.2016.03.037 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
external pressure because of the strong covalent bonding and low self-diffusion [6]. Methods of enhancing densification of borides include reactive hot pressing and spark plasma sintering (SPS) [7– 9]. Sintering temperature can also be reduced by adding metal additives such as nickel [10]. However, metal additives lead to formation of nickel-rich phases at grain boundaries and triple junctions deteriorating the mechanical properties [10–11]. Preparation of highly pure and dense borides without any additives is desirable. Additionally, commercially available diboride powders made by carbothermic reduction of zirconia with boria have mostly coarse particles and poor sinterability [12]. SPS is commonly used to fabricate dense UHTCs mainly by concurrent application of high temperature, axial pressure and field-assisted sintering [13]. The field component is associated with an electric current which passes through the powder. SPS has advantages over conventional sintering or hot pressing, such as denser final materials, higher heating rates, shorter holding times, fine product grain size and cleaner grain boundaries [13–17]. The objective of this work was to synthesize ultrafine ZrB2 particles by a solution-based polymer pyrolysis technique which gives the advantages of intimate mixing of the reactants at the atomic level and then to validate the applicability of using the assynthesized submicron powder as an additive for densification of commercial ZrB2 powder.
9566
N. Patra et al. / Ceramics International 42 (2016) 9565–9570
2. Experimental The starting materials were gum karaya (GK), zirconyl oxychloride octahydrate (ZOO) and boric acid (BO) obtained from Sigma-Aldrich Co (Dorset, UK). A flow chart for the synthesis of ZrB2 is shown in Fig. 1. GK (3 M) was dissolved in distilled water with continuous stirring by a magnetic stirrer at 60 °C. Then, BO (3 M) and ZOO (1 M) were mixed and refluxed under continuous stirring with a magnetic stirrer for 2 h followed by drying at 110 °C for 24 h in an oven and crushed to powder in a mortar with a pestle to get ZrB2 precursor powder. Finally, pyrolysis and subsequent heat treatment were carried out in a temperature range of 1000–1400 °C, dwelling for 1 h in a horizontal alumina tube furnace (Lenton furnaces and ovens, Derbyshire, UK) under controlled flow of argon (99.99% purity, 100 mL min 1) to avoid oxidation. Commercial ZrB2 powder (Grade B) prepared by carbothermic reduction with an average particle size of 2.4 mm (H.C. Starck GmbH, Goslar, Germany) was used as a comparison with the laboratory synthesized ZrB2 powder of this work. Sintering of commercial ZrB2 alone and with laboratory synthesized ZrB2 as an additive was carried out using SPS (FCT HPD 25; FCT Systeme GmbH, Rauenstein, Germany) under vacuum (5 Pa). For compositions with laboratory synthesized ZrB2 addition, commercial ZrB2 (90 wt%) with 10 wt% lab synthesized ZrB2 were mixed in acetone followed by ultrasonication for 30 min. A rotary evaporator (Rotavapour R-124, Bucchi, Flawil, Germany) was used to remove the acetone and the powder was then grind in a glass mortar and sieved. For the sintering, 6 g of ZrB2 powder was poured into a 20 mm diameter graphite die containing a graphite paper used on the inner wall of the mold to prevent sticking. The sample was pressed between two graphite punches of 3 cm diameter. The temperature was probed by the top pyrometer focused on the inner graphite wall of the pressing punch at a distance of 4 mm from the sample. Graphite felt was wrapped around the sintering mold during sintering to prevent heat loss by radiation from the sample. For comparison, the commercial samples with and without additive were sintered at 1900 °C under a uniaxial pressure of 50 MPa with a dwell time of 10 min. The heating and cooling rates were 100 °C and 50 °C min 1, respectively. X-ray diffraction (XRD) (PANAlytical Xpert3 diffractometer, Cambridge, UK) with Cu Kα1 radiation (λ ¼ 1.540598 Å) and a
Fig. 2. XRD of synthesized ZrB2 powders heat treated 1 h at 1000, 1100, 1200, 1300 and 1400 °C under Ar.
