Relation between thermal conductivity,sintering mechanism and microstructure of AIN with yttrium aluminate grain boundary phases

High thermal conductivity, polycrystalline, AIN ceramics are being considered as microelectronic packaging materials. Careful microstructural characterization of AIN with various Y₂O₃ contents has been used to determine the particular yttrium aluminate second phases formed on sintering. The presence and morphology of the aluminates explains the variation of thermal conductivity with Y₂O₃ content and gives an indication of the sintering mechanism.     

Thermal diffusivity of SiO2 and Y2O3 added AIN ceramics

The addition of both SiO₂ and Y₂O₃ to AIN led to decrease of 27R polytype in specimens sintered above 1600° C and also to an increase of thermal diffusivity of AIN ceramics. Furthermore, SiO₂ and Y₂O₃ added AIN ceramics were fully densified by liquid-phase sintering, and resulted in higher thermal diffusivity. The formation temperature of the liquid phase was lowered more by the addition of both SiO₂ and Y₂O₃ than only Y₂O₃ to AIN ceramics.     

Microstructure and thermal conductivity of spark plasma sintering AlN ceramics

Abstract

Dense aluminium nitride ceramics were prepared by spark plasma sintering at a lower sintering temperature of 1700°C with Y₂O₃, Sm₂O₃ and Dy₂O₃ as sintering additives respectively. The effects of three kinds of sintering additives on the phase composition, microstructure and thermal conductivity of AlN ceramics were investigated. The results showed that those sintering additives not only facilitated the densification via the liquid phase sintering mechanism, but also improved thermal conductivity by decreasing oxygen impurity. Sm₂O₃ could effectively improve thermal conductivity of AlN ceramics compared with Y₂O₃ and Dy₂O₃. Observation by scanning electron microscopy showed that AlN ceramics prepared by spark plasma sintering method manifested quite homogeneous microstructures, but AlN grain sizes and shapes and location of secondary phases varied with the sintering additives. The thermal conductivity of AlN ceramics was mainly affected by the additives through their effects on the growth of AlN grain and the location of secondary phases.     

Flame synthesis and effects of host materials on Yb/Er co-doped upconversion nanophosphors

The upconversion nanophosphors (UCNPs) of Yb³⁺/Er³⁺ co-doped into Y₂O₃, La₂O₃, and Gd₂O₃ were synthesized via the combustion method and characterized by powder X-ray diffractometer (XRD), scanning electron microscopy (SEM) and upconversion fluorescence spectroscopy. The characterization results showed that at the same flame temperature (2705 K) and precursor concentration (0.1 M), pure monoclinic and cubic-phase phosphors were achieved on Gd₂O₃ and Y₂O₃ hosted UCNPs, respectively; while the mixed phases were observed on La₂O₃ hosted UCNPs. Further annealing process at 850 °C produced pure cubic-phase La₂O₃:Yb³⁺,Er³⁺ UCNPs; while there was no phase transition observed on Gd₂O₃:Yb³⁺,Er³⁺ UCNPs. The dependence of upconversion luminescence on precursor concentrations and host materials was then examined. The La₂O₃ and Gd₂O₃ hosts were shown to be the promising alternates for the commonly used Y₂O₃ hosts for rare-earth doped phosphors.     

Raw material effect on AIN powder synthesis from Al2O3 carbothermal reduction

The effect of starting Al₂O₃ raw materials on the synthesis of AIN powder by Al₂O₃ carbothermal reduction was investigated. The reactivity of-Al₂O₃ among other materials, is excellent, at 1500° C. The skeleton of raw powders remains in the AIN particle shape. The thermal conductivity for hot-pressed AIN was severely affected by the oxygen content in AIN powder.     

