Preparation and heat-insulating properties of Al₂O₃–ZrO₂(Y₂O₃) hollow fibers derived from cogon using an orthogonal experimental design

Abstract

Bionic design is efficient to develop high-performance lightweight refractories with sophisticated structures such as hollow ceramic fibers. Here, we report a four-stage procedure for the preparation of Al₂O₃–ZrO₂(Y₂O₃) hollow fibers using the template of cogon—a natural grass. Subsequently, to optimize the thermal performance of the fibers, four sets of preparation parameters, namely, x(Al₂O₃), solute mass ratio of the mixture, dry temperature, and sintering temperature were investigated. Through an orthogonal design, the optimal condition of each parameter was obtained as follows: x(Al₂O₃) was 0.70, solute mass ratio of the mixture was 15 wt%, dry temperature was 80 °C, and sintering temperature was 1100 °C. Overall, Al₂O₃–ZrO₂(Y₂O₃) hollow fibers show relatively low thermal conductivity (0.1038 W m⁻¹ K⁻¹ at 1000 °C), high porosity (95.0%), and low density (0.05–0.10 g cm⁻³). The multiphase compositions and morphology of Al₂O₃–ZrO₂(Y₂O₃) hollow fibers, which may contribute to their thermal properties, were also discussed.

---

1 Introduction

It is acknowledged that natural species have developed specific structures, which perform more efficiently than artificial materials.¹ Recently, fabricating structures and functions similar to those like species in nature, such as morphology genetic materials (MGMs)¹ and functional biomimetic synthesis (FBS),² has received considerable interest. In addition, there are a wide variety of natural structures that can be imitated, including butterfly wings,^1,3 leaves,¹ cotton fiber,⁴ metallic wood⁵ and so on.² Assisted by the unusual microstructures of metallic wood, Wan et al. fabricated a composite material with anisotropic thermal conductivity and electrical conductivity,⁵ which has sufficiently shown the value of MGMs and FBS.

Alumina (Al₂O₃) is extensively used in lightweight refractories^6,7 and structural elements⁸ for the modification of polymers,⁹ batteries,¹⁰ and so forth. Similarly, zirconia (ZrO₂) possesses a relatively low thermal conductivity, a high melting point,¹¹ and high temperature strength,⁷ thereby leading to its significant potential in thermal applications.^11,12 In many cases, composite materials with certain structures show properties beyond those of single compositions.⁵ Herein, it is, in principle, feasible to develop novel lightweight refractories by synthesizing Al₂O₃–ZrO₂ composites.^13–17 Using solution blow spinning and atomic layer deposition (ALD), Xu et al. produced novel Al₂O₃ nanotube aerogels having a pore size of 1–2 μm, whose thermal conductivity is only 0.022 W m⁻¹ K⁻¹ at room temperature.⁷ Nevertheless, few studies have reported the thermal performance of Al₂O₃–ZrO₂ composites at a high temperature (900–1600 °C) till now. In addition, there are some universal problems in ceramic preparation. For example, it is a tedious and expensive process to design ceramic materials with certain components, as there are always much varying factors, including dry temperature and sintering temperature.^18,19 Hence, to solve this problem, we applied an orthogonal experimental design, i.e., an experimental arrangement in which orthogonal arrays and mathematical analysis is used to determine the optimal condition with limited experiments^20–24 for the preparation of Al₂O₃–ZrO₂(Y₂O₃) hollow fibers.

In this study, we developed a four-stage procedure for preparing Al₂O₃–ZrO₂(Y₂O₃) hollow fibers using the template of cogon—Imperata cylindrica (the scientific name). Cogon is commonly known as cogon grass or kunai grass and is a species of grass in the family Poaceae.²⁵ More importantly, cogon is mainly made of cellulose, hemicellulose, and lignin,²⁶ most of which would burn out between 300 and 1000 °C.

Fig. 1a and b shows the hollow structure and micromorphology of cogon fibers, which act as a template for fabricating Al₂O₃–ZrO₂(Y₂O₃) hollow fibers. It is obvious that cogon fibers have a length of 2–3 mm and an internal diameter of about 20 μm. Given Hu's theory about hollow-structured materials (HSMs),²⁷ it can be feasible to decrease the thermal conductivity of Al₂O₃–ZrO₂(Y₂O₃) hollow fibers using a hollow structure.

Fig. 1 Microstructure of natural cogon ((a) hollow structure, (b) micromorphology).

