Effect of core-shell microspheres as pore-forming agent on the properties of porous alumina ceramics

Accepted Manuscript Effect of core-shell microspheres as pore-forming agent on the properties of porous alumina ceramics

Meikang Han, Xiaowei Yin, Laifei Cheng, Sa Ren, Zhengkun Li PII: DOI: Reference:

S0264-1275(16)31340-5 doi: 10.1016/j.matdes.2016.10.043 JMADE 2400

To appear in:

Materials & Design

Received date: Revised date: Accepted date:

14 July 2016 21 September 2016 19 October 2016

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ACCEPTED MANUSCRIPT Effect of core-shell microspheres as pore-forming agent on the properties of porous alumina ceramics Meikang Han1, Xiaowei Yin*1, Laifei Cheng1, Sa Ren1, Zhengkun Li2 1

Science and Technology on Thermostructural Composite Materials Laboratory,

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Northwestern Polytechnical University, Xi’an, 710072, China 2

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Jiangsu Jingxin New Materials Co., Ltd., Yangzhou, 225265, China

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Abstract

The porous alumina ceramics with closed pores were fabricated using C@Al2O3

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microspheres as pore-forming agent. The closed pores with a diameter about 5μm

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were successfully obtained in the dense matrix, owing to the additive C@Al2O3 microspheres. Different amount of C@Al2O3 microspheres was added to investigate

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the effects of closed pores on thermal, dielectric and mechanical properties. With the

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increasing microspheres amount, the closed porosity of alumina ceramics ranges from 5.82 to 14.5%, the thermal conductivity decreases continuously at the temperature

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ranging from 30 to 1200 °C, and the flexural strength and fracture roughness decrease.

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As the closed porosity increases, the real permittivity decreases, while the tangent loss has a slight increase. When the mass ratio of C@Al2O3 microspheres is 10 wt.%, the porous alumina ceramics with the thermal conductivity (13.07-3.8 W·m-1·K-1), the flexural strength (97.05 ± 18 MPa), and the fracture roughness (2.65 ± 0.13 MPa·m1/2) can be obtained. 1

*

Corresponding author. Tel.: +86 29 88494947. Fax: +86 29 88494620.

E-mail: [email protected] (Xiaowei Yin). 1

ACCEPTED MANUSCRIPT Keywords: porous alumina ceramics; core-shell microspheres; thermal conductivity; permittivity; mechanical properties 1. Introduction The porous alumina ceramics, which are characterized by light weight, low

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dielectric loss, low thermal conductivity and good thermal shock resistance, are

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widely applied in many fields, such as heat insulator, electromagnetic (EM)

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transparent layers, biomedical substitutes and so on [1-5]. It is well known that high porosity enormously decreases the thermal conductivity, but the anti-erosion ability

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and mechanical strength also decrease for defects-sensitive ceramics. This is adverse

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to some refractories such as furnace linings. It is still a challenge to prepare alumina ceramics with low thermal conductivity, low permittivity and good anti-erosion

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performance at the same time. Porous ceramics with closed pores have obvious

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advantages in erosion resistance, low permittivity and mechanical strength. The closed pores are regarded as the dispersed phase distributed in the continuous matrix,

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which are different from the open pores. Compared with the conventional porous

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ceramics, the closed pore structure can not only decrease heat transfer and permittivity, but also maintain good erosion resistance and high mechanical strength. It is attributed to the rigid skeleton formed by the interconnected pore walls of closed pores [6, 7]. Up to now, many methods have been developed to prepare porous ceramics, including freeze casting, sacrificial template and foaming methods [8-13]. Generally, direct foaming process is considered to be the most effective approach to obtain 2

ACCEPTED MANUSCRIPT closed pores in porous ceramics. However, due to the thermodynamic instability of gas bubbles, they are easy to coalesce, resulting in large pores in the final ceramics with wide pore size distribution [14, 15]. Sacrificial template method was also frequently used to obtain the pores with various shapes. All kinds of templates were

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explored to use as pore-forming agents, including polymer microspheres, inorganic

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additives (ice, carbon and glass beads, etc.) and natural organic matters (wheat flour,

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poppy seed and cotton, etc.) [3, 16-22]. Typically, Zhou et al. prepared porous yttria-stabilized zirconia ceramics with an average diameter of pores about 15.7 μm,

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using polymethylmethacrylate (PMMA) microspheres as pore-forming agent [23].

