Anisotropic, biomorphic cellular Si3N4 ceramics with directional well-aligned nanowhisker arrays based on wood-mimetic architectures

Inspired by the transport behavior of water and ions through the aligned channels in trees, we demonstrate a facile, scalable approach for constructing biomorphic cellular Si3N4 ceramic frameworks with well-aligned nanowhisker arrays on the surface of directionally aligned microchannel alignments. Through a facile Y(NO3)3 solution infiltration into wood-derived carbon preforms and subsequent heat treatment, we can faultlessly duplicate the anisotropic wood architectures into free-standing bulk porous Si3N4 ceramics. Firstly, α-Si3N4 microchannels were synthesized on the surface of CB-templates via carbothermal reduction nitridation (CRN). And then, homogeneous distributed Y−Si−O−N liquid phase on the walls of microchannel facilitated the anisotropic β-Si3N4 grain growth to form nanowhisker arrays. The dense aligned microchannels with low-tortuosity enable excellent load carrying capacity and thermal conduction through the entire materials. As a result, the porous Si3N4 ceramics exhibited an outstanding thermal conductivity (TC, kR ≈ 6.26 W·m−1·K−1), a superior flexural strength (σL ≈ 29.4 MPa), and a relative high anisotropic ratio of TC (kR/kL = 4.1). The orientation dependence of the microstructure-property relations may offer a promising perspective for the fabrication of multifunctional ceramics.


Introduction 
Porous Si 3 N 4 ceramics with a cellular structure, especially with unidirectionally aligned channels are promising preparation of the aforementioned green body. Unfortunately, a multitude of investigations on honeycomb Si 3 N 4 ceramics exhibited less satisfactory permeability and mechanical properties, due to the heterogeneous architecture, loosely contacted grains on the cell wall of aligned channels, and relative low cell density. On the other hand, freeze-drying method through the unidirectional growth of ice has a high requirement for temperature and temperature gradient control [8][9][10]. The fabrication processes by selforganization method using an alginate template are complicate, expensive, and time-consuming. Ceramics that are ultralightweight, strong, and tough are in demand for a range of applications, requiring architectures and components carefully designed from the micrometer down to the nanometer scale. Biological materials can inspire new alleys to design sophisticated materials with intelligent and hierarchical microstructure [11][12][13][14]. Biotemplating technique, where natural grown structures are used as bulk templates for high-temperature conversion into ceramics, has attracted plenty of attention because of the unique biomorphic microstructures and high purity of the products.
Anisotropic structures are ubiquitous in biological materials. The structure of a tree features a large number of microchannels (i.e., lumina of tracheids, fibres, and vessels) along the longitudinal direction for mass (water, ions, and other components) transport. It has inspired the fabrication of hierarchically structured engineering ceramics [15][16][17][18][19][20]. Many kinds of woodderived biomorphic ceramics such as TiC [18], Fe 2 O 3 [19], Al 2 O 3 , TiO 2 , and ZrO 2 [20] have been prepared by physical vapor infiltration-reaction (PVI-R) method, inorganic precursor-impregnation-calcination method, and sacrificial template method, respectively. However, high-performance wood-derived ceramics can only be achieved when the wood tissue is totally converted into the ceramic phase [21]. Vapor-phase infiltration by utilizing a solid-vapor reaction between the bio-carbon template and silicon-containing gaseous is an effective strategy for fabricating biomorphic wood-derived SiC ceramics with nanocrystalline structures [15,[22][23][24].
Owing to the unique interlocking microstructure consisting of elongated β-Si 3 N 4 grains, porous Si 3 N 4 exhibits high strength and toughness. Silicon nitride powders have been synthesized by the carbothermal reduction nitridation (CRN) of rice husk-derived carbon and SiO 2 powder [25]. Luo et al. [26] have been fabricated wood-derived Si 3 N 4 ceramics by the CRN reaction of wood-derived carbon infiltrated with Y 2 O 3 and SiO 2 sol. Despite the imposing results reported for biomass-derived Si 3 N 4 ceramics synthesized by the existing routes, some drawbacks still remain unaddressed. Unfortunately, degraded microstructure of the cell wall with Si-containing impurities or carbon always exists in the products due to the uncontrollable reaction [25,26]. Subsequently, the mechanical properties and reliability of the bulk ceramics are considerably deteriorated due to the degraded architecture compared with the original biomass microstructure. Besides degraded microstructure, complicate fabrication process is required for repetitious SiO 2 sol infiltration into woodderived carbon preforms.
