Micrometer-Scale Kirkendall Effect in the Formation of High

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Micron Scale Kirkendall Effect in the Formation of High Temperature Resistance Cr2O3/Al2O3 Solid Solutions Hollow Fibers Shuwei Cao, Yue Zhang, Dahai Zhang, Jinpeng Fan, Jingyi Zhang, Jun Zhou, and Juan Zhang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b02127 • Publication Date (Web): 14 Aug 2018 Downloaded from http://pubs.acs.org on August 15, 2018

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Chemistry of Materials

Micron Scale Kirkendall Effect in the Formation of High Temperature Resistance Cr2O3/Al2O3 Solid Solutions Hollow Fibers Shuwei Cao,†,‡ Yue Zhang, † Dahai Zhang,‡ Jinpeng Fan,‡ Jingyi Zhang,‡ Jun Zhou,‡ Juan Zhang‡ †

Key Laboratory of Aerospace Advanced Materials and Performance, School of Materials Science and Engineering, Beihang University, Beijing, 100191, China ‡

National Key Laboratory of Advanced Functional Composite Materials, Aerospace Research Institute of Materials and Processing Technology, Beijing, 100076, China ABSTRACT: The Kirkendall effect in the formation of hollow structures mainly focus on nano-scaled metal-metal reactions, few have been performed to fabricate the micron scale ceramic hollow structures due to the diffusion difficulty. Here, through introducing the liquid mesophase to accelerate the diffusion rates, we identify that a micron scale ceramic hollowing process could be achieved via the Kirkendall effect. The formation mechanism of the Cr2O3/Al2O3 solid solutions hollow fibers is analysed. The small Kirkendall voids appear at the interface between Cr2O3 and Al2O3 via a bulk diffusion process. Then the core material diffuses along the pore surface through the liquid mesophase, which leads to the depletion of the center matter and forms the hollow structures. The introduction of the liquid mesophase is the key factors in the formation of Cr2O3/Al2O3 hollow fibers. The asfabricated Cr2O3/Al2O3 hollow fibers have a pore size of 8µm and a shell thickness of 2µm. The hollow structure can be remained well after heat-treated at 1400 °C for 100h in air, which indicates that the hollow fibers have excellent property of high temperature resistance. This method confirms that the micron scale ceramic hollow fibers can be fabricated in a simple and low-cost method using commercial raw materials without pollutant emissions. We expect that our findings could offer new perspectives in fabricating micron scale ceramic hollow structures, like hollow sphere, tube and heterotypic structure.

INTRODUCTION Ceramic hollow fibers, which have many special advantages including high temperature resistance, high surface area, high chemical resistance and low thermal conductivity, are widely applied to fuel cells, insulating material, catalyst carrier, gas separation, microchannel reactor and wastewater treatment.(1)-(9) Increasing attention has been paid to the development of ceramic hollow fiber for its distinct merits. By now, most of the ceramic hollow fibers which are reported are fabricated by the phase inversion/sintering method, the organic template method and the static filature method.(10)-(12) These kinds of methods have complicated techniques and include such disadvantages as having a high cost and taking a long time, which limit the practical applications of the hollow fibers. The Kirkendall effect, which was discovered in 1947 by Kirkendall and Smigelskas(13), can cause the formation of pores near the interface of dissimilar materials during the interdiffusion of different atomic species showing an imbalance of diffusivities. The Kirkendall pores are conventionally regarded as being detrimental to the mechanical, thermal or electrical properties of the materials. In 2004, the Kirkendall effect was firstly exploited to fabricate hollow nanoparticles where a single pore is formed by the interdiffusion of Co and O, S, or Se. (14) The hollow structures start with a dense core–shell structure in which the shell components diffuse slower than the core components, resulting in an inward flux of vacancies toward the center.(15)-(16) The pores can coalesce into a single Kirkendall pore due to