In short, we prepared lightweight Al₂O₃–ZrO₂(Y₂O₃) hollow fibers with relatively low thermal conductivity (0.1038 W m⁻¹ K⁻¹ at 1000 °C) and high porosity (95.0%). In addition, we also discussed two main mechanisms that may affect Al₂O₃–ZrO₂(Y₂O₃) hollow fibers' thermal properties, including their multiphases and structures.

2 Experimental

2.1 Preparation of Al₂O₃–ZrO₂(Y₂O₃) hollow fibers

Table 1 lists the chemical compositions of the mixture of AlCl₃·6H₂O (AR), ZrOCl₂·8H₂O (AR), and Y(NO₃)₃·6H₂O (99.5%) made with ion-free water and ethanol, which was prepared initially. Cogon was immersed in the mixture for 5–15 min to obtain precursors, which were readily modified into Al₂O₃–ZrO₂(Y₂O₃) hollow fibers. Next, the precursors were dried at a temperature ranging from 80 °C to 120 °C in a vacuum drying oven for 14–16 h. In the last stage, the arid precursors were sintered from room temperature to a certain temperature ranging from 1100 °C to 1300 °C at a rate of 10 °C min⁻¹ using a muffle furnace.

Table 1 Chemical compositions of the mixed solution (wt%)

Sample no.	AlCl3·6H2O (wt%)	ZrOCl2·8H2O (wt%)	Y(NO3)3·6H2O (wt%)
1, 2, 3	47.8	44.6	7.6
4, 5, 6	62.0	30.8	7.2
7, 8, 9	74.7	18.5	6.8
V1, 2, 3	74.7	18.5	6.8
V4	82.3	11.0	6.7
V5	89.3	4.0	6.7
V6	100.0	0	0

---

2.2 Orthogonal experimental arrangement

Based on our previous studies^18,19 and some references,^13,14,28 the composition of the ceramic fiber, solute mass ratio of the mixture, dry temperature, and sintering temperature were the major contributing factors in the preparation of Al₂O₃–ZrO₂(Y₂O₃) hollow fibers. Additionally, each parameter has a different effect on the thermal performance of the hollow fibers. Consequently, the theoretical molar fraction of Al₂O₃ (x) in the composite xAl₂O₃–(1 − x − 0.04)ZrO₂(0.04Y₂O₃), solute mass ratio of the mixture (wt%), dry temperature, and sintering temperature labeled as A, B, C, and D, respectively, were investigated in this study using an orthogonal table L₉(3⁴) designed by the Orthogonal Designing Assistant II. In the orthogonal table L₉(3⁴) (see Table S1, ESI†), 9 refers to the experimental times and 3 is the level of the factors set in the work.²¹

In our preceding experiments, the density of Al₂O₃–ZrO₂(Y₂O₃) hollow fibers drastically increased if the sintering temperature was above 1300 °C, thus leading to the increase in the thermal conductivity. However, it is difficult to get partly stabilized ZrO₂ (t-ZrO₂) below 1100 °C. In addition, with regard to the dry temperature, it can be time-consuming to dry the precursors below 80 °C. Overall, taking the influence of the above parameters into account, Table 2 lists the levels of the factors set in this study (more experimental details of the fiber preparation are listed in Table S2, ESI†).

Table 2 Factors and levels for orthogonal experimental design

Levels	Factors
	A (x)	B (wt%)	C (°C)	D (°C)
1	0.40	5	80	1100
2	0.55	15	100	1200
3	0.70	25	120	1300

---

2.3 Characterization

Thermal conductivity. Several samples of Al₂O₃–ZrO₂(Y₂O₃) hollow fibers were prepared with an apparent density of 0.2 g cm⁻³. Then, the thermal conductivities of these samples were estimated using a thermal conductivity measuring apparatus (TPS 2500S, Hot Disk, Sweden) at 500 and 1000 °C.

Microstructures. Microstructures of Al₂O₃–ZrO₂(Y₂O₃) hollow fibers were observed with a scanning electron microscope (SEM, Quanta 250 FEG, FEI, USA). Energy-dispersive spectroscopy (EDS, Quantax_400, Bruker, Germany) was applied to detect the elemental distribution of Al₂O₃–ZrO₂(Y₂O₃) composite with the original WD (15.0 mm) and HV (20.0 kV).