Herein, we demonstrate a facile, scalable approach for fabricating biomorphic cellular Si 3 N 4 ceramics with directional microchannel alignments to reduce the tortuosity, for improving the thermal conductivity (TC) and the mechanical properties. Through the facile Y(NO 3 ) 3 solution infiltration and subsequent high temperature sintering, we can faultlessly duplicate the anisotropic wood tissue microstructure into free-standing bulk porous Si 3 N 4 ceramics with devisable shape and size. The obtained biomorphic Si 3 N 4 ceramics possessed peculiar hierarchical microstructure and anisotropic properties. In addition, the grain growth mechanism of directional β-Si 3 N 4 nanowhisker arrays on the vertically aligned microchannels is investigated. The successful fabrication route, with its inner synergistic effect, offers an excellent approach to design and process advanced, strong, and integrated wood-like ceramics, which may have great potential applications in hot gas or molten metal filtration in the near future.

1 Preparation of biomorphic cellular Si 3 N 4 ceramics
Nature spruce woods were purchased from a furniture factory in Guizhou, China. SiO powders (particle size < 0.1 μm, Shanghai Aladdin Biochemical Technology Co., Ltd., China) were used as silicon source. The rectangular specimens in both the longitudinal (L) and radial (R) orientations with dimensions of 40 mm × 20 mm × 20 mm were cut from spruce woods which were fully naturally dried. Then, the specimens were dried at 110 ℃ for 24 h. The dried specimens were pyrolyzed at 1000 ℃ for 4 h under N 2 atmosphere with a low heating rate of 3 ℃·min −1 , resulting in a porous bio-carbon template (C B -template). The C Btemplate was machined into blocks with dimensions of 20 mm × 4 mm × 3 mm and 10 mm × 10 mm × 3 mm in R-and L-directions, respectively. Subsequently, the C B -templates were immersed in a 0.2 mol·L −1 Y(NO 3 ) 3 solution for 4 h under vacuum condition. After drying and heat treatment at 1000 ℃ for 2 h, the C B -templates attached with Y 2 O 3 nanoparticles were obtained. Finally, the C B -templates were put on a graphite plate with holes located in the gas outlet direction in the furnace (High Multi-5000, Fujidempa Co., Ltd., Osaka, Japan) and sintered at 1750 ℃ for 4 h under an N 2 pressure of 0.225 MPa. Silicon monoxide gas generated via sublimation of SiO powders, and reacted with the C B -templates with and without Y 2 O 3 to form bulk Si 3 N 4 ceramics.

2 Characterization
The porosity and density were measured by the Archimedes' method. Phase identification was performed by the X-ray diffractometer (X'Pert PRO, the Netherlands). Raman spectra were detected by the Thermo Scientific DXR Smart Raman spectrometer (Thermo Fisher DXR2xi, Thermo Fisher Scientific, USA). The morphology and quantitative elemental analyses of the bio-Si 3 N 4 ceramics and the C B -templates were performed by the scanning electron microscope (SEM, S-4800, Japan). The flexural strength was measured by a three-point bending method with a 16 mm span at an across-head speed of 0.5 mm·min −1 at room temperature. Each final value was averaged over five measurements. The thermal diffusivity (α) and specific heat capacity (C p ) of specimens were measured by a laser-flash diffusivity instrument (Netzsch LFA467, Germany). The TC (k) of the Si 3 N 4 samples was calculated according to the equation: where ρ is the density of the specimens. Each final value was averaged over three measurements.  pronounced linear shrinkage (23%-33%) occurs during the high temperature pyrolyzation processing (1000 ℃), the macroscopic characteristics of spruce wood, such as the annual growth rings, are clearly visible in the biological carbon preforms (C B -templates). Comparing with the C B -templates, the bio-Si 3 N 4 ceramics still retained the macroscopic bulk shape after sintering at 1750 ℃ for 4 h. No cracks and almost no shape changes are observed on the porous ceramics. The only observed change was a body-color transformation from black (the natural carbon color) to gray (the Si 3 N 4 color), confirming that the CRN reaction occurred. Figure 1(c) shows the XRD patterns of C B -templates and bio-Si 3 N 4 ceramics. Amorphous carbon phase in C B -template is confirmed by the two broad diffraction bands at approximately 2θ = 23.9° and 43.98°. For the C B -template, after the SiO and N 2 gas infiltration at 1750 ℃ for 4 h, primarily α-Si 3 N 4 (No. 41-0360) and a trace of β-Si 3 N 4 (No. 33-1160) phases were identified, which is also indicating the CRN reaction occurred according to the following reaction: 3C(s) + 3SiO(g) + 2N 2 (g) → Si 3 N 4 (s) + 3CO(g). The strong and sharp peaks indicate that the products have high crystallinity. Whereas, for the C B -template coating with Y 2 O 3 additives, pure β-Si 3 N 4 phase is achieved at the same sintering condition, indicating a phase transformation from α-to β-Si 3 N 4 occurred through the dissolutionprecipitation process in the Y-Si-O-N liquid phase [27]. The Y-Si-O-N secondary phase content is not detected, which may be due to the extremely low content. Except for the Si 3 N 4 , no other phase of silicon carbide (SiC), nor graphite, or other impurities were detected, indicating a high purity of the sintered products. It is noteworthy that SiC phase can be synthesized via the carbothermal reduction reaction between the SiO vapor and carbon materials. However, no SiC phase was found in the product in the present study.

