Secondary Crystalline Phases Research Articles

Dynamic crack nucleation, propagation, and interactions with crystalline secondary phases in aluminum alloys subjected to large deformations

The major objective of this work was to model within a continuum framework the dynamic nucleation and evolution of failure surfaces in aluminum alloys with complex microstructures, using a recently developed compatibility-based fracture criterion for large deformations. Computational analyses were conducted to understand how Mn-bearing dispersoids, Ω and θ′ precipitates affect dynamic fracture processes in an Al–Cu–Mg–Ag alloys (2139-Al). High strain-rate simulations were based on a rate-dependent dislocation-density-based crystalline plasticity formulation and a nonlinear explicit dynamic finite-element approach. Results indicate that the fracture criterion elucidated how dispersoids and precipitates have a dominant role in dynamic crack blunting, branching and arrest. Rationally orientated precipitates result in overall dynamic microstructural strengthening and enhanced uniformity of deformation. These precipitates, however, accelerated unstable crack propagation, and this is amplified in the presence of a pre-crack. In contrast, dispersoids decreased microstructural toughness and ductility, but greatly improved dynamic damage tolerance, especially in the presence of a pre-crack. It can also be predicted that low angle boundaries can change the propagation direction of ductile cracks, and contribute to damage tolerance without crack initiation. Collectively, rationally oriented precipitates and dispersoids can significantly improve the combined dynamic strength, toughness and damage tolerance of crystalline aluminum alloys.

The dual effect of Mg on the long-term alteration rate of AVM nuclear waste glasses

Inactive Mg-containing nuclear waste glasses simulating actual HLW glasses produced at the AVM facility since 1995 (Marcoule, France), were leached in aqueous solution in order to assess their long term behaviour. The focus was on the effect of Mg. Our findings show that the distribution of Mg between the gel and the secondary crystalline phases strongly influences the glass dissolution rate.The glasses were leached in initially pure water (T=50°C, surface/volume ratio (S/V)=55cm−1) with and without addition of Mg2+ in the solution. “Mg-free” AVM glasses were also leached in initially pure water (50°C, 200cm−1) with and without addition of Mg2+ in the solution. Accurate identification of Mg-smectite secondary phases and gel composition calculations enable us to explain the different observed behaviours.Glass AVM 10 was the less altered glass in pure water. Its gel is more protective than the other probably because it is mainly balanced by Mg2+. The addition of Mg2+ in the solution triggers the precipitation of smectite (not observed in pure water experiments), which consumes silicon from the gel, leading finally to a significant increase of the glass alteration.We also focused on the AVM 6 glass which was the most altered glass in pure water of available AVM glasses. Contrary to AVM 10, the gel of AVM 6 is mainly balanced by Na+. The addition of Mg2+ in the solution allows the replacement of Na by Mg within the gel. This reaction clearly improves the gel properties and allows the rate to decrease more rapidly, in spite of the precipitation of smectite (also observed in pure water experiments).Finally, the two glasses were altered in synthetic groundwater (SGW) with a high Mg–Ca content. As expected from the previous observations, AVM 10 was insensitive to the presence of alkaline earths in the leaching solution whereas AVM 6 glass exhibited a lower rate than in pure water thanks to the incorporation of Mg and Ca within the gel.

Secondary crystallization in (Fe65Co35)79.5+xB13Nb4−xSi2Cu1.5 and (Fe65Co35)83B10Nb4Si2Cu1 nanocomposite alloys

Here, secondary crystallization kinetics of high induction, low loss HTX002-type nanocomposite alloys with the compositions (Fe65Co35)79.5+xB13Nb4−xSi2Cu1.5 (x = 0-4) and (Fe65Co35)83B10Nb4Si2Cu1 are reported. The magnetization of the alloys was measured through the thermal cycle of 50 °C-700 °C-300 °C-800 °C-300 °C-900 °C-200 °C by vibrating sample magnetometry. In (Fe65Co35)79.5+xB13Nb4−xSi2Cu1.5 alloys, the stability of the (Fe,Co,Nb)23B6 (23-6) phase is increased with increasing Nb content. In the x = 4 alloy, (Fe,Nb)2B is the only secondary crystalline phase to form, demonstrating that Nb is necessary for the 23-6 phase to form. The (Fe65Co35)83B10Nb4Si2Cu1 alloy forms the 23-6 phase more readily than the x = 0 alloy, likely due to the lower B content. The kinetics of secondary crystallization are important to assess long-term ageing effects on the metastable microstructure at elevated temperatures.