Fig. 3. FT-IR of (a) starting materials (b) as-synthesized sample dried at 110 °C.
Fig. 1. Flow chart for the synthesis of ZrB2 from solution based process.
Fig. 4. TG (M represents mass) and DTA (Q represents heatflow) of as-synthesized hybrid precursor in argon.
N. Patra et al. / Ceramics International 42 (2016) 9565–9570
Fig. 5. TG-DTA plot of commercial and as-synthesized ZrB2 in air obtained at 1400 °C.
secondary monochromator scanned from 20° to 90° 2θ was used to identify the phase composition of the synthesized powders. The crystallite size was calculated from the XRD diffractogram based on the Scherrer equation. The morphology of the phases in the composite powders and sintered samples was investigated using secondary electron imaging (SEI) in a scanning electron microscope (SEM, JEOL JSM 6010 LA Tokyo, Japan) for fractography and back scattered electron imaging (BSI) for grain orientation sharp contrast using Carl Zeiss AURIGA crossbeam (FIB-SEM) workstation. Transmission electron microscopy (TEM, using a JEOL JEM 2000 FX, Tokyo, Japan) with an acceleration voltage up to 200 kV was used to analyze the morphology of fine particles using brightfield imaging. Selected area diffraction patterns (SADP) were solved using the reciprocal distance and angles between the
9567
reflections and then comparing the theoretical calculated dhkl values and in this study FT-IR spectra of as-synthesized composite precursors were acquired using a Nicolet iS10 spectrometer (Thermo Scientific Company Ltd, Waltham, USA) in the range of 4000–400 cm 1 to identify structural changes and interactions of organic ligands with the metal salt ions. FT-IR spectroscopy has been used to investigate the structural changes in materials by various investigators [18–20]. All spectra were baseline corrected and normalized thereafter to the highest peak. Thermal analysis (thermogravimetry-differential thermal analysis, TG-DTA) of the as-synthesized powder dried at 110 °C was performed at a heating rate of 10 °C min 1 in a range of 30–1550 °C under flowing argon (60 mL min 1) using NETZSCH STA 449 F1 Jupiter, Selb, Germany. TG oxidation of the synthesized and commercial ZrB2 powder was also performed in air at a heating rate of 10 °C min 1 in a range of 30–1500 °C. The density was measured using Archimedes principle according to ASTM C373-88 with water as the immersion medium using theoretical density of ZrB2 of 6.119 g cm 3.
3. Results and discussion Fig. 2 shows XRD of synthesized powders heat treated at different temperatures to identify the formation of ZrB2 (phase compositions and crystalline state). After heat treatment at 1000 °C, t-ZrO2 and m-ZrO2 were detected. Initial formation of ZrB2 was observed after heating to 1200 °C along with some monoclinic zirconia (m-ZrO2). The intensity of m-ZrO2 decreases with ZrB2 peaks appearing at 1200 °C. Strong ZrB2 crystalline peaks appear in powder pyrolysed at 1300 °C. Well defined ZrB2 XRD peaks were observed in pyrolysed precursors after 1 h at 1400 °C, which is lower than the temperature required for conventional solid state synthesis of ZrB2 ( 2000 °C) generally employed to synthesize diborides [21]. The stable polymorph of
Fig. 6. SEM of (a) commercial ZrB2 and (b) as-synthesized ZrB2 obtained after 1 h heat treatment at 1400 in Ar.
9568
N. Patra et al. / Ceramics International 42 (2016) 9565–9570
Fig. 7. (a) EDX (b)BF-TEM and (c) SADP of the synthesized ZrB2 powder obtained after heating 1 h at 1400 °C.
Fig. 8. XRD of commercial and as-synthesized powder, sintered commercial and sintered commercial with 10 wt% of fine as-synthesized powder as an additive.
