Effect of β–Si3N4 seeds on microstructure and properties of porous Si3N4 ceramics prepared by gelcasting using DMAA system

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Effect of β–Si3N4 seeds on microstructure and properties of porous Si3N4 ceramics prepared by gelcasting using DMAA system Shuang Yina,b, Limei Pana,b, Yun Liua,b, Yang Wanga,b, Tai Qiua,b, Jian Yanga,b,∗ a b

Jiangsu Collaborative Innovation Center for Advanced Inorganic Function Composites, Nanjing Tech University, Nanjing, 210009, China College of Materials Science and Engineering, Nanjing Tech University, Nanjing, 210009, China

A R T I C LE I N FO

A B S T R AC T

Keywords: Porous Si3N4 ceramics β–Si3N4 seeds Gelcasting Microstructure Mechanical properties Dielectric properties

Porous Si3N4 ceramics fabricated by gelcasting with DMAA demonstrated excellent mechanical and dielectric properties after introducing β–Si3N4 seeds into ceramic matrix. The microstructure and properties of slurries, green and sintered bodies were highly correlated with β–Si3N4 seed size, content and morphology. The slurry viscosity reached the maximum of 0.87 Pa s when introducing 10 wt% elongated seeds, but overall, exhibiting good fluidity. The green body strength dropped remarkably after adding small equiaxial seeds, but improved within 6 wt% elongated seeds and afterwards declined to the minimum of 25.99 MPa, which was quite ideal for processing. The elongated seeds impeded liquid phase flow and particle rearrangement more pronouncedly than equiaxial ones during sintering, thereby causing the highest porosity of 41.77% and the largest pore size of 1.04 μm when employing 10 wt% elongated seeds. The elongated seeds were more helpful in developing the strong self–reinforced bimodal microstructure due to the original hexagonal prism morphology and achieved the highest bending strength of 378.50 MPa and fracture toughness of 8.54 MPa m1/2 at 2 wt% and 6 wt% elongated seeds, respectively. The dielectric constant and loss tangent depended strongly on the porosity and separately varied in 4.70–4.82 and 0.041–0.047 ranges in 8.2–12.4 GHz when using 6 wt% elongated seeds, demonstrating superior microwave–penetrating properties.

1. Introduction Porous Si3N4 ceramics formed from β–Si3N4 grains typically have low density, low thermal expansion coefficient, high bending strength, high fracture toughness, acceptable dielectric properties and excellent resistance to oxidation, creep and thermal shock [1–3]. These attractive characteristics make porous Si3N4 ceramics one of the most promising structural–functional ceramic materials which are widely applied to engineering fields, including thermal insulators, separation membranes, catalyst carriers, high temperature gas/liquid filters, radomes and antenna windows [2–6]. Various molding methods have thus far been developed to produce porous Si3N4 ceramics, like direct foaming method [7,8], freeze–casting [9,10], freeze drying [11], tape casting [12], extrusion [13], adding pore former agent [14–16], oxidation bonding [17], combustion synthesis [18], carbothermal reaction [19] and gelcasting [20,21]. Among these methods, gelcasting has received extensive attention and has been applied to many structural [22–24] and functional ceramics [25–27], since gelcasting is appropriate for preparing green bodies with flexible shape and sufficient strength for non–destructive preprocessing

∗

[28]. Nonetheless, the monomer acrylamide (AM) is widely used in gelcasting and results in severe neurotoxicity, although AM imparts green bodies with excellent mechanical properties necessary for satisfying machining requirements. Recently, gelcasting with the low–toxic N, N–dimethyl acrylamide (DMAA) gelling system has been developed and widely used for the preparation of various ceramics [29–34], because DMAA endows green bodies with properties similar to those derived from AM. Nevertheless, the in–depth study and application of DMAA for fabricating Si3N4 ceramics have been neglected. To expand the application of Si3N4 ceramics in advanced engineering, further improvement in fracture toughness is extremely more urgent [35,36]. The fracture toughness of pressureless sintered Si3N4 ceramics typically varies within 4–6 MPa m1/2 range, which is superior to other structural ceramics (3–4 MPa m1/2) [35]. Lange et al. [37] investigated the relationship between the microstructure and mechanical properties and found that fracture toughness was highly correlated with elongated β–Si3N4 grain characteristics. Many studies [35–38] have demonstrated that high fracture toughness typically occurs in the microstructure containing some large elongated β–Si3N4 grains, because large elongated β–Si3N4 grains are preferable for crack deflection and bridging

