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![Conventional TEM and atomic-resolution STEM observations. (a) a bright-field TEM image of c-Si 3 N 4 synthesized at 15.6 GPa and 1800 °C. The average grain size is 143 ± 59 nm with the maximum grain size of ~400 nm. The inset shows an electron diffraction pattern indicating that the grains are randomly oriented in this polycrystalline material. (b) a low-angle annular dark-field (LAADF) STEM image of a triple junction (the center of the image) showing that no pore and no triple pocket exists. (c) an annular bright-field scanning transmission electron microscopy (ABF-STEM) image at a disordered/amorphous IGF (indicated by the orange arrow) between two grains, where the left grain is viewed along the [110] orientation. (d) EELS spectra at an IGF (top) and in a grain interior (bottom). The decrease of the N-K edge peak and the increase of O-K edge peak were observed at the IGF.](publication/315348384/figure/fig2/AS:473251672530945@1489843399729/Conventional-TEM-and-atomic-resolution-STEM-observations-a-a-bright-field-TEM-image-of.png)
Conventional TEM and atomic-resolution STEM observations. (a) a bright-field TEM image of c-Si 3 N 4 synthesized at 15.6 GPa and 1800 °C. The average grain size is 143 ± 59 nm with the maximum grain size of ~400 nm. The inset shows an electron diffraction pattern indicating that the grains are randomly oriented in this polycrystalline material. (b) a low-angle annular dark-field (LAADF) STEM image of a triple junction (the center of the image) showing that no pore and no triple pocket exists. (c) an annular bright-field scanning transmission electron microscopy (ABF-STEM) image at a disordered/amorphous IGF (indicated by the orange arrow) between two grains, where the left grain is viewed along the [110] orientation. (d) EELS spectra at an IGF (top) and in a grain interior (bottom). The decrease of the N-K edge peak and the increase of O-K edge peak were observed at the IGF.
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Glasses and single crystals have traditionally been used as optical windows. Recently, there has been a high demand for harder and tougher optical windows that are able to endure severe conditions. Transparent polycrystalline ceramics can fulfill this demand because of their superior mechanical properties. It is known that polycrystalline ceramics...
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... observed the microstructure of the transparent polycrystalline c-Si 3 N 4 . Figure 2a shows an example of the bright-field TEM images. The presence of equigranular texture was observed and the average grain size is 143 ± 59 (one standard deviation) nm. ...
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... observed the microstructure of the transparent polycrystalline c-Si 3 N 4 . Figure 2a shows an example of the bright-field TEM images. The presence of equigranular texture was observed and the average grain size is 143 ± 59 (one standard deviation) nm. No residual pore and no triple pocket was observed at multi-grain junc- tions (Fig. 2b), which is consistent with the results of the density measurements. Most of the c-Si 3 N 4 grains have straight grain boundaries and they are almost dislocation-free, which is consistent with the presence of sharp peaks in the XRD patterns ( Supplementary Fig. S1). Atomic-resolution STEM observations at grain boundaries between two ...
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... have straight grain boundaries and they are almost dislocation-free, which is consistent with the presence of sharp peaks in the XRD patterns ( Supplementary Fig. S1). Atomic-resolution STEM observations at grain boundaries between two grains show the presence of disordered/amorphous intergranular films 9 (IGFs) with a thickness of less than 1 nm (Fig. 2c). The results of electron energy-loss spectroscopy (EELS) measurements at the IGFs and grain interiors show that oxygen atoms preferentially segregate to the IGFs (Fig. ...
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... STEM observations at grain boundaries between two grains show the presence of disordered/amorphous intergranular films 9 (IGFs) with a thickness of less than 1 nm (Fig. 2c). The results of electron energy-loss spectroscopy (EELS) measurements at the IGFs and grain interiors show that oxygen atoms preferentially segregate to the IGFs (Fig. ...
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... nitrogen vacan- cies with electrical neutrality being obtained by an appropriate number of silicon vacancies or Si 3+ species 1 18 . Thus, the cation vacancy model (Si 23 N 30 O) might be preferable for the hypothetical oxygen-bearing c-Si 3 N 4 . Actually, our EELS spectrum at grain interior might show the pres- ence of oxygen in the structure (Fig. 2d). The silicon and nitrogen vacancies might cause the pale grey color of this transparent c-Si 3 N 4 (Fig. 1a). Further studies are required to elucidate the mechanism of oxygen incorporation into c-Si 3 N 4 and the resultant optical ...
