Intergranular Liquid Phase Research Articles

Effect of HfSi2‐induced intergranular liquid phase on densification behavior of ZrB2

AbstractThe densification behavior and mechanical properties of ZrB2‐based composites were investigated. The results demonstrated that the fully dense ZrB2‐based composites could be obtained at lower sintering temperature (1600°C) and pressure (30 MPa) when the content of HfSi2 was above 20 vol.%. The as‐sintered composite was a special core–shell structure, with ZrB2 as the core and (Zr, Hf)B2 solid solution as the shell. The core–shell structure resulted from the diffusion of Hf atom into the boride matrix, which could accelerate the densification. In addition, the intergranular liquid phase induced by the HfSi2 addition filled the micropores of the composites effectively during the sintering. When the content of HfSi2 increased to 20 vol.%, its compressive strength, hardness, and fracture toughness all reached the maximum values, which were 1617 MPa, 15.99 GPa, and 2.44 MPa m1/2, respectively.

Quasi‐Solid Electrolyte Design and In Situ Construction of Dual Electrolyte/Electrode Interphases for High‐Stability Zinc Metal Battery

AbstractInterfacial stability and compatibility in rechargeable metal batteries (RMBs) is still made difficult by deterioration under electrochemical dynamic operation due to the activeness of the metallic anodes and their spontaneous reaction with the liquid electrolyte. Herein, robust quasi‐solid zinc metal batteries enabled by in situ formation of stable dual electrolyte/electrode interphases with an electrochemical stability during cycling are reported. The quasi‐solid electrolyte sufficiently transfers the zinc ions due to the construction of the unobstructed ions transportation network composed of intergranular liquid phase migration and interlayer diffusion. The distinctive in situ formation of dual interphases, specially with heterojunction charge aggregation, cleverly settles the issues of interfacial compatibility, which achieves a stable Zn2+ plating/stripping of more than 3000 h and far exceeding the transient 250 h of its liquid counterpart. This design strategy for quasi‐solid electrolytes is expected to advance the development of the quasi‐solid battery field.

Effect of Zn content on hot tearing susceptibility of LPSO enhanced Mg–Znx–Y2–Zr0.06 alloys with different initial mold temperatures

The critical temperature (Thci) corresponding to the load relaxation measured during the solidification of alloy in a ‘T′-shaped mold is introduced into the Clyne-Davies’ model, so that it can be used to predict the hot tearing susceptibility (HTS) of alloy under non-equilibrium cooling. Then, the improved model is applied to study the HTS of Mg–Znx–Y2–Zr0.06 (x = 0.5, 1 and 1.5 at%) alloys under two initial mold temperatures, i.e., two cooling rates. It is found that HTS is very sensitive to both Zn content and initial mold temperature, and the increase in Zn content or initial mold temperature can reduce HTS of the alloy. However, their micro mechanisms are different. The reason for the decrease in HTS with the increase in Zn content is the decrease in the dendrite coherency temperature. Specifically, the ratio of unconstrained feeding stage to intergranular feeding stage increases, and the intergranular liquid phase with eutectic composition increases in the later stage of solidification. The HTS decreases with the increase in the initial mold temperature because the solidification shrinkage stress is relieved. Consequently, the residual liquid phase between the grains can be fed in a more timely manner.

Correlation between the Liquid Fraction, Microstructure and Tensile Behaviors of 7075 Aluminum Alloy Processed by Recrystallization and Partial Remelting (RAP)

The recrystallization and partial remelting (RAP) method was applied to obtain the semisolid 7075 aluminum alloy with different liquid fractions. The effects of liquid fraction on the microstructure and tensile properties were determined in detail. The results show that during the semisolid isothermal treatment, the number of the intra-granular liquid droplets increased initially with the melting of the eutectic phases. Extension of isothermal soaking led to the coarsening and spheroidization of the intra-granular droplets. Finally, these liquid droplets merged and moved towards the grain exterior. The room temperature tensile strength of the RAP-processed AA7075 alloy, which were isothermally soaked at 600 and 610 °C, increased with the holding time from 5 to 15 min and then decreased dramatically from 15 to 25 min, whilst that soaked at 620 °C decreased monotonously. The fracture morphology exhibited intra-granular fracture mode at low liquid fractions. However, it transformed to a completely brittle and inter-granular type at high liquid fractions and the cohesive force of the liquid-solid interfaces at the grain boundaries determined the strength of the alloys. The transfer of the intra-granular liquid droplets into the inter-granular liquid phase played a significant role for the different fracture behaviors of the RAP-processed AA7075 alloy. The paper provides some reference for better controlling the microstructure and mechanical properties in semisolid processing.