radial symmetry and spatial confinement. However, most of the investigations which involve the formation of hollow structures are only rare examples of nanoscaled metal-metal reactions, few have been performed to fabricate the micron scale ceramic hollow structures.(17)-(24) The application of the Kirkendall effect in micron scale is a difficult technical problem due to the slow diffusion. Solid solution reaction between different compounds, which exits different diffusion rates, can also generate the Kirkendall effect in theory. If the diffusion rates are accelerated through some special methods, the hollow structure can be fabricated. As we know, Alumina ceramic is one kind of widely used engineering ceramics, which has the merits of a high melting point (2050 °C), high chemical stability, high thermal conductivity(30W/m.K), high strength (300MPa) and high hardness (≥80 HRA).(25) Chromium oxide (Cr2O3, 2265 °C) and aluminium oxide (α-Al2O3), which have the same corundum structure with the space group R-3c consisting of a hexagonal close packed array, are the highly stable sesquioxides having isostructural crystal.(26) Besides Cr3+ and Al3+ having nearer ion radii, they can be mixed in atomic level and form complete substitutional solid solution ((Al2O3)x(Cr2O3)1−x in the entire range from x=0 to x=1) over the entire composition range without formation of any eutectic at high temperature, which have superior mechanical properties, thermal shock resistances and corrosion resistances.(27) -(30)

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Based on these characteristics, this work presents a novel methodology to fabricate Cr2O3/Al2O3 solid solution (Cr2O3/Al2O3 SS) ceramic hollow fibers via the micron scale Kirkendall effect. We report herein, to our best knowledge, the first fabrication of the ceramic hollow fibers via the micron scale Kirkendall effect, which solves the problems of the general methods mentioned above. The formation and evolution of Kirkendall porosity is discussed together with the microstructure, the composition and the high temperature resistance.

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goniometer -1°/min. The thermal behaviour in the transformation processes from solid fiber to hollow fiber is evaluated by the simultaneous thermal analyser (TGA–DSC, STA 449 F3 Jupiter, NETZSCH, Germany) from room temperature to 1600°C at a heating rate of 10 °C /min in air. The microstructure of the ceramic hollow fiber has been further examined by transmission electron microscopy (HRTEM) technique, using a Philips CM-200 microscope operated at 200 kV. RESLUTS AND DISCUSSION

EXPERIMENTAL SECTION

Figure 1. Schematic of the Cr2O3/Al2O3 SS hollow fiber fabrication approaches.

Fabrication of the Cr2O3/Al2O3 SS ceramic hollow fibers. This method which is based on a fiber coatings process uses commercially available aluminosilicate fibers (85 wt. % Al2O3 + 15 wt. % SiO2) with a diameter of 10µm and CrO3 reagents as raw materials. As is shown in Figure 1, this method consists of three key concepts: Firstly, aluminosilicate fibers are heat-treated at 600 °C in air for 1h to remove the surface organic treatment agent. Secondly, the aluminosilicate fibers are put into the CrO3 aqueous solution with a concentration of 5wt% for 2h under constant magnetic stirring. After drying at 150 °C for 2h, CrO3 coatings are formed on the surface of the aluminosilicate fibers. This procedure can also be repeated for several times to produce the desired coating thickness. Then the aluminosilicate fibers with CrO3 coatings are heat-treated at 1200 °C at 5 °C /min in air for 1h to make the CrO3 decompose into Cr2O3 completely. Finally, when the aluminosilicate fibers with Cr2O3 coatings are heat-treated at 1550 °C at 5 °C /min in air for 1h, the Cr2O3/Al2O3 SS ceramic hollow fibers are fabricated. General Characterization. The microstructure of the fabricated materials is characterised by scanning electron microscopy (SEM, Quanta 650, FEI, America). The elemental analysis of the ceramic hollow fibers is characterised by energy dispersive X-ray analysis (EDX, Quanta 650, FEI, America). The phase compositions of the fabricated materials are characterised by X-ray diffraction (XRD, D8 Advance, Germany) with CuKα radiation. Voltage on Cuanode -40 kV, current intensity 40 mA, range of measurement angle 10-80°, speed of

Figure 2. a, b, c, d: SEM micrographs (surface and cross-section) of the as-fabricated Cr2O3/Al2O3 SS hollow fibers. e, f: The raw aluminosilicate fibers heat-treated at the same temperature.