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... 12.5%), could be responsible for the discrepancy recognized here in the G 0 values, even though the oxygen content appears to be moderately higher than that in our sample, 1.3 wt.% (corresponds to O : (O + N) = 5.4%). It is known from the literature that a similarly moderate increase of the O : (O + N) ratio in another nitride having spinel structure, namely γ-Si 3 N 4 , caused a significant change of the elastic moduli [35,55]. In particular, G 0 = 148(16) GPa was reported for the γ-Si 3 N 4 sample with O : (O + N) = 9% (corresponds to the reported 4 wt.%) ...
... In particular, G 0 = 148(16) GPa was reported for the γ-Si 3 N 4 sample with O : (O + N) = 9% (corresponds to the reported 4 wt.%) [55] while G 0 = 248(1) GPa was measured for another γ-Si 3 N 4 sample with O : (O + N) = 5.5% (corresponds to the reported 2.5 wt.%) [35]. This reduction of G 0 of γ-Si 3 N 4 by 40% is even higher than the 33% reduction we recognized for γ-Ge 3 N 4 . ...
... The normalized elemental composition was calculated to be 42.6 at.%, 54.3 at.% and 3.1 at.%, respectively. The standard deviation of the measurement for each of the elements was typically approximately 0.6% for Ge, approximately 3.5% for N and approximately 8.3% for O. Interestingly, the elements ratio O : (N + O) = 5.4% obtained for our γ-Ge 3 N 4 was below the ratios O : (N + O) = 5.6% and O : (N + O) = 6.9% reported in the works on the high-pressure multi-anvil synthesis of γ-Si 3 N 4 where special precautions were undertaken to minimize the possible sample oxidation[34,35]. Thus, the graphite capsule used here provided an effective barrier against the diffusion of oxygen from the oxide components of the multi-anvil assembly into the sample volume. ...
Germanium nitride, having cubic spinel structure, γ-Ge3N4, is a wide band-gap semiconductor with a large exciton binding energy that exhibits high hardness, elastic moduli and elevated thermal stability up to approximately 700°C. Experimental data on its bulk and shear moduli (B0 and G0, respectively) are strongly limited, inconsistent and, thus, require verification. Moreover, earlier first-principles density functional calculations provided significantly scattering B0 values but consistently predicted G0 much higher than the so far available experimental value. Here, we examined the elasticity of polycrystalline γ-Ge3N4, densified applying high pressures and temperatures, using the techniques of laser ultrasonics (LU) and Brillouin light scattering (BLS) and compared with our extended first-principles calculations. From the LU measurements, we obtained its longitudinal- and Rayleigh wave sound velocities and, taking into account the sample porosity, derived B0 = 322(44) GPa and G0 = 188(7) GPa for the dense polycrystalline γ-Ge3N4. While our calculations underestimated B0 by approximately 17%, most of the predicted G0 matched well with our experimental value. Combining the LU- and BLS data and taking into account the elastic anisotropy, we determined the refractive index of γ-Ge3N4 in the visible range of light to be n = 2.4, similarly high as that of diamond or GaN, and matching our calculated value.
This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 1)'.
... Nitrides of the group 14 elements having spinel structure, γ -M 3 N 4 (where M = Si, Ge or Sn), and their solid solutions [1][2][3][4][5] are promising multi-functional materials which are not only hard and stiff [6][7][8][9][10][11][12][13] but have been predicted to exhibit interesting optoelectronic properties 2023 The Author(s) Published by the Royal Society. All rights reserved. ...
... Using the experimental value of H V = 20 GPa for the densified polycrystalline β-Si 3 N 4 sintered without any additives [80] and γ th = 0.59, measured at room temperature [81], we obtained H V /γ th = 34 GPa. Similarly, applying the experimental H V = 35 GPa measured for the densified polycrystalline γ -Si 3 N 4 [8] and the above-calculated γ th = 1.3, we obtained H V /γ th = 27 GPa. Thus, γ -Sn 3 N 4 is expected to exhibit moderate thermalshock resistance when compared with the two phases of Si 3 N 4 estimated to have comparable thermal-shock resistances. ...