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Although uniform pores and ultra-high porosity can be obtained using these templates, the pores usually constructed three dimensional interconnected structures in undense

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matrix phase. The connected pores will destroy the structure of closed pores, and

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finally reduces the flexural strength of the sintered body. On the other hand, when the microspheres (PMMA, polystyrene and carbon microspheres et al.) are used as

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pore-forming agent, the sintering processes have to be carried out at low heating rates

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to avoid crack formation which arises from strong exothermic reactions [24]. To obtain closed pores dispersed in dense bodies, which contribute to maintain the balance among mechanical strength, erosion resistance and thermal conductivity, suitable pore-forming agents are needed to be further explored. In this work, core-shell structured C@Al2O3 microspheres (carbon core and Al2O3 shell) were used as pore-forming agent to prepared porous alumina ceramics with closed pores structure. The alumina shell of microspheres playes an important 3

ACCEPTED MANUSCRIPT role in achieving dense sintered matrix, and forming regular closed pores at a normal sintering rate. The pore-forming mechanism is discussed. The effects of different closed porosity on thermal conductivity, dielectric properties, flexural strength and fracture toughness of the as-prepared alumina ceramics were investigated.

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2. Experimental

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2.1 Preparation of C@Al2O3 microspheres

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C@Al2O3 microspheres were synthesized by a hydrothermal process. In a typical procedure, 10 g of glucose and 4 g of Al(NO3)3•9H2O were dissolved in 140 mL of

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deionized water. The solution was transferred into a 200 mL Teflon-sealed autoclave

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and maintained at 160 °C for 24 h. After the reaction, the precipitate was filtrated, and then washed with absolute alcohol and deionized water. The washed precipitate was

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dried at 80 °C for 2 h.

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2.2 Preparation of porous alumina ceramics Commercially available alumina powder (α-Al2O3; Jingxin Co., Ltd., Yangzhou,

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China) was used as the raw material, and MgO powder (AR) was used as the sintering

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aid. The as-prepared C@Al2O3 microspheres were used as the pore-forming agent. Firstly, the alumina slurry was prepared by a planetary ball milling for 24 h with a rotate speed of 250 rpm after adding MgO powders (1 wt.% of ceramic powder). After the ball milling procedure, the ultrasonicated C@Al2O3 microspheres with different weight ratio (2.5-10 wt.%) were added into the slurry with stirring for 4 h. Then, the mixture was dried at 80 °C for 2 h. The powders were compacted by cold isotatic pressing at 250 MPa for 2 min. Finally, the green bodies were calcined at 600 °C for 2 4

ACCEPTED MANUSCRIPT h to remove carbon core of the pore-forming agent, and then sintered at 1650 °C for 2 h to obtain the porous ceramics, the heating rate is 3 ℃/min. 2.3 Characterization The thermal analysis was carried out on Thermogravimetry-differential scanning

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calorimeter (TG-DSC; STA449F3, Netzsch, Germany) under air atmosphere with a

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heating rate of 10 K·min-1. Phase compositions were analyzed by X-ray diffraction

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(XRD; D8 Avance, Bruker, Germany) measurements, using Cu Kα (λ = 0.154 nm) radiation. The microstructure of the as-prepared samples was observed by scanning

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electron microscopy (SEM; S-4700, Hitachi, 15 kV, Japan). The thermal conductivity

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at the temperature ranging from 30 to 1200 °C was measured by laser flash apparatus (LFA 427, Netzsch, Germany), with a sample dimension of φ 12.7 × 3 mm2. The

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permittivity of the samples with dimensions of 22.86 × 10.16 × 3 mm3 were measured

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by the vector network analyzer (VNA; MS4644A, Anritsu, Japan). The bending strength was measured by using a three-point bending test (SANS CMT 4304, Sans

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Testing Machine, China) with a span of 30 mm, and the loading rate was 0.5 mm/min.

