carbothermal reduction method

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Oxidation resistance of ZrB2 and ZrB2-SiC ultrafine powders synthesized by a combined sol-gel and boro/carbothermal reduction method ⁎

Faliang Lia,1, Yingnan Caoa,b,1, Jianghao Liua, Haijun Zhanga, , Shaowei Zhangc, a b c

⁎

The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China State Key Laboratory of Advanced Refractories, Sinosteel Luoyang Institute of Refractories Research Co., Ltd., Luoyang 471039, China College of Engineering, Mathematics and Physical Sciences, University of Exeter, Exeter, UK

A R T I C L E I N F O

A BS T RAC T

Keywords: ZrB2 ZrB2-SiC Powder Oxidation Oxidation activation energy

ZrB2 and ZrB2-SiC powders were prepared by a combined sol-gel and boro/carbothermal reduction method, and their oxidation kinetics was studied by using a non-isothermal thermogravimetric technique. The results showed that the Mample power law (n=1) was the most probable mechanism function, and the incorporation of SiC into ZrB2 greatly enhance the latter's oxidation resistance. The oxidation activation energy values of phase pure ZrB2 and ZrB2-SiC powders were respectively 249 and 308 kJ/mol.

1. Introduction Because of its high thermal conductivity, excellent thermal shock resistance and high melting point (3250 °C), ZrB2 is regarded as one of the most important members of ultra-high temperature ceramics (UHTCs) used in hypersonic flight vehicle, atmospheric re-entry, cutting tools and crucibles for molten metal refining [1,2]. Unfortunately, it suffers from poor oxidation resistance at high temperatures in an oxidizing atmosphere. Several recent investigations have found that this problem could be largely alleviated by incorporating SiC into ZrB2 [3–7]. Non-isothermal thermogravimetric analysis (TGA) was often used to investigate the oxidation behavior of non-oxide materials [3,8–13]. For example, Sarin et al. [3] studied the oxidation behavior of ZrB2 and ZrB2-SiC composites at RT ~1923 K by using both non-isothermal and isothermal methods, and found that the oxidation of ZrB2 could be reduced more significantly via incorporating higher levels of SiC. Tu et al. [13] investigated the oxidation behavior of ZrB2 -SiC (60 mol%) composite at 1573–1873 K with low pressures using non-isothermal thermogravimetric analysis (TGA) combined with a multiple paralinear model fitting method. According to them, the oxidation of ZrB 2-SiC composite was mainly affected by the outward diffusion of Si and/or SiO. Silvestroni et al. [12] evaluated the oxidation of ZrB2 toughened with 20 vol% SiC short fibers at 1473–1773 K using a non-isothermal thermogravimetric analysis (TGA) method, and determined the corresponding oxidation apparent activation energy as about 253 kJ/mol. ⁎

1

In most cases, ZrB2-SiC composite materials used for oxidation studies were prepared by simply mixing commercial ZrB2 and SiC powders and subsequent high temperature sintering. Unfortunately, ZrB2-SiC bulk composites so prepared often exhibited poor oxidation resistance arising from poor dispersion and inhomogeneous distribution of SiC in ZrB2. To overcome this, one-step preparation methods for high quality ZrB2-SiC composite powders have been suggested [4–6]. In this work, ZrB 2 and ZrB2 -SiC powders were fabricated via a one-pot boro/carbothermal reduction route, and their oxidation processes investigated by using a non-isothermal TG method. The Kissinger method was used to calculate the corresponding oxidation activation energy (E) values [14,15], and the Flyy – Wall – Ozawa (FWO) method to determine the mechanism functions at 473–1673 K. 2. Experimental procedure Raw materials used were analytical reagent grade ZrOCl2·8H2O (≥99%, Guoyao Chem. Co. Ltd., Shanghai, China), H3BO3 (≥99.5%, Tianli Chem. Co. Ltd., Tianjin, China), tetraethoxysilane (TEOS, SiO2 ≥28.5%, Guoyao Chem. Co. Ltd., Shanghai, China), C6H12O6·H2O (Bodi Chem. Co. Ltd., Tianjin, China), C6H8O7·H2O (≥99.5%, BodiChem. Co. Ltd., Tianjin, China), and C2H6O2 (Bodi Chem. Co. Ltd., Tianjin, China). All of the chemicals were used directly without further purification treatment. ZrB 2 powders and ZrB2 -SiC composite powders were prepared