We report on the synthesis of tin(IV) nitride with spinel structure, γ-Sn3N4, from the elements at high pressures and temperatures using a laser-heated diamond anvil cell, and on the Rietveld refinement of the product structure. The procedure described here is, in our opinion, the most reliable method of obtaining high-purity nitrides which are thermodynamically stable only at high pressures. Raman spectroscopy and powder X-ray diffraction were used to characterize the synthesis products. Pressure dependences of the Raman-band frequencies of γ-Sn3N4 were measured and used to determine its average mode Grüneisen parameter, 〈γ〉 = 0.95. Using this value, we estimated the thermal-shock resistance of γ-Sn3N4 to be about half that of γ-Si3N4, which, in turn, is moderately surpassed by β-Si3N4, known to be highly thermal-shock resistant.
This article is part of the theme issue ‘Exploring the length scales, timescales and chemistry of challenging materials (Part 1)’.
... The spinel-type structure is characterized by one third of the silicon (Si) atoms being fourfold coordinated by nitrogen (N) atoms, and two thirds are sixfold coordinated by N atoms. The increase in the coordination number of silicon in γ-Si 3 N 4 (as compared to that in α-/β-Si 3 N 4 ) results in a significant increase in the density by 26%, which consequently results in a higher elastic modulus and hardness in comparison to those of the low-pressure hexagonal phases [2][3][4][5][6]. In addition, the high-temperature stability of γ-Si 3 N 4 in an oxidizing environment is far beyond that of diamond [7,8]. ...
... Therefore, γ-Si 3 N 4 is a potential candidate material for the application in harsh environments (e.g., drill head and abrasive). However, the major shortcoming of the γ-Si 3 N 4 ceramic with respect to the above-mentioned applications relates to its low fracture toughness (K IC ), which is 3.5 MPa·m 1/2 [4]. The trade-off between hardness and fracture toughness in superhard materials makes it difficult to improve both properties simultaneously. ...
... (1)) and ~6.0±0.6 MPa·m 1/2 in the Evans equation (Eq. (2)), far exceeding that of pure γ-Si 3 N 4 (3.5 MPa·m 1/2 ), as reported in Ref. [4]. In order to identify the potential mechanisms accounting for the enhanced toughness of γ-Si 3 N 4 /Hf 3 N 4 ceramic, the radial cracks generated by the Vickers indentation were further characterized. ...
Cubic silicon nitride (γ-Si3N4) is superhard and one of the hardest materials after diamond and cubic boron nitride (cBN), but has higher thermal stability in an oxidizing environment than diamond, making it a competitive candidate for technological applications in harsh conditions (e.g., drill head and abrasives). Here, we report the high-pressure synthesis and characterization of the structural and mechanical properties of a γ-Si3N4/Hf3N4 ceramic nanocomposite derived from single-phase amorphous silicon (Si)–hafnium (Hf)–nitrogen (N) precursor. The synthesis of the γ-Si3N4/Hf3N4 nanocomposite is performed at ~20 GPa and ca. 1500 ℃ in a large volume multi anvil press. The structural evolution of the amorphous precursor and its crystallization to γ-Si3N4/Hf3N4 nanocomposites under high pressures is assessed by the in situ synchrotron energy-dispersive X-ray diffraction (ED-XRD) measurements at ~19.5 GPa in the temperature range of ca. 1000–1900 ℃. The fracture toughness (KIC) of the two-phase nanocomposite amounts ~6/6.9 MPa·m1/2 and is about 2 times that of single-phase γ-Si3N4, while its hardness of ca. 30 GPa remains high. This work provides a reliable and feasible route for the synthesis of advanced hard and tough γ-Si3N4-based nanocomposites with excellent thermal stabililty.
... The stress-strain curves of the c-Si3N4 were obtained by applied tensile and shear stress, in which the ideal tensile and shear strength were determined to be 45 and 49 GPa, respectively [11]. However, these first-principles calculations have been carried out on the perfect structure of the -Si3N4 while the experiments showed that the structure of the -Si3N4 contains grain boundaries and defects [4,12]. At the atomistic scale, molecular dynamics (MD) simulations represent a powerful complement to experimental techniques with providing mechanistic insight into experimentally observed processes [13]. ...
