Crystal structure of rare-earth silicon-oxynitride J-phases,Ln4Si2O7N2

Rare-earth silicon-oxynitride J-phases, Ln₄Si₂O₇N₂ (Ln=Y, La, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu), were prepared by the N₂ gas-pressured sintering method at 1 MPa of N₂ and 1500–1700 °C. The Rietveld analysis was carried out for X-ray powder diffraction data measured at room temperature. The crystal structures of Ln₄Si₂O₇N₂ were refined with the structure model of La₄Si₂O₇N₂ for Ln=La, Pr, Nd, and Sm, and with that of Lu₄Si₂O₇N₂ for Ln=Y, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. The refined monoclinic unit-cell parameters (lengths a, b, c, angles β, and volume V) increased linearly in their two series of Ln with increasing ionic radii of rare-earth atoms. Discontinuities of the unit-cell parameters were found between the two Ln series.     

Si3N4-ZrB2 ceramics prepared at low temperature with improved mechanical properties

Si₃N₄-ZrB₂ ceramics were hot-pressed at 1500 °C using self-synthesized fine ZrB₂ powders containing 2.0 wt% B₂O₃ together with MgO-Re₂O₃ (Re = Y, Yb) additives. Both Si₃N₄ and ZrB₂ grains in the hot-pressed ceramics were featured with elongated and equiaxed morphology. The presence of elongated Si₃N₄ and ZrB₂ grains led to the partial texture of the ceramics under the applied pressure. Vickers hardness and fracture toughness of Si₃N₄-ZrB₂ ceramics with MgO-Re₂O₃ additives prepared at low temperature were about 19–20 GPa and 9–11 MPa m^1/2, respectively, higher than the reported values of Si₃N₄-based ceramics prepared at high temperature (1800 °C or above) under the same test method.     

Influences of rare-earth oxide additives on the formation and properties of porous Si2N2O ceramic

Here we prepared porous silicon oxynitride (Si₂N₂O) ceramics by reaction sintering of SiO₂ and Si₃N₄ using five different rare-earth oxides (RE₂O₃, RE = Lu, Yb, Y, Sm, and La) as sintering aids. The influences of RE₂O₃ on the formation, densification, microstructure, and mechanical properties of Si₂N₂O ceramics have been investigated in detail. The results have indicated that with the increase in RE ionic radius, the formation temperature of Si₂N₂O decreases, and the densification process could be promoted by RE₂O₃ with larger RE³⁺ ionic radius. In addition, microstructures and mechanical properties are highly dependent on the RE₂O₃ additives. With the increase in RE³⁺ ionic radius, Si₂N₂O changes from platelike crystals to elongated crystals. The samples doped with La₂O₃ and Sm₂O₃ with elongated crystals exhibit higher flexural strength and higher Vickers hardness.     

Intergranular Nanostructure Effects on Strength and Toughness of Si3N4

Strength, toughness, microstructure, and atomic adsorption arrangement in silicon nitrides with MgO and RE₂O₃ additions (RE = La, Gd, Y, Lu) were examined. Mechanical properties were high for La, Gd, and equal La–Lu additions, but surprisingly were progressively lower for Y‐ and Lu‐doped samples. The lower strength and toughness were associated with fewer visible crack deflections and grain bridges. Detailed microstructural analysis of the Lu‐doped material revealed a complex intergranular nanostructure with variable Lu content and Si₃N₄ nanocrystals. Furthermore, the Lu‐rich areas showed an extra Lu‐adsorption site on the Si₃N₄ prismatic planes not previously observed in other studies. This inhomogeneous structure was attributed to grain growth impingement and higher viscosity of the Lu‐doped oxynitride glass that slows homogenization. The Y‐doped material with nearly identical glass viscosity demonstrates intermediate behavior. Finally, substituting half of the Lu₂O₃ with La₂O₃ resulted in a homogenous intergranular structure, attributed to a lower viscosity of the oxynitride glass phase, and high mechanical properties. Overall, care must be taken when adapting Si₃N₄ processing parameters for the smaller ionic radius rare earth dopants such as Lu and Y.     

