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 with a spinel structure in compositions of MgAl
O
and aluminum oxynitride (γ-AlON) show high optical transparency. Here we report the synthesis of the hardest transparent spinel ceramic, i.e. polycrystalline cubic silicon nitride (c-Si
N
). This material shows an intrinsic optical transparency over a wide range of wavelengths below its band-gap energy (258 nm) and is categorized as one of the third hardest materials next to diamond and cubic boron nitride (cBN). Since the high temperature metastability of c-Si
N
in air is superior to those of diamond and cBN, the transparent c-Si
N
ceramic can potentially be used as a window under extremely severe conditions.
Hardness of polycrystalline SiO2 coesite Kulik, Eleonora; Nishiyama, Norimasa; Higo, Yuji ...
Journal of the American Ceramic Society,
20/May , Letnik:
102, Številka:
5
Journal Article
Recenzirano
We measured elastic moduli and hardness of polycrystalline SiO2 coesite. Translucent polycrystalline bulk coesite with a grain size of about 10 micrometers was fabricated at 8 GPa and 1600°C using a ...Kawai‐type multianvil apparatus. The obtained bulk and shear moduli are 94(1) and 60.2(3) GPa, respectively. The resulting Vickers and Knoop hardness values are 10.9(7) and 9.6(4) GPa, respectively, at an indentation load of 4.9 N. Coesite is as hard as other fourfold coordinated silica materials such as quartz and densified silica glasses. The hardness values of coesite and the fourfold coordinated silica materials are about one‐third of those of sixfold coordinated silica materials, stishovite, and seifertite, which are the hardest known oxides.
Thermal expansion of synthetic coesite was studied with synchrotron powder X-ray diffraction in the temperature range of 100–1000 K. We determined the unit cell parameters of monoclinic coesite (
a, ...b, c
, and
β
) every 50 K in this temperature range. We observed that
a
and
b
parameters increase with increasing temperature, while
c
decreases. The
β
angle also decreases with temperature and approaches 120°. As a result, the unit cell volume expands by only 0.7% in this temperature range. Our measurements provide thermal expansion coefficients of coesite as a function of temperature: it increases from 3.4 × 10
−6
K
−1
at 100 K to 9.3 × 10
−6
K
−1
at 600 K and remains nearly constant above this temperature. The Suzuki model based on the zero-pressure Mie–Grüneisen equation of state was implemented to fit the unit cell volume data. The refined parameters are
V
0
= 546.30(2) Å
3
,
Q
= 7.20(12) × 10
6
J/mol and
θ
D
= 1018(43) K, where
θ
D
is the Debye temperature and
V
0
is the unit cell volume at 0 K with an assumption that
K
′
is equal to 1.8. The obtained Debye temperature is consistent with that determined in a previous study for heat capacity measurements.
Phase relations in silicon and germanium nitrides (Si3N4 and Ge3N4) were investigated using a Kawai‐type multianvil apparatus and a laser‐heated diamond anvil cell combined with a synchrotron ...radiation. The pressure‐induced phase transition from the β to γ (cubic spinel‐type structure) phase was observed in both compositions. We observed the coexistence of the β and γ phases in Si3N4 at 12.4 GPa and 1800°C, while the appearance of single phase γ‐Ge3N4 was observed at pressures above 10 GPa. Our observations under higher pressures revealed that γ‐Si3N4 and γ‐Ge3N4 have wide stability fields and no postspinel transition was observed up to 98 GPa and 2400°C in both compositions. Using the room‐temperature compression curves of these materials, the bulk moduli (K0) and their pressure derivatives (K′0) were determined: K0 = 317 (16) GPa and K′0 = 6.0 (8) for γ‐Si3N4; K0 = 254 (13) GPa and K′0 = 6.0 (7) for γ‐Ge3N4.
We report the synthesis of alumina/stishovite nano‐nano composite ceramics through a pressure‐induced dissociation in Al2SiO5 at a pressure of 15.6 GPa and temperatures of 1300°C‐1900°C. Stishovite ...is a high‐pressure polymorph of silica and the hardest known oxide at ambient conditions. The grain size of the composites increases with synthesis temperature from ~15 to ~750 nm. The composite is harder than alumina and the hardness increases with reducing grain size down to ~80 nm following a Hall–Petch relation. The maximum hardness with grain size of 81 nm is 23 ± 1 GPa. A softening with reducing grain size was observed below this grain size down to ~15 nm, which is known as inverse Hall–Petch behavior. The grain size dependence of the hardness might be explained by a composite model with a softer grain‐boundary phase.
Transparent polycrystalline nanoceramics consisting of triclinic Al2SiO5 kyanite (91.4 vol%) and Al2O3 corundum (8.6 vol%) were fabricated at 10 GPa and 1200‐1400°C. These materials were obtained by ...direct conversion from Al2O3‐SiO2 glasses fabricated using the aerodynamic levitation technique. The material obtained at 10 GPa and 1200°C shows the highest optical transparency with a real in‐line transmission value of 78% at a wavelength of 645 nm and a sample‐thickness of 0.8 mm. This sample shows equigranular texture with an average grain size of 34 ± 13 nm. The optical transparency increases with decreasing mean grain size of the constituent phases. The relationship between real in‐line transmission and grain size is well explained by a grain‐boundary scattering model based on a classical theory.
Silicon dioxide has eight stable crystalline phases at conditions of the Earth's rocky parts. Many metastable phases including amorphous phases have been known, which indicates the presence of large ...kinetic barriers. As a consequence, some crystalline silica phases transform to amorphous phases by bypassing the liquid via two different pathways. Here we show a new pathway, a fracture-induced amorphization of stishovite that is a high-pressure polymorph. The amorphization accompanies a huge volume expansion of ~100% and occurs in a thin layer whose thickness from the fracture surface is several tens of nanometers. Amorphous silica materials that look like strings or worms were observed on the fracture surfaces. The amount of amorphous silica near the fracture surfaces is positively correlated with indentation fracture toughness. This result indicates that the fracture-induced amorphization causes toughening of stishovite polycrystals. The fracture-induced solid-state amorphization may provide a potential platform for toughening in ceramics.
Hardness of polycrystalline SiO 2 coesite Kulik, Eleonora; Nishiyama, Norimasa; Higo, Yuji ...
Journal of the American Ceramic Society,
05/2019, Letnik:
102, Številka:
5
Journal Article
Recenzirano
Abstract
We measured elastic moduli and hardness of polycrystalline SiO
2
coesite. Translucent polycrystalline bulk coesite with a grain size of about 10 micrometers was fabricated at 8 GPa and ...1600°C using a Kawai‐type multianvil apparatus. The obtained bulk and shear moduli are 94(1) and 60.2(3) GPa, respectively. The resulting Vickers and Knoop hardness values are 10.9(7) and 9.6(4) GPa, respectively, at an indentation load of 4.9 N. Coesite is as hard as other fourfold coordinated silica materials such as quartz and densified silica glasses. The hardness values of coesite and the fourfold coordinated silica materials are about one‐third of those of sixfold coordinated silica materials, stishovite, and seifertite, which are the hardest known oxides.
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