Transparent polycrystalline cubic silicon nitride

At a Glance

Metadata	Details
Publication Date	2017-03-17
Journal	Scientific Reports
Authors	Norimasa Nishiyama, Ryo Ishikawa, Hiroaki Ohfuji, Hauke Marquardt, Alexander Kurnosov
Institutions	Deutsches Elektronen-Synchrotron DESY, Life Science Institute
Citations	67
Analysis	Full AI Review Included

Technical Analysis and Documentation: Transparent Cubic Silicon Nitride

Executive Summary

This research successfully synthesized bulk, transparent polycrystalline cubic silicon nitride (c-Si₃N₄) via high-pressure/high-temperature (HPHT) methods, positioning it as a leading candidate for extreme optical windows.

Ultimate Hardness Category: c-Si₃N₄ is officially categorized as the third hardest material, surpassed only by MPCVD Diamond (SCD) and cubic Boron Nitride (cBN).
Superior Thermal Stability: The transparent c-Si₃N₄ exhibits excellent thermal metastability in air, maintaining stability up to 1400 °C, which is notably superior to both diamond and cBN.
Exceptional Mechanicals: Achieved Vickers Hardness (H_v) of 34.9 GPa and high Fracture Toughness (K_IC) of 3.5 MPa-m^1/2, making it tougher than many common transparent ceramics (e.g., MgAl₂O₄ spinel).
Intrinsic Optical Transparency: The material shows high real in-line transmission (18-38% RIT in the visible spectrum) and an intrinsic optical transparency below its bandgap energy (258 nm / 4.8 eV).
Microstructural Mechanism: Transparency is achieved due to the suppression of light scattering centers (pores and amorphous triple pockets) via the incorporation and segregation of oxygen atoms into ultra-thin (< 1 nm) silicon oxynitride intergranular films (IGFs).
Synthesis Method: Bulk samples were synthesized using HPHT at a fixed pressure of 15.6 GPa and optimal temperature of 1800 °C.

Technical Specifications

Parameter	Value	Unit	Context
Synthesis Pressure	15.6	GPa	Fixed synthesis condition
Optimal Temperature	1800	°C	Yielded single-phase, transparent c-Si₃N₄
Vickers Hardness (H_v)	34.9 ± 0.7	GPa	Measured at 9.8 N indentation load
Fracture Toughness (K_IC)	3.5 ± 0.2	MPa-m^1/2	Average measured value
Bulk Modulus (B)	303.4 ± 4.0	GPa	Calculated elastic property
Young’s Modulus (E)	583.8 ± 10.1	GPa	Calculated elastic property
Poisson’s Ratio (ν)	0.1793 ± 0.0056	Dimensionless	Calculated elastic property
Measured Bulk Density	4.07 ± 0.08	g/cm³	Supports negligible porosity
Bandgap Energy	4.8 ± 0.2	eV	Corresponds to 258 nm wavelength
Real In-line Transmission (RIT)	18 - 38	%	Measured across visible spectrum (400-800 nm)
Max Operating Temperature (Air)	Up to 1400	°C	Thermal stability reference
Average Grain Size	143 ± 59	nm	Nanocrystalline structure
Sample Thickness	0.464	mm	Measured for transmission analysis

Key Methodologies

The transparent polycrystalline c-Si₃N₄ was synthesized under extreme conditions using a Kawai-type HPHT apparatus and characterized using advanced microstructural and mechanical techniques.