Corresponding author. College of Materials Science and Engineering, Nanjing Tech University, Nanjing, 210009, China. E-mail address: [email protected] (J. Yang).

https://doi.org/10.1016/j.ceramint.2019.10.229 Received 1 August 2019; Received in revised form 17 October 2019; Accepted 24 October 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Shuang Yin, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.229

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Fig. 1. SEM morphologies of Si3N4 powders and β–Si3N4 seeds (a Si3N4 powders; b equiaxial seeds with d50 = 5.89 μm; c elongated seeds).

matrix [35,53–57]. However, the clusters of large elongated β–Si3N4 grains in ceramic matrix tend to generate more defects, which deteriorates the bending strength [52,58]. For examples, Chen et al. [59] illustrated that the fracture toughness of hot–press sintered Si3N4 ceramics increased from 4.0 to 6.7 MPa m1/2 when 8 wt% β–Si3N4 seeds were used, which was accompanied by a drastic drop in the bending strength from 1005 to 670 MPa. Lu et al. [44] reported that when applying 5 wt% β–Si3N4 seeds, the fracture toughness of gas–pressure sintered Si3N4 ceramics increased from approximately 7.2 to 7.6 MPa m1/2, but the bending strength decreased from nearly 920 to 850 MPa. To deeply understand the microstructural evolution of Si3N4 ceramics in the presence of β–Si3N4 seeds, Lee [36], Vučković [46], Becher [60] and Hirao et al. [61] have conducted extensive research and found that β–Si3N4 seed size and content were vitally important to the final microstructure. Additionally, the β–Si3N4 seeds possessing diameter and aspect ratio greater than those of precursor α–Si3N4 powders are generally more conducive to the development of such a self–reinforced bimodal microstructure. It is now clear that preferential grain growth in the [001] crystallographic direction and well–faceted [210] planes endows β–Si3N4 grains with typical hexagonal prism shape [35]. From the perspective of β–Si3N4 nucleation and β–Si3N4 grain growth, the introduced β–Si3N4 seeds should promote the α→ β–Si3N4 transition while ensuring the sufficient growth of β–Si3N4 grains without space limitations. The studies on self–reinforced Si3N4 ceramics with β–Si3N4 seeds have mainly focused on β–Si3N4 seed size and content in dense Si3N4 ceramics, which have been prepared via high–cost sintering methods, such as gas pressure sintering (GPS) [35,44,62] and hot pressing sintering (HPS) [47,48]. However, β–Si3N4 morphology studies have been extremely limited, especially for porous Si3N4 ceramics. To further expand the range of applications, the gelcasting with low–toxic DMAA and the economical pressureless sintering (PLS) were employed in this work. More crucially, the β–Si3N4 seed morphology was deeply discussed with respect to microstructure, mechanical and dielectric properties in addition to β–Si3N4 seed size and content.

