Anomalous Grain Research Articles

Texture development and anomalous grain growth in Sb-doped MnPt 3 thin films by solid-state reaction of multilayered films

Antimony-doped MnPt 3 thin films have been successfully fabricated by solid-state reaction of thin multilayered Mn/Sb/Pt films. It was found that the (100) preferential orientation of the MnPt 3 phase was developed with increasing antimony content. In addition to this, an anomalous crystalline growth with (100) oriented grains was observed, which resulted in a wide distribution of the grain size of the annealed samples. This behavior is considered to be secondary grain growth, which is usually caused by a surface free energy minimization rule. However, the (100) planes of MnPt 3 do not have the lowest surface free energy. We discuss the mechanism of this anomalous grain growth from the view-point of strain energy minimization, in conjunction with the anisotropy in the strain energy density for each crystallographic plane of MnPt 3.

Monte Carlo simulation of abnormal grain growth in two dimensions

Abnormal grain growth in the presence of second-phase particles is investigated with the help of a two-dimensional Monte Carlo simulation. An aggregate of equiaxed grains is considered with constant grain boundary energy and mobility. The only driving force accounted for stems from the grain boundary curvature. The process of abnormal grain growth is investigated as a function of two governing parameters, the initial degree of pinning of the matrix grains by the particles and the initial size advantage of the anomalous grain. In such conditions, moderate growth is obtained whose specific features are discussed with respect to the available models. It is shown that it is possible to obtain drastic grain growth by introducing the thermally activated unpinning of grain boundaries from particles. For this purpose, a simplified but effective procedure is proposed and discussed that includes the influence of the capillary force on the height of the local energy barrier for grain unpinning.

Investigations on the liquid phase in barium titanate ceramics with silica additives

Abstract Additives of SiO 2 lower the sintering temperature of BaTiO 3 ceramics and promote anomalous grain growth. However, the nature of the working liquid phase is not clear. A binary eutectic is out of the question because the system BaTiO 3 –SiO 2 is not a binary one, as shown by Robbins [Robbins, C. R., Synthesis and growth of fresnoite (Ba 2 TiSi 2 O 8 ) from a TiO 2 flux and its relation to the system BaTiO 3 –SiO 2 . Journal of Research of the National Buereau of Standards — A. Physics and Chemistry , 1970, 74A, 229–232]. Guha and Kolar [Guha, J. P. and Kolar, D., Phase equilibria, sintering characteristics and dielectric properties in the BaTiO 3 -rich portion of the system BaO–TiO 2 –SiO 2 . In 5th Conference on Ceramics for Electronics , Liblice, 1974, pp. 1–9] found a ternary eutectic with a composition of 33 mol% BaO, 54 mol% TiO 2 and 13 mol% SiO 2 . We examined powder tips of this composition and ones of pure SiO 2 in contact with BaTiO 3 green bodies by heating microscopy. The results suggest that the ternary eutectic develops in sintering bodies with SiO 2 addition. During further sintering the composition of the liquid phase is changed and this modified melt is able to trigger anomalous grain growth.

Anomalous grain growth in commercial lead

Anomalous grain growth in commercial lead

Regional and local-scale gold grain and till geochemical signatures of lode Au deposits in the western Abitibi Greenstone Belt, central Canada

Abstract This paper is an overview of drift exploration methods for lode Au deposits in areas of thin and thick cover of glacial sediments within the Abitibi Greenstone Belt of central Canada. It summarizes a large volume of data produced by government regional surveys and case studies as well as that from industry-led gold exploration programs. Regional till surveys can be used as targeting mechanisms for further Au exploration. Anomalies are defined by a series of samples with elevated Au concentrations that lie along significant bedrock structures, occurring in clusters or as isolated samples in areas of low sample density. Thresholds between background and anomalous Au grain abundances or Au concentrations are variable and depend on location within the Abitibi Greenstone Belt. Case studies around known deposits provide examples of geochemical and mineralogical signatures of Au deposits that can be expected in till down-ice. These serve as sources of information on appropriate sampling methods and size fractions to analyse, and on ice flow patterns, local glacial stratigraphy and suitable till units for sampling. Two methods for measuring the Au content of till are commonly used: (1) a count of visible Au grains and (2) geochemical elemental analysis. Close to source, till contains thousands to hundreds of thousands ppb Au and several hundred Au grains. The Au grains vary from coarse sand to silt sizes and have pristine shapes. The presence of high Au concentrations in till indicates that the ore zones subcrop and that glacial processes have produced Au dispersal trains down-ice.