Fig. 9. SPS event profile of (a) commercial ZrB2 and (b) commercial with 10% assynthesized ZrB2.
zirconia at room temperature and atmospheric pressure is monoclinic, which transforms to tetragonal at 1170 °C on heating. Polymorphic transformation is also a function of the crystallite size [22]. The crystallite size calculated for ZrB2 was 70 nm. When the particle size reaches 1 mm, metastable tetragonal zirconia can exist at room temperature [23]. This means that at low temperature metastable tetragonal ZrO2 is formed along with stable
monoclinic phase at 1000 °C. Fig. 3a and b shows the normalized FT-IR absorption spectral band of starting materials used for synthesis of the hybrid precursor i.e. GK, ZOO, BO and the hybrid complex after drying at 110 °C to determine the structural changes and interactions of organic ligands with the metal salt ions (e.g. Zr and B). For GK (Fig. 3a), the major bands observed correspond to vibrations of characteristic groups; the broad band at 3330 cm 1 corresponds to the O–H stretching band of the hydroxyl group; the band at 1725 cm 1 is due to the C ¼O stretching vibration; that at 1595 cm 1 to the C ¼C band of CO2 asymmetric stretching from carboxylic acid salts; that of 1417 cm 1 to the CO2 symmetric stretching and 1371 cm 1 represents the C–H deformation. The band at 1244 cm 1 is due to the CO2 symmetrical stretching from carboxylic acid salts or C–O–C asymmetrical stretching. For ZOO (Fig. 3a) the band at 1623 cm 1 can be assigned to the “scissor” bending mode of coordinated water. The wide peak at 3330 cm 1 can be attributed to the stretching vibration of the –OH group. For BO (Fig. 3a) the characteristic O–H stretching band of the hydroxyl group appears at 3183 cm 1. The absorption band near 1410 cm 1 was assigned to the anti-symmetric stretching vibration of the B–O band. The intense peak at 545 cm 1 was attributed to vibration of B–O band. Comparing the FT-IR results of the starting materials with the as-synthesized dried hybrid precursor reveals the structural changes occurring. For the as-synthesized hybrid complex (Fig. 3b) the O–H stretching band appears at 3180 cm 1 which is among the O–H bands of BO and ZOO. The narrow O–H band for the as-synthesized hybrid complex (Fig. 3b) is an indication of the association of Zr, B with the organic polymer. The existence of a B–O–B linkage was indicated by the peak located at 495 cm 1. The organic ligands ν(C ¼O) chelate to zirconium which is evident from the differences in the FT-IR spectra [24]. The peaks at 1723 and 1604 cm 1 assigned to ν(C ¼O) of GK indicate it chelates to the zirconium ion which is shifted to lower wave number of 1711 cm 1 forming Zr–O–C. The B–O bond of boric acid chelates to the Zr–O–C which induced condensation so forming a Zr–O–C–B complex network evident from the band shifting to lower wave numbers (1385 and 1338 cm 1). Thermogravimetric analysis has been used to investigate the decomposition, stability, purity and yield by various investigators [25–28]. Fig. 4 shows representative TG and DTA curves of dried samples as synthesized under argon atmosphere. TG reveals the mass loss for the hybrid occurs in three steps with respect to temperature. The first major mass loss (30%) occurs from 30 to 400 °C, the second (of 12%) from 400 to 1200 °C and the third mass
N. Patra et al. / Ceramics International 42 (2016) 9565–9570
9569
Fig. 10. SEM BSI of the polished surface of (a) commercial ZrB2 and (b) commercial with 10 wt% as-synthesized ZrB2 powder spark plasma sintered at 1900 °C for 10 min under 50 MPa pressure.