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Fracture toughness (KIC) was measured by a single-edge notched beam (SENB) test with a cross head speed of 0.05 mm/min. The support span was 30 mm, and the notch length was half of the sample height. Three samples were measured for each test. The porosity and density of the sintered ceramics were measured using the Archimedes method (GB/T2997-1982), the equations are as follows:

q1 

m 3  m1  100% m3  m2

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(1)

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D 

m1

(2)

m 3 m 2

q 1

D 0

(3)

q2  q  q1

(4)

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where q, q1 and q2 are the total, open and closed porosity of the material, respectively;

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m1 is the weight of dry sample; m2 and m3 are the weight of water-saturated sample in

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water and air, respectively; D is volume weight of the material; ρ0 is the theoretical density of the pure alumina ceramics (3.95 g·cm-3).

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3. Results and discussion

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3.1 Characterizations of C@Al2O3 microspheres

The SEM micrographs of as-prepared C@Al2O3 microspheres are shown in Fig.

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1a. It can be observed that C@Al2O3 microspheres have spherical structure with a

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diameter about 3-7 μm. Fig. 1b shows the mono-dispersed microsphere after annealing at 700 °C. The entire spherical structure after the removal of carbon phase

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indicates the core-shell structure of the microspheres, whose core and shell are carbon

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and Al2O3, respectively. The formation of core-shell microspheres is attributed to a self-assembled process. Firstly, the colloidal carbon spheres nucleated and grew by the polymerization of glucose monomers during the hydrothermal process [25, 26]. The surface of carbon spheres has a distribution of hydroxyl and carbonyl groups. Al3+ in the solution was absorbed and reacted with hydroxyl groups. Then, the surface of carbon spheres was wrapped by the colloidal Al(OH)3. More details on thermal decomposition and phase transformation of C@Al2O3 microspheres are provided by 6

ACCEPTED MANUSCRIPT the TG-DSC curves ranging from 30 to 1450 °C. As shown in Fig. 1c, the obvious weight loss about 92%, which corresponds to the exothermic peak located at 463.5 °C in DSC curve, is attributed to the oxidation of colloidal carbon core. It is noteworthy that a slight exothermic peak occurs at 1039 °C without weight loss. This is ascribed

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to the phase transformation from γ-Al2O3 to α-Al2O3. The result is further identified

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by XRD patterns of C@Al2O3 microspheres annealed at different temperatures, as

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shown in Fig. 1d. The sole diffraction peak around 20° corresponds to the amorphous colloidal carbon microspheres [25]. After annealing at 700 °C, the peaks of γ-Al2O3

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phase (JCPDS no. 02-1420) with weak degree of crystallinity can be observed,

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indicating the existence of alumina shell. When the annealed temperature increased to 1200 °C, γ-Al2O3 transformed into high-temperature stable α-Al2O3 completely, which

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is in good agreement with TG-DSC analysis.

Fig. 1 SEM images of the core-shell structured C@Al2O3 microspheres (a) and a mono-dispersed hollow Al2O3 microspheres after annealing at 700 °C (b); (c) the TG-DSC curves of C@Al2O3 microspheres at air atmosphere; (d) XRD patterns of C@Al2O3 microspheres and annealed samples at 700 and 1200 °C. 3.2 Characterizations of porous alumina ceramics 7

ACCEPTED MANUSCRIPT As shown in Fig. 2a, pure Al2O3 ceramics with MgO as sintering aids present a relatively dense structure after sintering at 1650 °C. The typical microstructure of porous Al2O3 ceramics fabricated using C@Al2O3 microspheres as pore-forming agent is shown in Fig. 2b, it can be observed that regular closed pores distribute in the

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ceramics. Fig. 2c presents the approximately spherical pore shape with a diameter

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about 5 μm, and the matrix is dense. This is attributed to the addition of C@Al2O3

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microspheres. When the amount of C@Al2O3 microspheres changed from 2.5 to 10 wt.%, the closed porosity in the ceramics increased from 5.82 to 14.5%, and the open

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porosity had a few fluctuations, as shown in Fig. 2d. It is proved that porous alumina

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ceramics with small-scale closed pores can be obtained using core-shell structured

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C@Al2O3 microspheres as pore-forming agent.