... Consequently, the narrow bands including hcp and d-N structures are considered as the stacking faults and defects in the fcc N structure of c-Si3N4. The narrow bands of hcp and d-N structures are observed as the grain boundaries which are observed by STEM images [12]. This suggests that MD simulations can aid to understand more insight of structural grain boundaries at the atomic level. ...
... The energy and volume of the system were determined at the same time. The energy-volume curve is showed in Figure 5. Based on this curve, the system energy as a function of the volume (V -2/3 ) can be estimated by a function using the equation (3) with the following coefficients obtained through curve fitting: Now, we used these coefficients in Eq. (7) to calculate the bulk modulus in Eq. (1), B=308 GPa and this value is good agreement with experimental data [5,12] and other calculations [11]. ...
The molecular dynamics simulations have been used to study the microstructure as well as mechanical behavior of cubic silicon nitride (c-Si3N4) under the extended deformation. The silicon nitride sample was simulated under the cooling process and high pressure. At T=300 K, dominant nitrogen (N) atoms arrange into fcc lattice, and the rest of N atoms have hexagonal close-packed (hcp) and disordered structures. The hcp and disordered N atoms gather into the narrow bands. The phonon spectra of this sample are calculated and discussed. In this work we also present a molecular dynamics prediction for the elastic moduli in strained cubic silicon nitride as functions of the volumetric strain. Young’s modulus and Poisson’s ratio are also calculated for the c-Si3N4.
... This result strongly suggests that low-pressure nitrides could be transformed into high-density materials under HPHT conditions, which may have high hardness or other functionalities. 6,7 Similarly, c-Zr 3 N 4 and c-Hf 3 N 4 were also found by his group. 8 After the discovery of γ-Si 3 N 4 , the International Workshop on Spinel Nitrides and Related Materials was launched in 2002 by Prof. Riedel, and the 10th meeting is planned for 2022, where researchers can freely and deeply discuss functional nitrides. ...
Wide bandgap III‐nitrides, such as cubic boron nitride and wurtzite‐type aluminum nitride, are promising systems for optoelectronics. To extend their luminescent properties, we doped single Ce atoms into III‐nitride single crystals using reactive flux with a temperature gradient method at high‐pressure and high‐temperature conditions. To fully understand such properties in a large size‐mismatch system, it is critically important to determine the point‐defect structures of single dopants, their spatial distribution in three dimensions, and their atomistic dynamics at an atomic level. This review discusses point defect structures and their dynamics in III‐nitrides using single‐atom‐sensitive scanning transmission electron microscopy, and the recent progress in the related field of electron microscopy.
... Following the nitride deposition, the silicon wafer is wet etched straight through to reveal the 50 nm thick SiN membrane on the opposite surface. While commonly used as a waveguide material for atomic sensors 189,199 , SiN membrane windows have been used in vacuum window formation 285 , with polycrystalline cubic silicon nitride, c-Si 3 N 4 , being demonstrated with a wide optical transparency and material toughness comparable to diamond 286 . Importantly, SiN is compatible with anodic bonding, and its simple deposition from low-pressure chemical vapor deposition (LPCVD) favours integration with silicon frame based cells 287 . ...
Laser cooled atoms have proven transformative for precision metrology, playing a pivotal role in state-of-the-art clocks and interferometers, and having the potential to provide a step-change in our modern technological capabilities. To successfully explore their full potential, laser cooling platforms must be translated from the laboratory environment and into portable, compact quantum sensors for deployment in practical applications. This transition requires the amalgamation of a wide range of components and expertise if an unambiguously chip-scale cold atom sensor is to be realized. We present recent developments in cold-atom sensor miniaturization, focusing on key components that enable laser cooling on the chip-scale. The design, fabrication and impact of the components on sensor scalability and performance will be discussed with an outlook to the next generation of chip-scale cold atom devices.
... TCs are highly transparent in the visible and mid-IR ranges (0.25-5.5 m), which suggests the utilization of TCs as windows and domes in electro-optical and infrared sensors in military systems [74]. Promising recent transparent materials used for military systems are MgAl 2 O 4 [35] , Al 2 O 3 [75,76], AlON [77], Lu 2 O 3 [78,79], c-BN [80], Y 2 O 3 [81,82], SiC [27], Si 3 N 4 [65,83], and SiAlON [84][85][86] because of their high strength, hardness, and fracture toughness. Different mechanical, thermal, and optical properties of these transparent ceramics are listed in Table 2. ...