Low loss alumina dielectrics by aqueous tape casting: The influence of composition on the loss tangent

Polycrystalline aluminas with various concentrations of oxide dopants CaO, MgO, and TiO2, ranging from 0.05wt.% to 5wt.% (3wt.% in case of MgO), as well as pure alumina references were prepared by tape casting of aqueous suspensions and sintered in air at 1600°C for 4h for applications as low dielectric loss electroceramics. Loss tangents were measured at room temperature in the frequency range between 1 and 100kHz as the key parameter for the intended application. The values of loss tangents of doped materials were influenced by the concentration of dopants. The addition of 0.5 and 5wt.% of TiO2 and 3wt.% of MgO decreased the value of loss tangent in the whole frequency range. The addition of these dopants eliminated abnormal grain growth and decreased the amount of residual porosity. By doing so, the dopants compensated the negative influence of process impurities and decreased the loss tangent values. The cations (Ti4+) with high solubility in the Al2O3 crystal lattice were preferably built into the grain boundary glass, thus efficiently reducing the concentration of polarizable defects in corundum matrix; the formation of vitreous phase had a positive effect on the value of loss tangent in TiO2 doped samples. The increased values of loss tangent were related to lower density of prepared materials, and the presence of residual porosity. Other contributing factors were especially the formation of calcium-containing secondary crystalline phases, and the increased concentration of lattice defects due to incorporation of atoms with different valencies to alumina crystal lattice.

Low Temperature Sintering and Dielectric Properties of Alumina‐Filled Glass Composites for LTCC Applications

Glass + ceramic composites using lead‐free ultralow‐softening point glass matrices (brand names G018‐249 and G018‐250) with alumina fillers are investigated. The composites show good densification with near zero shrinkage at very low sintering temperature and do not show either the formation of secondary crystalline phases or dissolution of alumina. They further show no reactivity with the silver electrodes at a sintering temperature of 650°C and have permittivities of 9.5 and 8.8, loss tangents of 0.0068 and 0.0087 at 1 MHz for the samples containing G018‐249 glass and G018‐250 glass, respectively.

Steady-state conduction behaviour of 100 MeV Ag ion-irradiated polyetherimide

The effect of high-energy ion irradiation on the steady-state conduction behaviour of polyetherimide (PEI) has been investigated. PEI samples of 25 μm thickness were irradiated with 100 MeV Ag ions (fluences of 5.6×1010, 1.8×1011 and 1.8×1012 ions/cm2) at the Inter University Accelerator Centre, New Delhi. Measurements under steady-state conditions at different temperatures (40–250 °C) were made at different electric fields ranging from 40 to 600 kV/cm. The ionic jump distance estimations indicate the possibility of ionic conduction at low and moderate temperatures. At high temperatures, the Schottky and Poole–Frenkel-type conduction mechanisms occur, depending on the fluence of irradiance. A decrease in charge carrier mobility (μ) with field has been associated with the radiation-induced disorders in PEI. An anomalous variation in μ with fluence shows the occurrence of a secondary radiation-induced crystalline phase. The activation energies were determined to be 0.05 and 2.3 eV below and above the glass transition, temperature, indicating the presence of more than one type of trapping centre in irradiated PEI.

Synthesis, Structural and Magnetic Properties of Ni-Doped In2O3 Nanoparticles

Pure and Ni-doped In2-xNixO3 (x = 0.0, 0.01, 0.05, 0.1) nanocrystalline powders were synthesized using sol-gel process. The XRD patterns revealed the formation of nanocrystalline particles with cubic bixbyte crystal structure. The average crystal size was calculated from XRD patterns using Debye-Scherer's Equation for (222) peak and it was found to be 8 nm. This observation agrees well with the particle size estimated from the transmission electron microscope (TEM). The observed Raman peaks in the doped materials were negligibly shifted towards the lower frequency. This clearly indicates that Ni ions are incorporate into the host In2O3 structure, in agreement with the XPS measurements. The Raman spectra show that the nanoparticles are free of secondary crystalline phase. The temperature dependent magnetization showed no evidence of ferromagnetism at room temperature.

