Effects of N2 pressure and Si particle size on mechanical properties of porous Si3N4 ceramics prepared via SHS

Porous silicon nitride ceramics with high flexural strength and high porosity were directly fabricated by self-propagating high temperature synthesis (SHS). The effects of N₂ pressure and Si particle size on the phase composition, microstructure, and mechanical property were investigated. N₂ influences not only the thermodynamics but also the kinetics of the SHS as initial reactant. Flexural strength ranged between 67 MPa and 134 MPa with increasing N₂ pressure. On the other hand, flexural strength ranged from 213 MPa to 102 MPa with different Si particle sizes. This plays an important role on the final diameter and length of β-Si₃N₄ grains and the formation mechanism of porous Si₃N₄ ceramics.     

Fabrication and properties of Si2N2O-Si3N4 ceramics via direct ink writing and low-temperature sintering

Direct ink writing (DIW) and low-temperature sintering methods were applied to prepare Si₂N₂O-Si₃N₄ ceramics for radome materials. Lattices of Si-SiO₂ green body were printed by DIW with 78 wt % solid portion of water-based Si-SiO₂ slurry, in which silicon particles and silica fume were used as the solid portion and Methylcellulose (HPMC) was used as the dispersant. Effects of HPMC addition on stability and silica fume content on rheological properties of the slurry were studied, respectively. The pseudoplastic mechanism of the slurry was analyzed. The Si-SiO₂ green bodies were sintered at 1250 °C–1400 °C in nitrogen. The effect of temperature on phase composition, microstructure, mechanical and dielectric properties of samples was investigated. With the HPMC addition of 0.12 wt% and the silica fume proportion of 30 wt% in solid portion, a stable and pseudoplastic slurry with the yield stress of 110.9 Pa was obtained, which is suitable for DIW. With the decrease of initial holding temperature, more N₂ enters the sample and reacts with silicon and silica fume, promoting the generation of Si₂N₂O and Si₃N₄. The optimal condition yields Si₂N₂O-Si₃N₄ ceramics with apparent porosity of 42.73%, compressive strength of 24.7 MPa, dielectric constant of 4.89 and loss tangent of 0.0054. It is found that columnar Si₃N₄ comes from a direct reaction between silicon and N₂, and fibrous Si₂N₂O is mainly generated by the reaction between silicon, SiO(g), and N₂ through the chemical vapor deposition mechanism. Good dielectric properties are achieved due to high porosity, high proportion of Si₂N₂O phase and no residual silicon.     

The effect of fabrication parameters on the mechanical properties of sintered reaction bonded porous Si3N4 ceramics

Porous silicon nitride ceramics were prepared via sintered reaction bonded silicon nitride at 1680 °C. The grain size of nitrided Si₃N₄ and diameter of post-sintered β-Si₃N₄ are controlled by size of raw Si. Porosity of 42.14–46.54% and flexural strength from 141 MPa to 165 MPa were obtained. During post-sintering with nano Y₂O₃ as sintering additive, nano Y₂O₃ can promote the formation of small β-Si₃N₄ nuclei, but the large amount of β-Si₃N₄ (>20%) after nitridation also works as nuclei site for precipitation, in consequence the growth of fine β-Si₃N₄ grains is restrained, the length is shortened, and the improvement on flexural strength is minimized. The effect of nano SiC on the refinement of the β-Si₃N₄ grains is notable because of the pinning effect, while the effect of nano C on the refinement of the β-Si₃N₄ grains is not remarkable due to the carbothermal reaction and increase in viscosity of the liquid phase.     

In situ synthesis of Si2N2O/Si3N4 composite ceramics using polysilyloxycarbodiimide precursors