Pore size distribution. Mercury porosimetry (AutoPore Iv 9510, Micromeritics, USA) was used to measure the pore size distributions and porosities of the samples composed of fibers, whose actual density was 0.11 g cm⁻³ during the test.

Raman spectrum. The Raman spectrum was recorded on a Raman spectrometer (LabRam HR Evolution, Horiba Jobin Yvon, France) from 100 cm⁻¹ to 1200 cm⁻¹. The crystal structures of the samples were characterized with a time of exposure of 20 s.

Thermogravimetry and differential scanning calorimetry. Thermogravimetric (TG) analysis and differential scanning calorimetry (DSC) analysis of the sample from natural cogon and that immersed in the mixture of ZrOCl₂·8H₂O, AlCl₃·6H₂O, and Y(NO₃)₃·6H₂O were performed using a thermogravimetric-differential scanning calorimeter (449 F3, Netzsch, Germany).

Phase compositions. Phase compositions of the samples with different components were investigated using X-ray diffraction (XRD, Bruker-AXS D8 Advance, Bruker, Germany) from 10 to 80 degrees at a scanning speed of 6° min⁻¹.

3 Results and discussion

3.1 The optimal preparation condition

Table 3 lists the evaluation index (thermal conductivity) for the orthogonal design experiment as well as the corresponding mathematical analysis. According to the R value range, in which a higher R value means more significant impact,^20,21 it is apparent that factor D, i.e., the sintering temperature, has a maximum R value of 0.025, which represents the most important effect among these four factors set in this study. In contrast, the R value of factor A, B, and C ranges from 0.04 to 0.07. Correspondingly, the order of these factors' importance is D > B > A > C.

Table 3 Results and analysis of the orthogonal L₉(3⁴) experimental design^a

Sample no.	Factors				Results
	A (x)	B (wt%)	C (°C)	D (°C)	λ (W m^−1 K^−1, 500 °C)
a 1: ; 2: ; 3: Ri = m^maxi − m^mini.^20,21
1	1	1	1	1	0.09648
2	1	2	2	2	0.1172
3	1	3	3	3	0.2020
4	2	1	2	3	0.1760
5	2	2	3	1	0.1028
6	2	3	1	2	0.1287
7	3	1	3	2	0.1129
8	3	2	1	3	0.1515
9	3	3	2	1	0.1038

---

	A	B	C	D
K1	0.139	0.128	0.126	0.101
K2	0.136	0.124	0.132	0.120
K3	0.123	0.145	0.139	0.176
m1	0.046	0.043	0.042	0.034
m2	0.045	0.041	0.044	0.040
m3	0.041	0.048	0.046	0.059
R	0.005	0.007	0.004	0.025
Order of importance	D > B > A > C
Optimal lever	A3	B2	C1	D1

---

Furthermore, Table 3 also shows K₁, K₂, and K₃, which represent the mean values of evaluation index for each level of one factor. For example, when the factor is C (dry temperature) and the level is 1 (80 °C), then K₁ = (0.09648 + 0.1287 + 0.1515)/3 = 0.126. Herein, by comparing dissimilar K values, the optimized level corresponding to each factor to prepare Al₂O₃–ZrO₂(Y₂O₃) hollow fibers can be obtained: D1, B2, A3, and C1. Thus, the optimal criterion was confirmed: x(Al₂O₃) was 0.70, solute mass ratio of the mixture was 15 wt%, dry temperature was 80 °C and sintering temperature was 1100 °C.

Fig. 2 demonstrates the relationship between average thermal conductivity with the four factors and levels. It is obvious that most factors are monotonously increasing or decreasing with the increase in levels except for factor B, which reaches its valley at level 2 (15 wt%). As for factor A, the factors of mole fraction of Al₂O₃ (x), i.e., the average thermal conductivity, drops with the increase of x(Al₂O₃). Moreover, when it comes to factor C and D, they share a similar tendency, i.e., both increase with the increase in levels. However, it is noteworthy that factor D increased much more drastically with the growth in the sintering temperature, leading to the highest R value (0.025) in Table 3.

Fig. 2 Relationship between average thermal conductivity with the four factors and levels.