Microwave dielectric properties of silico-carnotite Ca3M2Si3O12 (M= Yb,Y) ceramics synthesized via high energy ball milling

The Ca₃M₂Si₃O₁₂ (M = Yb, Y) ceramics with orthorhombic silico-carnotite structure were fabricated via high-energy ball milling and solid-state reaction route. Dense Ca₃Yb₂Si₃O₁₂ and Ca₃Y₂Si₃O₁₂ ceramics sintered at 1260 °C and 1240 °C revealed promising microwave dielectric properties with ε_r = 9.2 and 8.7, Q×f = 56,400 GHz and 29,094 GHz, τ_f = −77.5 ppm/°C and −76.8 ppm/°C, respectively. The connection between crystal structure and Q×f values of Ca₃M₂Si₃O₁₂ (M = Yb, Y) ceramics was discussed with respect to the packing fraction, and their intrinsic microwave dielectric properties were examined using the infrared reflectivity spectra analysis. The thermal stability of Ca₃Yb₂Si₃O₁₂ was improved successfully by forming 0.91Ca₃Yb₂Si₃O₁₂‐0.09CaTiO₃ composite ceramics with τ_f = +2.9 ppm/°C, ε_r = 12.93 and Q×f = 26,729 GHz.     

On the fabrication and mechanical properties of Si3N4 ceramics with low content sintering additives

Si₃N₄ ceramics were prepared by hot pressing (HP) and spark plasma sintering (SPS) methods using low content (5 mol%) Al₂O₃–RE₂O₃(RE = Y, Yb, and La)–SiO₂/TiN as sintering additives/secondary additives. The effects of sintering additives and sintering methods on the composition, microstructures, and mechanical properties (hardness and fracture toughness) were investigated. The results show that fully density Si₃N₄ ceramics could be fabricated by rational tailoring of sintering additives and sintering method, and TiN secondary additive could promote the density during HP and SPS. Besides, SN-AYS-SPS possesses the most competitive mechanical properties among all the as-prepared ceramics with the Vickers hardness as 17.31 ± .43 GPa and fracture toughness as 11.07 ± .48 MPa m^1/2.     

Variable thermochemical stability of RE2Si2O7 (RE = Sc,Nd, Er,Yb, or Lu) in high-temperature high-velocity steam

Five rare-earth (RE) disilicates (RE₂Si₂O₇, RE = Sc, Nd, Er, Yb, or Lu) were synthesized and exposed to high-velocity steam (up to 235 m/s) for 125 hours at 1400°C. Water vapor reaction products, mass loss, average reaction depths, and product phase microstructural evolution were analyzed for each material after exposure. Similar to steam testing results in the literature, RE₂Si₂O₇ (RE = Er, Yb, Lu) underwent silica depletion producing gaseous silicon hydroxide species, RE₂SiO₅, and RE₂O₃ product phases. Sc₂Si₂O₇ reacted with high-velocity steam to produce only a Sc₂O₃ product layer with no stable Sc₂SiO₅ phase detected by X-ray diffraction or microscopy techniques. Further, Nd₂Si₂O₇ rapidly reacted with steam to produce with no Nd₂SiO₅ or Nd₂O₃ reaction products. All RE₂Si₂O₇ that produced a silicate reaction product (RE = Nd, Er, Yb, Lu) showed densification of the product phase at steam velocities above 150 m/s that resulted in enhanced resistance. The results presented in this work demonstrate that rare-earth silicates show diverse steam reaction products, reaction product microstructures, and total reaction depths after high-temperature high-velocity steam exposure. Of the materials in this study, RE₂Si₂O₇ (RE = Yb, Lu) were most stable in high-temperature high-velocity steam, making them most desirable as environmental barrier coating candidates.     

Thermal and mechanical properties of Si3N4 ceramics obtained via two-step sintering

Si₃N₄ ceramics were prepared using novel two-step sintering method by mixing α-Si₃N₄ as raw material with nanoscale Y₂O₃–MgO via Y(NO₃)₃ and Mg(NO₃)₂ solutions. Si₃N₄ composite powders with in situ uniformly distributed Y₂O₃–MgO were obtained through solid–liquid (SL) mixing route. Two-step sintering method consisted of pre-deoxidization at low temperature via volatilization of in situ-formed MgSiO₃ and densification at high temperature. Variations in O, Y, and Mg contents in Si₃N₄–Y₂O₃–MgO during first sintering step are discussed. O and Mg contents decreased with increasing temperature because SiO₂ on Si₃N₄ surface reacted with MgO to form low-melting-point MgSiO₃ compound, which is prone to volatilize at high temperature. By contrast, Y content hardly changed due to high-temperature stability of Y–Si–O–N quaternary compound. In the second sintering step, skeleton body was densified, and the formation of Y₂Si₃O₃N₄ secondary phase occurred simultaneously. Two-step sintered Si₃N₄ ceramics had lower total oxygen content (1.85 wt%) than one-step sintered Si₃N₄ ceramics (2.51 wt%). Therefore, flexural strength (812 MPa), thermal conductivity (92.1 W/m·K), and fracture toughness (7.6 MPa?m^1/2) of Si₃N₄ ceramics prepared via two-step sintering increased by 28.7%, 16.9%, and 31.6%, respectively, compared with those of one-step sintered Si₃N₄ ceramics.     