In situ synthesis of Si₂N₂O/Si₃N₄ composite ceramics was conducted via thermolysis of novel polysilyloxycarbodiimide ([SiOSi(NCN)₃]_n) precursors between 1000 and 1500 °C in nitrogen atmosphere. The relative structures of Si₂N₂O/Si₃N₄ composite ceramics were explained by the structural evolution observed by electron energy-loss spectroscopy but also by Fourier transform infrared and ²⁹Si-NMR spectrometry. An amorphous single-phase Si₂N₂O ceramic with porous structure with pore size of 10–20 μm in diameter was obtained via a pyrolyzed process at 1000 °C. After heat-treatment at 1400 °C, a composite ceramic was obtained composed of 53.2 wt.% Si₂N₂O and 46.8 wt.% Si₃N₄ phases. The amount of Si₂N₂O phase in the composite ceramic decreased further after heat-treatment at 1500 °C and a crystalline product containing 12.8 wt.% Si₂N₂O and 87.2 wt.% Si₃N₄ phases was obtained. In addition, it is interesting that residual carbon in the ceramic composite nearly disappeared and no SiC phase was observed in the final Si₂N₂O/Si₃N₄ composite.     

硅灰-氮化硅体系材料高温反应的研究

将硅灰(w(SiO2)=94.5%,平均粒度0.08μm)和氮化硅(粒度≤0.074mm)按1:1质量比混合后成型,在空气中埋炭条件下分别经1300℃、1450℃、1500℃、1550℃、1600℃处理3h后水冷,对其显微结构及物相进行了分析。结果表明:在1550℃以上,以硅灰和氮化硅为原料反应生成Si2N2O比较明显,氮化硅颗粒的边角变得圆滑,而且分布在含Si2N2O的连续胶结相中,形成胶结相包裹Si3N4的致密结构;1500℃以下,氮化硅仍然棱角分明,基本上未形成Si2N2O,只是硅灰中的SiO2析晶,析晶比较显著的温度为1300℃。     

Novel porous Si3N4 ceramics prepared by aqueous gelcasting using Si3N4 poly-hollow microspheres as pore-forming agent

In this paper, novel porous Si₃N₄ ceramics were prepared by aqueous gelcasting using Si₃N₄ poly-hollow microspheres as pore-forming agent. The effect of Si₃N₄ poly-hollow microsphere content on the phase composition, microstructure, shrinkage, porosity and mechanical properties of the prepared porous Si₃N₄ ceramics were investigated. It is found that there is only β-Si₃N₄ phase in all the prepared porous Si₃N₄ ceramics. Meanwhile, the SEM results show that the pores in the porous Si₃N₄ ceramics distribute uniformly, the added Si₃N₄ poly-hollow microspheres and the basal body contact closely. With the increase of Si₃N₄ poly-hollow microsphere content, the shrinkage of the porous Si₃N₄ ceramics decreases gradually, and the porosity of the porous Si₃N₄ ceramics decreases firstly and then increases. Furthermore, the flexural strength and fracture toughness of the porous Si₃N₄ ceramics decrease with the increase of the Si₃N₄ poly-hollow microsphere content.     

The effect of oxidation on the mechanical properties and dielectric properties of porous Si3N4 ceramics

The effect of oxidation temperature and time on the microstructures, phase compositions, mechanical properties, and dielectric properties of porous Si₃N₄ ceramics was investigated in the temperature range from 900 °C to 1300 °C for 1 h, 5 h, and 24 h. The weight gain measured either at lower temperature (900 °C) for long time (24 h) or at higher temperature (1300 °C) for 1 h demonstrated that the porous Si₃N₄ ceramics were easily oxidized under the current test conditions. Results showed that the amount of open pores, flexural strength, compressive strength, and dielectric constant all decreased with the increase of oxidation temperature independent upon the oxidation time. The oxidation product SiO₂ was low-temperature quartz in mild condition (low temperature, short time) and cristobalite in severe condition (high temperature, long time). The existence of cracks on the oxide scale was due to the phase transformation of SiO₂ and thermal expansion coefficient mismatch between SiO₂ and Si₃N₄.     

Low-temperature preparation of Si3N4 whiskers bonded/reinforced SiC porous ceramics via foam-gelcasting combined with catalytic nitridation

Porous α-Si₃N₄ whiskers bonded/reinforced SiC (Si₃N_4(w)/SiC) ceramics were successfully prepared at as low as 1473 K for 2 h, via a combined foam-gelcasting and catalytic nitridation route using commercial Si and SiC powders containing some Fe impurity as the main raw materials. Small pores (0.03–5 μm) left by the packing of raw material particles and interlocking of in-situ formed Si₃N₄ whiskers coexisted with large ones (8–400 μm) resultant mainly from the foaming process. The impurity Fe from the raw materials Si and SiC acted as an internal catalyst, accelerating the nitridation of Si by increasing the bond length and weakening the bond strength in the N₂ molecules adsorbed on it. As-prepared Si₃N_4(w)/SiC porous ceramics contained 71.53% porosity and had flexural and compressive strengths of 5.60 ± 0.69 MPa and 12.37 ± 1.05 MPa, respectively.     