Fig. 3a shows dissimilar morphologies of nine types of Al₂O₃–ZrO₂(Y₂O₃) hollow fibers (Sample 1–9) prepared using the parameters in the orthogonal experiment in Table 3, while Fig. 3b shows thermal conductivities at 500 °C of Sample 1–9 and V1-3 (prepared using the optimal condition: D1, B2, A3, and C1). Obviously, Sample V1-3 has the lowest average thermal conductivity (0.08395 W m⁻¹ K⁻¹), which effectively proves the rationality of the optimal preparation condition obtained by R values and K-analysis. However, further experiments are required to explain why these samples' thermal conductivities were distinct.

Fig. 3 (a) Morphologies of Al₂O₃–ZrO₂(Y₂O₃) hollow fibers (Sample 1–9) and (b) their thermal conductivities.

3.2 Microstructures and pore size distribution

Fig. 4 shows the morphologies and microstructures of Al₂O₃–ZrO₂(Y₂O₃) hollow fibers prepared at optimal conditions (Fig. 4a–c) and other conditions (Fig. 4d–l). The ceramic fibers maintained the microstructures and hollow structures of the cogon fibers (Fig. 1). Moreover, their internal diameters varied from 5 to 10 μm. Due to the sintering process, the pores of Al₂O₃–ZrO₂(Y₂O₃) hollow fibers shrink in contrast to that of the cogon fiber (20 μm) (see Fig. 1). In addition, various chemical compositions may lead to various thermal conductivities for the Al₂O₃–ZrO₂(Y₂O₃) hollow fibers.

Fig. 4 Morphologies and microstructures of Al₂O₃–ZrO₂(Y₂O₃) hollow fibers ((a–c): x(Al₂O₃) = 0.70; (d–f): x(Al₂O₃) = 0.80; (g–i): x(Al₂O₃) = 0.90; (j–l): x(Al₂O₃) = 1.00).

Fig. 5 illustrates the pore-size distributions of Al₂O₃–ZrO₂(Y₂O₃) hollow fibers, whose actual density was 0.11 g cm⁻³ during the test. It is noticeable that their pore sizes are mainly distributed between 6.6 μm and 0.4 μm, which connected well with the hollow structures of the fibers shown in Fig. 4. Moreover, the porosity of Al₂O₃–ZrO₂(Y₂O₃) hollow fibers is high (95.0%), which may contribute to its ultra-low thermal conductivity at a high temperature.²⁷

Fig. 5 Pore-size distributions of Al₂O₃–ZrO₂(Y₂O₃) hollow fibers.

3.3 Phase analysis

Fig. 6 demonstrates the XRD patterns of Al₂O₃–ZrO₂(Y₂O₃) hollow fibers prepared with different x(Al₂O₃). Evidently, when x(Al₂O₃) is 0.70, the major phases of Al₂O₃–ZrO₂(Y₂O₃) composite are c-ZrO₂ and t-ZrO₂, whose thermal stability is better than that of m-ZrO₂.^13,26–28 In addition, as the intensity of ZrO₂ is quite stronger than that of Al₂O₃, it is only when x(Al₂O₃) is 1.00 can its peak appear. It is also noteworthy that in such composite ceramic, SiO₂ exists as well, which may be mainly due to the natural cogon. Ref. 19 has explained its positive effect with the decrease in thermal conductivities of such hollow fibers.

Fig. 6 XRD patterns of Al₂O₃–ZrO₂(Y₂O₃) hollow fibers (▽: PDF#47-1292 Al₂O₃; △: PDF#50-1089 t-ZrO₂; ◆; PDF#49-1642 c-ZrO₂; ●: PDF#46-1045 SiO₂).

Fig. 7 shows a wide variety of elements diffusing in the Al₂O₃–ZrO₂(Y₂O₃) composite such as O, Al, Y, Zr, and Si. As confirmed by the XRD results (Fig. 6), Al₂O₃–ZrO₂(Y₂O₃) hollow fibers are supposed to contain SiO₂, Y₂O₃, Al₂O₃, Al₂SiO₅, t-ZrO₂ and c-ZrO₂ when x(Al₂O₃) is 0.70. More importantly, the coexistence of multiphases including ZrO₂, Al₂O₃ and SiO₂ might hinder the transport of phonons—the main carrier of heat in ceramic materials^27,31—thereby leading to the decrease in thermal conductivity of Al₂O₃–ZrO₂(Y₂O₃) hollow fibers. In addition, ref. 19 has demonstrated the effect of silicon-based compounds from cogon, which can help decrease the thermal conductivity of Al₂O₃–ZrO₂(Y₂O₃) hollow fibers to some degrees. However, it is difficult to distinguish t-ZrO₂ and c-ZrO₂ merely from EDS and XRD patterns.²⁸ Therefore, the Raman spectra of Sample V2, V4, V5 and V6 was detected.