Results and discussion
In the SiO-C-N 2 system, these following reactions may take place: where T is the sintering temperature.
Phase transformation from carbon to silicon nitride is most likely occurred in the SiO-C-N 2 system rather than silicon carbide because ΔG of Reaction (1) is lower than that of Reaction (3) at 1750 ℃. Whereas, SiC would be preferentially formed as the sintering temperature is higher than 1900 ℃.
As shown in Fig. 1(d), the peaks at about 1350 and 1580 cm −1 in the pyrolyzed C B -template are characteristic of the D (defect) and G (graphite) bands of graphitic carbon, respectively [28,29]. After the SiO gas and N 2 infiltration, for the C B -template, the peaks of products at approximately 510, 560, 664, 763, 972, and 1032 cm −1 are observed, which are related to the lattice vibration of the α-Si 3 N 4 crystal [30,31]. It is worth emphasizing that unreacted graphite phase is confirmed. Whereas, for the C B -template coating with Y 2 O 3 additives, seven sharp peaks at approximately 446, 615, 727, 862, 925, 936, and 1043 cm −1 are obviously observed, which can be well indexed to the β-Si 3 N 4 [32]. The absence of graphite peaks is attributed to the complete CRN reaction. The weight gain of Si 3 N 4(R) and Si 3 N 4(L) ceramics fabricated by C B -template without Y 2 O 3 additives are 192.4% and 127.3%, indicating unreacted carbon retained in the products are ~33.1% and 55.9%, respectively. Whereas, the CRN reaction ratios of Si 3 N 4(R) and Si 3 N 4(L) ceramics fabricated by the C B -template with Y 2 O 3 additives are 93.7% and 89.3%, respectively, indicating nearly full CRN reaction. The results indicated that the phase transformation from αto β-Si 3 N 4 through the dissolution-precipitation process facilitated the CRN reaction. Figure 2 shows the SEM images of the C B -templates and the anisotropic bio-Si 3 N 4 ceramics sintered at 1750 ℃ for 4 h. It is worth mentioning that although large shrinkage occurs, the unique anisotropic structures of nature wood are well preserved after hightemperature carbonization. As shown in Fig. 2(a), the pyrolyzed C B -template directly inherits the anisotropic structure of the natural wood. The diameters of the large and small lattice-shaped subunits are approximately 15-20 and 30-40 μm, respectively. Cross-sectional view shows a vertically carbon alignment with very thin channel walls of 2-3 μm, which is duplicated the sophisticated microstructure of the natural wood, in which the long channels that extend throughout the material for directional transportation of water and nutrient ( Fig. 2(b)). After the CRN reaction, the asobtained bio-Si 3 N 4 (α phase) ceramics directly inherit the anisotropic structure of the C B -template, including lattice-like channels of 15-40 μm in size and very thin channel walls of 2-3 μm (Figs. 2(c) and 2(d)). The macropore distribution is similar to the C B -template, and equiaxed α-Si 3 N 4 grains on the cell wall are observed. The above results indicate that the vaporsolid CRN reaction allows the intrinsic unique structure of wood, featuring multiple aligned channels (i.e., vessels and lumina), to be well retained [33]. The homogeneous vertically aligned long channels within the C B -template play a vital role in the inheritance of wood tissue honeycomb microstructure during the phase transformation from carbon to Si 3 N 4 . The long microchannels guarantee intimate contacts between the solid carbon and the SiO gaseous together with N 2 atmosphere, which leads to a spatially homogeneous phase transformation toward a perfect honeycomb structure.