Corresponding authors. E-mail addresses: [email protected] (H. Zhang), [email protected] (S. Zhang). Faliang Li and Yingnan Cao contributed equally to this work and should be considered co-first authors.

http://dx.doi.org/10.1016/j.ceramint.2017.03.080 Received 10 January 2017; Received in revised form 9 March 2017; Accepted 12 March 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Li, F., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.03.080

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5–7 h under vigorous stirring to form a wet gel. The wet gel was then oven-dried at 373 K for 24 h, and the resultant dry gel was further heated in flowing Ar (purity: 99.999%) to obtain ZrB2 and ZrB2 -SiC powders in a microwave furnace (Model: HAMiLabV3000, 3 kW, 2.45 GHz, by Changsha Longtech CO., Ltd., Hunan province, China). The precursors were initially heated to 1273 K at a heating rate of 20 K/min and then heated at a rate of 10 K/min up to 1573 K immediately. After keeping at 1573 K for 3 h, the samples were cooled to room temperature naturally. The as-prepared powders were characterized by X-ray diffraction (XRD) (Philips X′Pert PRO diffractometer, PANalytical, NETHERLANDS, 40 kV, 40 mA), field emission scanning electron microscopy (Nova400NanoSEM, Philips, Netherlands, 15 kV) equipped with energy dispersive spectroscopy, and transmission electron microscopy (JEM-2100UHR- STEM, JEOL, Japan, 200 kV). The particle size distribution was measured using a Mastersizer 2000 laser particle size analyzer (Malvern Instruments Ltd., UK). Oxidation behaviors of ZrB2 and ZrB2-SiC powders were examined by using a STA 449F3 Thermal Analysis System (NETZSCH Co., German) with a heating rate of 10, 15 and 20 K/min respectively in flowing air at a rate of 30 mL/min. Using alpha-alumina as the reference material, about 6 mg as-prepared powders was loaded in the alumina crucible in the TG furnace and the instantaneous weight was recorded by the Thermal Analysis System when the powders was heated from 298 to 1673 K. The oxidation ratio α was calculated according to the following equation [16,17],

1: ZrB2

1

2: SiC

1 1

2

ZrB2-SiC

1 11 1 1 2 2

1

ZrB2

10

20

30

40

50

60

70

80

90

2 Theta/Degree Fig. 1. XRD patterns of ZrB2 and ZrB2-SiC powders prepared at 1573 K for 3 h via microwave heating (ZrB2: ICDD 89-3826; SiC: ICDD 73-1708).

by following the procedures described in our previous work [4,5]. Typically, the raw materials were dissolved in distilled water to obtain a homogeneous solution which was then heated at 353 K for

Fig. 2. FE-SEM and TEM images of ZrB2 and ZrB2-SiC powders whose XRD patterns are shown in Fig. 1((a) (b) ZrB2, (c) (d) ZrB2-SiC).

2

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composite powders was about 35 nm. The average size and size distribution of as-prepared ZrB2 and ZrB2-SiC powders measured by the Mastersizer 2000 laser particle size analyzer (Fig. 3), reveals that in both cases, the average size was about 9 µm, suggested the low levels of agglomeration in both cases. 3.2. TG-DSC analysis of ZrB2 and ZrB2-SiC powders ZrB2 and SiC were oxidized according to the following equations, ZrB2(s) + 5/2O2(g) = B2O3(l) + ZrO2(s)

(2)

Δr G2θ = − 1992800 + 370.34T

SiC(s) + 3/2O2(g) = SiO2(s) + CO(g)

(3)