Electrical properties and structure of grain boundaries in n-conducting BaTiO3 ceramics

The electrical properties of positive temperature coefficient (PTC) ceramics are expected to strongly correlate with the potential barrier height at grain boundaries, which in turn may be influenced by the grain boundary structure and chemistry. In this study, n-conducting BaTiO3 ceramics co-doped by La and Mn were prepared, and the electrical properties were determined by impedance spectroscopy and dc four-point van der Pauw measurements. Detailed analysis of the grain boundary structure was performed by electron microscopy techniques across different length scales. The study revealed that the randomly oriented polycrystalline microstructure was dominated by large angle grain boundaries, which in the present case were dry although a secondary crystalline and glass phase formed at triple junctions. The relationship between the observed grain boundary atomic structures and electrical properties is briefly discussed.

Copper sulfide (CuxS) thin films as possible p-type absorbers in 3D solar cells

Abstract Films of Cu x S with 217-502 nm thickness were deposited by Automatic Spray Pyrolysis technique, at T = 275 − 325 ° C , from water:ethanol:glycerol= 7:2:1 (in volumes) solutions with molar ratio Cu:S=1:2.5−3.5. Relative dense, homogenous and uniform films of CuxS ( x = 1 − 2 ), containing copper-rich phase digenite (single, predominant or secondary crystalline phase), with E g = 2.41 − 3.1 eV and exhibiting electric resistance behavior were obtained. The optimized CuxS layers were integrated in 3D solar cells which show the diode behavior but the photovoltaic response is very low, in the experimental conditions.

Nanocystalline ZnO films prepared via polymeric precursor method (Pechini)

The polymeric precursor method (Pechini) was employed to prepare high-quality nanocrystalline zinc oxide (ZnO) films. Briefly, the process started off with the preparation of a coating solution by the Pechini process followed by a coating of the glass substrates by a dip-coating technique and subsequent heat-treatment of the as-deposited films up to 550 °C for 30 min. The Rietveld profile analysis of the X-ray diffraction (XRD) spectra revealed the würzite structure as expected for ZnO with a P6 3mc symmetry. No additional peaks were observed that would correspond to any secondary crystalline phase. The average crystallites size was 20 nm as calculated by Sherrer’s equation. UV–vis spectroscopy showed sharp ultraviolet absorption edges at ∼380 nm. The absorption edge analysis yielded optical band gap energy of 3.24 eV with electronic transition of the direct transition type. The Fourier transform infrared (FTIR) analysis showed asymmetric and symmetric stretching modes of the carboxyl group (CO). Scanning electron microscope (SEM) analysis revealed a crack-free surface morphology indicating that coating of the amorphous glass substrates was homogeneous on large surface areas. The temperature dependent conductivity featured a typical semiconducting-like behavior with resistivity approaching 3×10 −1 Ω cm at 220 K.

Effect of microstructure on the conductivity of a NASICON-type lithium ion conductor

Sintering temperature is used to control the microstructure of Li 1 + x + y Al x Ti 2 − x Si y P 3 − y O 12 ( x = 0.3, y = 0.2), a NASICON-type glass-ceramic. Scanning Electron Microscope imaging, X-Ray Diffraction, and Electrochemical Impedance Spectroscopy are employed to show that increase in sintering temperature increases conductivity while generating secondary crystalline phases. Total conductivity is as high as 3.81 × 10 −4 S cm −1 for sintering temperatures above 1000 °C. Crystallization of dielectric phases places the optimal sintering temperature in the 900 °C to 1000 °C range. Thermal analysis of the glass precursor reveals the glass transition, and crystallization temperatures.

Direct Evidence of Al-Rich Layers around Nanosized ZrTiO4 in Glass: Putting the Role of Nucleation Agents in Perspective

In the glass industry, nucleation agents are broadly used to obtain homogeneously dispersed and nanograined crystals during heat treatment leading to glass ceramics. In this paper, the role of zirconia and titania used as nucleation agents in a lithium alumosilicate glass is studied on the nanoscale using cutting edge analytical and imaging techniques performed in the transmission electron microscope. It is found that the formation of ZrTiO4 nanocrystals upon heat treatment is accompanied by the formation of a diffusion barrier surrounding these precipitates. At the interface between ZrTiO4 nanocrystals and the residual glass, an enrichment of alumina is observed, making the glass network more rigid and eventually preventing further growth or ripening of the nanosized crystals. Within the compositional gradient formed around the homogeneously distributed ZrTiO4 crystallites, nucleation of the secondary crystalline phase, a lithium alumosilicate high-quartz solid solution, is likely to be much facilitated rather than the never doubtlessly reported on epitaxial overgrowth assumed to act so far.