Fig. 7 (a and b) EDS selected area of Al₂O₃–ZrO₂(Y₂O₃) hollow fibers (x(Al₂O₃) = 0.70); (c–g) EDS mapping of O, Al, Y, Zr, and Si elements, respectively; (h) EDS spectrum of the sample (bar scale: 2 μm).

Fig. 8 demonstrates the Raman spectra of Al₂O₃–ZrO₂(Y₂O₃) hollow fibers. The reciprocal effect between ZrO₂ and Al₂O₃ is significant due to the similar peaks at 462 cm⁻¹ and 639 cm⁻¹. For example, with the increase in x(Al₂O₃), the gap between different peaks drastically decreases and the intensity of the peak at 600 cm⁻¹ marginally increases. Furthermore, a band at 269 cm⁻¹ that only relates to t-ZrO₂ moved to a shorter wavelength in the presence of Al₂O₃.²⁹ It is noticeable that the peak at 639 cm⁻¹ has higher intensity than that at 462 cm⁻¹ in t-ZrO₂, while the opposite is true when it comes to c-ZrO₂.^27,30 Moreover, when x(Al₂O₃) = 0.70, two major crystalline structures of ZrO₂ can be successfully distinguished from their Raman intensities.

Fig. 8 Raman spectra of Al₂O₃–ZrO₂(Y₂O₃) hollow fibers [x(Al₂O₃) = 0.70, 0.80, 0.90, and 1.00].

Overall, it can be concluded that with varying amounts, t-ZrO₂ and c-ZrO₂ are the two primary phases for the Al₂O₃–ZrO₂(Y₂O₃) composite. In addition, their first-order phase transformation also helps to reduce the thermal conductivity to some extent because of the extra energy consumed and scattering of phonons during the phase transition.^32,33

3.4 Thermal conductivity

We conducted further investigations for thermal conductivities at 1000 °C on the variation of x(Al₂O₃) between 0.70 and 1.00; the samples' apparent density was set at 0.2 g cm⁻³. Thereafter, the testing results are shown in Fig. 9. It can be noted that Sample V2 (x(Al₂O₃) = 0.70) has the lowest thermal conductivity, which is only 0.1038 W m⁻¹ K⁻¹ at 1000 °C. More importantly, this value is approximately 60% lower than that of traditional ZrO₂ solid fibers¹⁹ and 34% lower than that of Al₂O₃ hollow fibers (prepared by the identical four-stage procedure). When the x(Al₂O₃) increases from 0.80 to 1.00, thermal conductivities of Al₂O₃–ZrO₂(Y₂O₃) hollow fibers fluctuate between 0.1564 W m⁻¹ K⁻¹ and 0.1636 W m⁻¹ K⁻¹, which adequately shows the effect of compositions in this composite.

Fig. 9 Thermal conductivities of Al₂O₃–ZrO₂(Y₂O₃) hollow fibers.

To summarize, this study presents two mechanisms of thermal conductivity drop: (1) the hollow structure and high porosity hinder the transportation of phonons at a high temperature, thus leading to the low thermal conductivity. (2) Both the coexistence of multiphases and the phase transition between t-ZrO₂ and m-ZrO₂ could contribute to the decrease in the thermal conductivity of Al₂O₃–ZrO₂(Y₂O₃) hollow fibers.

3.5 TG-DSC

Fig. 10a shows the TG-DSC thermograms of natural cogon fibers. Having experienced a sintering process in the range of 0–1100 °C, they have lost 87.3% mass in total, leaving stable remnants such as SiO₂, MgSiO₃, Ca₂SiO₄, and Al₂SiO₅.^19,25,26 In this course, the reaction was exothermic at 340.2 °C and 440.4 °C due to the burning of cogon fibers,^18,19 with an approximate weight loss of 64.5% and 22.8%, respectively.

Fig. 10 TG-DSC thermograms ((a) cogon fibers; (b) cogon fibers immersed in a mixed solution of ZrOCl₂, AlCl₃ and Y(NO₃)₃).