Novel technique for synthesis and consolidation of aluminium nitride nanopowders

In this paper, the use of a microwave plasma method for the synthesis of aluminium nitride nanopowders is described. The powders were consolidated to near theoretical densities using the unique rapid consolidation technique, plasma pressure consolidation (P2C), developed by MMI. Rapid consolidation of nanopowders is an ideal requirement for better mechanical and thermal properties in the consolidated part, as it retains the fine microstructure preventing anomalous grain growth. Microwave plasma synthesis resulted in aluminium nitride nanopowders (85–200 nm), which were consolidated to near theoretical density using P2C in <5 min without sintering additives. The effect of yttria (3 wt-%) as a sintering additive on the thermal conductivity (TC) of aluminium nitride was also evaluated and compared with TC values obtained from additive free AlN consolidated samples.

Containerless rapid solidification of Ag 28.1Cu 41.4Ge 30.5 ternary alloy

Droplets of Ag 28.1Cu 41.4Ge 30.5 ternary alloy with different sizes are solidified during containerless processing in a 3 m drop tube. The experimental result shows that the microstructural evolution depends mainly on droplet size. Generally, droplet undercooling is inverse proportional to its diameter. When the droplet diameter is large, the (Ge) primary phase and intermetallic compound η phase show granular and strip shapes, respectively. In the medium-sized droplets, the (Ge) phase grows in a dendritic mode whereas η phase has little change. Meanwhile, both the grain size and interphase spacing also decrease with the reduction of droplet size. Moreover, it is found that η phase can nucleate directly from alloy melt and does not depend on primary (Ge) phase during the formation of binary eutectic structure. If the droplet diameter is smaller than 250 μm, a spherical anomalous eutectic grain structure forms, which consists of three very fine phases and is surrounded by coarse (Ge) and η phases. There is no evident feature of macrosegregation of semiconducting (Ge) phase and it distributes quite homogeneously, which is ascribed to the reduced gravity condition and rapid cooling in the drop tube.

Effect of oxygen on sintering plasmachemical powders of nonoxide high-melting compounds

Electron microscope studies and X-ray structural microanalysis have shown that sintering in the temperature range 800–2000°C proceeds in the solid phase. The granular structure of specimens is due to secondary recrystallization. Development of TiC and TiN recrystallization shows common features but also differences. A common feature is that the centers of grain growth in sintering are not initial nanodispersed powder particles but monocrystalline formations based on pore-free aggregates. The formations are due to coalescence in aggregates at T sint>1000°C, caused by the activation of diffusional mobility in TiC and TiN. The latter is due to the oxygen dissolution (which is always present in plasmachemically synthesized powders) in TiC and TiN and the development of neutral highly mobile complexes which include ions of Ti and O in combination with Ti vacancies. At the initial stage of TiC and TiN recrystallization pores are always present at the grain boundaries. Differences are found in the initial recrystallization temperature ( T r) and the influence of grain boundary pores on its development. For TiN T r is about 1300°C, while for TiC it is about 1400°C. In growing TiN grains, the separation of pores from boundaries takes place at T=1400°C. This causes the anomalous grain growth. At T⩾1400°C the grain growth takes its normal development. In the case of TiC, in the whole range of sintering temperature, the boundaries migrate with pores.

Sintering and properties of highly donor-doped barium titanate ceramics

The electrical properties of donor (La3+) doped BaTiO3 samples with a donor concentration in the range from 0.3 to 1.5 mol.% of La were studied. Samples were sintered at a low partial pressure of oxygen in order to facilitate anomalous grain growth and donor incorporation. In order to optimise the PTCR anomaly, the samples were annealed in air at 1100°C. Results show that with the use of a specific sintering profile PTCR ceramics containing an amount of donor dopant >0.3 mol.%, can be prepared. Heavily doped samples which do not exhibit anomalous grain growth show a core shell structure.