The typical Cr2O3/Al2O3 SS ceramic hollow fibers are fabricated at 1550 °C in air for 1h. Figure 2 shows the scanning electron microscopy (SEM) micrographs of the ceramic hollow fibers and the raw aluminosilicate fibers which are heat-treated at the same temperature. The asfabricated fibers, which have a pore size of 8µm and a shell thickness of 2µm respectively, apear the uniform hollow structure (Figure 2a, b). As is shown in Figure 2c, these fibers are hollow all the way from one end to another. The diameter of the fibers is about 10µm, which is accordance with the raw aluminosilicate fibers. Crystal can be seen obviously on the surface of the fibers, which is due to the formation of the Cr2O3/Al2O3 SS (Figure 2d). As is shown in Figure 2e, f, the surface of the raw aluminosilicate fibers becomes unsmooth after being heat-treated at 1550°C in air for 1h, which indicates that the grain-growth in the fibers is very

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obvious (Figure 5a). However, the cross-section of the raw aluminosilicate fibers is still solid and no hollow structure appears.

endothermic peak at 87.5 °C is caused by the vaporisation of water, which is very well corresponded to the TGA curve. The endothermic peak at 202.2 °C without mass loss in TGA curve is due to the melting of CrO3. Then the CrO3 begins to decompose into Cr2O3 with oxygen releasing and mass loss. The endothermic peak at 484.6 °C is due to the fast decomposition of CrO3 with a mass loss of 6.08%. At this temperature, all the CrO3 decompose completely for the no mass loss after 484.6 °C in TGA curve. There is no exothermic peak or endothermic peak from 600 °C to 1200 °C. The exothermic peak at 1269.5 °C is due to the fast crystallization of SiO2 (Figure 3b). From 1350 °C to 1600 °C, there exists a slowly endothermic peak for the Cr2O3/Al2O3 SS reaction and the melt of SiO2. The weak endothermic peak at 1467.9 °C is due to the formation of the Cr2O3/Al2O3 SS, which is corresponds to the XRD data (Figure 5). According to the appearance of the hollow structure in SEM micrographs at 1500 °C

Figure 3. a: Thermogravimetric analysis(TGA) data, differential thermal gravity(DTG) data and different scaning calorimetry(DSC) data of the fibers with CrO3 coatings upon heating up to 1600 °C in air. b: Local zoom of the graph a.

Thermogravimetric analysis along with differential scanning calorimetry is employed to study the thermal behaviour in the transformation processes. Heating aluminosilicate fibers with CrO3 coatings under air gas produce mass changes that are recorded by TGA (Figure 3a), which demonstrates a mass loss of 6.76% at 83.5 °C, followed by a mass loss of 3.9% and 3.26% in the temperature range from 310 °C to 450 °C. A large mass loss of 6.08% is then observed, ranging from 450 °C to 500 °C. The significant decrease in the mass of the fiber which is observed upon heating up to 83.5 °C due to the loss of adsorbed water for the moisture absorption of CrO3. The mass loss of 3.9% and 3.26% is due to the decomposition of CrO3, which releases oxygen. During the decomposition, different decomposition reactions exist to form intermediate compounds between CrO3 and Cr2O3, which results in the appearance of the peaks at 343.9 °C and 390.4 °C in DTG curve. From 484.6 °C to 1600 °C, there is little mass loss. From the DSC curve we can see, there are three clear endothermic peaks and two exothermic peaks that can be observed. The

Figure 4. The microstructure evolution of the ceramic hollow fibers (surface and cross-section) heat-treated at 1000 °C, 1200 °C, 1400 °C, 1450 °C, 1500 °C, 1550 °C at 5 °C /min in air for 1h respectively.