Fig. 10b shows the TG-DSC thermograms of precursors (the cogon fibers immersed in a mixed solution of 74.7 wt% AlCl₃, 18.5 wt% ZrOCl₂, and 6.8 wt% Y(NO₃)₃). In contrast to natural cogon fibers, the burning process of the cogon fiber in the precursors changed to 341.8 °C and 536.3 °C, with a weight loss of 39.8% and 16.7%, respectively, as a result of the effect of the inorganic salts.²⁸ The endothermic peak at 415.3 °C corresponding to a 12.4% mass decrease might be caused by the loss in the water of crystallization in ZrOCl₂·8H₂O, AlCl₃·6H₂O, and Y(NO₃)₃·6H₂O. The process associated with the phase change from ZrOCl₂·2H₂O (non-crystal) to t-ZrO₂ (unstable at this temperature) contributed to the endothermic peak at around 617.8 °C. Finally, the transition between t-ZrO₂ and m-ZrO₂ could lead to the endothermic peak at 917.3 °C.³⁴ Thereafter, there was no further mass loss and heat exchange, which indicates the thermal stability of Al₂O₃–ZrO₂(Y₂O₃) hollow fibers.

4 Conclusion

In this study, we reported a four-stage procedure for the preparation of Al₂O₃–ZrO₂(Y₂O₃) hollow fibers. Using an orthogonal design, the optimal combination was found as follows: the x(Al₂O₃) was 0.70, solute mass ratio of the mixture was 15 wt%, dry temperature was 80 °C and the sintering temperature was 1100 °C.

Through the optimized conditions, the synthesized Al₂O₃–ZrO₂(Y₂O₃) hollow fibers have high porosity (95.0%), low thermal conductivity of 0.1038 W m⁻¹ K⁻¹ (at 1000 °C) and low density (0.05–0.10 g cm⁻³). This adequently showed the effect of orthogonal design in the biomimetic synthesis of natural species. In addition, we discussed the hollow structure and multiphases in the hollow fibers, which are supposed to assist in the decrease of thermal conductivity. Herein, it is worthwhile to apply an orthogonal design for the preparation of Al₂O₃–ZrO₂(Y₂O₃) hollow fibers and other lightweight refractory fibers with multiphases.

However, as there is no detailed experimental data on the thermal conductivity of Al₂O₃–ZrO₂(Y₂O₃) hollow fibers, it is difficult to confirm the findings of the study. We hope that our exploratory experiment will help the development of Al₂O₃–ZrO₂(Y₂O₃) lightweight refractory fibers. Importantly, it is highly desirable to investigate the dynamic process of thermal conductivity of Al₂O₃–ZrO₂(Y₂O₃) hollow fibers, which could help generalize this four-stage procedure for the preparation of other ceramic fibers.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors wish to express thanks to the National Natural Science Foundation of China (51672131) and the Extracurricular Academic Research Foundation for Undergraduate of NJUST.