Silica reactivity during reaction‐sintering of Nd:YAG transparent ceramics

AbstractSilica (SiO2) is widely used as sintering aid during vacuum sintering of YAG (Y3Al5O12)‐based transparent ceramics. These ceramics are mainly used for laser applications when they are doped with rare‐earth luminescent elements such as Yb3+ or Nd3+. By means of microstructural, chemical, dilatometry, and thermogravimetry analyses, this study has evidenced that sufficiently high amount of silica (ie above the solubility limit in YAG) forms intergranular transient liquid phase of mixed composition Y‐Al‐Si‐O that vaporizes rapidly for temperatures higher than 1350°C. As a result, silica content after sintering remains always lower than the solubility limit in YAG ceramics (ie lower than 900 ppm). Finally, vacuum sintering with an external source of gaseous Si was proven to be suitable to manufacture highly transparent Nd:YAG ceramics.

An automatic granular structure generation and finite element analysis of heterogeneous semi-solid materials

The quality of cast metal products depends on the capacity of the semi-solid metal to sustain the stresses generated during the casting. Predicting the evolution of these stresses with accuracy in the solidification interval should be highly helpful to avoid the formation of defects like hot tearing. This task is however very difficult because of the heterogeneous nature of the material. In this paper, we propose to evaluate the mechanical behaviour of a metal during solidification using a mesh generation technique of the heterogeneous semi-solid material for a finite element analysis at the microscopic level. This task is done on a two-dimensional (2D) domain in which the granular structure of the solid phase is generated surrounded by an intergranular and interdendritc liquid phase. Some basic solid grains are first constructed and projected in the 2D domain with random orientations and scale factors. Depending on their orientation, the basic grains are combined to produce larger grains or separated by a liquid film. Different basic grain shapes can produce different granular structures of the mushy zone. As a result, using this automatic grain generation procedure, we can investigate the effect of grain shapes and sizes on the thermo-mechanical behaviour of the semi-solid material. The granular models are automatically converted to the finite element meshes. The solid grains and the liquid phase are meshed properly using quadrilateral elements. This method has been used to simulate the microstructure of a binary aluminium–copper alloy (Al–5.8 wt% Cu) when the fraction solid is 0.92. Using the finite element method and the Mie–Grüneisen equation of state for the liquid phase, the transient mechanical behaviour of the mushy zone under tensile loading has been investigated. The stress distribution and the bridges, which are formed during the tensile loading, have been detected.

Local impedance spectroscopic and microstructural analyses of Al-in-diffused Li7La3Zr2O12

A cylindrical Li7La3Zr2O12 specimen in contact with an Al2O3 crucible is sintered and the effect of Al diffusion on the microstructure, phase composition, and Li-ion conductivity of Li7La3Zr2O12 is investigated. The decrease in the Li-ion conductivity and Al concentration with increasing distance from the contact area with the Al2O3 crucible is confirmed using spatially resolved local-impedance spectroscopy and compositional analysis. The region close to the Al2O3 crucible shows abnormally large grains (size > 0.7 mm) within a matrix of fine grains (size: ∼10 μm), abundant intergranular liquid, the Li7La3Zr2O12 phase mostly in cubic form, and a high Li-ion conductivity (8.5 × 10−5 S cm−1). In contrast, the region far from the Al2O3 crucible exhibits uniform fine grains (size: ∼10 μm), scarce intergranular liquid, the tetragonal form of the Li7La3Zr2O12 phase, and a low Li-ion conductivity (1.1 × 10−6 S cm−1). The incorporation of Al into the Li7La3Zr2O12 lattice and the resulting increase in the number of Li-ion vacancies is suggested to be the main reason for the increase in the Li-ion conductivity. The origins of the abnormal grain growth induced by the diffusion of Al are discussed in relation to the atomic surface structures of Li7La3Zr2O12 and the presence of intergranular liquid phase.