... VIckers Hardness (1 kg) of cubic silicon nitride compared to other super-hard ceramics[83]. ...
... Transparency vs. a) Hardness, and b) Fracture Toughness for different TCs[27,37,59,63,65,75,83,86,[92][93][94][95][96][97][98][99][100][101]111,112]. ...
Owing to superior properties, i.e. high hardness, high wear resistance, and weight reduction of transparent ceramics (TCs) over glasses, TCs have shown promising tribological potential for applications such as face shields, explosive ordnance visors, windows for aircraft, spacecraft and, re-entry vehicles, electromagnetic windows, laser igniter windows, screens for smartphones and more. Researchers globally have been attracted to explore more about TCs, considering the tremendously increasing demand over different other transparent materials. The optical quality of TCs is mostly characterized by the in-line transmittance, and the effect of various processing parameters on transmittance has already been studied by various researchers. In this review, the current research progress regarding tribological performance of TCs is compiled. TCs with potential in tribological applications include MgAl2O4, Al2O3, AlON, Lu2O3, c-BN, Y2O3, Si3N4, and SiAlON. The relevant strategies to improve the tribological properties, including microstructures and mechanical properties are comprehensively discussed. In addition, the mechanisms of material removal of different transparent ceramics are also presented. It is well observed that surface fracture comprising three stages is found as one of the dominant wear mechanisms during wear. This review aims to provide some meaningful guidelines for development of transparent ceramics with enhanced wear resistance, while identifying the wear mechanisms in particular wear conditions.
... The advantage of transparent ceramics is the possibility of producing reasonably priced and large-size materials within a relatively short process time, especially for applications in harsh and extreme environments. Many transparent ceramics, including oxides and nonoxides, have been reported in the open literature, such as ZrO 2 (Peuchert et al., 2009), MgAl 2 O 4 (Reimanis and Kleebe, 2009;Shi et al., 2020), YAG (Ikesue et al., 1995) (Ikesue et al., 1995a), Gd 3 (Ga,Al) 5 O 12 (GAGG) (Luo et al., 2013), Y 3 Fe 5 O 12 (YIG) (Ikesue and Aung, 2018), Y 2 O 3 Lu et al., 2002), Lu 2 O 3 , Pb 1Àx La x (Zr y Ti z )O 3 (PLZT) (Choi et al., 2001;Haertling, 1971), AlON (McCauley et al., 2009;McCauley and Corbin, 1979), AlN (Kuramoto and Taniguchi, 1984), SiN (Nishiyama et al., 2017), CaF 2 (Basiev et al., 2008), ZnS (Chlique et al., 2013), among others. They have been explored for applications in many fields, such as windows, domes, laser media, scintillators, phosphors, electrooptic or magnetooptic devices, optical components, and so on (Goldstein and Krell, 2016;Ikesue and Aung, 2008;Wang et al., 2013a;Wei, 2009). ...
The family of materials with a pyrochlore structure exhibits a wide variety of technologically important physical properties, including ionic conductivity, photocatalytic activity, radiation resistance, and many others. However, the number of reliably established bulk ferroelectrics with a pyrochlore structure is extremely small (with the exception of cadmium niobate and related compounds). Instead, a large number of pyrochlores (primarily bismuth-containing) based on their temperature dependence of dielectric permittivity have a relaxor-like anomaly, which is not connected with the macroscopic ferroelectric phase transition. This chapter discusses the crystal-chemical and structural features of bismuth-containing pyrochlores that can cause the manifestation of nonferroelectric relaxor dielectric properties. In addition, a comparative analysis is given of the dielectric relaxation parameters of various compounds with the pyrochlore structure, discussed within the model of atomic hopping between crystallographically equivalent positions.