behaviors. Based on this viewpoint, a series of toughening methods, including introducing SiC whiskers and β–Si3N4 seeds (or whiskers) to the starting materials, were proposed to regulate β–Si3N4 grain size, content, morphology and distribution in ceramic matrix [35,39–45]. Related studies [36] showed that β–Si3N4 seeds are more favorable than SiC whiskers for enhancing fracture toughness, since β–Si3N4 seeds can promote the reconstructive phase transition of α→β–Si3N4 by heterogeneous nucleation, which reduces the barrier potential required for system to generate new crystal nucleus and thereby facilitates the α→ β–Si3N4 transition and β–Si3N4 grain growth. Hirao et al. [35] introduced β–Si3N4 seeds with diameter of 1 μm and length of 4 μm into the precursor α–Si3N4 powders and found that the fracture toughness of samples with bimodal microstructure obtained from gas–pressure sintering was improved from 6.3 to 8.4–8.7 MPa m1/2, and simultaneously, the flexural strength remained at high levels of 1 GPa. Vučković et al. [46] demonstrated that the β–Si3N4 seeds with diameter of 2.22 μm and length of 5.43 μm helped to achieve the bimodal microstructure consisting of small matrix grains and large elongated β–Si3N4 grains with approximate diameter of 2 mm and length of greater than 10 mm, wherein the fracture toughness reached 8.4 MPa m1/2. Bučevac et al. [47] indicated that the precursor α–Si3N4 powder size was extremely important to microstructural evolution. When 5 wt% β–Si3N4 seeds were introduced into the ultra–fine α–Si3N4 powders (d50 = 0.3 μm), the resulting bimodal microstructure from hot–press sintering contained many large diameter and high aspect ratio β–Si3N4 grains, which conferred upon the Si3N4 ceramics a high fracture toughness of 9.0 MPa m1/2. Wang et al. [48] obtained hot–press sintered Si3N4 ceramics with a fracture toughness of up to 9.0 MPa m1/2 by adding 5 wt% β–Si3N4 seeds. Additionally, it has been revealed that high fracture toughness was attributed to bimodal microstructure, since large elongated β–Si3N4 grains therein promoted crack deflection and bridging mechanisms. The abovementioned studies confirmed that introducing β–Si3N4 seeds to starting materials is an effective method for preparing high toughness Si3N4 ceramics through developing ‘in situ composites’ or ‘self–reinforced’ bimodal microstructure [46–48]. Commonly, such a bimodal microstructure–reinforced fracture toughness fluctuates within 8–11 MPa m1/2 range [49–52] and the reinforcing effect mainly depends on the large elongated β–Si3N4 grains in ceramic 2

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2. Experimental section 2.1. Experimental procedure Commercially available Si3N4 powders (α phase content > 93 wt%; purity > 99.9%; d50 = 1.0 μm) and β–Si3N4 seeds (purity > 99.9%; both from Yi–Nuo Material Co., Ltd., China) were employed as the starting powders. Fig. 1(a–c) demonstrates the SEM micrographs of Si3N4 powders and β–Si3N4 seeds with different morphologies. Y2O3 and Al2O3 (purity ≥ 99.9%; both obtained from Aladdin Industrial Co., Ltd., China) were employed as sintering additives. N, N–dimethylacrylamide (DMAA, CH2]CHCON(CH3)2; purity ≥ 99.4%; Kowa Co., Ltd., Japan), N, N–methylenebis acrylamide (MBAM, CH2]CHCONHCH2NHCOCH]CH2; purity ≥ 99%; Aladdin Industrial Co., Ltd., China), ammonium persulfate (purity ≥ 98%; Aladdin Industrial Co., Ltd., China) and TH–908 (purity ≥ 40%; Taihe Water Treatment Co., Ltd., China) were separately used as monomer, crosslinker, initiator and dispersant. The premix solution composed of 79.21 wt% deionized water, 19.41 wt% DMAA, 1.39 wt% MBAM was first ball milled with Si3N4 powders, 1.5 wt% TH–908, 2 wt% Y2O3 and 1 wt% Al2O3 powders (both based on Si3N4) for 4 h to prepare 40 vol% slurries. Next, the β–Si3N4 seeds with different sizes (d50 = 2.74 μm, 5.89 μm and 9.84 μm, ɸ = 1–3 μm and L = 5–20 μm), morphologies (equiaxed and elongated shapes) and contents (0 wt%, 2 wt%, 6 wt% and 10 wt%) were added to the slurries and ball milled again for 2 h to mix the slurries evenly. After defoaming, the initiator (ammonium persulfate aqueous solution with a 0.1 g/ml concentration) was introduced. Afterwards, the slurries were cast into mold and later consolidated into green bodies at 70 °C for 90 min. After drying and polymer burning–out, the green bodies were sintered at 1750 °C for 2 h in a powder bed composed of 50 vol% Si3N4 and 50 vol% BN powders, which was protected by flowing N2 introduced with a pressure of 0.1 MPa.