Low-temperature sintering and electrical properties of strontium- and magnesium-doped lanthanum gallate with V2O5 additive

The effects of a V2O5 additive on the low-temperature sintering and ionic conductivity of strontium- and magnesium-doped lanthanum gallate (LSGM: La0.8Sr0.2Ga0.8Mg0.2O2.8) are studied. The LSGM powders prepared by the glycine nitrate method are mixed with 0.5–2at.% of VO5/2 and then sintered at 1100–1400°C in air for 4h. The apparent density and phase purity of the LSGM specimens are increased with increasing sintering temperature and VO5/2 concentration due to the enhanced sintering and mass transfer via the intergranular liquid phase. The 1at.% VO5/2-doped LSGM specimen sintered at 1300°C exhibits a high oxide ion conductivity of ∼0.027Scm−1 at 700°C over a wide range of oxygen partial pressure (PO2=10−27−1 atm), thereby demonstrating its potential as a useful electrolyte for anode-supported solid oxide fuel cells (SOFCs) without the requirement for any buffer layer between the electrolyte and anode.

Inter-granular liquid phase aiding grain boundary sliding in superplastic deformation of fine-grained ZK60 Mg alloy

For 7475 Al alloy, there were micrographs showing filaments or whiskers formation during the separation stage of superplastic elongation. This indicates the presence of liquid phase which accommodates grain boundary sliding to reach superplasticity. On the other hand, there is no such phenomenon reported regarding Mg alloy in literatures. Scanning electron microscopic (SEM) fractography exceptionally exhibits a mark of grain boundary sliding and its accommodating mechanism of inter-granular liquid phase. Under the testing conditions of 350 °C and 1×10 −4 s −1, the initially fine-grained structure (3.7 μm) yields 642% superplastic elongation and exhibits fluffy appearance on the fractured surface. For other specimens showing less superplasticity, their fractured surfaces exhibit partial fluffy appearance.

Investigation on the decomposable process and the secondary liquid phase effect on the dielectric properties of CaCu3Ti4O12 ceramics

CaCu3Ti4O12 (CCTO) single-phase powder synthesized by the solid-state reaction method was used to investigate the decomposition reaction after different sintering conditions, in which copper (Cu) atoms played a key role. A secondary intergranular liquid phase (Cu-rich, especially CuO-rich), which is related to both the abnormal grain and the domain boundary growths, started to appear at the grain boundaries when the CCTO pellet was sintered at 1000 °C. The liquid phase improved the dielectric properties of the 1000 °C sample, such as dielectric constant as high as 17 000 and dielectric loss as low as 0.074 at the measured frequency of 1 kHz. Meanwhile, the CuO-rich phase existed only on the surface layers and disappeared from the surface of the pellets sintered at 1100 °C for 10 h. The grain size increased significantly with the sintering temperature. The liquid phase also played an important role in the microstructure evolution and in the improvement of the dielectric properties. In addition, CCTO completely decomposed when sintered for 10 h at temperatures higher than 1100 °C.

The effect of silica doping on neodymium diffusion in yttrium aluminum garnet ceramics: implications for sintering mechanisms

The Nd 3+ cation diffusion into transparent polycrystalline YAG (Y 3Al 5O 12) was investigated as a function of temperature and silica content. Thin neodymium oxide layers were deposited on sintered YAG substrates prior to annealing under air at temperatures from 1400 to 1600 °C. Bulk and grain boundary neodymium diffusion coefficients were measured by secondary ion mass spectrometry. The experimental results show that silica addition increases the diffusivity of Nd 3+ by a factor 10 whatever the diffusion path, probably as a result of extrinsic point defects formation, especially rare-earth vacancies. The experimental diffusion data were used to elucidate the sintering mechanism of Nd:YAG ceramics in the temperature range 1450–1550 °C. Firstly, it appeared that the intermediate stage of solid-state sintering should be controlled by the rare-earth diffusion along the grain boundary with an activation energy of about 600 kJ mol −1. Secondly, grain growth mechanism at the final stage of liquid-phase sintering was investigated for silica-doped Nd:YAG samples. Thus, the grain growth should be limited by the reaction at interfaces at a temperature lower than 1500 °C, with an activation energy of about 880 kJ mol −1. At higher temperature, it seems to be limited by the ionic diffusion through the intergranular liquid phase, with an activation energy of 250 kJ mol −1.