Δr G3θ = − 948450 + 82.3T Based on Eqs. (2) and (3), the theoretical total mass gains of ZrB2 and SiC after their complete oxidation can be calculated as about 70.33 and 50 wt%, respectively. Fig. 4 presents TG and DSC curves corresponding to different heating rates. The TG curve of ZrB2 at 473–1673 K (Fig. 4(b)) can be divided into four different stages. No obvious weight change was observed with increasing oxidation temperature from 473 to 973 K, indicating that the oxidation of ZrB2 had not yet started. Upon increasing oxidation temperature to 1173 K, the mass gain became evident, indicating that ZrB2 was oxidized to B2O3 and ZrO2 (Eq. (2)). Upon further increasing oxidation temperature from 1173 to 1473 K, no obvious mass change was observed, this was probably due to the formation of a glassy B2O3 barrier layer on ZrB2, which reduced the diffusion of oxygen and thus inhibited the further oxidation of ZrB2. Interestingly, upon further increasing oxidation temperature to above 1473 K, instead of weight gain, weight loss was seen evidently on the TG curves, which can be ascribed to the evaporation of B2O3. The DSC results of ZrB2 at different heating rates (Fig. 4(b)) revealed two exothermic peaks at 960 and 1080 K. Based on the published experimental data and thermodynamic calculations [18,19], the first peak was attributed to the oxidation of residual carbon (since excessive carbon was used in the fabrication process), and the second peak to the oxidation of ZrB2. The oxidation of ZrB2-SiC composite powder also experienced four different stages (Fig. 4(c)). In the initial stage (at 473–973 K), no weight change was detected, indicating that the oxidation of ZrB2-SiC composite powder had not been started yet. In the second stage (at 973–1273 K), the weight gain increased sharply with the temperature, indicating the rapid oxidation of ZrB2-SiC composite powder. In the third stage (at 1273–1573 K), the weight gain increased slowly with the temperature, which could be attributed to the formation of a boronsilicate glass layer from the reaction between SiO2 (oxidation product of SiC) and B2O3 (oxidation product of ZrB2). This viscous glass layer inhibited the further oxidation of the powder. Finally, in the fourth stage (at 1573–1673 K), minor weight loss caused by the evaporation of B2O3 was observed. Comparison of Fig. 4(a) with Fig. 4(c) reveals that the weight loss of ZrB2-SiC composite powder at a given temperature > 1473 K was much less than that of ZrB2 powder, although their oxidation processes were similar. Based on this it can be reasonably concluded that the incorporation of SiC significantly enhanced the oxidation resistance of ZrB2. Fig. 4(d) further gives DSC results of as-prepared ZrB2-SiC powder at different heating rates, revealing three exothermic peaks. As discussed above, the first two exothermic peaks (at about 960 K and 1020 K) were caused by the oxidation of residual carbon and ZrB2 respectively. And the third peak at about 1120 K was attributed to the oxidation of SiC [18,19].

Fig. 3. Particle size distributions of as-prepared ZrB2 and ZrB2-SiC powders measured by a laser particle size analyzer.

α = (wi − wt )/(wi − wf )

(1)

where wi, wt, and wf are initial, instantaneous and final weights of the sample, respectively. 3. Results and discussion 3.1. Crystalline phases and microstructures of as-prepared ZrB2 and ZrB2-SiC powders When ZrB2 and ZrB2-SiC powders were prepared at 1573 K for 3 h with the n(C)/n(Zr) molar ratios of 5.0 and 8.0, respectively (the B/Zr molar ratio (n(B)/n(Zr)) was fixed at 2.5). As shown in Fig. 1, except for the ZrB2 and SiC, no other phase was detected in the product samples, indicating that phase pure ZrB2 powders and ZrB2-SiC composite powders were obtained. Shown in Fig. 2 are FE-SEM and TEM micrographs of as-prepared ZrB2 and ZrB2-SiC powders. The dot-EDS mapping shown in Fig. 2(a) reveals that the sample contained only B and Zr, further confirming the formation of phase pure ZrB2. Fig. 2(b) further reveals that the particle size of ZrB2 was smaller than 100 nm. The dot-EDS mapping shown in Fig. 2(c) demonstrates that the sample was composed of B, C, Zr and Si, verifying the formation of phase pure ZrB2-SiC composite powder. Since the size of EDS electron beam (about several microns) is bigger than that of the as-prepared ZrB2-SiC powders (less than one micron), more than one particle would be detected by the characterization one time, thus, it can be concluded that SiC particles were evenly distributed among ZrB2 particles. According to Fig. 2(d), the average size of prepared ZrB2-SiC