Fig. 2. Viscosity of slurries with different β–Si3N4 seed contents.

3.2. Microstructure and properties of green bodies

The porosity was determined by the Archimedes method. The bending strength and fracture toughness were examined via the three–point bending and single–edge–notched beam (SENB) methods. The crystalline phase was identified by XRD (Rigaku, Cu Kα, Japan). The fracture surface morphologies were observed by SEM (JSM–5900, Japan). The pore size distribution was determined using a mercury intrusion porosimeter (Quantachrome, GT–60, U.S.A). The dielectric constant and loss tangent (22.86 mm × 10.16 mm × 2.5 mm) in X–band (8.2–12.4 GHz) were examined by a wave–guide method using a vector network analyzer (PNA–N5244A, Agilent, U.S.A).

Considering the crucial role of β–Si3N4 seed size, content, morphology and distribution in regulating the microstructure and properties of final ceramics, the fracture surfaces of green bodies with different β–Si3N4 seeds were characterized, as shown in Fig. 3(a–c). It is apparent that the Si3N4 particles and the introduced equiaxial and elongated seeds were homogeneously distributed without obvious agglomeration, which helps to eliminate the elongated β–Si3N4 grain clusters in final ceramic matrix. To facilitate mechanical processing, a green body strength greater than 10 MPa is usually required. Fig. 4 demonstrates the bending strength of green bodies at different β–Si3N4 seed contents. The bending strength decreased after adding equiaxial seeds, and the smaller the seed size, the lower the strength. Fig. 2 indicates that the small equiaxial seeds caused the slurry viscosity to increase rapidly and consequently increased the difficulty of defoaming. These residual bubbles in slurries eventually formed defects in green bodies, i.e., pores, which sacrificed the mechanical properties. Nevertheless, the bending strength was enhanced for elongated seed contents of up to 6 wt% and reached the maximum of 32.06 MPa, even though the elongated seeds were more prone than equiaxial ones to increase the viscosity of slurries. It was believed herein that the large elongated seeds pullout behavior strengthened the mechanical properties, because these large elongated seeds can dissipate more energy. By further increasing the seed content, the bending strength dropped to the minimum of 25.99 MPa due to the large numbers of pores in green bodies, which were derived from the residual bubbles in slurries. However, in general, all the prepared green bodies had strength greater than 25 MPa, demonstrating their superior mechanical properties for subsequent processing.

3. Results and discussion

3.3. Phase composition and microstructure of sintered bodies

3.1. Slurry properties

Fig. 5 demonstrates the phase composition of samples with different β–Si3N4 seeds. Clearly, only β–Si3N4 was detected in all the samples, indicating that the α→β–Si3N4 transition was vitally sufficient, which helps to reinforce the mechanical properties [66–68]. Fig. 6(a and b) shows the low–magnification and high–magnification fracture surface microstructure of the sample without β–Si3N4 seeds. Apparently, as shown in Fig. 6(a), a relatively homogeneous microstructure appeared. Fig. 6(b) suggests that this microstructure consisted primarily of β–Si3N4 grains with small aspect ratio. Fig. 7(a–d) illustrates the low–magnification fracture surface microstructure of samples with 6 wt % equiaxial and 6 wt% elongated β–Si3N4 seeds. All the samples possessed a distinct and homogeneous crosslinked interlocking microstructure without visible agglomeration, which was considered to strengthen the mechanical properties [69,70]. Notably, the samples with seeds exhibited larger grain width, length and aspect ratio and

