Effect of Si3N4 solid contents on mechanical and dielectric properties of porous Si3N4 ceramics through freeze-drying

Accepted Manuscript Effect of Si3N4 solid contents on mechanical and dielectric properties of porous Si3N4 ceramics through freeze-drying Ling Li, Qinggang Li, Juan Hong, Mengyong Sun, Jie Zha, Shaoming Dong PII:

S0925-8388(17)33608-3

DOI:

10.1016/j.jallcom.2017.10.172

Reference:

JALCOM 43569

To appear in:

Journal of Alloys and Compounds

Received Date: 11 September 2017 Revised Date:

19 October 2017

Accepted Date: 21 October 2017

Please cite this article as: L. Li, Q. Li, J. Hong, M. Sun, J. Zha, S. Dong, Effect of Si3N4 solid contents on mechanical and dielectric properties of porous Si3N4 ceramics through freeze-drying, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.10.172. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Effect of Si3N4 solid contents on mechanical and dielectric properties of porous Si3N4 ceramics through freeze-drying Ling Lia,d, Qinggang Li b,c,d,*, Juan Hongf, Mengyong Sune, Jie Zhaa,*, Shaoming Dongc School of Material Science and Engineering, Harbin Institute of Technology, Harbin 150001,

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a

China

Shandong Provincial Key Laboratory of Preparation and Measurement of Building Materials,

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b

University of Jinan, Jinan 250022, China

State Key Laboratory of High Performance Ceramics and Superfine Microstructure,

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c

Shanghai Institute of Ceramics, Chinese Academy of Science, Shanghai 200050, China d

Shandong Industrial Ceramics Research & Design Institute Co., Ltd, Zibo 255000, China

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Science and Technology on Thermostructural Composite Materials Laboratory, Northwestern

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Polytechnical University, Xi,an, 710072, China

College of Field Engineering, PLA Army Engineering University, Nanjing 210007, China

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Abstract

Porous Si3N4 ceramics with uniform pore distributions were fabricated by freeze drying

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with liquid N2 as refrigerant, and then sintered at 1700 °C under an N2 pressure of 0.3 MPa. The Si3N4 dispersant slurries were prepared by ball milling for 12h and casting into a mold using liquid nitrogen as a freezing medium with the top surface of the sample exposed to air at room temperature. The temperature gradient contributed to bimodal peaks of pore size distribution. Effects of solid concentration on the porosity, mechanical, and dielectric

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** Corresponding author at: School of Material Science and Engineering, University of Jinan, Jinan 250022, China E-mail: [email protected]; [email protected]. Tel: +86 531 82767655

ACCEPTED MANUSCRIPT properties of the Si3N4 sample were systematically investigated. As the solid content increased from 15 vol.% to 40 vol.%, the porosity decreased from 83.9% to 40.58%. Therefore, flexural strength and compressive strength increased from 0.1 MPa to 94.7 MPa,

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and 1.3 MPa to 314 MPa, respectively. Moreover, the dielectric constant increased from 1.4 to 4.0. This study indicated that the freeze-drying process is a useful method for fabricating novel porous Si3N4 ceramics with unidirectional channels.

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Keywords: Freeze-drying; Pore structure; Porous Si3N4; Mechanical property; Dielectric

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constant 1. Introduction

Owing to their excellent thermal shock resistance and good corrosion resistance, porous ceramics have been widely used in various fields, such as industrial hot-gas filters, catalyst

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supports, and gas membranes [1-3]. Various methods, such as adding a pore-forming agent, gel casting and carbothermal reduction, and freeze casting, are employed for preparing highly porous ceramics [4,5]. In the recent years, freeze casting, a novel, simple, and

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environmentally friendly fabrication method, has attracted increasing attention for the

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preparation of superior ceramics with controllable porosity, favorable mechanical properties, and a wide range of pore distributions [6-8]. Porous Si3N4 ceramics are well known to be low-density

materials

with

high-temperature

strength,

good

oxidation

resistance,

thermal-chemical corrosion resistance, thermal-shock resistance, and low thermal expansion coefficient [9,10]. Furthermore, the porous Si3N4 ceramics prepared through freeze-drying process exhibits high fluid permeability owing to their interconnected pore channels [11]. Moreover, porous Si3N4 ceramics possess low dielectric constant and loss tangent, values, and 2

ACCEPTED MANUSCRIPT could be used as wave-transmitting materials [12-15]. Li reported that the porous Si3N4-SiC/SiO2 materials in which Si3N4 was used as the wave-transparent matrix exhibited high-temperature electromagnetic wave absorption [16].