Effects of binary additives B2O3–Y2O3 on the microstructure and thermal conductivity of aluminum nitride ceramics

Aluminum nitride (AIN) ceramics, with binary additives B₂O₃-Y₂O₃, were sintered at temperatures from 1700 to 1850 °C. The microstructure and sintering characteristics were studied by XRD, HREM, SEM and TEM/EDS, which showed that Y₂O₃ gave different yttrium aluminates through the reaction with Al₂O₃ under different conditions. With the increase of sintering temperature, the yttrium-to-aluminum atomic ratio Y/Al decreased in the secondary phases of the sintered bodies. It was discovered that B₂O₃ could dissolve in the yttrium aluminates, forming some ordered structure with a superlattice. After sintering at 1850 °C for 4 h, a specimen with a fine microstructure and a thermal conductivity of 190 Wm^–1K^–1 was obtained.     

Y3−xErxAl5O12 Aluminate Ceramics: Preparation,Thermal Properties and Theoretical Model of Thermal Conductivity

Rare‐earth aluminate ceramics for thermal‐barrier coatings (TBCs) are synthesized. The Young's modulus and thermal properties decrease with erbium additive increasing. The Y_3?xEr_xAl₅O₁₂ ceramics (x = 1, 3) possess a much‐lower thermal conductivity compared with 8YSZ. The lower Young's modulus and thermal‐expansion coefficient are due to the larger atomic weight of the Er substitutional atom. Additional phonon‐scattering effects also contribute to the lower thermal conductivity. The results indicate that Y_3?xEr_xAl₅O₁₂ can be explored as a candidate material for TBC systems. A theoretical model that describes the influence of point defects on the thermal conductivity is discussed.     

Upconversion and infrared luminescences in Er/Yb codoped Y2O3 and (Y0.9La0.1)2O3 transparent ceramics

Er³⁺ and Yb³⁺ codoped Y₂O₃ and (Y_0.9La_0.1)₂O₃ transparent ceramics were fabricated by the conventional ceramics processing with nanopowders. Compared to Er/Yb:Y₂O₃, Er/Yb:(Y_0.9La_0.1)₂O₃ ceramics had higher transmittance. Intense upconversion (UC) and infrared emission (1543 nm) were observed under excitation of 980 nm. According to three intensity parameters Ω₂, Ω₄, and Ω₆ fitted by the Judd-Ofelt theory, the spectroscopic quality parameters (X), radiative lifetimes (τ_rad), and emission cross-sections (α_em) were determined. Er/Yb:(Y_0.9La_0.1)₂O₃ ceramics owned broader peaks and longer lifetime (12.3 ms) at 1548 nm due to the glass-like structure of (Y_0.9La_0.1)₂O₃ ceramics. The results showed Y₂O₃ and Y_1.8La_0.2O₃ transparent ceramics are promising gain media for developing the solid-state 1.5 μm optical amplifiers and tunable UC lasers.     

Local thermal effect at luminescent spot on upconversion luminescence in Y2O3:Er,Yb nanoparticles

Y₂O₃:Er³⁺,Yb³⁺ nanoparticles were synthesized using Pechini type sol-gel method and then characterized by XRD, TEM, SEM, Raman spectroscopy, and fluorescence spectrophotometer. Local temperature effect on upconversion luminescence intensities was theoretically analyzed and experimentally tested. These results indicate that a competition process between local temperature at luminescent spot and laser pump power density decides the development trend of upconversion luminescence intensity. Therefore, it can be concluded that the most intensive upconversion luminescence in Y₂O₃:Er³⁺,Yb³⁺ nanoparticles can be achieved at a certain pump power density, which should be slightly below a given constant value (the corresponding threshold of temperature).     

Phase transformations during the synthesis and sintering of Y<Subscript>2 − <Emphasis Type="Italic">x</Emphasis></Subscript>Yb<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>3</Subscript> nanopowders

The formation of an Y₂O₃-Yb₂O₃ solid solution (8 mol % Yb₂O₃) during the thermal decomposition of (Y,Yb)₂(CO₃)₃ · 2H₂O mixed carbonates obtained by coprecipitation from a nitrate solution has been studied by X-ray diffraction, thermal analysis, and optical microscopy. The results demonstrate that the formation of a cubic yttria-based solid solution begins in the range 470–500°C and reaches completion at temperatures above 1100°C. The unit-cell parameter a of the cubic solid solution and the X-ray density of the corresponding ceramic are 10.5910 Å and 5.349 g/cm³, which corresponds to the intended chemical composition of the isovalent substitutional solid solution: Y_1.84Yb_0.16O₃.     

