Figure 1 (a, b) display the optical photographs of CB-templates and cellular bio-Si3N4 ceramics. Although pronounced linear shrinkage (23%~33%) occurs during 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 (CB-templates). Comparing with the CB-template, the bio-Si3N4 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 Si3N4 color), confirming that the CRN reaction occurred.
Figure 1(c) shows the XRD patterns of CB-template and bio-Si3N4 ceramics. Amorphous carbon phase in CB-template is confirmed by the two broad diffraction bands at approximately 2θ = 23.9° and 43.98°. For the CB-template, after the SiO and N2 gas infiltration at 1750°C for 4 h, primarily α-Si3N4 (No.41–0360) and a trace of β-Si3N4 (No.33-1160) phases were identified, which is also indicating the CRN reaction occurred according to the the following reaction: 3C(s) + 3SiO(g) + 2N2(g)→Si3N4(s) + 3CO (g). The strong and sharp peaks indicate that the products have high crystallinity. Whereas, for the CB-template coating with Y2O3 additives, pure β-Si3N4 phase is achieved at the same sintering condition, indicating a phase transformation from α to β-Si3N4 occurred through the dissolution-precipitation process in the Y-Si-O-N liquid phase[22]. Except for the Si3N4, 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, in this study, no SiC phase were found in the product. The Si3N4 phase is thermodynamically more stable than the SiC phase in N2, and Si3N4 phase formation is preferred compared to SiC phase.
As shown in Fig. 1(d), the peaks at about 1350 cm− 1 and 1580 cm− 1 in the pyrolyzed CB-template are characteristic for the D (defect) and G (graphite) bands of graphitic carbon[23, 24]. After the SiO gas and N2 infiltration, for the CB-template, the peaks at approximately 510 cm− 1, 560 cm− 1, 763 cm− 1, 848 cm− 1, 914 cm− 1, 972 cm− 1, 1032 cm− 1 are observed, which are related to the lattice vibration of the α-Si3N4 crystal[25, 26]. It is worth emphasizing that unreacted graphite phase is confirmed. Whereas, for the CB-template coating with Y2O3 additives, seven sharp peaks at approximately 450 cm− 1, 624 cm−1, 736 cm− 1, 865 cm− 1, 930 cm− 1, 936 cm− 1 and 1047 cm− 1 are obvious observed, which can be well indexed to the β-Si3N4[27]. The absence of graphite peaks is attributed to the complete CRN reaction. The results indicated that the α to β-Si3N4 phase transformation through the dissolution-precipitation process facilitating the CRN reaction.
However, for the CB-templates coating with with Y2O3 nanoparticles, the as-obtained β-Si3N4 ceramics after sintering at 1750°C for 4 h also inherits the anisotropic microstructure of the CB-templates (Fig. 2e, f). Meanwhile, vertically well-aligned β-Si3N4 nanowhisker arrays are observed on the microchannel walls, indicating the phase transformation from α to β-Si3N4 occurred during liquid sintering.
Figure 3(a) displays the enlarged SEM images of β-Si3N4 nanowhisker arrays on the microchannel walls. The single crystalline β-Si3N4 nanofibers are uniformly aligned normal to the microchannel wall surface. The length of β-Si3N4 nanowires is less than 8 µm and their diameter is smaller than 700 nm. Conventional solution-precipitation route is an effective approach to grow β-Si3N4 whiskers[29]. The entire nucleation-growth of β-Si3N4 nanowhisker arrays consists of four steps: (I) Long microchannels formed by equiaxed α-Si3N4 crystallines are synthesized according to the vapor-solid CRN reaction. (II) Homogeneous distributed Y2O3 nanoparticles attaching to the walls surface of long microchannel facilitates the formation of Y-Si-O-N liquid phase during heat treatment processing. (III) The Y-Si-O-N liquid phase attaching to the microchannel wall surface facilitates the anisotropic β-Si3N4 growth along [0001] direction to form one-dimensional nanowires, following the crystal prolongation mechanism[30–32]. Large amount of β-Si3N4 nuclei are formed during the α to β-Si3N4 phase transformation process through the dissolution-precipitation mechanism in liquid phase. (IV)The nanowhisker growth starts with the large scale nucleation on the wall surface and clusters of nanowhiskers quickly develop at high temperature of 1750°C. As a result, the walls became thinner and vertically well-aligned β-Si3N4 nanowhisker arrays are successfully synthesized. As reaction proceeds these clusters get densely packed leading to large scale nano-arrays. For confirmation of microstructural characteristics of bio-Si3N4 ceramics, EDS patterns of elemental mappings of bio-Si3N4 ceramic is shown in Fig. 3(b). As shown in it, quantitative analysis exhibits that the average atomic ratio of Si/N is approximately 1:1, which further confirms the formation of Si3N4 nanocrystals.
Figure 4(a) shows the anisotropic flexural strength of wood-derived bio-Si3N4 ceramics sintered at 1750°C for 4 h. In contrast to conventional, isotropic Si3N4 ceramics derived from powder mixtures, a distinct anisotropy of the mechanical properties in different loading directions can be found. The flexural strength in the L-direction (σL=29.4 MPa, along the oriented pores) is considerably higher than that in the R-direction (σR=13.6 MPa), as indicated in Fig. 4 (a). The considerable anisotropy of σ because of the anisotropic cellular microstructure. The strength of the Si3N4 channel walls and struts is largely governed by their defect distribution. When the loading direction is parallel to microchannels, large micropores (40 µm) would result in low strength. Whereas, few defects exist on the microchannels 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 by sol infiltration and carbothermal reduction (1 ± 0.25 MPa)[33]. Low test results can be attributed to high porosity in the cell walls caused by the volatilization of two-thirds of the carbon mass of the template[20]. Moreover, the defect size of the microchannel walls was confined, resulting in the strong bio-Si3N4 struts that can withstand larger force without rupture. As a consequence, the bio-Si3N4 ceramics exhibit superior specific flexural strength. Figure 4(b) shows the stress-strain curves of wood-derived Si3N4 ceramics. Under all test conditions, the specimens finally fail in a typically brittle mode. Loading in the radial direction causes stepwise cracking between individual microchannels, which leads to a jagged curve shape. In contrast, a single, clear maximum curve are obtained under longitudinal loading.
Figure 5 shows the anisotropic thermal conductivity of wood-derived bio-Si3N4 ceramics. The bio-Si3N4 ceramics also exhibit a distinct anisotropy of the TC. The unidirectionally aligned dense microchannels served as a thermal conductive “expressway” with low-tortuosity enable excellent thermal conduction through the entire materials. As a consequence, the TC value of Si3N4(R) ceramics (6.26 W·m− 1·K− 1, porosity = 68.9%) is much higher than that of Si3N4(L) ceramics (1.51 W·m− 1·K− 1, porosity = 73.8%). 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. As a result, a high anisotropic ratio of thermal conductivity (kR/kL) of 4.1 in bio-Si3N4 ceramics could be achieved. Many applications can benefit from the excellent anisotropic thermal properties of the bio-Si3N4 ceramics. Based on the orientation of the bio-Si3N4 ceramics and its thermal boundary conditions, bio-Si3N4 ceramics can be used in either thermal insulation or dissipation applications.