(Figure 4), the weak endothermic peak at 1503.0 °C is due to the production of liquid phase, which indicates that the solid solution reaction continues. This kind of method is environmentally friendly as no pollutant emissions are released in the whole process. Figure 4 shows the microstructure evolution of the ceramic hollow fibers which are heat-treated at 1000°C, 1200°C, 1400°C, 1450°C, 1500°C and 1550°C at 5°C /min in air for 1h respectively. From the micrographs of 1000°C, we can see that the coatings on the aluminosilicate fibers are irregular amorphous phase for the decompositon of CrO3. After being heat-treated at 1200 °C, CrO3 decompose into Cr2O3 completely, which



has a structure of fine crystal.(31) A homogeneous Cr2O3 coatings can be seen on the surface of the aluminosilicate fibers from the micrographs of 1200 °C, which has a shell thickness of about 2µm (Figure 8a, b). After heat-treated at 1400 °C, the particles of the Cr2O3 crystal become much bigger and appear sintering phenomenon, but the interface between the coatings and the fiber is still clear. After being heat-treated at 1450 °C, the sintering phenomenon of the fiber becomes much seriously for the formation of the Cr2O3/Al2O3 SS. The interface between the coatings and fiber cannot be recognized and the fibers become brittle. On the contrary, after being heat-treated below the temperature of 1450 °C, the surface of the raw aluminosilicate fibers which have a diameter of 10µm is very smooth and the cross-section of the fibers is homogenous solid (Figure S1, Supporting Information). When the fibers with Cr2O3 coatings are heat-treated at 1500 °C, the size of the crystal particles on the surface becomes un-uniform and the liquid phase appears. Voids can be found at the interface between the coatings and fibers at this temperature, which obviously have the net-shaped and skin-core structure. This kind of structure indicates that the internal matters of the fibers diffuse into the shell and result in the production of the Kirkendall voids for the different diffusion rates between Cr2O3 and Al2O3. The generation of liquid phase can accelerate the solid solution reaction between Al2O3 and Cr2O3 which can promote the Kirkendall effect appear in micron scale. When heat-treated at 1550 °C, all the internal Al2O3 of the fibers diffuse into the outside shell to form solid solution with Cr2O3 and the Cr2O3/Al2O3 SS hollow fibers are fabricated. The appearance of liquid phase makes the crystals grow much further. However, at this temperature, the surface of the raw aluminosilicate fibers becomes unsmooth for the obvious grain-growth and no hollow structure appears (Figure S1, Supporting Information). These results indicate that the hollow structure appears at about 1500 °C, and there exists liquid phase reaction while the internal matter of the aluminosilicate fibers diffuses into the fiber shell. Changing the heat-treatment temperature and time can

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Figure 5. XRD patterns of the raw aluminosilicate fibers(a) and the fibers with CrO3 coatings(b) heat-treated at 1000 °C, 1200 °C, 1400 °C, 1450 °C, 1500 °C, 1550 °C respectively. c, d, e, f: Local zoom of the XRD patterns(b). g: Rhomb-centered hexagonal structure of the Pure Cr2O3 and Al2O3. h: Unit cell parameters of the Cr2O3/Al2O3 SS fibers at different temperature, a and c.

control the hollow structure of the fibers (Figure S2, Supporting Information). The discussion results of SEM are in accordance with the results of TGA-DSC. The XRD patterns of the raw aluminosilicate fibers indicate that the mullite crystal structure is formed at 1200 °C and the characteristic peaks of the mullite become stronger as the temperature increases (Figure


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5a). From the XRD patterns of the fibers with coatings, we can see that there are only Cr2O3 characteristic peaks at 1000 °C (Figure 5b). At 1200 °C, the characteristic peaks of mullite begin to appear. Above 1400 °C, the main phase structure in the XRD patterns is mullite and Cr2O3. With the increase of the heat-treated temperature of the specimens, there exists obvious shifts to higher 2Theta of the characteristic peaks corresponding to the pure Cr2O3 obviously (Figure 5c, 5d, 5e, 5f). As the ionic radius of Cr3+ (0.615Å) is larger than that of Al3+ (0.535Å)(31), the shift of the corresponding peaks from lower angles to higher angles with the increasing temperature indicates the formation of the Cr2O3/Al2O3 SS. Pure Cr2O3 and Al2O3 are both rhomb-centered hexagonal structural with the lattice constants of a = 4.91Å, c = 13.47Å (JCPDS NO. 84-0315) and a = 4.76Å, c = 12.99Å (JCPDS NO. 10-0173) respectively (Figure 5g). Though the differences between the corresponding lattice constants a and c are both JB, JV=JA - JB) (Figure 8d)(15),(17). This is the reason why most of the voids produce just at the interface between the coatings