Processing of micro-components made of sintered reaction-bonded silicon nitride (SRBSN). Part 2: Sintering behaviour and micro-mechanical properties

This second part of the report deals with, how the sintering additives Y 2O 3, Al 2O 3, and MgO influence the sintering behaviour of SRBSN. Paraffin-based feedstocks with varying sintering aid compositions and silicon grain size were used for moulding macro- and micro-scale samples. It was observed that compositions with smaller Si grain size (with correspondingly high SiO 2 content) and containing Al 2O 3 as sintering additive exhibit higher shrinkage and lower residual porosity when sintered at 1700–1800 °C after nitridation. The mechanical properties determined for micro-scale samples were obtained by three-point bending tests, with the resulting characteristic strength values σ 0 ranging from 500 MPa up to 1200 MPa. Surprisingly residual porosity did not play the role of a strength limiting factor; rather it was observed that the presence of crystalline secondary phases – mainly Y 2Si 3O 3N 4 – was responsible for reducing the micro-bending strength. As micro-samples exhibit a large surface-to-volume ratio they are in particular affected by decomposition of Si 3N 4 and volatilization of SiO 2 which is considered to be responsible for the occurrence of secondary phases preferred at the sample surface. The powder bed condition was also found to play a prominent role in the development of the secondary phases during liquid phase sintering.

Lead Zirconate Titanate-Magnetoplumbite Composites: A First Step Toward Multiferroic Ceramics?

When lead zirconate titanate (PZT) is acceptor doped way above the solubility limit of Fe3+, crystalline secondary phases become thermodynamically stable that are antiferromagnetic and ferromagnetic, i.e., plumboferrite, PbFe4O7, and magnetoplumbite (MP), PbFe12O19, respectively. Three materials were studied by X-ray diffractometry and transmission electron microscopy, with 3 mol% Fe (B-site) and high volume fractions of iron, which corresponds to a 1.5 and 6 mol% of MP addition, with emphasis placed on the phase and microstructure evolution, depending on the dopant level. Although the addition of the high iron content resulted in the formation of the desired ferromagnetic phase MP, homogeneously dispersed within the PZT host matrix, the densification kinetics became quite sluggish, resulting in rather porous multiferroic ceramics.

Thermal Expansion Matching via Framework Flexibility in Zinc Dicyanometallates

The thermal expansion properties of two isostructural zinc dicyanometallates that crystallize with and without the inclusion of a weakly interacting secondary crystalline phase have been investigated using variable temperature single-crystal and powder X-ray diffraction. The guest-free Zn[Au(CN)(2)](2) framework was found to show very strong anisotropic positive and negative thermal expansion. In contrast, its cocrystal analogue Zn[Ag(CN)(2)](2) x xAgCN exhibited much more moderate behavior, such that the coefficient of thermal expansion for the host Zn[M(CN)(2)](2) framework now matched that of crystalline AgCN. It was proposed that this correlation points to a more general ability of highly flexible framework materials to "match" the thermal expansivity of adhered phases (e.g., substrates, sorbates, or cocrystallized species), suggesting a methodology of eliminating thermal strain in multicomponent assemblies.

Formation of magnetic grains in ferroelectric Pb[Zr0.6Ti0.4]O3 ceramics doped with Fe3+ above the solubility limit

The effect of excess iron-doping in piezoelectric lead-zirconate-titanate (PZT) on the microstructure was investigated by means of transmission electron microscopy in conjunction with electron diffraction and energy dispersive x-ray spectroscopy analysis. Electron paramagnetic resonance (EPR) spectroscopy has been used to characterize the site of the iron functional centers. A crystalline secondary magnetic phase formed upon sintering, magnetoplumbite (PbFe12O19), has been unequivocally identified. Furthermore, the phase assemblage observed in the Fe3+-doped PZT 60/40 sample could be correlated with an exchange-coupled resonance in the EPR spectra.