Synthesis and Crystal Structure of Na2Ba9Si20O50—An Intermediate Phase Along the Join Na2Si2O5‐BaSi2O5

Single crystals of Na₂Ba₉Si₂₀O₅₀ were obtained from solid state reactions performed along the join Na₂Si₂O₅‐BaSi₂O₅. The crystal structure has been determined from a data set collected at ambient temperatures and subsequently refined to a residual of R(|F|) = 0.0328 for 2211 independent reflections. The compound belongs to the group of phyllosilicates and adopts the monoclinic space group C2/m with the following lattice parameters: a = 39.111(3) Å, b = 7.6566(6) Å, c = 8.2055(6) Å, β = 97.319(6)°, V = 2437.2(3) Å³, Z = 2. Furthermore, weak one‐dimensional diffuse streaks running parallel to a* as well as a very small number of low intensity reflections at b*/3, indicating the presence of a superstructure, were observed. Basic buiding units are silicate layers parallel to (40‐1) which can be obtained from the condensation of single chains with a periodicity of four running along [010]. The sheets can be partitioned into two kinds of consecutive strips containing (i) a sequence of four‐ and eight‐membered rings and (ii) a four‐ring wide “zig‐zag shaped” unit consisting of exclusively six‐membered rings. The sodium and barium cations—distributed among six crystallographically independent positions—are sandwiched between subsequent layers and are linked to seven to nine nearest oxygen neighbors. The structure of Na₂Ba₉Si₂₀O₅₀ is closely related to that of K₂Ba₅Si₁₂O₃₀ and K₂Ba₇Si₁₆O₄₀, respectively. There are strong arguments that the previously claimed phase Na₄Ba₈Si₂₀O₅₀ is actually misinterpreted Na₂Ba₉Si₂₀O₅₀ and that the composition of the intermediate phase along the join Na₂Si₂O₅–BaSi₂O₅ is slightly different from that described in the literature.     

M2(Si,Al)4(N,C)7 (M = La,Y,Ca) carbonitrides: II. The crystal structure of Ca0.8Y1.2Si4N6.8C0.2

A new (Ca,Y)Si₄(N,C)₇ phase has been characterised lying between the two end-members Y₂Si₄N₆C and CaYSi₄N₇. This phase is similar to BaYbSi₄N₇, which is made up of a network of [N(SiN₃)₄] structural units linked together in a three-dimensional network, with the large cations located in the interstices, but (Ca,Y)Si₄(N,C)₇ is a disordered variant, with nitrogen atoms partially occupying two sets of equivalent sites related by the combined operations of rotation and tilt. The crystal of (Ca,Y)Si₄(N,C)₇ used for structure determination contained Ca and Y in the atomic ratio 2:3, the excess positive charge in the cation sites being balanced by the partial replacement of nitrogen by carbon in the central non-metal site of the [N(SiN₃)₄] unit. Powder diffraction data are listed for Ca_0.8Y_1.2Si₄N_6.8C_0.2, which is hexagonal with a=5.9874(4), c=9.7849(8) Å at ambient temperature. The crystal structure has been determined from single crystal data; Z=2; S.G. P6₃mc (no. 186); R_int=0.0274, R1=0.0384, wR2=0.0993 for all data.     

Synthesis and phosphorescence mechanism of yellow-emissive long-afterglow phosphor BaAl2Si3O4N4: Yb2+

A series of novel Ba_1-xAl₂Si₃O₄N₄: xYb²⁺ yellow-emissive long-afterglow phosphors was successfully synthesized via a high-temperature solid-state reaction with two sintering steps. The phosphors exhibited broad band emission (full width at half maximum, FWHM = 107 nm) with a peak at 518 nm. With an increase in the Yb²⁺ doping content in Ba_1-xAl₂Si₃O₄N₄: xYb²⁺ phosphors, the luminescence intensity reached a maximum at x = 0.15. The long-afterglow phosphor can be activated effectively by 254 nm UV light, and the afterglow can persist for more than 7 h. From the thermoluminescence (TL) spectrum, there are three types of electron traps, with average evaluation depths of 0.485, 0.592, and 0.681 eV. The afterglow is caused by the capture and re-release of free electrons by the three trap energy levels. Finally, the thermal and chemical stability of the phosphor was tested, its luminescence intensity was 78.7% at 150 °C and 99.5% after 5 h soak in water compared to that at room temperature and original, respectively.     