Fig. 2 SEM images of the porous alumina ceramics without (a) and with (b) addition of C@Al2O3 microspheres; (c) the closed pore with dense wall; (d) dependence of porosity on the additive mass ratio of C@Al2O3 microspheres. Based on the above results, the closed pore-forming mechanism in the ceramics is 8

ACCEPTED MANUSCRIPT illustrated in Fig. 3. At the first stage, the colloidal carbon spheres were oxidized, and the external amorphous layer transformed into γ-Al2O3. The original hollow structure was formed in green body. As the temperature increased, γ-Al2O3 with better activity gave priority to grow up, and react with MgO to form magnesium aluminate spinel

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(MgAl2O4). This is ascribed to that the crystal structure of γ-Al2O3 is similar to that of

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MgAl2O4 [27]. When the temperature elevated continually, unreacted γ-Al2O3 phase

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transformed into α-Al2O3, and the crystal grain grew up with crystal boundary migration. At last, the ceramics achieved densification with closed micropores. It is

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noticed that the pore size is larger than that of C@Al2O3 microsphere. The reason is

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that the hollow spheres undergo tension during the densification process, and the

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increasing temperature results in inner gas expansion.

Fig. 3 Schematic illustration of the closed pore-forming mechanism in the porous

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alumina ceramics.

3.3 Thermal conductivity of porous alumina ceramics Fig. 4a shows the thermal conductivity of porous alumina ceramics with different mass ratio of C@Al2O3 microspheres at the temperature ranging from 30 to 1200 °C. It can be seen that the thermal conductivity of all the samples decreases with the increasing temperature. In general, the effective thermal conductivity of porous materials is a result of a combined heat transfer by conduction, convection and 9

ACCEPTED MANUSCRIPT thermal radiation [28]. Convection can clearly be neglected, because the pore size is below 10 μm. At low temperatures, heat transfer is mainly governed by the conduction.

The excitation of phonons is the main mechanism which causes a

temperature dependence of the thermal conductivity [29]. The phonon-phonon

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scattering increases with the elevated temperature, and the increasing phonon density

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shortens the mean free path. This causes the reduction of thermal conductivity. When

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the temperature exceeds 1000 °C, the effective thermal conductivity tends to be constants. This can be explained by the increasing thermal radiation and the specific

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heat capacity which is near constant value. Especially, the strong thermal radiation is More importantly, the

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mainly attributed to optical and near-infrared regions [30, 31].

thermal conductivity decreases with the increasing mass ratio of C@Al2O3

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microspheres at any temperature. Typically, when the addition of microspheres is 10

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wt.%, the measured value decreases from 28.08 to 13.07 W·m-1·K-1 at room temperature, and from 6.76 to 3.8 W·m-1·K-1 at 1200 °C. The obvious change indicates

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the influence of closed porosity on the thermal conductivity of porous alumina

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ceramics. At high temperatures, the spectra emittance is also reduced with the increasing porosity. This is ascribed to the increasing scattering of radiation which mainly occurs at the interface between the medium and the air-filled pores within the ceramic. Fig. 4b presents the relative thermal conductivity (λr) as a function of porosity at room temperature, together with the predicted values fitted by the well-known Maxwell-Eucken model and the effective medium theory (EMT) equation. The Maxwell-Eucken model assumes that neighboring inclusions of the 10

ACCEPTED MANUSCRIPT dispersed phase cannot be connected. Maxwell-Eucken 1 equation is the case that the thermal conductivity of the dispersed phase is lower than that of the continuous phase, while that of Maxwell-Eucken 2 equation is opposite [32, 33]. The EMT equation represents a heterogeneous two-phase material in which the two phases are distributed

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randomly [34, 35]. The equations are as follows:

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Maxwell-Eucken 1: 21 +2 -2  1 -2 V2 1 21 +2 +  1 -2 V2

c =

22 +1 -2  2 -1 V1 2 22 +1 +  2 -1 V1

(5)

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c =

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Maxwell-Eucken 2:

EMT equation:

(6)

  c =  3V2 -1 2 + 3 1-V2  -1 1 +  3V2 -1 2 + 3 1-V2  -1 1  +812  (7) 4  2

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D

1

where λc, λ1 and λ2 are the thermal conductivity of the composite, high thermal

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conductive phase, and low thermal conductive phase, respectively; V1 and V2 are the volume fraction of the phase with high and low thermal conductivity, respectively.