2.2. Characterization and test

The viscosity of slurries used for gelcasting is normally required to be less than 1.0 Pa s (shear rate 100 s−1) to promote slurry casting [63–65]. Fig. 2 demonstrates the viscosity of slurries as a function of β–Si3N4 seed content. Both equiaxial and elongated seed addition increased the viscosity of slurries. For equiaxial seeds, the small seeds were more inclined than the large ones to increase viscosity of slurries due to the large specific surface area. Furthermore, the elongated seeds increased the viscosity of slurries more dramatically than the equiaxial ones due to the poor fluidity of elongated seeds with irregular geometries. Therefore, the highest viscosity value of 0.87 Pa s occurred when the elongated seed content reached 10 wt%, but overall, this slurry was well suited for slurry casting.

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Fig. 3. Fracture surface SEM morphologies of the green bodies with different β–Si3N4 seeds (a without seeds; b 6 wt% equiaxial seeds with d50 = 5.89 μm; c 6 wt% elongated seeds).

Fig. 4. Bending strength of green bodies with different β–Si3N4 seeds. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 5. XRD patterns of sintered bodies with different β–Si3N4 seeds (a without seeds; b–d 6 wt% equiaxial seeds, d50 = 2.74 μm, 5.89 μm, 9.84 μm, respectively; e 6 wt% elongated seeds).

lower bulk density than those without ones, and this was especially true for samples with elongated seeds. It can thus be concluded that the elongated seeds were highly effective in developing such coarsening interlocking microstructure and impeding densification, which contributed to improving the mechanical and electromagnetic wave–penetrating performance of the resulting ceramics. Fig. 8(a) and (b) reveals the microstructural evolution as a function of β–Si3N4 seed size, content and morphology. Clearly, the bimodal microstructure appeared in each sample after introducing β–Si3N4 seeds. Concerning the samples in Fig. 8(a), β–Si3N4 grain width, length and aspect ratio increased for β–Si3N4 seed content up to 6 wt% but significantly decreased when the seed content reached 10 wt%, this phenomenon was correlated with grain growth mechanism. It is generally recognized that the solubility and dissolution rate of α–Si3N4 are higher than those of β–Si3N4 in the glass phase containing Si–N–Y–Al–O [48,58,71]. At approximately

1400 °C, α–Si3N4 is dissolved in the glass phase generated by the reaction of Y2O3, Al2O3, and SiO2 layers on Si3N4 surface and is then precipitated as β–Si3N4 nucleus from the local saturation area. The α–Si3N4 dissolved in the glass phase is later gradually precipitated on the β–Si3N4 nucleus by mass transfer of diffusion, thereby promoting β–Si3N4 growth. After introducing β–Si3N4 seeds, the Si–N group in the glass phase can directly diffuse onto the existing β–Si3N4 nucleus provided by the seeds, thus accelerating the α→β–Si3N4 transition and subsequent β–Si3N4 growth process. Consequently, some large elongated β–Si3N4 grains appeared in ceramic matrix within 6 wt% seed content. However, when 10 wt% seeds were introduced, a great number of nucleation sites were available for α→β–Si3N4 transition and β–Si3N4 grain growth. The resulting large amounts of β–Si3N4 grains grew 4

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Fig. 6. Low–magnification and high–magnification fracture surface SEM morphologies of sintered bodies without β–Si3N4 seeds.

3.4. Sintering, mechanical and dielectric properties of sintered bodies

competitively and accordingly reduced the free growth space, ultimately lowering the width, length and aspect ratio. With regards to the c2, c6, d2, and d6 samples shown in Fig. 8(b), the β–Si3N4 grains had larger width, length and aspect ratio, indicating that the number of nucleation sites provided by seeds coordinated well with the phase transition and grain growth. Similarly, with respect to the c10 and d10 samples, the β–Si3N4 grains were slightly refined, suggesting that the added seeds were still somewhat excessive. Additionally, some abnormally grown β–Si3N4 grains appeared in sample d10, which were mainly derived from some oversized elongated seeds in the starting powders. Besides, at the same seed content, the large equiaxial seeds were more inclined than small ones to realize the coarsening bimodal microstructure. More importantly, the elongated seeds performed the best at developing this bimodal microstructure. In general, the bimodal microstructure containing more large elongated β–Si3N4 grains is highly desirable, since these large elongated β–Si3N4 grains can strengthen fracture toughness through facilitating crack deflection and bridging [53–57,69,70]. These phenomena demonstrate that β–Si3N4 seed size, content and morphology are extremely critical for the microstructural evolution of final Si3N4 ceramics.