3

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140

Exo

Mass/%

130

DSC/ (mw mg )

=10 K/min =10 K/min

120 =20 K/min

110

=20 K/min

1079 K

=15 K/min

100 200 473

400 673

600 800 1000 873 1273 1073 Temperature/K

1200 1673 1400 1473

960 K

=15 K/min 1102 K

971 K 963 K

1096 K

673

473

1273

1673

1473

Temperature/K

(b) DSC curves

(a) TG curves 150

Exo

=10 K/min

DSC/mw mg

=15 K/min

130

=10 K/min =20 K/min

-1

140

Mass/%

1073

873

=20 K/min

120

=15 K/min

1016 K

961 K

110

1035 K

971 K 969 K

100

1027 K

473

673

873

1073

1273

1473

473

1673

673

873

1073

1100 K 1131 K 1119 K

1273

Temperature/K

Temperature/K

(c) TG curves

(d) DSC curves

1473

1673

Fig. 4. TG-DSC curves of ZrB2 and ZrB2-SiC ultrafine powders at RT ~1673 K and different heating rates ((a)(b): ZrB2, (c)(d) ZrB2-SiC).

3.3. Calculation of oxidation activation energy by the Kissinger method

Table 1 Peak temperatures shown on DSC curves of ZrB2 and ZrB2-SiC powders (with various heating rates). β/ K min−1

10 K min−1 15 K min−1 20 K min−1

ZrB2 powders

ZrB2-SiC composite powders

Tp1/K

Tp2/K

Tp1/K

Tp2/K

Tp3/K

960 964 971

1079 1096 1102

961 969 971

1016 1027 1035

1100 1119 1131

Based on the peak temperatures shown on the DSC curves (Tp) and listed in Table 1, the oxidation activation energies of the asprepared ZrB2 and ZrB2-SiC powders can be calculated by using a differential isoconversional method based on the Kissinger model (Eq. (4)) [20,21],

⎛ ⎞ ⎛ AR ⎞ ⎡ −df (a ) ⎤ β E ln⎜⎜ 2 ⎟⎟ = ln⎜ ⎟ + ln ⎢ ⎥ − ⎣ dα ⎦ β ⎝ E ⎠ RTp ⎝ Tp ⎠ p

(4)

where α is oxidation conversion ratio, β is heating rate, E oxidation activation energy, A preexponential factor, f(α) the differential form Table 2 ln(β/Tp2) and Tp−1 in the cases of ZrB2 and ZrB2-SiC composite powders at different heating rates. β/ K min-1

ln (β/Tp2)

ZrB2 powders Tp1-1

10 15 20

2.375 2.771 3.054

−3

(×10 ) /K

1.042 1.037 1.030

ZrB2-SiC composite powders −1

Tp2

−1

−3

(×10 ) /K

−1

0.927 0.912 0.907

4

Tp1−1 (×10−3) /K−1

Tp2−1 (×10−3) /K−1

Tp3−1 (×10−3) /K−1

1.040 1.032 1.030

0.984 0.974 0.966

0.909 0.894 0.884

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Table 4 Calculated oxidation conversion ratio α of ZrB2 powder at different heating rates.