loss (30%) occurs from 1200 to 1550 °C. The first major mass loss is due to the decomposition of organic polymer forming carbon along with evaporation of physisorbed water and dehydration of boric acid [29]. The second mass loss of 12% is due to the melting of amorphous boron oxide and evaporation of some impurities during polymorphic changes of boron and zirconium oxides. The third major mass loss of 30% is due to the carbothermic reaction started following evaporation of CO gas. DTA in argon of the hybrid complex shows five major reactions. The first endothermic peak at 180 °C is due to the dehydration of boric acid and physisorbed water. The second exothermic peak was due to the decomposition of organic polymer to form carbon. The third endothermic peak at 550 °C was due to the melting of amorphous boron trioxide [29]. The fourth peak at 1025 °C is due to evaporation of boron oxide and the last endothermic peak is due to the carbothermic reaction to form ZrB2 powder. To complement the structural and compositional information, the oxidation behavior of the commercial ZrB2 was compared with our laboratory synthesized ZrB2 powder by TG-DTA in air atmosphere. The changes in mass as well as the reaction profile as a function of temperature are shown in Fig. 5. From the TG curve, there is no change in mass up to 550 °C for both as-synthesized and as-received commercial powder. The onsets of oxidation starts at about 560 and 680 °C with gain in mass for the commercial and laboratory synthesized ZrB2 powder which corresponds to the temperature at which ZrB2 started to oxidize [30] while the commercial ZrB2 powder shows a sharp increase in mass at 650 °C as can be clearly seen from the onset of the corresponding DTA peak, the laboratory-synthesized powder shows a broad oxidation range from 680 to 1390 °C. Both powders show the same amount of gain in mass of 57%. The laboratory synthesized powder shows a plateau after 1390 °C whereas the commercial powder still shows slight increment in mass. From the DTA curve, the lower
temperature exothermic peak is due to the oxidation of Zr and B of ZrB2 to form ZrO2 and B2O3. The SEM micrographs of commercial and synthesized ZrB2 powder at 1400 °C for 1 h are shown in Fig. 6a and b. Fig. 6a and b shows that both ZrB2 particles having similar grain sizes whereas synthesized ZrB2 particles (Fig. 7b have rounded morphology with mean particle (crystallites) of about 200 nm. Fig. 7 shows EDX, and a BF-TEM image and the corresponding diffraction pattern of the ZrB2 powder prepared by solution technique. TEM imaging revealed rounded nanosized particle morphology. TEM investigation of the powders revealed no evidence of the presence of amorphous carbon consistent with the results obtained from the XRD. SADP reveals mainly ring type pattern indicating nanocrystallinity of the produced material. The ZrB2 crystallites were in the range of 30–100 nm. The crystallite sizes estimated from the TEM micrographs are roughly in agreement with the XRD. XRD analysis (Fig. 8) did not show any difference in the phases present between as-synthesized powder and commercial SPS sintered samples with and without as-synthesized ZrB2 additives. However, the lattice parameter increased slightly with as-synthesized powder addition which is believed to be due to the buckling of the boron planes. The commercial ZrB2 powder and the commercial powder with 10 wt% of as-synthesized fine ZrB2 powder were sintered at 1900 °C. The times, temperature, force and displacement (shrinkage) profiles are given in Fig. 9. The pressure was kept constant at 50 MPa in both cases. The densification of both commercial and as-synthesized ZrB2 appeared to start around 900 °C. However, the densification increases when 10 wt% as-synthesized fine powder was added which is indicated by lower displacement. Fig. 10 shows BSI of the sintered commercial powders (Fig. 10a) and with 10 wt% of as-synthesized powder as an additive (Fig. 10b)
9570
N. Patra et al. / Ceramics International 42 (2016) 9565–9570
sintered at 1900 °C for 10 min. The relative density of the ceramic SPS for the commercial powder was 87%. There is a notable increase in density with fine grain size for 10 wt% fine as-synthesized powder used as an additive. The ceramics relative density increases from 87% to 93% at 1900 °C while retaining a fine grain size due to the fine grain size of the synthesized powder the shrinkage rate increases. The smallest grain size ( 2 mm) and highest density (93%) was attributed to the absence of surface oxidation or oxide contamination.
[9]
[10]
[11]
[12]
4. Conclusions [13]
ZrB2 powder was successfully synthesized by boro/carbothermal reduction of GK, ZOO and boric acid, respectively. It was found out that the formation of ZrB2 started at 1200°C and pure ZrB2 phase was obtained at 1400 °C. As-synthesized ZrB2 fine powder was used as an additive to assist the densification of commercial ZrB2 powder. Ceramic samples were produced using commercial powder with 10 wt% of fine synthesized powder and SPS'd at 1900 °C for 10 min with an applied pressure of 50 MPa. The addition of synthesized powder to the system improved the relative density from 87 to 93%. Additionally, the obtained microstructure consists of fine grains with size of 1–2 mm compared to 5–10 mm for commercial powders sintered without added synthesized powder.