Selective Area Excimer‐Laser Crystallization of Amorphous Silicon Thin Films

Amorphous silicon films deposited on high quality fused‐silica wafers were crystallized using an excimer laser. A unique configuration was used, which is referred to as selective‐area crystallization. This was done by using surface masking of the film during standard laser irradiation. The resulting microstructure of the films was then characterized by transmission electron microscopy. The samples processed using the selective‐area crystallization method resulted in one of three microstructures. Samples processed with the lower laser energy densities demonstrated a standard microstructure consistent with regular laser annealing (grain size ∼ 200 nm). Samples processed with higher energy densities demonstrated either enlarged grain growth (a factor of three greater than standard microstructure) or anomalous grain growth (unusual aspect ratio or geometry). © 1999 The Electrochemical Society. All rights reserved.

Hydrothermal synthesis of dy-doped BaTiO3 powders

Submicron-sized (∼200 nm), monodisperse, and spherical powders of pure and dysprosium (Dy)-doped (0.8 at. pct) BaTiO3 have been prepared by “hydrothermal synthesis” at 90 °C in an air atmosphere. The powder preparation procedure developed in this work did not necessitate the use of strict and expensive processes, which were commonly required for the removal of free CO2 present in the atmosphere. The prepared powders were found to be crystalline, pure, and contained no BaCO3 as an impurity phase. Pure and Dy-doped BaTiO3 powders synthesized at 90 °C had the pseudocubic (space group: Pm-3m) crystal structure. Grain growth characteristics of pure and Dy-doped BaTiO3 pellets were compared during sintering in air over the temperature range from 1200 °C to 1500 °C. The Dy doping was found to be significantly effective in inhibiting the anomalous grain growth in BaTiO3 samples heated at or above 1200 °C.

Extended crystal defects in perovskite related materials: Characterisation and technological relevance

In perovskite related materials grain boundary phenomena exist that differ significantly from bulk properties. In BaTiO3 ceramics these properties have been used to design and fabricate a whole family of devices based on the positive temperature coefficient (PTC) effect, whereas for high-Tc superconductors grain boundary properties have delayed so far applications with large critical currents in the magnet and energy technology. The role of extended defects, particularly of {111} twins, for the anomalous grain growth was investigated. It was found that twinning alone does not trigger anomalous grain growth in BaTiO3 ceramics. Superconducting material relevant for applications in the magnet and energy technology should have critical current densities of 105–106 A/cm2. In truly polycrystalline YBa2Cu3O7 material the critical current density is reduced by several orders of magnitude, i.e. 102–103 A/cm2 at 77 K, due to the presence of grain boundaries. Bi-epitaxially textured material is required and can be produced for power applications yielding critical current densities of 106 A/cm2 at 77 K. Among the high-T c superconducting materials (Bi, Pb) 2Sr2Ca2Cu3O10+δ is the most complicated but also the one with the largest potential for high-T c superconducting components and devices. High-resolution TEM combined with image simulation was used to image and analyse the structure of the crystal defects. The anisotropy and granular microstructure limits the critical current densities in silver sheathed (Bi, Pb)2Sr2Ca2-Cu3O10+δ tapes to about 4.104 A/cm2 at 77 K.

PTCR behaviour of highly donor doped BaTiO 3

The electrical properties of donor doped BaTiO 3 samples with a donor concentration >0·3 mol% were studied. Samples were sintered at a low partial pressure of oxygen in order to facilitate the anomalous grain growth and donor incorporation. In order to optimise the PTCR anomaly the samples were annealed in an oxidising atmosphere. The samples were characterised using impedance spectroscopy and SEM. Results show that by the use of a specific sintering profile PTCR ceramics containing a higher amount of donor dopant can be prepared.

Observations of and model for insular grains and grain clusters formed during anomalous grain growth in N18 superalloy

Anomalous grain growth has been studied in the solid state in an N18 superalloy. An unexpected observation was the presence of small grains, either isolated or clustered, embedded in the large grains. The misorientations of the grains and clusters with respect to the large grain have been measured by electron backscattering diffraction. It is found that a majority of the isolated grains exhibit boundaries of low interfacial energy, while in the clusters the misorientations mainly correspond to high energy random boundaries. A simple wetting model in terms of interfacial energies is proposed and discussed, which explains the occurrence of the low energy insular grains. A specific mechanism for the occurrence of the large energy grains is proposed, which qualitatively explains the experimental distributions of misorientations.