Catalytic nitridation preparation of high-performance Si3N4(w)-SiC composite using Fe2O3 nano-particle catalyst: Experimental and DFT studies

The complete conversion from Si into Si₃N₄ was achieved after 2 h nitridation at 1400 °C by using in-situ formed Fe₂O₃ nano-particles (NPs) as a catalyst. Such a synthesis condition was remarkably milder than that (>1450 °C for many hours) required by the conventional Si nitridation method. Density functional theory (DFT) calculations suggest that Fe₂O₃ catalyst accelerates the Si nitridation via weakening the bond strength of absorbed N₂ molecule. Furthermore, Si₃N_4(w)-SiC composites prepared by the present catalytic nitridation method showed excellent high-temperature properties including modulus of rupture (MOR of 29.9 MPa at 1400 °C), thermal shock resistance (residual MOR percentage of 50% at ΔT = 1300 °C), as well as good oxidation resistance and cryolite corrosion resistance against molten cryolite. It can be concluded that, Fe₂O₃ NPs not only greatly accelerated the Si nitridation and Si₃N₄ formation, but also facilitated the epitaxial growth of reinforcement phase of Si₃N₄ whisker in the Si₃N_4(w)-SiC composites.     

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.     

Crystal structural and microwave dielectric properties of Ba4B'Nb3O12 (B′=Yb,Tm, Er,Y, Ho,Dy, Gd) ceramics

The influence of substitution of rare-earth ion (RE = Yb, Tm, Er, Y, Ho, Dy, Gd) for B′-site on phase composition, crystal structure, micromorphology, and microwave dielectric properties of Ba₄RENb₃O₁₂ ceramics are investigated. The results of XRD and Rietveld refinement indicate that the Ba₄RENb₃O₁₂ ceramic is composed of Ba(RE_1/12Nb_9/12)O₃ and Ba (RE_1/2Nb_1/2)O₃ phases. Porosity analysis shows that the relative density of Ba₄RENb₃O₁₂ ceramics can reach more than 95.25%. The variations of permittivity and temperature coefficient are associated with ionic polarization and cell polarization, respectively. The A_1g Raman‐active modes at 754 cm^?1 are oxygen-octahedron stretch vibration, reflecting the variation of the structure. Optimal microwave dielectric properties (ε_r = 38.75, Q × f = 41251 GHz, τ_f = 71.57 ppm/°C) of Ba₄RENb₃O₁₂ ceramics for RE = Yb are obtained at 1425 °C for 6 h.     

Mechanical properties and microstructure of Al2O3-SiCw ceramic tool material toughened by Si3N4 particles

Al₂O₃-SiC_w toughened ceramic tools play vital role in high-speed machining of nickel-based superalloys due to their superior mechanical properties. Herein, owing to synergistic toughening mechanism, α-Si₃N₄ particles are employed as reinforcement phase into Al₂O₃-SiC_w ceramic composite to optimize mechanical properties of Al₂O₃-SiC_w ceramic tools. Moreover, the influence of Si₃N₄ content and sintering parameters on microstructure and mechanical properties of Al₂O₃-20 vol%SiC_w ceramic tool material is systematically investigated. Results reveal that appropriate amount of Si₃N₄ particles is required to effectively increase the density of Al₂O₃-SiC_w ceramic composites. The presence of Si₃N₄ particles leads to formation of novel β-sialon phase during hot-press sintering, which effectively enhances fracture toughness and flexural strength of Al₂O₃-SiC_w ceramic composites. It is observed that grain size of newly formed β-sialon phase is extremely sensitive to hot-pressing sintering conditions. The degree of chemical transformation of α-Si₃N₄ into Si_6-zAl_zO_zN_8-z (β-sialon) and z-value of Si_6-zAl_zO_zN_8-z are significantly influenced by sintering temperature. Overall, Al₂O₃-20 vol%SiC_w-15 vol%Si₃N₄ ceramic tool material, with 1.5 vol%Y₂O₃-0.5 vol%La₂O₃-0.5 vol%CeO₂ (YLC) sintering additive, rendered optimal mechanical properties after sintering at 1600 °C under 32 MPa for 30 min. Improved mechanical performance can be ascribed to synergistic toughening and strengthening influence of whiskers and particles.     