Starting Material:
- Commercially available $\alpha$-Si₃N₄ powder (SN-E10, Ube Industries) was used, characterized by >95 wt% $\alpha$-phase content and an oxygen content of <2 wt%.
- Powder was dried in an oil-free vacuum oven (~8hPa) at 200 °C for 12 hours prior to use.
Sample Encapsulation:
- Dried powder was enclosed in a Platinum (Pt) sleeve and disks, which were then embedded into an outer MgO sleeve with MgO lids.
- Pt and MgO parts were pre-heated at 1000 °C for 10 minutes before final assembly and further vacuum drying (150 °C for >2 hours).
HPHT Synthesis:
- Synthesis runs were conducted using a Walker-module (mavo press LPR 1000-400/50).
- Pressure was fixed at 15.6 GPa.
- Temperatures tested were 1600 °C, 1700 °C, and 1800 °C.
- The 1800 °C samples yielded single-phase c-Si₃N₄ that was fully sintered and transparent.
Microstructural and Compositional Analysis:
- X-ray Diffraction (XRD): Used to confirm the single-phase c-Si₃N₄ structure and determine the unit cell parameter ($a$ = 7.7373 ± 0.0006 Å).
- STEM-EDS (Atomic Resolution): Used to observe grain boundaries and triple junctions, confirming the absence of amorphous triple pockets and the existence of ultra-thin (<1 nm) silicon oxynitride intergranular films (IGFs).
- EELS Spectroscopy: Confirmed oxygen atom segregation to the IGFs, elucidating the mechanism for achieving optical transparency by minimizing scattering centers.
Mechanical and Optical Testing:
- Vickers/Knoop Indentation: Used to measure hardness (H_v) and calculate fracture toughness (K_IC).
- Brillouin Scattering: Employed to determine elastic wave velocities (Vp and Vs) and calculate bulk, shear, and Young’s moduli.
- Light Transmission: Real In-line Transmission (RIT) was measured using a double-beam spectrophotometer (240 to 1600 nm) on samples polished down to 1 µm surface finish.

6CCVD Solutions & Capabilities

The synthesis of transparent c-Si₃N₄ demonstrates a critical industry demand for materials offering extreme mechanical properties combined with high-temperature optical performance. While c-Si₃N₄ ranks third in hardness, MPCVD Diamond remains the superior choice for absolute maximum performance in applications demanding the highest strength, broadest transparency, and unmatched thermal conductivity.

6CCVD provides the SCD/PCD materials and engineering services necessary to replicate or extend high-performance material research utilizing diamond, the hardest known material.

Application Requirement	6CCVD MPCVD Diamond Solution	Technical Capability Alignment
Ultimate Hardness & Toughness: Research demands materials harder than $c$-Si₃N₄ (34.9 GPa) for survivability.	Optical Grade Single Crystal Diamond (SCD): Offers the highest H_v (80-100 GPa) and K_IC, ideal for next-generation protective and industrial windows.	6CCVD delivers optical SCD wafers up to 500µm thickness, with ultra-low nitrogen incorporation for broad spectral transmission (UV to IR).
Transparent Substrate Dimensions: Research used small disks (2mm); industrial application requires larger optics.	Large-Area Polycrystalline Diamond (PCD): 6CCVD synthesizes large, homogeneous PCD wafers suitable for scalable industrial and defense applications.	Custom Dimensions: Plates/wafers up to 125mm (PCD). Polishing: Inch-size PCD wafers polished to Ra < 5nm, ensuring minimal light scattering comparable to transparent ceramics.
Extreme Environment Integration: Need for custom metal contacts for sensors operating in high-pressure/high-temperature fields (e.g., HPHT anvils, geothermal probes).	Integrated Metalization Services: We offer internal capabilities to pattern and deposit complex metal stacks directly onto diamond surfaces.	Metalization Options: Standard and custom layers including Au, Pt, Pd, Ti, W, and Cu, supporting advanced device fabrication and contacting.
High Density / Low Defect Rate: c-Si₃N₄ success depended on eliminating pores and minimizing IGF thickness.	High Purity CVD Synthesis Control: 6CCVD maintains strict control over the MPCVD environment, ensuring high-purity, low-defect density SCD and PCD suitable for high-power laser optics.	Thickness Control: Precise material thicknesses for SCD and PCD ranging from 0.1µm to 500µm, plus robust Substrates up to 10mm.
Electrochemical Sensors in Harsh Environments: Boron-doped materials offer stability in corrosive/HPHT systems.	Boron-Doped Diamond (BDD) Wafers: Ideal for electrochemically stable sensors and electrodes required in extreme synthesis or analytical chemistry projects.	Offers high-quality, heavily B-Doped (BDD) MPCVD materials.

Engineering Support: 6CCVD’s in-house PhD team can assist with material selection, polishing requirements (Ra < 1nm for SCD), and custom metalization protocols for projects similar to high-pressure window design or extreme environment sensor technology.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

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Original Source

DOI: https://doi.org/10.1038/srep44755