Effect of zirconia (3 mol% yttria) additive on mechanical properties and structure of alumina ceramics

The effects of ZrO₂-3 mol% Y₂O₃ additives containing 7.3, 15, 23.3 and 32 vol% of ZrO₂ on _f, K _IC, H _v and the microstructure of hot-pressed alumina-based ceramics were investigated. The presence of the m-, t- and t-ZrO₂ phases was discovered by using X-ray diffraction and transmission electron microscopy. An inhomogeneous distribution of Y₂O₃ in the ZrO₂ grains was observed. The variation of the mechanical properties of the ceramics is explained by the influence of different toughening mechanisms and by a change in the structure of the material.     

Potential use of only Yb2O3 in producing dense Si3N4 ceramics with high thermal conductivity by gas pressure sintering

Yb₂O₃ is an efficient sintering additive for enhancing not only thermal conductivity but also the high-temperature mechanical properties of Si₃N₄ ceramics. Here we report the fabrication of dense Si₃N₄ ceramics with high thermal conductivity by the gas pressure sintering of α-Si₃N₄ powder compacts, using only Yb₂O₃ as an additive, at 1900 °C under a nitrogen pressure of 1 MPa. The effects of Yb₂O₃ content, sample packing condition and sintering time on the densification, microstructure and thermal conductivity were investigated. Curves of the density plotted against the Yb₂O₃ content exhibited a characteristic ‘N’ shape with a local minimum at 3 mol% Yb₂O₃ and nearly complete densification below and above this concentration. The effects of the sample packing condition on the densification, microstructure and thermal conductivity strongly depended on the Yb₂O₃ content. The embedded condition led to more complete densification but also to a decrease in thermal conductivity from 119 to 94 W m^-1 K⁻¹ upon 1 mol% Yb₂O₃ addition. The sample packing condition had little effect on the density and thermal conductivity (102–106 W m⁻¹ K⁻¹) at 7 mol% Yb₂O₃. The thermal conductivity value was strongly related to the microstructure.     

Infrared Luminescence of Y2O2S:Er3+ and Y2O3:Er3+

The diffuse reflectance and luminescence spectra of Y₂O₂S:Er³⁺ and Y₂O₃:Er³⁺ are studied under selective and polarized laser excitation. The results indicate that the Er³⁺ luminescence bands of yttrium oxysulfide in the 1.54-m region are one order of magnitude stronger and broader than those of yttria. Y₂O₂S:Er³⁺ is shown to contain two types of Er-related emission centers differing in the anion environment of the Er³⁺ ion.     

Effects of oxidation on intergranular phases in silicon nitride ceramics

The effects of oxidation on changes in the secondary phases of two Si₃N₄ ceramics were investigated by transmission electron microscopy. The Si₃N₄ materials were oxidized at 1400 °C for 168 h in laboratory air. One material, sintered with 5 vol% Yb₂O₃+0.5 vol% Al₂O₃, containing a Yb₂Si₂O₇ crystalline secondary phase, displayed no gross changes following oxidation. However, the thickness of the amorphous intergranular film was observed to have decreased by 20% from its initial thickness of 1.0 nm. The second Si₃N₄ material, sintered with 5 wt% Y₂O₃+1 wt% MgO, had a completely amorphous secondary phase. Devitrification of the secondary phase at multiple-grain junctions to -Y₂Si₂O₇ accompanied the outward diffusion of additive and impurity cations occurring in the residual amorphous intergranulàr films during oxidation. Substantial cavitation and intergranular phase depletion was observed at both multiple-grain junctions and two-grain boundaries. The equilibrium thickness of the amorphous intergranular film consequently decreased from 1.2 to 0.9 nm following oxidation. Purification of the amorphous intergranular films by diffusion of cations to the surface led to a reduction in impurity concentration, resulting in the observed thinning of grain-boundary films.     