The two-tier tissue reinforcing and toughening mechanism caused by dual-granularity Si powders ingredient,and dielectric properties in reaction bonded and sintered reaction bonded Si3N4 porous ceramics

With the rapid development of hypersonic vehicles and broadband wave-transparent radome, Si₃N₄ porous ceramics (Si₃N₄-PC) have attracted attention due to their excellent intrinsic properties of Si₃N₄ and high porosity. However, its high porosity results in low strength and toughness, which are fundamental properties for radome. Reaction bonded (RB) Si₃N₄-PC has advantages of dielectric properties and cost over general phase transformation sintering (PTS) and sintered reaction bonded (SRB) Si₃N₄-PC while it has been neglected in recent years. In this study, RB and SRB Si₃N₄-PC prepared by non-aqueous gelcasting and the influence of Si powders ingredient on their properties are discussed in order to illuminate the potential of RB Si₃N₄-PC in wave-transparent materials. The results show that RB Si₃N₄-PC with dual-granularity ingredients of 5 μm & 45 μm produces a two-tier tissue of framework of coarse whiskers enhanced by a network of tiny whiskers. SRB Si₃N₄-PC evolves into a two-tier tissue of framework of columnar and rod-like grains joining together to brace each other. The two-tier microscopic tissue strongly reinforces and toughens the structure and results in higher σ_F and γ_wof. As a result, the RB and SRB Si₃N₄-PC of dual-granularity of 5 μm & 45 μm obtain the maximum σ_F of 109.94 MPa and 119.56 MPa as well as maximum γ_wof of 990.74 J m^-2 and 1167.88 J m^-2, respectively. Furthermore, the ε′ and tanδ of RB and SRB Si₃N₄-PC of dual-granularity of 5 μm & 45 μm are about 4.20 and 4.52 as well as 7.01 × 10^-3 and 22.90 × 10^-3, respectively. It is concluded that RB Si₃N₄-PC of dual-granularity has good mechanical and dielectric properties, which are favorable for radome.     

Tuning the dielectric properties of porous BN/Si3N4 composites by controlling porosity

To solve the shortcomings of traditional layered structure for broadband wave-transmitting applications, a novel porosity gradient structure is proposed. The precise turning of the relative permittivity of each layer and their simultaneous shrinkage during sintering are the keys to design and fabrication. Porous BN/Si₃N₄ composites with different porosities were prepared by incorporation of pore former, tape casting and gas pressure sintering. Results show that the obtained composites with different porosities exhibit low and similar shrinkage after sintering at 1900 °C, which is mainly due to the bridging effect between rapidly growing β-Si₃N₄ rod. When the porosity increases from 33.2% to 59.4%, the relative permittivity decreases from 4.00 to 2.38. Moreover, the composites with different porosities have excellent mechanical properties, with bending strength and fracture toughness of 180 ± 10–36 ± 4 MPa and 3.2 ± 0.30–0.85 ± 0.10 MPa·m^?1/2, respectively, which is attributed to the bridging and pull-out strengthening effects of the elongated β-Si₃N₄ grains.     

SnO2 modified PNN-PZT ceramics with ultra-high piezoelectric and dielectric properties