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In the present study, porous Si3N4 ceramics with uniform pore distributions were fabricated by freeze-drying process with water as the solvent. To produce small crystals, we used liquid nitrogen as the freezing medium. This study aims to explore the effects of

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microstructure and pore distribution regularity of Si3N4 solid content on the mechanical and

2. Experimental procedure 2.1. Raw materials

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dielectric properties of Si3N4 porous ceramics fabricated by freeze casting.

The precursors were α-Si3N4 (purity > 99.9 %, average particle size of 0.8 µm, Shanghai

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Shuitian technology co., LTD, Shanghai, China), Y2O3 powders (Aladdin Industrial Corporation, Shanghai, China, 99.999 % purity, an average particle size of 0.7 µm), and Al2O3 powders (purity >99.8 %, average particle size of 1.5 µm, Kermel co., LTD, Tianjin, China ).

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Polyacrylic amine (PAA-NH4) was used as the dispersant, deionized water was used as

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solvent, polyvinyl alcohol (PVA) was used as the binder in freeze casting. 2.2. Fabrication procedures Starting powders were α-Si3N4 (92 wt.%), Y2O3 (6 wt. %) and Al2O3 (2 wt.%). Contents of PVA and PAA-NH4 were 1 and 0.8 wt.%, respectively, based on the mass of powders. Slurries with different Si3N4 solid contents (15 vol.%, 20 vol.%, 25 vol.%, 30 vol.%, 35 vol.%, 40 vol.%) were added into the starting powders, followed by ball milling with Si3N4 balls for 12 h. The pH values of the slurries were adjusted to approximately 10 by adding ammonia. 3

ACCEPTED MANUSCRIPT Before freeze casting, the slurries were de-aired in a vacuum machine (THINKY-ARV310). Then, the slurry was poured into a polyethylene mold with a brass bottom plate immersed in liquid N2. The top of the mold was open so that the upper surface of the slurry could be

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maintained at room temperature. Owing to the temperature gradient, the ice crystals grew unidirectionally from the bottom to the upper surface. Then, the frozen samples were freeze dried under a vacuum using liquid nitrogen as a freezing medium for 50 h. After sublimation,

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the green body, which buried in Si3N4/BN powder beds, was sintered in a graphite furnace at

temperature at a rate of 5 oC/min. 2.3. Characterizations

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1700 oC at a rate of 10 oC/min for 2 h under N2 pressure of 0.3 MPa and cooled to room

The flexural strength was measured via a three point bending test on 3 mm × 4 mm × 36

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mm bars with the support distance of 30 mm and a cross-head speed of 0.5 mm/min, and the compressive strength was measured on a Φ20 mm × 20 mm cylinder using Electromechanical Universal Testing Machine (CMT5504, MTS SYSTEMS, co., LTD, China). The bulk density

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was measured by the Archimedes method. The pore size distribution was measured by

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mercury porosimetry (IV9510, Micrometics Co.,USA). Microstructures were characterized by Scanning Electron Microscope (SEM, FEI QUANTA FEG250, USA) with backscattered electron mode (BSE). Phase analysis was carried out by the X-ray diffraction analysis (XRD, Bruker D8 Advance, Germany). Dielectric constants were analyzed by using PNA-L network analyzer (Model N5234A, Agilent, USA ). 3. Results and discussion Fig. 1 shows an increased bulk density from 0.58 g/cm3 to 1.9 g/cm3 and shrinkage from 4

ACCEPTED MANUSCRIPT 16.88 % to 26 % with the solid contents of porous Si3N4 ceramics increasing from 15 to 40 vol.%. However, porosity decreased from 83.9 % to 40.58 % with solid contents increasing from 15 vol.% to 40 vol.%.