[14]
[15]
[16]
[17] [18]
[19]
Acknowledgment Financial support provided by Office of Naval Research Global (ONRG), USA under contract number N62909-13-1-N055 is gratefully acknowledged. The authors would like to thank Dr Ben Milsom at Queen Mary University London for help with spark plasma sintering.
[20] [21]
[22]
[23] [24]
References [25] [1] S.R. Levine, E.J. Opila, M.C. Halbig, J.D. Kiser, M. Singh, J.A. Salem, Evaluation of ultra-high temperature ceramics for aeropropulsion use, J. Eur. Ceram. Soc. 22 (2002) 2757–2767. [2] S. Ran, O. Van der Biest, J. Vleugels, ZrB2 powders synthesis by borothermal reduction, J. Am. Ceram. Soc. 1590 (2010) 1586–1590. [3] E.Y. Jung, J.H. Kim, S.H. Jung, S.C. Choi, Synthesis of ZrB2 powders by carbothermal and borothermal reduction, J. Alloy. Compd. 538 (2012) 164–168. [4] S. Guo, C. Hu, Y. Kagawa, Mechanochemical processing of nanocrystalline zirconium diboride powder, J. Am. Ceram. Soc. 94 (2011) 3643–3647. [5] N. Patra, D.D. Jayaseelan, W.E. Lee, Synthesis of ZrB2/SiC composite powders by single-step solution process from organic–inorganic hybrid precursor, Adv. Appl. Ceram. 115 (2016) 36–42. [6] W.G. Fahrenholtz, G.E. Hilmas, I.G. Talmy, J.A. Zaykoski, Refractory diborides of zirconium and hafnium, J. Am. Ceram. Soc. 90 (2007) 1347–1364. [7] W.-W. Wu, G.-J. Zhang, Y.-M. Kan, P.-L. Wang, Reactive hot pressing of ZrB2– SiC–ZrC composites at 1600 °C, J. Am. Ceram. Soc. 91 (2008) 2501–2508. [8] W.-W. Wu, G.-J. Zhang, Y.-M. Kan, P.-L. Wang, K. Vanmeensel, J. Vleugels, et al., Synthesis and microstructural features of ZrB2–SiC-based composites by
[26]
[27]
[28]
[29]
[30]
reactive spark plasma sintering and reactive hot pressing, Scr. Mater. 57 (2007) 317–320. X.-J. Zhou, G.-J. Zhang, Y.-G. Li, Y.-M. Kan, P.-L. Wang, Hot pressed ZrB2–SiC–C ultra high temperature ceramics with polycarbosilane as a precursor, Mater. Lett. 61 (2007) 960–963. J.J. Melendez-Martınez, A. Domınguez-Rodrıguez, F. Monteverde, C. Melandri, G. de Portu, Characterisation and high temperature mechanical properties of zirconium boride-based materials, J. Eur. Ceram. Soc. 22 (2002) 2543–2549. E. Wuchina, M. Opeka, S. Causey, K. Buesking, J. Spain, A. Cull, J. Routbort, F. Guitierrez-Mora, Designing for ultrahigh-temperature applications : the mechanical and thermal properties of HfB2, HfCx, HfNx and αHf (N), J. Mater. Sci 39 (2004) 5939–5949. S. Zhu, W.G. Fahrenholtz, G.E. Hilmas, Enhanced densification and mechanical properties of ZrB2–SiC processed by a preceramic polymer coating route, Scr. Mater. 