In this work, a two-step solid-state reaction method is used to prepare the 0.55 Pb(Ni_1/3Nb_2/3)O₃-0.135PbZrO₃-0.315PbTiO₃/xSnO₂ (PNN-PZT/xSnO₂) ceramics. The influences of SnO₂ on the crystalline structure, electromechanical properties, and temperature stability of PNN-PZT ceramics were studied in detail. The results demonstrate that the Sn⁴⁺ ions are successfully introduced into the PNN-PZT crystalline lattice and substitute B-site Ni²⁺ and Zr⁴⁺. The x = 0.0025 ceramic with the coexistence of rhombohedral, tetragonal, and pseudocubic phases exhibits the optimized comprehensive properties: the quasi-static piezoelectric constant d₃₃, large-signal d₃₃*, electromechanical coupling coefficients k_p and k_t, free dielectric constant ε_r, and mechanical quality factor Q_m are 1123 pC/N, 1250 p.m./V, 0.63, 0.54, 9529, and 57, respectively. Meanwhile, the Curie temperature for this composition is 103 °C, almost maintaining the same level as the PNN-PZT matrix. After annealing at 75 °C, the retained d₃₃ of x = 0.0025 ceramic is as high as 975 pC/N, superior to the PNN-PZT matrix (retained d₃₃ ≈ 873 pC/N). Our results provide a promising piezoelectric material for board bandwidth, high sensitivity, and miniaturized medical ultrasonic transducers applications.     

Colored Si3N4 ceramics via constructing Nd3+ doped core-shell structures

Si₃N₄ ceramics have been attractive casing materials for electronics devices under the 5th generation mobile communication technology because of its outstanding comprehensive properties. However, single color cannot meet the needs of most consumers. Here, a typical strategy, involving forming core-shell structures by utilizing Nd₂O₃-MgO-YAG system, is proposed to rich color. Scanning transmission electron microscope-energy dispersive X-ray spectrometry results confirm the presence of core-shell structures in the silicon nitride matrix, and the core is pore and the shell is Nd-enriched liquid phase. The Nd-rich core-shell structure as the color center makes the silicon nitride appear cyan or blue.     

Effects of slurry composition on the properties of 3-1 type porous PZT ceramics prepared by ionotropic gelation

In this study, 3-1 type lead zirconate titanate (PZT) ceramics with one-dimensional pore channels were produced by ionotropic gelation process of alginate/PZT suspensions. By increasing the sodium alginate concentration from 1.0wt% to 3.0 wt%, the alginate/PZT suspensions turned from Newtonian to non-Newtonian behavior with substantial increase in apparent viscosity. Accordingly, 3-1 type PZT ceramics with porosity decreasing from 56.78% to 41.44% were obtained, while the pore size distribution became non-uniform gradually. Based on the structural features, the 3-1 type PZT ceramics possessed much higher relative permittivity (ε_r) than that of 3-0 or 3-3 type PZT ceramics with similar porosities. Increase in the porosity led to a moderate decline in the longitudinal piezoelectric strain coefficient (d₃₃), a reduction in the dielectric loss factor (tan δ), and a high value of hydrostatic strain coefficient (d_h). As a result, the 3-1 type PZT ceramics possessed a maximal hydrostatic figure of merit (HFOM) value of 5597×10^–15 Pa⁻¹ when the porosity was 56.78%, which may be of help for low frequency hydrophones applications.     

Si3N4-BN-SiCN ceramics with unique hetero-interfaces for enhancing microwave absorption properties

SiCN-based ceramics with broadband and strong microwave absorption properties are desired for the structural and functional integration of ceramic matrix composites. The elemental composition and thermal expansion coefficients of the ceramics matrix crucially affect its microstructure and electromagnetic wave (EMW) absorption properties. BN layer with lower electrical conductivity and higher specific area, exhibits both the impedance matching characteristic and EMW attenuation in the process of multiple reflections, electrical conductivity loss, dipole polarization and interfacial polarization. Therefore, Si₃N₄-BN-SiCN ceramics, which were synthesized using chemical vapor infiltration (CVI) method, construct unique hetero-interface of Si₃N₄-BN, Si₃N₄–SiCN and BN-SiCN. Therefore, the Si₃N₄-BN-SiCN ceramics have outstanding EMW absorption performance and realize an effective absorption bandwidth (EAB) that covers the whole X band and the minimum reflection coefficient (RC) reaches -18.43 dB at a thickness of 3.37 mm.     