Fig. 9 illustrates the change in porosity of samples with different β–Si3N4 seeds. The porosity of the sample without seeds was 38.36%. It is quite apparent that the addition of seeds, including equiaxial and elongates ones, remarkably increased the porosity, and the higher the seed content, the higher the porosity. Notably, regarding these samples with large equiaxial or elongated seeds, the porosity maintained at high levels and reached the maximum of 41.77% when elongated seed content was 10 wt%, which accorded with the studies reported by Vučković et al. [46] and Peillon et al. [62]. The reasons for the tendency of porosity to vary resulted mainly from two aspects. (1) As demonstrated in Fig. 2, the addition of seeds, especially elongated ones, significantly increased the viscosity of slurries, thus increasing the difficulty of defoaming. The residual bubbles in slurries ultimately transformed into pores in the sintered bodies, which subsequently enhanced the porosity. (2) Regarding the same phase, the small grains are more soluble and have a higher dissolution rate than the large ones. Additionally, the β–Si3N4 seeds are less soluble and have a lower dissolution rate than the starting α–Si3N4 powders having similar dimensions [48]. Therefore, the large equiaxial seeds greatly prevented liquid phase flow and particle rearrangement during liquid phase sintering,

Fig. 7. Fracture surface SEM morphologies of sintered bodies with 6 wt% β–Si3N4 seeds (a–c equiaxial seeds, d50 = 2.74 μm, 5.89 μm, 9.84 μm, respectively; d elongated β–Si3N4 seeds). 5

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Fig. 8. Fracture surface SEM morphologies of sintered bodies with different β–Si3N4 seeds (a–c equiaxial seeds, d50 = 2.74 μm, 5.89 μm, 9.84 μm, respectively; d elongated seeds; 2, 6 and 10 represent seed content).

Fig. 10. Pore size distribution of sintered bodies with different β–Si3N4 seeds (seed content 10 wt%).

Fig. 9. Porosity of sintered bodies with different β–Si3N4 seed contents.

consequently reducing densification. In particular, the elongated seeds with irregular geometries and small specific surface area had stronger steric hindrances than equiaxial ones, thus impeding densification to a greater extent. Fig. 10 shows the pore structure of samples with 10 wt% β–Si3N4 seeds. For the samples without seeds, the average pore size was 0.07 μm. After adding seeds, the pore size varied in 0.43–1.04 μm range. Apparently, the larger the seed size, the larger the pore size, which agreed well with the microstructure shown in Fig. 7. Based on the analysis in Fig. 9, it can be concluded that the increase in pore size was primarily due to the residual bubbles in slurries and the resistance of seeds to densification. Fig. 11(a and b) illustrates the mechanical properties of samples with different β–Si3N4 seeds. For the samples without seeds, the bending strength and fracture toughness were 300.50 MPa and 4.28 MPa m1/2, respectively. When 2 wt% equiaxial or elongated seeds

were introduced, all the samples demonstrated the highest bending strength varying in 315.4–378.50 MPa range, suggesting that the resulting coarsened bimodal microstructure induced by β–Si3N4 seeds enhanced the bending strength, albeit with an increasing porosity as proved in Fig. 9. However, for the samples with seed contents greater than 2 wt%, the bending strength dropped, demonstrating that the further increasing porosity degraded the bending strength. The relationship between the bending strength and the porosity can be characterized using the following equation reported by Ryshkewite [72,73]:

δ = δ0 exp(−nP )