Anomalous grain boundary and carrier transport in cat-CVD poly-Si films

Polycrystalline silicon (poly-Si) films are obtained at temperatures less than 400°C by catalytic chemical vapor deposition (cat-CVD) method, often called `hot-wire CVD' method. Structural properties of the cat-CVD poly-Si films are studied by Raman spectroscopy and high resolution transmission electron microscopy, along with carrier transport properties measured by the Van der Pauw method. It is found that the cat-CVD poly-Si films have a microstructure which consists of columnar crystalline grains with tens of nanometers diameter and a few nm thick amorphous layers surrounding the columnar grains. It is also found that the Hall mobilities of some samples are several-tens cm 2/V s and the barrier heights at the grain boundaries become the order of 0.01 eV or less. These good carrier transport properties appear dependent on the grain boundary structure of the cat-CVD poly-Si films.

Nanocrystalline nickel and nickel-copper alloys: Synthesis, characterization, and thermal stability

Pulsed electrodeposition is a simple, yet versatile method for the production of nanostructured metals. Forn-nickel we determine the influence of the physical and chemical deposition parameters on the nanostructure of the deposits and demonstrate that the grain size can be tuned to values between 13 and 93 nm, with rather narrow grain size distribution. The thermal stability of ourn-nickel as studied by x-ray diffraction and differential thermal analysis exhibits no detectable grain growth up to temperatures of about 380 K and an initialbehavior at 503 K followed by a regime of anomalous grain growth. For nanocrystalline Ni1-xCux(Monel-metal™) we demonstrate that alloy formation occurs at room temperature and that both chemical composition and grain size can be controlled by the pulse parameters and by appropriate organic additives.

Novel Electromigration Failure Mechanism for Aluminium-Based Metallization on Titanium Substrate

AbstractIn this paper a phenomenological characterisation of an anomalous grain growth, observed after classical electromigration lifetests, on Al-l%Si-0.5%Cu multigrain stripes, deposited at high temperature (460°C) on a Ti substrate, is reported. Failure analysis, carried out by Scanning Electron Microscopy (SEM) and Focus Ion Beam (FIB), has detected an abnormal single-grain growth in the vertical direction, starting from the Ti/Al interface. Medium Time to Failure (MTF) data are compared with those of stripes on Ti/TiN substrate, on which no grain growth was observed. The growth of the anomalous grains has been related to the Ti/Al interface properties. Transmission Electron Microscopy (TEM), X-Ray Diffraction (XRD) and Atomic Force Microscopy (AFM) analyses concurrently show a higher texture of the film on the Ti, compared with the film on the Ti/TiN substrate. The tight columnar orientation of grain boundaries strongly limits the mechanism of grain boundary diffusion during electromigration stress. The atomic flux is then forced to take place at the Ti/Al interface, where the epitaxial growth of the Al single grains is favoured.

ChemInform Abstract: Dedensification and Anomalous Grain Growth During Sintering of Undoped Barium Titanate.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Characteristic of grain structure of transformer steel with 3% Si after secondary recrystallization

Anisotropic electromagnetic steels made in the form of strips 0.18–0.5 mm thick are commonly used in the production of transformers and other parts of powerful electric and magnetic circuits. The level of power loss in the operation of these devices depends to a considerable degree on the structure of the electromagnetic steels, specifically, on the grain size and the uniformness of their distribution. In the final form the grain structure in the steels is formed in high-temperature annealing as a result of an anomalous grain growth, which is also known as secondary recrystallization. As distinct from normal growth in which the mean grain size increases gradually and simultaneously in the entire volume, in secondary recrystallization only separate grains grow rapidly to a large size while absorbing the surrounding fine (matrix) grains, whose growth is impeded for this or that reason. The present paper considers the results of statistical analysis of the structure of a steel that contains 3% Si.

Dedensification and Anomalous Grain Growth during Sintering of Undoped Barium Titanate

Dedensification (up to 10% of the theoretical density in some instances) occurs at temperatures of .1320°C during the sintering of commercial undoped BaTiO3 powders. The expansion of the rigid, interconnected network of large, rounded grains that grow by the Ostwald ripening effect can explain the dedensification that is observed. The isotropic anomalous grain growth results from the appearance of a liquid phase at } 1320°C and induces tensile stresses on the residual porous fine‐grained matrix. No dedensification was observed at temperatures of, 1320°C, despite anisotropic anomalous grain growth.