Preparation and characterization of polymer-derived Zr/Si/C multiphase ceramics and microspheres with electromagnetic wave absorbing capabilities

Precursors for Zr/Si/C multiphase ceramics were synthesized by the reactions of dilithiozirconocene complex with dichlorodimethylsilane, methyltrichlorosilane and dichloromethylvinylsilane, respectively. The precursor-to-ceramic process of the precursor was investigated by TG-GC–MS and TG-FTIR analyses, confirming a complete transformation from organometallic polymers into ceramics below 800 °C. Annealing experiments of the derived ceramics at temperatures from 1000 °C to 2000 °C indicated the crystallization from ZrSiO₄, ZrO₂ to ZrC. Furthermore, micrometer-sized Zr/Si/C ceramic microspheres were successfully fabricated from the precursor at 1000 °C, showing surface morphology like wrinkled pea. According to the XRD, HRTEM and XPS analyses, such multiphase ceramic microspheres consist of ZrSiO₄, ZrO₂, and amorphous SiO_xC_y. Interestingly, the ceramic microspheres performed satisfactory electromagnetic wave absorbing capacity with the RL_max reaching −34 dB, which could be potential candidates for electromagnetic micro-devices.     

Self-propagating high temperature synthesis (SHS) of porous Si3N4-based ceramics with considerable dimensions and study on mechanical properties and oxidation behavior

The aim of present work is to fabricate porous Si₃N₄ ceramics with considerable dimensions and homogeneous microstructure by self-propagating high temperature synthesis (SHS) using Si, Si₃N₄ diluent and Y₂O₃ as raw materials. The results indicate that Si₃N₄ diluent with coarse particle sizes and appropriate β-phase content is beneficial to obtain porous Si₃N₄ ceramics with homogeneous microstructure and excellent mechanical property by controlling the shrinkage inside the sample. The produced Si₃N₄ ceramics possessed excellent flexural strength of 168 MPa～259 MPa, and high Weibull modulus of 11.0～17.2. Additionally, BN and SiC are added as second phase to modify the properties of Si₃N₄-based ceramics. Optimum flexural strength of 170 MPa and 137 MPa were obtained with 10 wt.% addition of BN and SiC respectively. After oxidation at 1100 °C～1300 °C, second phase-doped Si₃N₄ ceramics also presented higher residual strength than pure Si₃N₄ ceramics.     

氮化烧结制备Si3 N4 -SiC复相陶瓷

以酚醛树脂作为结合剂,以冷等静压方法成型制备氮化烧结Si3N4-SiC复相陶瓷,研究了结合剂对坯体强度和生成材料物相组成的影响。坯体强度随酚醛树脂含量增加而提高,最高强度达到23MPa,实现坯体可直接机械加工。经过氮化烧结,生成材料物相中含有SiC,含量达到7.1%～15.7%,并观察到细小的等轴颗粒αSi3N4、棒状晶粒βSi3N4以及少量针状和晶须状Si3N4。SiC颗粒与Si3N4结合在一起,被Si3N4包裹。Si3N4-SiC复相材料的生成机理:300～600℃,酚醛树脂发生裂解,形成单质C,残碳率为50%;1000～1100℃,C开始与Si发生固相反应,形成SiC;1100℃后,Si开始发生氮化反应,生成Si3N4。     

Preparation and characterization of porous mullite ceramics via foam-gelcasting

Porous mullite ceramics were prepared via foam-gelcasting using industrial grade mullite powder as the main raw materials, Isobam-104 as the dispersing and gelling agent, sodium carboxymethyl cellulose as the foam stabilizing agent, and triethanolamine lauryl sulfate as the foaming agent. The effects of processing parameters such as type and amount of additive, solid loading level and gelling temperature on rheological properties and gelling behaviors of the slurries were investigated. The green samples after drying at 100 °C for 24 h were fired at 1600 °C for 2 h, and the microstructures and properties of the resultant porous ceramic samples were characterized. Based on the results, the effects of foaming agent on the porosity level, pore structure and size and mechanical properties of the as-prepared porous mullite ceramics were examined. Porosity levels and pore sizes of the as-prepared samples increased with increasing the foaming agent content up to 1.0%, above which both porosity levels and pore sizes did not change. The compressive strength and flexural strength of the as-prepared sample with porosity of 76% and average pore size of 313 μm remained as high as 15.3±0.3 MPa and 3.7±0.2 MPa, respectively, and permeability increased exponentially with increasing the porosity.