For the C B -template coating with Y 2 O 3 nanoparticles, the as-obtained β-Si 3 N 4 ceramics after sintering at 1750 ℃ for 4 h also inherit the anisotropic microstructure of the C B -templates (Figs. 2(e) and 2(f)). Meanwhile, vertically well-aligned β-Si 3 N 4 nanowhisker arrays are observed on the microchannel walls, indicating the phase transformation from α-to β-Si 3 N 4 occured during the liquid sintering. Figure 3(a) displays the enlarged SEM image of β-Si 3 N 4 nanowhisker arrays on the microchannel walls. The single crystalline β-Si 3 N 4 nanofibers are uniformly aligned normal to the microchannel wall surface. The length of β-Si 3 N 4 nanowires is less than 8 μm and their diameter is smaller than 700 nm. Conventional solutionprecipitation route is an effective approach to grow β-Si 3 N 4 whiskers [34]. For the confirmation of microstructural characteristics of bio-Si 3 N 4 ceramics, EDS patterns of elemental mappings of bio-Si 3 N 4 ceramic are  Fig. 3(b). As shown in Fig. 3(b), the quantitative analysis exhibits that the average atomic ratio of Si/N is approximately 4 : 4 (1 : 1), and the atomic percentage of Si is slightly higher. It may be due to the formation of natural SiO 2 oxide layer on the bio-Si 3 N 4 ceramic surface.
As shown in Fig. 4, the entire nucleation-growth of β-Si 3 N 4 nanowhisker arrays consists of four steps: (1) Homogeneous distributed Y 2 O 3 nanoparticles are attached to the wall surface of long carbon microchannel after heat treatment at 1000 ℃.  [35][36][37]. (4) The nanowhisker growth starts with the large scale nucleation on the wall surface and the clusters of nanowhiskers quickly develop at a high temperature of 1750 ℃. As a result, vertically well-aligned β-Si 3 N 4 nanowhisker arrays are successfully synthesized. As reaction proceeds, the cell walls gradually become thinner, and these β-Si 3 N 4 nanowhisker clusters get densely packed, leading to the large scale nano-arrays. Figure 5(a) shows the anisotropic flexural strength of wood-derived bio-Si 3 N 4 ceramics sintered at 1750 ℃ for 4 h. In contrast to conventional and isotropic Si 3 N 4 ceramics derived from powder mixtures, a distinct anisotropy of the mechanical properties in different loading directions can be found. For the β-Si 3 N 4 ceramics via C B -template attaching with Y 2 O 3 additives, despite the porosity is similar, the flexural strength in the L-direction (σ L = 29.4 MPa, porosity = 66.5%) is considerably higher than that in the R-direction (σ R = 13.6 MPa, porosity = 65.6%). For the α-Si 3 N 4 ceramics derived from pure C B -template, the flexural strength in the L-direction (σ L = 5.34 MPa, porosity = 77.9%) is also higher than that in the R-direction (σ R = 1.79 MPa, porosity = 75.8%), as indicated in Fig. 5(a). The anisotropic cellular microstructure leads to a significant anisotropy of σ. The strength of β-Si 3 N 4 ceramics is much higher than that of α-Si 3 N 4 ceramics, which is due to the existence of unreacted carbon on the products. The strength of the Si 3 N 4 channel walls and the skeleton of Si 3 N 4 is largely governed by their defect distribution. When the loading direction is parallel to microchannels, large micropores (40 μm) would result in low strength. However, few defects exist on the microchannel surface, leading to improved strength as loading perpendicular to the microchannels. The strength in the present research is much higher than that of the bio-SiC ceramics prepared by sol infiltration and carbothermal reduction (1±0.25 MPa) [38]. Low test results can be attributed to high porosity  in the cell walls caused by the volatilization of twothirds of the carbon mass of the template [20]. Moreover, the defect size of the microchannel walls was confined, resulting in the strong skeleton of bio-Si 3 N 4 that can withstand larger force without rupture. As a consequence, the bio-Si 3 N 4 ceramics exhibit superior specific flexural strength. Figure 5(b) shows the stress-strain curves of wood-derived Si 3 N 4 