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and fiber at 1500 °C, rather than internal. The driving force for this process is minimisation of the interfacial energy by the reduction of internal grain surfaces, elimination of small angle grain boundaries and polygonization of crystals.(33) When heat-treated at 1550 °C, the core material diffuses along the pore surface through the liquid mesophase, which leads to the depletion of the center matter and forms the hollow structures. When all the internal Al2O3 of the fibers diffuses into the external shell through the liquid mesophase to form solid solution with Cr2O3, the micron scale Cr2O3/Al2O3 SS hollow fibers are fabricated (Figure 8e). The distinct merits of this method are the simple techniques, low-cost of raw materials, environment friendly and the short period of time it takes to implement. The as-fabricated micron scale Cr2O3/Al2O3 SS hollow fibers, which have distinct merits of high temperature resistance, high corrosion resistance and uniform aperture, can be applied up to 1400 °C. We firstly expand the application of the Kirkendall effect from nanoscale to micron scale. This method can also be widely applied to fabricate micron scale ceramic hollow sphere, tube and heterotypic structure. CONCLUSION In summary, the micron scale Kirkendall effect has been firstly applied in the formation of the ceramic hollow fibers. This method, which is based on a kind of fiber coatings process, uses commercial aluminosilicate fibers and CrO3 reagent as the raw materials. The formation mechanism of the Cr2O3/Al2O3 SS hollow fibers is analysed. The small Kirkendall voids appear at the interface between Cr2O3 and Al2O3 via a bulk diffusion process. Then the core material diffuses along the pore surface through the liquid mesophase, which leads to the depletion of the center matter and forms the hollow structures. The introduction of the liquid mesophase at the preparation temperature is the key factors in the formation of Cr2O3/Al2O3 SS hollow fibers. The as-fabricated ceramic hollow fibers have the mainly composition of the Cr2O3/Al2O3 SS with a pore size of 8µm and a shell crystal thickness of 2µm respectively. The hollow structure can be remained well after heat-treated at 1400 °C for 100h in air, which indicates that the as-fabricated hollow fibers have excellent property of high temperature resistance. This kind of micron scale Cr2O3/Al2O3 SS hollow fibers, which have distinct merits of high temperature resistance, high corrosion resistance and uniform aperture, can be applied to fuel cells, insulating material, catalyst carrier and microchannel reactor. This method confirms that the micron scale ceramic hollow fiber can be fabricated in a simple and low-cost method using commercial raw materials without


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pollutant emissions. We expect that our findings could offer new perspectives in fabricating high temperature resistance micron scale ceramic hollow structures, like hollow sphere, tube and heterotypic structure. ASSOCIATED CONTENT S Supporting Information. ○

The Supporting Information is available free of charge on the ACS Publications website. Figure S1. Scanning electron microscope micrographs of the raw aluminosilicate fibres heat-treated at 1000°C, 1200°C, 1400°C, 1550°C at 5°C /min in air for 1h. Figure S2. The different hollow structure of the Cr2O3/Al2O3 SS ceramic hollow fibre heattreating at different heating rate and different holding time.

AUTHOR INFORMATION Corresponding Author

*Shuwei Cao E-mail: [email protected] Address: Key Laboratory of Aerospace Advanced Materials and Performance, School of Materials Science and Engineering, Beihang University, Beijing, 100191, China *Yue Zhang E-mail: [email protected] Address: Key Laboratory of Aerospace Advanced Materials and Performance, School of Materials Science and Engineering, Beihang University, Beijing, 100191, China ORCID Shuwei Cao: 0000-0003-4186-6882 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors would like to thank the National Natural Science Foundation of China for their financial support (NSFC-Nos.51672014).

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Micrometer-Scale Kirkendall Effect in the Formation of High

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