Mechanical and dielectric properties of porous Si2N2O–Si3N4 in situ composites

Mechanical and dielectric properties of porous Si₂N₂O–Si₃N₄ in situ composites fabricated for use as radome by gel-casting process were investigated. The flexural strength of the Si₂N₂O–Si₃N₄ ceramics is 230.46 ± 13.24 MPa, the complex permittivity of the composites varies from 4.34 to 4.59 and the dissipation factor varies from 0.00053 to 0.00092 from room temperature to elevated temperature (1150 °C) at the X-band. In the porous regions, some Si₂N₂O fibers (50–100 nm in diameter) are observed which may improve the materials properties.     

Synthesis,Crystal Structure,and Luminescence Properties of Y4Si2O7N2: Eu2+ Oxynitride Phosphors

Y₄Si₂O₇N₂: Eu²⁺ phosphor has been prepared by a pretreatment method. Reduction in Eu³⁺ ions into Eu²⁺ by the use of hydrogen iodide (HI) is verified by X‐ray absorption near‐edge structure (XANES) and electrode potential analysis. Y₄Si₂O₇N₂: Eu²⁺ phosphor has a broad emission band in the range of 400–500 nm. Furthermore, the effect of Zr doping on the structure and luminescence properties of Y₄Si₂O₇N₂: Eu²⁺ phosphor is researched. It found that the Zr doping leads to an emission blueshift, and improves the luminescence intensity and thermal quenching behavior of Y₄Si₂O₇N₂: Eu²⁺ phosphors. Prospectively, the pretreatment approach could be extended to develop other Eu²⁺‐doped compounds.     

Enhanced thermal conductivity in Si3N4 ceramic with the addition of Y2Si4N6C

Si₃N₄ ceramic was densified at 1900°C for 12 hours under 1 MPa nitrogen pressure, using MgO and self‐synthesized Y₂Si₄N₆C as sintering aids. The microstructures and thermal conductivity of as‐sintered bulk were systematically investigated, in comparison to the counterpart doped with Y₂O₃‐MgO additives. Y₂Si₄N₆C addition induced a higher nitrogen/oxygen atomic ratio in the secondary phase by introducing nitrogen and promoting the elimination of SiO₂, resulting in enlarged grains, reduced lattice oxygen content, increased Si₃N₄‐Si₃N₄ contiguity and more crystallized intergranular phase in the densified Si₃N₄ specimen. Consequently, the substitution of Y₂O₃ by Y₂Si₄N₆C led to a great increase in ~30.4% in thermal conductivity from 92 to 120 W m^?1 K^?1 for Si₃N₄ ceramic.     

Some aspects of glass formation in R′2O−P2O5 and R″O−P2O5 systems

The limits of the glass-formation regions are established for the binary R′₂−P₂O₅ and R″O₅−P₂O₅ systems (R′=Li, Na, K: R″=Mg, Ca, Sr, Ba). It is shown that these boundaries closely correlate with those reported by other investigators despite the difference in the conditions in which these binary phosphate glasses were synthesized. The fact that all of the systems under study have about the same upper limits of glass-formation regions is attributed to the ratio R′₂O(R′O): P₂O₅, which for the systems in question is 1.0–1.5 rather than the individual crystal-chemical characteristics of the R⁺ and R²⁺ cations. Translated from Steklo i Keramika, No. 2, pp. 13–15 February, 1997.     

Effect of sintering additives on the oxidation behavior of Si3N4 ceramics at 1300 °C

This paper describes the results of systematic investigation of the oxidation behavior of Si₃N₄ based ceramics. The tests were carried out at 1300 °C for 2000 h in a high-temperature dry air environment. The Si₃N₄ specimens tested include the following: (a) S-1: Si₃N₄ added 8 mass% Y₂O₃, (b) S-2: Si₃N₄/SiC added 8 mass% Y₂O_3, (c) S-3: Si₃N₄ added 5 mass% Y₂O₃ and 3 mass% Al₂O_3, (d) S-4: Si₃N₄/SiC added 5 mass% Y₂O₃ and 3 mass% Al₂O_3. Several interesting conclusions were obtained as follows: (1) the thicknesses of oxidized layer of S-3 and S-4 were much thicker than S-1 and S-2, (2) oxidation kinetics of S-1 and S-2 obeyed a parabolic law on the whole, while those of S-3 and S-4 had a break, (3) the yttrium (Y) concentration under the oxidized layer decreased significantly. The Y-decreased zone was defined as a diffused layer. The thicknesses of the diffused layers of S-3 and S-4 samples were very large. (4) Primarily, the crystalline phases in the oxidized layer were SiO₂ and Y₂Si₂O_7. (5) The effect of SiC composition on the oxidation behavior was small.