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It can be seen that the relative thermal conductivity of as-prepared porous alumina ceramics is lower than the values fitted by the models, and deviate gradually from EMT curve along with increasing ratio of C@Al2O3 microspheres. This is ascribed to the spinel phase dispersed in sintered ceramics. The thermal conductivity of MgAl2O4 is lower than that of α-Al2O3. In addition, its spinel crystal structure destroys the periodic structure of alumina ceramics which decreases the mean free path of phonons. The smaller closed pore size than most of reported porous alumina 11

ACCEPTED MANUSCRIPT ceramics is considered to be another factor. The increasing number of pores arising from the smaller pore size will reduce the point contact between the solid phases, and

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finally decrease the heat transfer between the alumina matrixes [6].

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Fig. 4 (a) Dependence of thermal conductivity of the porous alumina ceramics with different addition ratio on temperature. (b) The measured thermal conductivity at

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room temperature compared to the Maxwell-Eucken and EMT models.

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3.4 Dielectric properties of porous alumina ceramics Fig. 5 shows the real and imaginary permittivity of the porous alumina ceramics with different mass ratio of C@Al2O3 microspheres in the X-band. Obviously, the real part decreases with the increasing porosity of the samples, and the average value ranges from 9.74 to 7.48, as shown in Fig. 5a. There is an opposite change that the imaginary part has a slight increase with the increasing porosity, although all the values of the samples are relatively low (Fig. 5b). The low imaginary part is owing to 12

ACCEPTED MANUSCRIPT the transparent character of alumina ceramics with weak dielectric loss. In order to evaluate the effects of closed pores on the permittivity of sintered samples, two equations were used to calculate the real part (ε') and tangent loss (tanδ = ε"/ε'), respectively. Firstly, the models consider the material as a composite system

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composing of two dielectrics with different permittivity. Heidinger et al. quoted an

 3P   m -1   2 m +1  

(8)

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’ = m 1-

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approximate model for εm-ε' << εm [36],

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where εm is the real permittivity of the matrix, P is the fractional porosity. In terms of tangent loss, the equation is as follow [37]:

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 P  tan   1  P  tan  0  A 'P    1 P 

2/3

(9)

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The data were fitted to this model to give tan δo=1.265×10-3, A'=0.3577.

Fig. 5 The real part (a) and imaginary part (b) of permittivity as a function of frequency for the porous alumina ceramics with different account of microspheres. 13

ACCEPTED MANUSCRIPT As shown in Fig. 6a-b, both the experimental real permittivity and tangent loss fit well with the values computed from the equations. The results indicate that the closed pores play a critical role on the change of the permittivity in the porous alumina ceramics. Although the closed pores in the ceramics reduce the real part, they also

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lead to an increase in tangent loss. Generally, Scattering occurs at pores, grain

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boundaries, cracks, and interfaces [30]. It is not unreasonable to consider that the

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increasing loss is related to the internal scattering effect of EM wave within the pores

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[38, 39].

Fig. 6 The average values of the real (a) and imaginary (b) permittivity compared to the data fitted by models. 3.5 Mechanical properties of porous alumina ceramics The influence of porosity on flexural strength and fracture toughness of the 14

ACCEPTED MANUSCRIPT porous alumina ceramics fabricated with different additions of C@Al2O3 microspheres is shown in Fig. 7. It can be seen that flexural strength decreases along with increasing porosity. The average value changes from 291.98 to 97.05 MPa when the porosity increases from 3.3% to 18.3%. The relationship between the bending

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strength (σf) and the porosity (P) is calculated by the Ryskewitsch expression as

(10)

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 f = 0exp  -nP 

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follow [40]:

where σ0 is flexural strength of the dense ceramic, and n is a constant between 4 and 7.