References

1. J. J. Gu, W. Zhang, H. L. Su, T. X. Fan, S. M. Zhu, Q. L. Liu and D. Zhang, Adv. Mater., 2015, 27, 464–478.
2. G. T. Zan and Q. S. Wu, Adv. Mater., 2016, 28, 2099–2147.
3. Z. Y. Chen, F. F. Fu, Y. R. Yu, H. Wang, Y. X. Shang and Y. J. Zhao, Adv. Mater., 2018, 8, 1805431.
4. P. Song, Q. Wang, Z. Zhang and Z. X. Yang, Sens. Actuators, B, 2010, 147, 248–254.
5. J. Y. Wan, J. W. Song, Z. Yang, D. Kirsch, C. Jia, R. Xu, J. Q. Dai, M. W. Zhu, L. S. Xu, C. J. Chen, Y. B. Wang, Y. L. Wang, E. Hitz, S. D. Lacey, B. Yang and L. Hu, Adv. Mater., 2017, 29, 1703331.
6. L. P. Fu, Y. S. Zhou, A. Huang, H. Z. Gu and H. W. Ni, J. Am. Ceram. Soc., 2018, 00, 1–10,  DOI:10.1111/jace.16205.
7. C. C. Xu, H. L. Wang, J. N. Song, X. P. Bai, Z. L. Liu, M. H. Fang, Y. S. Yuan, J. Y. Sheng, X. Y. Li, N. Wang and H. Wu, J. Am. Ceram. Soc., 2018, 101, 1677–1683.
8. A. N. Samant and N. B. Dahotre, J. Eur. Ceram. Soc., 2009, 29, 969–993.
9. S. J. Kwon, B. M. Jung, T. Kim, J. Byun, J. W. Lee, S. B. Lee and U. H. Choi, Macromolecules, 2018, 51, 10194–10201.
10. S. K. Das, Angew. Chem., Int. Ed., 2018, 57, 16606–16617.
11. W. L. Huo, X. Y. Zhang, Y. G. Chen, Y. J. Lu, W. T. Liu, X. Q. Xi, Y. L. Wang, J. Xu and J. L. Yang, J. Am. Ceram. Soc., 2016, 99, 3512–3515.
12. K. Ning, H. Ju and K. Lu, J. Am. Ceram. Soc., 2019, 102, 569–577.
13. J. LLorca, J. Y. Pastor and P. Poza, J. Am. Ceram. Soc., 2004, 87, 633–639.
14. C. P. Romao, B. A. Marinkovic, U. Werner-Zwanziger and M. A. White, J. Am. Ceram. Soc., 2015, 98, 2858–2865.
15. L. Fu, G. Chen, X. S. Fu and W. L. Zhou, J. Am. Ceram. Soc., 2019, 102, 498–507.
16. J. Luo, S. Luo, C. X. Zhang, Y. H. Xue and G. Q. Chen, J. Am. Ceram. Soc., 2018, 101, 5151–5156.
17. D. Sarkar, B. S. Reddy and B. Basu, J. Am. Ceram. Soc., 2018, 101, 1333–1343.
18. T. C. Wang, Q. K. Yu and J. Kong, Int. J. Appl. Ceram. Technol., 2018, 15, 472–478.
19. T. C. Wang, Z. H. Zhang, C. H. Dai, Q. Li, Y. C. Li, J. Kong and C. P. Wong, Ceram. Int., 2019, 45, 7120–7126.
20. C. M. Pang and H. Huang, Optimal Design of Testing and Data Analysis, Southeast University Press, Nanjing, 2018.
21. G. Xie, Z. Chen, S. Ramakrishna and Y. Liu, J. Appl. Polym. Sci., 2015, 132, 42574,  DOI:10.1002/app.42574.
22. X. Yang, M. Yang, B. Hou, S. Q. Li, Y. Zhang, R. H. Lu and S. B. Zhang, J. Sep. Sci., 2014, 37, 1996–2001.
23. W. G. Cui, X. H. Li, S. B. Zhou and J. Weng, J. Appl. Polym. Sci., 2007, 103, 3105–3112.
24. D. C. Liu, S. X. Xia, H. W. Tang, D. Zhong, B. H. Wang, X. Cai and R. Lin, Int. J. Energy Res., 2018, 1–12,  DOI:10.1002/er.4131.
25. M. A. Haque, D. N. Barman, M. K. Kim, H. D. Yun and K. M. Cho, J. Sci. Food Agric., 2016, 96, 1790–1797.
26. Y. S. Lin and W. C. Lee, Bioresources, 2011, 6, 2744–2756.
27. F. Hu, S. Wu and Y. G. Sun, Adv. Mater., 2018, 1801001,  DOI:10.1002/adma.201801001.
28. M. Kogler, E. M. Kock, S. Vanicek, D. Schmidmair, T. Gotsch, M. S. Pollach, C. Heiny, B. Klotzer and S. Penner, Inorg. Chem., 2014, 53, 13247–13257.
29. A. Sobolev, A. Musin, G. Whyman, K. Borodianskiy, O. Krichevski, A. Kalashnikov and M. Zinigrad, J. Am. Ceram. Soc., 2018, 1–8,  DOI:10.1111/jace.16232.
30. A. R. Krause, H. F. Garces, C. E. Herrmann and N. P. Padture, J. Am. Ceram. Soc., 2017, 100, 3175–3187.
31. H. G. Zhu and X. L. Wang, Research and Testing Methods of Material Science, Southeast University Press, Nanjing, 2nd edn, 2015.
32. H. Chen, Z. Yue, D. Ren, H. R. Zeng, T. R. Wei, K. P. Zhao, R. G. Yang, P. F. Qiu, L. D. Chen and X. Shi, Adv. Mater., 2018, 31, 1806518.
33. Z. Q. Hu, Fundamental Course of Inorganic Material Science, Chemical Industry Press, Beijing, 2nd edn, 2011.
34. D. M. Jiang, L. J. Wang and X. K. Che, Preparation and Application of Zirconium Oxychloride, Metallurgical Industry Press, Beijing, 2012.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ra01176e

---