ceramics. Under all test conditions, the specimens finally fail in a typically brittle mode. Loading in the longitudinal direction causes stepwise cracking between individual microchannels, which leads to a jagged curve shape. In contrast, a single and clear maximum curve is obtained under radial loading. Figure 6 shows the anisotropic TC of wood-derived bio-Si 3 N 4 ceramics. The bio-Si 3 N 4 ceramics also exhibit a distinct anisotropy of the TC. The TC value of the β-Si 3 N 4(R) ceramics via C B -template attaching with Y 2 O 3 additives (6.26 W·m −1 ·K −1 , porosity = 68.9%) is much higher than that of Si 3 N 4 ( L ) ceramics (1.51 W·m −1 ·K −1 , porosity = 73.8%). For the α-Si 3 N 4

Fig. 6
Anisotropic TC of wood-derived bio-Si 3 N 4 ceramics sintered at 1750 ℃ for 4 h. ceramics derived from pure C B -template, the TC value in the R-direction (1.98 W·m −1 ·K −1 , porosity = 79.9%) is also higher than that in the L-direction (0.39 W·m −1 ·K −1 , porosity = 81.8%). Due to the long aligned microchannels in the longitudinal direction being parallel to the direction of heat flow, the heat propagates more effectively in the vertical direction. The TC of β-Si 3 N 4 ceramics is much higher than that of α-Si 3 N 4 ceramics, which is due to the existence of unreacted amorphous carbon in the products.
We compared the TC of the bio-Si 3 N 4 ceramics with other porous silicon nitride ceramics prepared by other methods, such as freeze-drying method [39] and foaming method [40,41]. Although the porosity is similar (60%-80%), the TC of bio-Si 3 N 4 is considerably higher than that of porous Si 3 N 4 ceramics (0.08-0.1 W·m −1 ·K −1 ) by sol-gel and freeze drying [39]. It is mainly caused by the undesired thermal resistance between loosely contacted Si 3 N 4 grains on the vertically aligned microchannels. In addition, the TC is much larger than that of Si 3 N 4 ceramics prepared by the foaming method (2.427-3.154 W·m −1 ·K −1 at porosity of 83.32%-86.42%). In a word, for the Si 3 N 4 ceramics with randomly distributed whiskers, the only thermal conductive road access is a tortuous route. However, for our wood-derived bio-Si 3 N 4 ceramics, the unidirectionally aligned dense microchannels with low-tortuosity served as a thermal conductive "expressway" for the rapid transport of phonons throughout the rigid framework. As a result, a high TC of 6.26 W·m −1 ·K −1 in the R-direction could be achieved. A high anisotropic ratio of TC (k R /k L ) of 4.1-5.0 in bio-Si 3 N 4 ceramics could be achieved. Many applications can benefit from the excellent anisotropic thermal properties of the bio-Si 3 N 4 www.springer.com/journal/40145 ceramics. Based on the orientation of the bio-Si 3 N 4 ceramics and their thermal boundary conditions, bio-Si 3 N 4 ceramics can be used in either thermal insulation or dissipation applications.

Conclusions
Biomorphic cellular Si 3 N 4 ceramics with well-aligned nanowhisker arrays on the wall surface of directionally aligned microchannels were successfully fabricated by CRN and subsequent liquid phase sintering. Firstly, through a facile Y(NO 3 ) 3 solution infiltration into woodderived C B -templates and subsequent CRN reaction, long microchannels formed by equiaxed α-Si 3 N 4 crystalline coating with Y 2 O 3 nanoparticles were formed on the surface of C B -templates. And then, Y-Si-O-N liquid phase on the cell walls of microchannels facilitated the anisotropic β-Si 3 N 4 grain growth along [0001] direction to form nanowhisker arrays at 1750 ℃. The directionally aligned dense microchannels enable excellent load carrying capacity and thermal conduction through the entire materials. As a result, the porous Si 3 N 4 ceramics exhibited an outstanding TC (k R ≈ 6.26 W·m −1 ·K −1 ), a superior flexural strength (σ L ≈ 29.4 MPa), and a relative high anisotropic ratio of TC (k R /k L ≈ 4.1-5.0). The orientation dependence of the microstructure-property relation may offer a promising perspective for the fabrication of multifunctional ceramics.