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It is clear that the experimental data fit well with the formula. The closed pores in

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the dense body reduce the effective area bearing the load and result in the stress concentration in the ceramics. Similarly, the measured fractural toughness also

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decreases with the increasing porosity, ranging from 4.52 to 2.65 MPa·m1/2 (Fig. 7b).

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The morphology of fracture surface of the porous alumina ceramics shows the typical characteristics of intergranular and cleavage fracture. It is noteworthy that obvious

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river pattern can be observed in the pore walls, indicating the existence of closed

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pores weakens the mechanical strength of the porous ceramics. To perform the advantage of the as-prepared alumina ceramics with closed pores, the mechanical properties of the alumina ceramics with similar open porosity (q2 < 3%) were measured. As shown in Fig.7, both the flexural strength and fracture toughness of the samples with closed pores are higher than those of the alumina ceramics with the open pores. This is owing to the rigid skeleton formed by the isolated closed pores. It is believed that the as-prepared porous alumina ceramics are promising for the 15

ACCEPTED MANUSCRIPT refractories which require both low thermal conductivity and high mechanical

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strength.

Fig. 7 The flexural strength (a) and fracture toughness (b) of the porous alumina

4. Conclusions

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ceramics as a function of porosity.

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In this work, the porous alumina ceramics were fabricated by pressureless sintering process using core-shell structured C@Al2O3 microspheres as pore-forming

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agent. The closed pores with a diameter about 5μm dispersed in the dense sintered matrix. When the additive account of microspheres is 10 wt.%, the porosity of closed pores reaches 14.5%. With the increasing porosity of the porous alumina ceramics, the thermal conductivity decreases from 28.08 to 13.07 W·m-1·K-1 at room temperature, the average value of real permittivity decreases from to 9.74 to 7.48 in X-band, the average flexural strength changes from 291.98 to 97.05 MPa, and the fractural toughness decreases from 4.52 to 2.65 MPa·m1/2. Above all, this work paves the 16

ACCEPTED MANUSCRIPT possible way to fabricate the porous ceramics with closed pores using core-shell structured pore-forming agent. Acknowledgements This work was financially supported by the National Natural Science Foundation of

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China (Grant: 51332004 and 51372204), and the Innovation Foundation for Doctor

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Dissertation of Northwestern Polytechnical University (CX201604).

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ACCEPTED MANUSCRIPT Figure captions Figure 1 SEM images of the core-shell structured C@Al2O3 microspheres (a) and a mono-dispersed hollow Al2O3 microspheres after annealing at 700 °C (b); (c) the TG-DSC curves of C@Al2O3 microspheres at air atmosphere; (d) XRD patterns of

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C@Al2O3 microspheres and annealed samples at 700 and 1200 °C.

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Figure 2 SEM images of the porous alumina ceramics without (a) and with (b)

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addition of C@Al2O3 microspheres; (c) the closed pore with dense wall; (d) dependence of porosity on the additive mass ratio of C@Al2O3 microspheres.

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Figure 3 Schematic illustration of the closed pore-forming mechanism in the porous

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alumina ceramics.

Figure 4 (a) Dependence of thermal conductivity of the porous alumina ceramics with

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different addition ratio on temperature. (b) The measured thermal conductivity at

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room temperature compared to the Maxwell-Eucken and EMT models. Figure 5 The real part (a) and imaginary part (b) of permittivity as a function of

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frequency for the porous alumina ceramics with different account of microspheres.

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Figure 6 The average values of the real (a) and imaginary (b) permittivity compared to the data fitted by models. Figure 7 The flexural strength (a) and fracture toughness (b) of the porous alumina ceramics as a function of porosity.

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Graphical Abstract

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Hightlights

1. The closed pores in alumina ceramics were achieved using core-shell structured

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C@Al2O3 microspheres as pore-forming agent.

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2. The closed pores significantly decreased the thermal conductivity to 3.8 W·m-1·K-1

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at 1200 °C.

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increase of the imaginary permittivity.

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3. The closed pores decreased the real permittivity from 9.74 to 7.48 with a slight

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