Highly resistive intergranular phases in solid electrolytes: an overview

The siliceous intergranular phase in acceptor-doped zirconia and ceria and its effect on the ionic conduction across the grain boundaries were reviewed. Not only the abundant siliceous intergranular liquid phase, but also the monolayer-level siliceous intergranular segregation significantly deteriorates the grain-boundary conduction. To decrease the harmful effect of the resistive siliceous phase at the grain boundary, ‘additive scavenging’ or ‘precursor scavenging’ can be employed. The former involves the addition of a secondary phase or another acceptor material with a very high chemical affinity for the siliceous phase, while the latter involves the intergranular phase changing from having a continuous (blocking) configuration to having a discrete (non-blocking) configuration. The mechanisms of various scavenging reactions have been explained, compared, and discussed.

산화물 고체전해질의 입계전도

Grain-boundary conduction in the fluorite-structure solid oxide electrolytes such as acceptor-doped zirconia and ceria were reviewed. The siliceous impurity, even several hundreds ppm, affects the ionic conduction across grain boundary to a great extent. Various approaches to improve grain-boundary conduction in fluorite-structure oxide electrolytes have been investigated, which include (1) the scavenging of siliceous phase by the reaction with second phase, (2) the gathering of intergranular siliceous phase into a discrete configuration and (3) the dewetting of intergranular liquid phase by post-sintering heat treatmtent.

Microstructural evolution and dielectric properties of Cu-deficient and Cu-excess CaCu 3Ti 4O 12 ceramics

The microstructural evolution and dielectric properties of CaCu 3− x Ti 4O 12− x (3 − x = 2.8–3.05) ceramics were investigated. Normal grain growth behavior was observed at Cu/Ca ≤ 2.9, while abnormal grain growth was observed at Cu/Ca ≥ 2.95. A CuO-rich intergranular liquid phase at Cu/Ca ≥ 2.95 and angular grain morphology were the main reasons for abnormal grain growth. However, the abundant intergranular liquid at Cu/Ca = 3.05 significantly affected the relative dielectric permittivity and dielectric loss. The CuO composition is the key parameter that determines the microstructure and dielectric properties of CCTO ceramics.

Fundamental understanding of Na-induced high temperature embrittlement in Al–Mg alloys

Sodium is an undesirable impurity in aluminium–magnesium alloys. In trace amounts it leads to high temperature embrittlement (HTE), due to intergranular fracture, which results in edge cracking during hot rolling. In the present work, the results of a thermodynamic investigation to elucidate the mechanism are presented. Correlations between HTE, phase formation, temperature and composition in Al–Mg alloys were determined. It is suggested that: (i) HTE is related to the formation of an intergranular Na-rich liquid phase, which significantly weakens the strength of grain boundaries; (ii) for a given Mg content, there exists a maximum Na content above which HTE cannot be avoided; and (iii) for a given alloy, a proper hot-rolling temperature should be chosen with respect to Na and Mg contents to suppress HTE. The HTE sensitive zone and a hot-rolling safe zone of Al–Mg–Na alloys are defined as functions of processing temperature and alloy composition. The tendency of HTE formation was evaluated based on thermodynamic simulations of phase fraction of the intergranular Na-rich liquid phase.

Creating Dense, Constrained Ce0.9Gd0.1O1.95Films at Low Temperature for SOFC Applications

AbstractWe have investigated the effect of various dopants on the sintering characteristics of Ce0.9Gd0.1O1.95(CGO) and found that 99% dense electrolyte pellets can be produced at the record low temperature of 800°C (as opposed to the 1400°C typically needed) by sintering Ce0.9Gd0.1O1.95with as little 3mol% lithium. Our studies indicate that doping the CGO surface with lithium nitrate, as opposed to using alternative lithium salts, produces the largest decrease in sintering temperature. Unlike other dopants that lower the sintering temperature by altering the near grain boundary vacancy concentration, lithium lowers the sintering temperature through the formation of an intergranular liquid phase. This liquid phase allows fully dense, completely constrained CGO films to be produced on inert substrates at temperatures as low as 950°C.