(1)

where δ0 and δ denote the bending strength of the samples with porosity of 0 and P, respectively, and n refers to the structural parameter with values typically in 4–7 range. Hence, the bending strength tends to decrease in an exponential manner as the porosity increases. The pores 6

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where KC IC and EC refer to fracture toughness and elasticity modulus, respectively. Jm and Jcb refer to the energy consumed by crack deflection and bridging. As a result, the coarsened elongated β–Si3N4 grains within 6 wt% seed content promoted crack deflection and bridging, which dissipated more energy and accordingly enhanced the fracture toughness. With further increase in seed content, the fracture toughness of all the samples reduced, which was primarily due to the microstructural refinement and the increasing porosity, as demonstrated in Figs. 8 and 9. Regarding the samples with equiaxial seeds, those with seeds having d50 = 5.89 μm had the highest fracture toughness, which was similar to fracture toughness of Si3N4 ceramics studied by Liu et al. [76]. The SEM morphologies in Fig. 7 demonstrate that introducing the same seed content resulted in the number density of large and high aspect ratio β–Si3N4 grains increasing as seed size increased, which improved the fracture toughness. On the other hand, the large seeds increased porosity and pore size in ceramic matrix, which dominated fracture toughness and deteriorated it significantly as seed size exceeded 5.89 μm. These phenomena demonstrate that the resultant microstructure and porosity had the opposite effects on fracture toughness. When the seed content was within 6 wt% or the seed size was less than 5.89 μm, the formed bimodal microstructure had a major impact on the fracture toughness, but otherwise, the porosity and pore size primarily affected the fracture toughness. Additionally, these samples with elongated seeds had the highest fracture toughness despite having the highest porosity and pore size, as illustrated in Figs. 9 and 10, primarily due to the highly crosslinked and homogeneous interlocking microstructure formed by larger and higher aspect ratio β–Si3N4 grains, which strengthened the crack deflection and bridging toughening behaviors. Fig. 12 demonstrates the dielectric properties of sintered bodies with various β–Si3N4 seeds. Apparently, all the samples exhibited a relatively low and stable dielectric constant (ε) and loss tangent (tanδ) in 8.2–12.4 GHz range. For the samples without seeds, the ε and tanδ varied in 6.36–6.58 and 0.098–0.127 ranges, respectively. The introduction of seeds, including equiaxial and elongated ones, greatly reduced ε and tanδ. Notably, the sample with elongated seeds had the lowest ε and tanδ, in which the ε and tanδ fluctuated in the range of 4.70–4.82 and 0.041–0.047, respectively, demonstrating an excellent electromagnetic wave–penetrating performance. Porous Si3N4 ceramics can be considered as gas–solid type composite materials with low ε phase pores (ε = 1, tanδ = 0) distributed evenly in ceramic matrix, which consists of high ε phase β–Si3N4 grains (ε = 7.9, tanδ = 0.005). Based on the mixing rule proposed by Lichtenecker, the ε of the composite materials is highly dependent on the ε and content of the various components, which can be approximately quantified by the following equation [77,78]:

Fig. 11. Bending strength and fracture toughness of sintered bodies with different β–Si3N4 seeds (a bending strength; b fracture toughness).

in ceramic matrix can reduce the solid phase cross section area, thereby increasing the actual stress and concentrating stress, which deteriorates the mechanical properties. Furthermore, these samples with large equiaxial seeds had higher strength values than those containing small ones, even though the porosity and pore size were further increased, as shown in Figs. 9 and 10, which possibly resulted from the highly crosslinked and extremely homogeneous interlocking microstructure formed by the large and high aspect ratio β–S3N4 grains. But for samples with elongated seeds, the bending strength at all seed contents displayed higher values than that of samples with equiaxial ones, demonstrating that elongated seeds were more conducive to developing this unique interlocking microstructure beneficial for high strength. Clearly, a similar phenomenon was observed for fracture toughness. Nevertheless, the optimum seed content for achieving the peak values ranging from 6.62 to 8.54 MPa m1/2 was 6 wt%. It is generally recognized that the elongated β–Si3N4 grains favor the enhanced fracture toughness by crack deflection and bridging [35,53–57], and the toughening effect is highly correlated to elongated grain size, number, morphology and distribution in ceramic matrix [36,46,60,61]. The XRD patterns in Fig. 5 confirmed the presence of only β–Si3N4 in all samples. As demonstrated in Fig. 8, the β–Si3N4 grains were coarsened within 6 wt% seed content and afterwards refined. The coarsened elongated β–Si3N4 grains were universally believed to enhance fracture toughness, since more conditions were created for activating crack deflection and bridging [38,48,58,74]. Based on these toughening mechanisms of β–Si3N4 grains, the fracture toughness can be described according to the following equation proposed by Becher [75]: C KIC = [E C (J m + ΔJ cb)]1/2