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Pore size distributions of porous Si3N4 ceramics synthesized with different solid contents determined by mercury porosimetry are shown in Fig. 2. Two scale ranges of pore sizes, namely, small (< 500 nm) and large (> 1000 nm), were observed. Moreover, the relative

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content of small pores increased with increasing solid content. When the solid content

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increased to 40 vol.%, the large pore showed a decreased size of smaller than 1000 nm. These results showed that the porous ceramics fabricated by freeze drying mainly possessed two types of pore size. As the solid content increased from 15 vol.% to 40 vol.%, pore size decreased and pore size distribution changed gradually from bimodal to single peaks. These

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findings indicated that the solid contents of the slurry markedly affected the pore size distribution of the porous Si3N4 ceramics.

Table 1 shows that the experiments considered 1000 and 500 nm as the cut-off points for

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two types of pore size. When the solid content increased from 15 vol.% to 25 vol.%, the

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volume content of large pores decreased from 83 to 71.5%, whereas that of the small pores increased from 17 % to 28.5 %. When the solid content increased from 30 vol.% to 40 vol.%, the volume content of large pore deceased from 69 % to 35.5 %, whereas that of the small pores increased from 31 % to 64.5 % rapidly. Quantitative analysis further revealed the effect of solid content on the pore size distribution, which resulted from the increasing tendency of sintering driving force. Results showed more porosity and more uniform distribution of pores at the same Si3N4 solid content compared with the results of Takayuki Fukasawa [17]. 5

ACCEPTED MANUSCRIPT As shown in Fig. 3, when the solid contents increased from 15 vol.% to 40 vol.%, the flexural strength increased from 0.1 MPa to 94.7 MPa, and the compressive strength increased from 1.3 MPa to 314 MPa. In particular, the mechanical properties of porous ceramics rapidly

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increased when the solid content exceeded 25 vol.%. Thus, the solid contents exerted an important effect on the mechanical properties of the porous Si3N4 ceramics owing to the critical role of the solid content in determining porous structure and porosity.

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Fig. 4 shows the XRD results of porous ceramic after gas pressure sintering. The β-Si3N4

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phase was observed as the main phase in the ceramics with different solid contents. The results revealed that the α-Si3N4 phase was completely transformed into β-Si3N4 phase at the sintering temperature of 1700 oC.

The SEM images of the fracture surface of the porous ceramics sintered at 1700 oC by

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gas pressure sintering are shown in Figs. 5 and 6. As shown in Fig. 5, unidirectional channels were formed due to the directional distribution of temperature. Low solid content yielded high porosity and large pore size. When the solid content increased to 40 vol.%, the aligned

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channels disappeared. As shown in Fig. 6, the pore structure consists of aligned channels with

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dendrites and internal walls formed by abundant fibrous Si3N4 grains. Moreover, the solid content of 40 vol.% is the critical solid content. The novel pore structure fabricated by freeze-drying would disappear if the solid contents exceed the critical solid content. The dielectric constant of the porous Si3N4 ceramics with different solid content measured in the frequency range 12.4-18 GHz was exhibited in Fig. 7. The results indicate that the dielectric constant values of different solid contents are relatively stable in the whole frequency range. The dielectric constant of the porous ceramics with different solid contents 6

ACCEPTED MANUSCRIPT varied between 1.4 and 4.0 derived from the different porosity of porous ceramics. 4. Conclusions The experiment fabricated Si3N4 porous ceramics with unidirectional aligned channels

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by freeze-drying process using liquid N2 as refrigerant. The solid content of slurry played a critical role on pore structure, porosity, mechanical properties and dielectric constant of the as prepared Si3N4 porous ceramics. With the increasing of the solid content from 15 vol.% to 40

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vol.%, the porosity decreased from 83.9 % to 40.58 %, the flexural strength increased from

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0.1 MPa to 94.7 MPa, the compressive strength increased from 1.3 MPa to 314 MPa, the dielectric constant increased from 1.4 to 4.0. Acknowledgments

Authors appreciate the financial support of the National Natural Science Foundation of China