Potential use of only Yb2O3 in producing dense Si3N4 ceramics with high thermal conductivity by gas pressure sintering

Abstract

Yb₂O₃ is an efficient sintering additive for enhancing not only thermal conductivity but also the high-temperature mechanical properties of Si₃N₄ ceramics. Here we report the fabrication of dense Si₃N₄ ceramics with high thermal conductivity by the gas pressure sintering of α-Si₃N₄ powder compacts, using only Yb₂O₃ as an additive, at 1900 °C under a nitrogen pressure of 1 MPa. The effects of Yb₂O₃ content, sample packing condition and sintering time on the densification, microstructure and thermal conductivity were investigated. Curves of the density plotted against the Yb₂O₃ content exhibited a characteristic ‘N’ shape with a local minimum at 3 mol% Yb₂O₃ and nearly complete densification below and above this concentration. The effects of the sample packing condition on the densification, microstructure and thermal conductivity strongly depended on the Yb₂O₃ content. The embedded condition led to more complete densification but also to a decrease in thermal conductivity from 119 to 94 W m^-1 K^?1 upon 1 mol% Yb₂O₃ addition. The sample packing condition had little effect on the density and thermal conductivity (102–106 W m^?1 K^?1) at 7 mol% Yb₂O₃. The thermal conductivity value was strongly related to the microstructure.     

Uniform lanthanide-doped Y2O3 hollow microspheres: Controlled synthesis and luminescence properties

Lanthanide-doped uniform pure cubic phase Y₂O₃ hollow microspheres have been successfully synthesized via a facile, high yield urea-based coprecipitation route with assistant of carbon spheres templates. The diameter and shell thickness of the microspheres can be manipulated by adjusting carbon sphere templates. Under a 980 nm excitation, Yb³⁺/Er³⁺, Er³⁺, Yb³⁺/Tm³⁺-doped Y₂O₃ hollow microspheres emit bright upconversion red, green, blue light with high purity, respectively, while Eu³⁺, Eu³⁺/Tb³⁺-doped Y₂O₃ hollow microspheres exhibit intense downconversion red light under the excitation of 254 nm ultraviolet light. Especially, the 610 nm emission intensity of Eu³⁺ in the Eu³⁺/Tb³⁺-codoped Y₂O₃ hollow microspheres is almost 5 times of that in the Y₂O₃:Eu³⁺ hollow microspheres indicating the occurring of the energy transfer from Tb³⁺ to Eu³⁺ ions.     

Up-conversion luminescence of trivalent-rare-earth ion-doped LnTaO4 (Ln = Y, Gd, La)

The up-conversion luminescence properties of trivalent-rare-earth ion-doped orthotantalates are studied under 980 nm laser diode excitation. The results indicate that YTaO₄:Tm³⁺/Yb³⁺ emits blue and strong infrared emissions respectively corresponding to ¹G₄→³H₆ and ³H₄→³H₆ transitions of Tm³⁺ ions, while YTaO₄:Er³⁺/Yb³⁺ produces strong green and weak red emissions, resulting from ²H_11/2/⁴S_3/2→⁴I_15/2 and ⁴F_9/2→⁴I_15/2 transitions of Er³⁺ ions, respectively. In contrast to three excellent up-conversion phosphors, viz. NaYF₄:Er³⁺/Yb³⁺, Y₂O₂S:Er³⁺/Yb³⁺ and Y₂O₃:Er³⁺, YTaO₄:Er³⁺/Yb³⁺ shows not only strong green emission, but also larger intensity ratio of green to red emission. In addition, it is found that LaTaO₄: Er³⁺/Yb³⁺ shows the minimum quenching concentration of Yb³⁺ and weakest up-conversion emission intensity, and the main peak presents blue shift (8 nm/556 nm) as compared with YTaO₄:Er³⁺/Yb³⁺ and GdTaO₄:Er³⁺/Yb³⁺. The distinct luminescent characteristics of LaTaO₄: Er³⁺/Yb³⁺ can be attributed to the larger difference of radius between doped ion and La³⁺ ion.     