n

log ε =

∑ φi log εi i

(3)

where ϕi and εi separately represent the volume fraction and dielectric constant of i phase. Obviously, the resulting pores contributed to lowering the ε and tanδ. As demonstrated in Fig. 9, the porosity increased with increasing seed size and realized the highest values with regards to the samples containing elongated seeds. Combined with the dielectric property results, it was believed that the reduction in ε and tanδ was mainly attributed to the increasing porosity. 4. Conclusion Porous Si3N4 ceramics possessing superior mechanical and dielectric properties were prepared through gelcasting technique with low–toxic DMAA and simultaneous addition of β–Si3N4 seeds. The β–Si3N4 seed size, content and morphology were vitally important to the microstructure and properties of slurries, green and sintered bodies. After incorporating elongated seeds, the slurry viscosity increased

(2) 7

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Acknowledgments This work was sponsored by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), Qing Lan Project, the Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX19_0835), the Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT), IRT1146 and IRT15R35, and the Top-notch Academic Programs Project of Jiangsu Higher Education Institutions (TAPP, PPZY2015B128). References [1] S.S. Xiao, H. Mei, D.Y. Han, W.Y. Yuan, L.F. Chen, Porous (SiCw–Si3N4w)/ (Si3N4–SiC) composite with enhanced mechanical performance fabricated by 3D printing, Ceram. Int. 44 (2018) 14122–14127. [2] H. Mei, G.K. Zhao, G.X. Liu, Z. Wang, L.F. Chen, Effect of pore size distribution on the mechanical performance of carbon foams reinforced by in situ grown Si3N4 whiskers, J. Eur. Ceram. Soc. 35 (2015) 4431–4435. [3] X.J. Yang, B. Li, C.R. Zhang, S.Q. Wang, K. Liu, C.R. 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Fig. 12. Dielectric constant and loss tangent of sintered bodies with 6 wt% β–Si3N4 seeds (a dielectric constant; b dielectric loss tangent).

dramatically and reached the peak of 0.87 Pa s with 10 wt% seed addition, but generally exhibiting good casting fluidity. The green body strength deteriorated after introducing equiaxial seeds, however, increased within 6 wt% elongated seeds and afterwards decreased to the minimum of 25.99 MPa at 10 wt% seeds, but still had superior processability. The elongated seeds impeded liquid phase flow and particle rearrangement more pronounced than the equiaxial ones, thereby causing the maximum porosity (41.77%) and maximum pore size (1.04 μm) at 10 wt% elongated seeds. Moreover, the elongated seeds were more useful in developing the strong self–reinforced bimodal microstructure due to the original hexagonal prism morphology, and thus, the highest bending strength (378.50 MPa) and fracture toughness (8.54 MPa m1/2) were achieved at 2 wt% and 6 wt% elongated seeds, respectively. The dielectric constant and loss tangent in 8.2–12.4 GHz depended mainly on the porosity, which was controlled by the seeds, and separately varied in 4.70–4.82 and 0.041–0.047 ranges at 6 wt% elongated seeds. This study demonstrates that it is possible to prepare porous Si3N4 ceramics with high strength, high toughness and low dielectric constant by gelcasting with DMAA and simultaneously incorporating β–Si3N4 seeds.

Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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