ZrB2 powders ZrB2-SiC composite powders

2

lg( /Tp )

3.0

α

T/K

2.8 1083 1097 1123 1133

2.6

10 K min−1

15 K min−1

20 K min−1

0.567 0.692 0.862 0.902

0.461 0.565 0.760 0.821

0.379 0.465 0.653 0.722

2.4 2.2 0.90

0.95

By plugging G(α ) = − In(1 − α ) into Eq. (5), lgβ versus 1/T can be plotted, then from the slope of the regression line the oxidation activation energy E and preexponential factor A (Table 6) can be obtained (Fig. 7). The results (Table 6) showed that the average oxidation activation energy E calculated based on the FWO model was 232 kJ/mol which is close to that (266 kJ/mol) estimated by using the Kissinger model. By plugging the calculated preexponential factor A and oxidation activation energy E into Eq. (6), the oxidation kinetics equation in the case of ZrB2 powder can be written as follows,

1.00

Tp ( 10 )/K -1

-3

-1

Fig. 5. Relationship between ln(β/Tp2) and Tp−1×10−3 in the cases of ZrB2 and ZrB2-SiC composite powders.

of the mechanism function, R the ideal gas constant, and Tp the peak ⎛β⎞ temperature shown on the DSC curve. Obviously, the plot of ln⎜ 2 ⎟ ⎝ Tp ⎠ versus 1/Tp (Table 2) should be a straight line, from whose slope the apparent oxidation activation energy E can be evaluated (Fig. 5). The calculation results (Table 3) showed that the oxidation activation energy of ZrB2 and ZrB2-SiC composite powders was 266 and 307 kJ/ mol, respectively.

lg[− ln(1 − α )] = 12.772 − lg β −

The oxidation mechanism function G(α) of as-prepared powders can be determined based on the Flynn-Wall-Ozawa (FWO) model described as follows [20,21],

AE E − 2.315 − 0.4567 RG(α ) RT

(5)

Tp

activation energy E of ZrB2 in as-prepared composite powder was evaluated as 309 kJ/mol, which was close to that estimated using the Kissinger method (307 kJ/mol). For comparison, the oxidation activation energy of ZrB2 −20 vol% SiC composite prepared by mechanical mixing followed by a spark plasma sintering and hot-pressing process was reported as 125 and 253 kJ/mol, respectively [12,22], indicating that ZrB2-SiC composite powders prepared in this work exhibited a better oxidation resistance. Table 7 compares the calculated oxidation activation energy (E) values of ZrB2 and ZrB2-SiC composite powders based on the Kissinger and FWO models. The average oxidation activation energy E of ZrB2 in as-prepared composite powder was 308 kJ/mol, which was much larger than that in the case of ZrB2 (249 kJ/mol), further demonstrating that in-situ incorporation of SiC is an effective way to effectively improve the oxidation resistance of ZrB2.

Taking G(α) as a single component, Eq. (5) can be written as,

⎡ AE E ⎤ lg G(α ) = ⎢lg − 2.315 − 0.4567 ⎥ − lg β ⎣ R RT ⎦

(6)

where α is oxidation conversion ratio, β heating rate, E oxidation activation energy, A preexponential factor, T oxidation temperature, and R the ideal gas constant. AE E Assuming a = [lg R − 2.315 − 0.4567 RT ], b=−1, Eq. (6) can be written as the following simple form, (7)

lgG(α) = a + blgβ

(8)

Based on Eq. (8), the theoretical oxidation curves of prepared ZrB2 powders corresponding to different heating rates can be determined, and compared with those from experimental results (Fig. 8). As seen from Fig. 8, the former matched with the latter well. Therefore, it can be concluded that the oxidation kinetic parameters determined can be used to describe the oxidation behavior of ZrB2 with a reasonable reliability. The oxidation activation energy of ZrB2-SiC composite powder also can be similarly estimated by using the FWO method (Eq. (5)). After linear regression of the data listed in Table 2 using the least square method, the lgβ versus 1 can be plotted linearly (Fig. 9). The oxidation

3.4. Determination of oxidation mechanism function G(α)

lg β = lg

12783.38 T

Table 4 lists the calculated oxidation conversion ratio α of ZrB2 at different heating rates based on Eq. (1). Based on these data, the fitted lines of lgG(α) versus lgβ corresponding to different mechanism functions can be drawn (Fig. 6). As revealed by Fig. 6 and the corresponding kinetic equations given in Table 5, the Mample power law (n=1) appeared to be the most probable mechanism function (at 973–1133 K) since the fitted slope b was close to −1 (Eq. (7)) and the correlation coefficient (R2) close to 1.