Sol–gel synthesis of 2D and 3D nanostructured YSZ:Yb<Superscript>3+</Superscript> ceramics

This paper presents results of a detailed study of fundamental aspects of the formation of 2D and 3D nanostructured YSZ:Yb³⁺ ceramics with a cubic structure through a key synthesis step in aqueous solutions of zirconium-containing hydroxy nanoparticles (1–2 nm) modified by Y³⁺ and Yb³⁺ ions, with the use of a sol–gel method and subsequent calcination of the resultant xerogels at temperatures above 350°C. As starting chemicals for the synthesis of ceramic powders, we used zirconyl, yttrium, and ytterbium nitrates and chlorides and aqueous ammonia. Using mixed solutions of these salts and a procedure developed by us, we synthesized sols, gels, and xerogels. To examine the effect of temperature on solid-state transformations, the xerogels were calcined according to a predetermined program in a muffle furnace at temperatures in the range from 350 to 1350°C (rarely, up to 1650°C). We focused primarily on ceramic powders close in composition to 0.86ZrO₂ · 0.10Y₂O₃ · 0.04Yb₂O₃. The ceramics were characterized by high-resolution transmission electron microscopy, electron microdiffraction, electronic diffuse reflectance spectroscopy, energy dispersive X-ray microanalysis, and X-ray fluorescence analysis.     

Structure and up-conversion luminescence in sol–gel derived Er–Yb co-doped SiO2:PbF2 nano-glass–ceramics

Transparent oxyfluoride nano-glass–ceramics 90(SiO₂)10(PbF₂) co-doped with 0.3 Yb³⁺ and 0.1 Er³⁺ (mol%) have been prepared by thermal treatment of precursor sol–gel glasses. X-ray diffraction and high resolution transmission electron microscopy analysis pointed out a precipitation of cubic β-PbF₂ nanocrystals of certain diameter in nano-glass–ceramics varying from 10 to 20 nm depending on heat treatment conditions. The incorporation of Yb³⁺ and Er³⁺ dopants in these nanocrystals has been confirmed by signatures of luminescence spectroscopy. Up-conversion luminescence pumped at 980 nm has been detected. Colour tuneability of up-conversion luminescence varying pump power has been analyzed in terms of standard chromaticity diagram. This tuneability opens applications for up-conversion phosphors and three-dimensional optical recording.     

Yb3+ concentration dependence of upconversion luminescence in Y2Sn2O7:Yb3+/Er3+ nanophosphors

Pyrochlore Y₂Sn₂O₇ nanophosphors codoped with Er³⁺ (fixed 2 at.%) and Yb³⁺ ions (2–16 at.%) were synthesized via hydrothermal process followed by heat treatment. We investigate the infrared-to-visible upconversion (UC) luminescence properties of Er–Yb codoped Y₂Sn₂O₇. Upon 980 nm excitation at room temperature, green (at ~522 and 544 nm) and red (at ~661 nm) UC emissions were observed, which are ascribed to the (²H_11/2, ⁴S_3/2) → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions, respectively. It has been found that the Yb³⁺-doping concentrations have greatly influenced on the UC luminescence intensity and the emission ratio of the red and green in Y₂Sn₂O₇:Yb³⁺/Er³⁺ nanophosphors. The tunable emission is due to the energy back transfer from Er³⁺ to Yb³⁺ and the cross relaxation between the two neighboring Er³⁺ ions. It is expected that the achieved single and intense red emission band may have potential application for in vivo bioimaging.