59 (2008) 123–126. S. Grasso, T. Saunders, H. Porwal, O. Cedillos-Barraza, D.D. Jayaseelan, W.E. Lee, et al., Flash spark plasma sintering (FSPS) of pure ZrB2, J. Am. Ceram. Soc. 97 (2014) 2405–2408. S. Venugopal, A. Paul, B. Vaidhyanathan, J.G.P. Binner, A. Heaton, P.M. Brown, Synthesis and spark plasma sintering of sub-micron HfB2: effect of various carbon sources, J. Eur. Ceram. Soc. 34 (2014) 1471–1479. S.R. Bakshi, V. Musaramthota, D. Lahiri, V. Singh, S. Seal, A. Agarwal, Spark plasma sintered tantalum carbide: effect of pressure and nano-boron carbide addition on microstructure and mechanical properties, Mater. Sci. Eng. A 528 (2011) 1287–1295. V. Medri, F. Monteverde, A. Balbo, A. Bellosi, Comparison of ZrB2-ZrC-SiC composites fabricated by spark plasma sintering and hot-pressing, Adv. Eng. Mater. 7 (2005) 159–163. E. Khaleghi, Y.S. Lin, M.A. Meyers, E.A. Olevsky, Spark plasma sintering of tantalum carbide, Scr. Mater. 63 (2010) 577–580. S. Thorat, N. Patra, R. Ruffilli, A. Diaspro, M. Salerno, Preparation and characterization of a BisGMA-resin dental restorative composites with glass, silica and titania fillers, Dent. Mater. J. 31 (2012) 635–644. N. Patra, A.C. Barone, M. Salerno, Solvent effects on the thermal and mechanical properties of poly(methyl methacrylate) casted from concentrated solutions, Adv. Polym. Technol. 30 (2011) 12–20. N. Patra, D.D. Jayaseelan, W.E. Lee, Synthesis of biopolymer-derived zirconium carbide powder by facile one-pot reaction, J. Am. Ceram. Soc. 98 (2015) 71–77. R. Thompson, Production, fabrication, and uses of borides, in: R. Freer (Ed.), The Physics and Chemistry of Carbides, Nitrides and Borides, Kluwer Academic Publishers, Dordrecht, Dordrecht, 1990, pp. 113–120. N. Igawa, T. Nagasaki, Y. Ishii, K. Noda, H. Ohno, Y. Morii, Phase-transformation study of metastable tetragonal zirconia powder, J. Mater. Sci. 33 (1998) 4747–4758. X.-M. Liu, Z.-F. Yan, Phase transformation of nanosized zirconia, Chin. J. Struct. Chem. 25 (2006) 424–432. C. Yan, R. Liu, Y. Cao, C. Zhang, D. Zhang, Synthesis of submicrometer zirconium carbide formed from inorganic–organic hybrid precursor pyrolysis, J. Sol–Gel Sci. Technol. 64 (2012) 251–256. N. Mishra, N. Patra, S. Pandey, M. Salerno, M. Sharon, M. Sharon, Taguchi method optimization of wax production from pyrolysis of waste polypropylene, J. Therm. Anal. Calorim. 117 (2014) 885–892. N. Patra, J. Hladik, L. Martinova, Investigating the thermal properties of polyethylene plasma modified by using unconventional chemical vapors, J. Therm. Anal. Calorim. 117 (2014) 229–234. N. Patra, M. Salerno, P.D. Cozzoli, A. Athanassiou, Surfactant-induced thermomechanical and morphological changes in TiO2-polystyrene nanocomposites, J. Colloid Interface Sci. 405 (2013) 103–108. N. Patra, M. Salerno, P.D. Cozzoli, A.C. Barone, L. Ceseracciu, F. Pignatelli, et al., Thermal and mechanical characterization of poly(methyl methacrylate) nanocomposites filled with TiO2 nanorods, Compos. Part B: Eng. 43 (2012) 3114–3119. F. Sevim, F. Demir, M. Bilen, H. Okur, Kinetic analysis of thermal decomposition of boric acid from thermogravimetric data, Korean J. Chem. Eng. 23 (2006) 736–740. W.C. Tripp, H.C. Graham, Thermogravimetric study of the oxidation of ZrB2 in the temperature range of 800 °C to 1500 °C, J. Electrochem. Soc. 118 (1971) 1195–1199.