Direct measurement of oxygen in lead-based ceramics using the ζ-factor method in an analytical electron microscope

X-ray absorption of oxygen is significant in thin specimens of Pb(Mg1/3Nb2/3)O3-35 mol% PbTiO3 [PMN-35PT] due to the presence of heavy elements such as Pb and Nb. Therefore, direct measurement of the oxygen concentration in these types of systems can be difficult. Furthermore, assumption of the composition from stoichiometric considerations may not be feasible, particularly if the valence state of one or more of the cation species is variable. Using only XEDS data, the ζ-factor method provides absorption corrected compositional information. In the present study, it was shown that such data were in very good agreement with the nominal values for PMN-35 PT, whereas the uncorrected data underestimated the oxygen content by 300%. In previous work, it was theorized that the swelling of samples containing excess PbO was linked to changes in the composition of the intergranular liquid phase. The ζ-factor technique was used to show that the oxygen to lead ratio of this second phase changes upon annealing.

Penetration and characteristics of an intergranular-liquid phase during sintering of CaSi 2O 5-dropped 8 mol%-yttria-stabilized zirconia estimation by impedance spectroscopy

The grain-boundary resistivity of CaSi 2O 5-dropped 8 mol%-yttria-stabilized zirconia (8YSZ) was determined by impedance spectroscopy using sub-millimeter-scale electrodes. During sintering, a liquid that formed at the top surface of the specimen penetrated into the 8YSZ and induced enhanced grain growth near the surface region. The grain-boundary resistivity of the specimen surface was observed to be 150 times higher than that of the interior. The deterioration of the grain-boundary conductivity was explained in terms of the presence of an intergranular siliceous phase.

Microstructural Evolution in Liquid‐Phase‐Sintered SiC: Part III, Effect of Nitrogen‐Gas Sintering Atmosphere

Effects of N2sintering atmosphere and the starting SiC powder on the microstructural evolution of liquid‐phase‐sintered (LPS) SiC were studied. It was found that, for the β‐SiC starting powder case, there was complete suppression of the β→α phase transformation, which otherwise goes to completion in Ar atmosphere. It was also found that the microstructures were equiaxed and that the coarsening was severely retarded, which was in contrast with the Ar‐atmosphere case. Chemical analyses of the specimens sintered in N2atmosphere revealed the presence of significant amounts of nitrogen, which was believed to reside mostly in the intergranular phase. It was argued that the presence of nitrogen in the LPS SiC helped stabilize the β‐SiC phase, thereby preventing the β→α phase transformation and the attendant formation of elongated grains. To investigate the coarsening retardation, internal friction measurements were performed on LPS SiC specimens sintered in either Ar or N2atmosphere. For specimens sintered in N2atmosphere, a remarkable shift of the grain‐boundary sliding relaxation peak toward higher temperatures and very high activation energy values were observed, possibly due to the incorporation of nitrogen into the structure of the intergranular liquid phase. The highly refractory and viscous nature of the intergranular phase was deemed responsible for retarding the solution–reprecipitation coarsening in these materials. Parallel experiments with specimens sintered using α‐SiC starting powders further reinforce these arguments. Thus, processing of LPS SiC in N2atmosphere open the possibility of tailoring their microstructures for room‐temperature mechanical properties and for making high‐temperature materials that are highly resistant to coarsening and creep.

Liquid-phase redistribution during sintering of 8 mol% yttria-stabilized zirconia

The distribution of the intergranular liquid phase during the sintering of 8 mol% Y2O3-stabilized ZrO2 containing 0.5 mol% of SiO2 and Al2O3 was investigated using spatially resolved impedance spectroscopy. The variation in the grain-boundary resistivity as a function of the distance from the specimen surface indicated that the liquid phase coagulates at the specimen center in the initial stages of sintering, and spreads outward upon further sintering. The liquid redistribution was also examined by transmission electron microscopy.