Preparation and characterization of porous cordierite for potential use in cellular ceramics

AbstractCordierite porous ceramics Z, X, and K were prepared using three mixtures of clay minerals: Z from kaolinite, talc, and aluminum hydroxide, X from kaolinite, talc, vermiculite, and aluminum hydroxide, and K from kaolinite, talc, and magnesium oxide. Ceramics were different in porosity, specific surface area, cordierite polymorphs, and secondary crystalline phases. Vermiculite influenced textural architecture of calcined cordierite ceramics X and predestinated crystallization of the high-temperature hexagonal α-cordierite with secondary minerals enstatite, spinel and corundum. Ceramics Z contained low-temperature orthorhombic β-cordierite, enstatite, and corundum, K was diphase of β-cordierite and forsterite. Total pore area (TPA) and specific surface area (SSA) of X, in spite of the higher porosity and the pore size distribution in the range of 300–1000 nm, were smaller in comparison with TPA and SSA of Z. Ceramics K retained high porosity, two maxima at 300–1000 nm and 50–200 nm in the pores size distribution, and the highest TPA and SSA compared to those observed in ceramics Z and X.

Preparation and characterization of Zr-IV-containing Mg-Al-Cl layered double hydroxide

In order to contribute to the long-term safety analysis of direct disposed research reactor fuel elements, the corrosion behavior of these fuel-types in final repository relevant salt brines was determined. Due to the corrosion processes, radionuclides were mobilized first, but then trapped by the formed secondary phases. A Mg-Al layered double hydroxide (LDH) with chloride as interlayer anion was identified as one crystalline secondary phase component. The possibility to incorporate zirconium-IV into the lattice structure of the Mg-Al-Cl LDH was investigated by a co-precipitation process. The element zirconium was chosen because it has a low absorption cross section for neutrons, and is therefore used for nuclear energy applications, such as for cladding fuel elements. No zirconium release was detected, when the obtained LDH was treated with an ammonium carbonate solution. The molar stoichiometry with respect to Mg, Al and Zr remained stable and this is the first indication for the incorporation of zirconium. For further examination, the material was analyzed by powder X-ray diffraction (XRD) combined with a Rietveld refinement. Due to a recursive calculation inside the Rietveld refinement, it was possible to derive structural parameters of the disordered LDH. Further on, the Mg-Al-Zr-LDH was analyzed by EXAFS and the results did show that Zr is the central metal atom coordinated by 5 OH groups and by one chloride.

High level nuclear waste glass corrosion in synthetic clay pore solution and retention of actinides in secondary phases

The corrosion of the simulated high level waste glass GP WAK1 in synthetic clay pore solution was studied in batch-type experiments at 323 and 363K with special focus on the effect of high carbonate concentration in solution. The corrosion rate after 130 days was <10−4gm−2d−1 – no significant effect of the carbonate was identified. During glass corrosion, crystalline secondary phases (powellite, barite, calcite, anhydrite and clay-like Mg(Ca,Fe)-silicates) were formed. To obtain a molecular level picture of radionuclide speciation within the alteration layer, spectroscopic methods have been applied including grazing incidence X-ray absorption spectroscopy (XAS) to study the structural changes in the coordination of uranyl upon alteration layer formation. The number of equatorial oxygen atoms increases from 4 in the bulk glass to 5 in the alteration layer. Furthermore, reduced coordination symmetry was found. Hectorite, a frequently observed secondary clay mineral within the glass alteration layer, was synthesized in the presence of trivalent f-elements (e.g. Eu) and structurally characterized using time-resolved laser fluorescence spectroscopy. Structural incorporation into the octahedral layer is indicated.

Microstructure and boundary phases of Lu–Al-doped silicon nitride by pressureless sintering

Various silicon nitride materials were fabricated by pressureless sintering using lutetia and alumina as sintering additives. Densification behavior, microstructure, strength and formation of secondary crystalline phases were investigated. The combination of Lu 2O 3/Al 2O 3 sintering aids can promote the densification and evolution of a fine grain microstructure of Lu–Al-doped silicon nitride because of the low viscosity of the liquid. The J′ phase given by Lu 4Si 2− x Al x O 7+ x N 2− x was considered to be secondary crystalline phase at the grain pockets. The composition with a Lu 2O 3/Al 2O 3 weight ratio 10/10 had highest strength of 690 ± 50 MPa.