4. Conclusions Phase pure ZrB2 and ZrB2 -SiC composite powders were pre-

Table 3 Fitting results of the relationship between ln(β/Tp2) and Tp−1×10−3/K−1 in the cases of ZrB2 and ZrB2-SiC powders, based on the Kissinger model. Sample

Fitted equation/y=a+bx

Correlation coefficient / R2

Oxidation activation energy E/ kJ mol−1

Phase pure ZrB2 ZrB2-SiC composite powders

y=31.745–31.951x y=38.644–36.967x

0.999 1.000

266 307

5

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-1.2 -0.6

lgG( )

1033 K 1023 K

-1.2

lgG( )

-1.4 -0.9

1097 K

-1.5

1033 K 1023 K

-1.6 1097 K

-1.8

1083 K

1083 K -1.8

-2.0 1.0

1.1

1.2

1.3

1.0

1.4

1.1

1.2

1.3

1.4

lg (b) Inverse Jander equation

lg (a) Jander equation (n=2) -0.6 0.2 0.1

-1.2

1033 K 1023 K

-1.5

1097 K

-0.1

1083 K

-0.2

lgG( )

lgG( )

-0.9

1.1

1.2

1.3

0.0

1097 K

-1.8 1.0

1033 K 1023 K

1.4

1083 K 1.0

1.1

1.2

1.3

1.4

lg (d) Avrami - Erofeev equation (n=2/3)

lg (c) G-B equation 0.4

lgG( )

0.2 0.0

1033 K 1023 K

-0.2

1097 K 1083 K

-0.4 1.0

1.1

1.2

1.3

1.4

lg (e) Mample power law (n=1) Fig. 6. Calculated relationships between lgG(α) and lgβ in the case of ZrB2 powder.

to as high as 308 kJ/mol upon in-situ incorporation of SiC. ZrB2 SiC composite powders prepared in this work showed better oxidation resistance compared with those previously prepared by using the other techniques.

pared by a one-pot boro/carbothermal reduction method, and their oxidation behaviors investigated by using a non-isothermal method. The results indicated that the Mample power law (n=1) was the most probable oxidation mechanism function. The oxidation activation energy E in the case of ZrB2 was 249 kJ/mol, but increased 6

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Table 5 Linear regression results corresponding to different oxidation mechanism functions, in the case of ZrB2 powders. G (α)

y=a+bx

R2

Jander equation(n=2) Inverse Jander equation G-B equation Avrami-Erofeev equation(n=2/3) Mample power law (n=1)

[1-(1-α)1/3]2 [(1+α)1/3−1]2 1–2α/3-(1-α)2/3 [-ln(1-α)]2/3

y=0.577–1.412x y=0.580–0.797x y=0.249–1.220x y=0.689–0.577x

0.984 0.961 0.978 0.991

-ln(1-α)

y=1.034–0.866x

0.991

SiC

E (kJ mol−1)

lgA

0.3 0.4 0.5 0.6 0.7 0.8 0.9 Average

282 247 237 242 217 209 195 232

13.18 11.35 10.82 11.07 9.83 9.47 8.74 10.64

1.1

1.0 0.90

lg

1.2

0.95

1.00

1.05

Tp (10 )/K -1

-3

-1

Fig. 9. Relationship between lgβ and Tp−1×10−3 in the case of ZrB2 incorporated with SiC.