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under the Grant No. of 51172256 and No. 51621091, the Grant No. of 51142010, the Grant No. of 51372099. Authors also appreciate the financial supported by Program for Scientific research innovation team in Colleges and universities of Shandong Province and the financial

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supported by the Opening Project of State Key Laboratory of High Performance Ceramics

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and Superfine Microstructure (SKL201601SIC) and the Project funded by China Postdoctoral Science Foundation of 2016M602184. Supports from the 111 Project of International Corporation on Advanced Cement-based Materials (No. D17001) is greatly appreciated. Reference

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ACCEPTED MANUSCRIPT [3] Shifeng Liu, Yu-ping Zeng, Dongliang Jiang. Fabricaion and characterization of cordierite-bonded porous SiC ceramics[J]. Ceramics International. 2009,35(2):597-602. [4] Wen Zhang, H Wang, Zhihao Jin. Gel casting and properties of porous silicon

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ACCEPTED MANUSCRIPT [12] Xiangming Lia, Litong Zhang, Xiaowei Yin. Effect of chemical vapor infiltration of Si3N4 on the mechanical and dielectric properties of porous Si3N4 ceramic fabricated by a technique combining 3-D printing and pressureless sintering[J]. Scripta Materialia.

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of Si3N4-based sandwich ceramics[J]. Materials & Design. 2012,35:770-3. [16] Mian Li, Xiaowei Yin, Guopeng Zeng, et al. High-temperature dielectric and microwave absorption properties of Si3N4-SiC/SiO2 composites ceramics[J]. Journal of materials science.

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[17] Takayuki Fukasawa, Zhen-Yan Deng, and Motohide Ando. Synthesis of porous silicon nitride with unidirectionally aligned channels using freeze-drying process[J]. Journal of the American Ceramic Society. 2002, 85 [9]: 2151-55.

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Captions of figures and table Figure 1. The bulk density, porosity and shrinkage of porous Si3N4 ceramics with different

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solid contents. Figure 2. Pore size distribution of the porous Si3N4 ceramics with different solid content of the slurries.

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Figure 3. The flexural strength and compressive strength of porous ceramics with different

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solid contents.

Figure 4. XRD patterns of porous Si3N4 ceramics with different solid content sintered at 1700oC for 2h under 0.3MPa nitrogen atmosphere.

Figure 5. Backscattered electron mode SEM micrographs of the porous Si3N4 ceramics with

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different solid contents.

Figure 6. Magnification of SEM micrographs of the porous Si3N4 ceramics with different solid contents.

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by freeze-drying.

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Figure 7. Influence of solid content on dielectric constant of porous Si3N4 ceramics fabricated

Table 1. The influence of solid content on the pore size distribution.

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ACCEPTED MANUSCRIPT Table 1. The influence of solid content on the pore size distribution of Si3N4 ceramics. D<1000nm

D>1000nm

15

17%

83%

20

26%

74%

25

28.5%

31%

35

54%

D>500nm

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30

71.5%

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D<500nm

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Solid content(vol%)

64.5%

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40

11

69% 46%

35.5%

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Fig. 1. The bulk density, porosity and shrinkage of porous Si3N4 ceramics fabricated with different solid

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contents.

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slurries.

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Fig. 2. Pore size distribution of the porous Si3N4 ceramics prepared with different solid content of the

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Fig. 3. The flexural strength and compressive strength of porous ceramics with different solid contents.

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Fig. 4. XRD patterns of porous Si3N4 ceramics with different solid content sintered at 1700oC for 2h under

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a nitrogen pressure of 0.3MPa.

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solid contents.

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Fig. 5. Backscattered electron mode SEM micrographs of the porous Si3N4 ceramics with different

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Fig. 6. Magnification of SEM micrographs of the porous Si3N4 ceramics with different solid contents.

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Fig. 7. Influence of solid content on dielectric constant of porous Si3N4 ceramics fabricated by

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freeze-drying.

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ACCEPTED MANUSCRIPT Porous Si3N4 ceramics were fabricated by freeze drying with liquid N2 as refrigerant. Effects of solid concentration on the dielectric properties of the samples were

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The dielectric constant increased from 1.4 to 4.0.

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investigated.