... The advantage of transparent ceramics is the possibility of producing reasonably priced and large-size materials within a relatively short process time, especially for applications in harsh and extreme environments. Many transparent ceramics, including oxides and nonoxides, have been reported in the open literature, such as ZrO 2 (Peuchert et al., 2009), MgAl 2 O 4 (Reimanis and Kleebe, 2009;Shi et al., 2020), YAG (Ikesue et al., 1995) (Ikesue et al., 1995a), Gd 3 (Ga,Al) 5 O 12 (GAGG) (Luo et al., 2013), Y 3 Fe 5 O 12 (YIG) (Ikesue and Aung, 2018), Y 2 O 3 Lu et al., 2002), Lu 2 O 3 , Pb 1Àx La x (Zr y Ti z )O 3 (PLZT) (Choi et al., 2001;Haertling, 1971), AlON (McCauley et al., 2009;McCauley and Corbin, 1979), AlN (Kuramoto and Taniguchi, 1984), SiN (Nishiyama et al., 2017), CaF 2 (Basiev et al., 2008), ZnS (Chlique et al., 2013), among others. They have been explored for applications in many fields, such as windows, domes, laser media, scintillators, phosphors, electrooptic or magnetooptic devices, optical components, and so on (Goldstein and Krell, 2016;Ikesue and Aung, 2008;Wang et al., 2013a;Wei, 2009). ...
Special features of the pyrochlore structure A2B2O6O′ with two types of both cationic and anionic sites and a large range of cation radii ratios, where this structural type is preserved, provide for a multitude of compositions as well as a wide spectrum of properties and possible fields of application. In recent decades, oxide compounds with the pyrochlore-type structure have been intensely studied to search for new materials suitable for electrochemical use. Electrical properties and electrochemical behavior of oxide pyrochlore compounds depend on the composition, electronic structure, cation valence, and the defectiveness of cation and anion sublattices. This chapter presents the electrochemical behavior of complex oxide pyrochlores, the heightened interest in them being associated with their potential use as the basis for materials for electrochemical detectors, sensors, membranes, solid electrolytes, electrode material for solid oxide fuel cells (SOFCs), and electrochemical catalysis. The conductivity of binary oxide pyrochlores varies over a wide range depending on their composition, from dielectric to metallic. Electronic and ionic (oxygen, proton, cation) conductivity depends on the composition, temperature, and external atmosphere. Often, several types of conductivity show themselves in various temperature ranges. By doping into both A and B cation sites, both electronic and ionic components of conductivity may be regulated. A group of complex pyrochlores may be set off with a high doping level, where cationic sites are occupied by cations with different valence or with variable valence in comparable ratios. The control of the defect structure by varying the composition offers the prospect of essentially varying the electrochemical properties of compounds. The main factors determining the electrochemical properties of complex oxide pyrochlores and the trends resulting from investigations are given in this chapter. Topics explored include general conductivity aspects, structural disordering, defects, oxygen migration pathways, and regulation of the electrochemical behavior of oxide pyrochlores through doping.
... The advantages of transparent ceramics include not only their optical merits but also their improved mechanical properties. [1][2][3][4] Owing to its high hardness and thermal stability, Si 3 N 4 is used in cutting-tool inserts. 5 Although a-Si 3 N 4 has a higher hardness than b-Si 3 N 4 , the latter is preferable for cutting tool applications because the stability of the former reduces at high temperatures. ...
... The Vickers hardness was 39.7 (0.73) GPa at 4.9 N load and 37.3 (0.67) GPa at 9.8 N load. The highest Vickers hardness for c-Si 3 N 4 at 9.8 N reported so far is 34.9 (0.7) GPa by Norimasa et al. 2 Our specimen achieved a hardness of approximately 7% higher than this value, approaching the threshold for superhard materials (40 GPa). It even exceeds the well-known fourthhardest material, SiO 2 -stishovite, after diamond, BC 2 N, and cubic boron nitride. ...
Transparent polycrystalline ceramics exhibit improved mechanical and optical properties. However, synthesizing transparent ceramics without additives is nontrivial. Herein, we report the synthesis of two transparent ceramics (β-Si3N4 and γ-Si3N4) under high pressure and high temperature from a pure Si3N4 precursor with nano-/micro-dual grain sizes. Synthesized β-Si3N4 exhibited a significantly enhanced Vickers hardness reaching 24.2 GPa (at 10 N load) when transparency was achieved. Transparent nano-grained γ-Si3N4 exhibited a Vickers hardness of 37.3 GPa. These are the highest hardness values reported for these two phases at a 10 N load. Density and microstructure measurements suggest that the hardness and transparency of the specimens correlate with both the grain size and porosity/density. The negligible amount of pores accounts for the superior optical transparency and high hardness of two Si3N4 allotropes. As higher pressures can effectively suppress grain growth and minimize pores between grains, high-pressure sintering is demonstrated as an effective way to realize highly dense transparent ceramics.