Table 7 Oxidation activity energy (E) values in the cases of ZrB2 and ZrB2-SiC composite powders, calculated by using the Kissinger and FWO models. Model

Kissinger FWO Average

0.3 0.4 0.5 0.6 0.7 0.8 0.9

1.3

C

1.2

Table 6 Calculated oxidation activation energy E and lgA in the case of ZrB2 powder, based on the FWO method. α

ZrB2

1.3

Linear regression of lgG (α) versus lgβ

lg

Oxidation mechanism

1.4

Oxidation activation energy E, kJ mol−1 Phase pure ZrB2 powders

ZrB2 in ZrB2-SiC composite powders

266 232 249

307 308 308

Acknowledgements This work was financially supported by National Natural Science Foundation of China (General program, 51272188, 51502211, 51472184, 51472185), the China Postdoctoral Science Foundation (2016M590721, 2015M572209) and Program for Innovative Teams of Outstanding Young and Middle-aged Researchers in the Higher Education Institutions of Hubei Province (T201602).

1.1

1.0 References

0.86

0.88

0.90 0.92 -3 -1 -1 Tp 10 /K

0.94

0.96 [1] R.B. Zhang, X.M. Cheng, D.N. Fang, L.L. Ke, Y.S. Wang, Ultra-high-temperature tensile properties and fracture behavior of ZrB2-based ceramics in air above 1500 °C, Mater. Des. 52 (2013) 17–22. [2] S.W. Zhang, M. Khangkhamano, H.J. Zhang, H.A. Yeprem, Novel synthesis of ZrB2 powder via molten-salt-mediated magnesiothermic reduction, J. Am. Ceram. Soc. 97 (2014) 1686–1688. [3] P. Sarin, P.E. Driemeyer, R.P. Haggerty, D.K. Kim, J.L. Bell, Z.D. Apostolov, W.M. Krivcn, In situ studies of oxidation of ZrB2, and ZrB2-SiC composites at high temperatures, J. Eur. Ceram. Soc. 30 (2010) 2375–2386. [4] Y.N. Cao, H.J. Zhang, F.L. Li, L.L. Lu, S.W. Zhang, Preparation and characterization of ultrafine ZrB2-SiC composite powders by a combined sol-gel and microwave boro/carbothermal reduction method, Ceram. Int. 41 (2015) 7823–7829. [5] Y.N. Cao, J.K. Wang, H.J. Zhang, F.L. Li, L.L. Lu, S.W. Zhang, Preparation of ZrB2SiC ultrafine composite powders by a combined sol-gel and boro/carbothermal reduction method, Rare Met. Mater. Eng. 44 (Suppl. 1) (2015) S706–S709. [6] X.G. Deng, S. Du, H.J. Zhang, F.L. Li, J.K. Wang, W.G. Zhao, F. Liang, Z. Huang, S.W. Zhang, Preparation and characterization of ZrB2-SiC composite powders from zircon via microwave-assisted boro/carbothermal reduction, Ceram. Int. 41 (2015) 14419–14426. [7] P.A. Williams, R. Sakidja, J.H. Perepezko, P. Ritt, Oxidation of ZrB2-SiC ultra-high temperature composites over a wide range of SiC content, J. Eur. Ceram. Soc. 32 (2012) 3875–3883. [8] J. Rychlý, L. Matisová-Rychlá, K. Csomorová, I. Janigová, M. Schilling, T. Learner, Non-isothermal thermogravimetry, differential scanning calorimetry and chemiluminescence in degradation of polyethylene, polypropylene, polystyrene and poly(methyl methacrylate), Polym. Degrad. Stab. 96 (2011) 1573–1581. [9] P.Z. Gao, H.N. Xiao, H.J. Wang, Z.H. Jin, A study on the oxidation kinetics and mechanism of three-dimensional (3D) carbon fiber braid coated by gradient SiC, Mater. Chem. Phys. 93 (2005) 164–169. [10] A. Biedunkiewicz, A. Strzelczak, G. Mozdzen, J. Lelatko, Non-isothermal oxidation of ceramic nanocomposites using the example of Ti-Si-C-N powder: kinetic analysis

Fig. 7. Relationships between lgβ and Tp−1×10−3 in the case of ZrB2 powder.

1.0 -1

0.8

10 K min

-1

15 K min

0.6 0.4

-1

20 K min

0.2 experimental data calculated data

0.0

873

1073 Temperature/K

1273

Fig. 8. Comparison of theoretically calculated oxidation curves and experimental ones in the case of ZrB2 powder.

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