Increase In Average Grain Size Research Articles

Nonlinear Magnetic Properties of Mn-Modified Ba<tex>$_3$</tex>Co<tex>$_2$</tex>Fe<tex>$_23$</tex>O<tex>$_41$</tex>Hexaferrite

We have investigated the structural and magnetic properties of a novel type of hexaferrite with composition of Ba/sub 3/M/sub 2/Fe/sub 23/O/sub 41/ (where M=Co and Mn) (Z hexaferrite) with tunable permeability. The material is promising for very high frequency (VHF) control when used in a voltage-controlled oscillator (VCO) as a tunable inductor. We substituted Mn for Co in the composition Ba/sub 3/Co/sub 2-x/Mn/sub x/Fe/sub 23/O/sub 41/ from x=0.0 to 1.2 to improve the inductance tunability and quality factor of the inductor material. Mn substitution had significant influence on the grain size of the sintered hexaferrites, and the coercivity was greatly reduced. The initial permeability and quality factor increased with increasing substitution of Mn for Co until x reached 0.5, an effect attributed to the decrease of the magnetocrystalline anisotropy constant of the hexaferrite and increase of average grain size. The tunability of the samples also increased with increasing Mn substitution, reaching a maximum at x=0.5, and then decreased with further substitution. This behavior is similar to that of the initial permeability and coercivity, and can be explained by the same mechanism.

Density of states near Fermi level in PbS nanoparticle films

Temperature-dependent DC-conductivity of PbS nanoparticle films prepared by solution growth technique has been studied. Variable range hopping is identified as a predominant conduction mechanism. Density of localized states near the Fermi level is observed to decrease exponentially with increase of average grain size in the films.

Density of states near Fermi level in PbS nanoparticle films

Temperature-dependent DC-conductivity of PbS nanoparticle films prepared by solution growth technique has been studied. Variable range hopping is identified as a predominant conduction mechanism. Density of localized states near the Fermi level is observed to decrease exponentially with increase of average grain size in the films.

Basis for Paleoenvironmental Interpretation of Magnetic Properties of Sediment from Upper Klamath Lake (Oregon): Effects of Weathering and Mineralogical Sorting

Studies of magnetic properties enable reconstruction of environmental conditions that affected magnetic minerals incorporated in sediments from Upper Klamath Lake. Analyses of stream sediment samples from throughout the catchment of Upper Klamath Lake show that alteration of Fe-oxide minerals during subaerial chemical weathering of basic volcanic rocks has significantly changed magnetic properties of surficial deposits. Titanomagnetite, which is abundant both as phenocrysts and as microcrystals in fresh volcanic rocks, is progressively destroyed during weathering. Because fine-grained magnetite is readily altered due to large surface-to-volume ratios, weathering causes an increase in average magnetic grain size as well as reduction in the quantity of titanomagnetite both absolutely and relative to hematite. Hydrodynamic mineralogical sorting also produces differences in magnetic properties among rock and mineral grains of differing sizes. Importantly, removal of coarse silicate and Fe-oxide grains by sorting concentrated extremely fine-grained magnetite in the resulting sediment. The effects of weathering and sorting of minerals cannot be completely separated. These processes combine to produce the magnetic properties of a non-glacial lithic component of Upper Klamath Lake sediments, which is characterized by relatively low magnetite content and coarse magnetic grain size. Hydrodynamic sorting alone causes significant differences between the magnetic properties of glacial flour in lake sediments and of fresh volcanic rocks in the catchment. In comparison to source volcanic rocks, glacial flour in the lake sediment is highly enriched in extremely fine-grained magnetite.

Development of zirconia-titania porous ceramics for humidity sensors

Porous ZrTiO4 ceramic specimens were prepared by pressing and sintering at different temperatures (1150–1500°C range) powders synthesized by the polymeric precursor technique. X-ray diffraction analysis shows that all specimens are orthorhombic single-phase ZrTiO4. Average pore diameter had approximately 73% reduction for increasing the sintering temperature from 1150 to 1500°C. Scanning electron micrographs of polished and thermally etched surfaces show also intergranular porosity reduction at the expenses of the increase of average grain size. Impedance spectroscopy measurements in specimens under controlled humidity conditions gave evidences of the ability of the porous ZrTiO4 specimens to be used as humidity sensors.

Microstructural Evolution of Calcium‐Doped α‐Alumina

Effect of different calcium doping levels on the microstructure of high‐purity α‐alumina was studied as a function of sintering time and temperature using scanning electron microscopy (SEM). Microstructural evolution was related to hypothetical calcium excess at the grain boundaries (ΓCa) that was calculated assuming zero solubility of calcium in bulk α‐alumina. Under all sintering conditions, grains were uniform in size and equiaxed for low calcium concentrations (<3 Ca atoms/nm2). The grain morphology became elongated when the calcium concentration at the grain boundaries reached calcium excess of ΓCa= 3–3.5 Ca atoms/nm2 in all samples. The average grain sizes of undoped samples were ∼10% larger than the average grain sizes of low‐calcium‐doped samples. This decrease is believed to be due to solute drag effect of segregated Ca impurities on the grain boundary mobility. For the samples that were sintered at 1500° and 1600°C, slablike abnormally grown grains appeared for critical calcium excess concentrations of ΓCa= 4.5–8 Ca atoms/nm2. With abnormally grown grains a dramatic increase in average grain size was observed. However, when the calcium concentration was increased further, above certain calcium excess concentration depending on sintering temperature, a significant decrease in grain size was observed. In contrast to samples sintered at 1500° and 1600°C, when the samples sintered at 1400°C, although the hypothetical calcium coverage exceeded ΓCa= 11 Ca atoms/nm2, only few grains grew abnormally without significantly affecting the average grain size. Observations clearly indicated that calcium impurities caused elongated (slablike) grain morphology when their excess concentrations reached a critical level at the grain boundaries.

Evolution of microstructure and mechanical properties of in situ consolidated bulk ultra-fine-grained and nanocrystalline Zn prepared by ball milling

Abstract The evolution of the microstructure and mechanical properties of ultra-fine-grained and nanocrystalline Zn induced by ball milling at room temperature are studied systematically. The yield stresses measured from miniaturized disk bend tests and tensile tests are consistent with the microhardness results and generally increase with the decrease of average grain size. A dramatic decrease of hardness during milling from 1 to 3 h is a reflection of the increase of average grain size from 80 to 240 nm due to the initial unstable grain size and therefore, grain growth in this period. Young's modulus remains almost the same for samples milled for different times and is that for conventional grain size Zn. A transition from bending to membrane stretching is observed in the force–displacement curves for Zn ball milled for ≤18 h. The variation of transition strain with milling time could be related to the evolution of grain size distribution and therefore hardness during milling.

Dome Concordia ice microstructure: impurities effect on grain growth

AbstractWe present a detailed analysis of the microstructure in the shallow part (100–580m) of the European Project for Ice Coring in Antarctica (EPICA) ice core at Dome Concordia. In the Holocene ice, the average grain-size increases with depth. This is the normal grain-growth process driven by a reduction of the total grain-boundary energy. Deeper, associated with the Holocene–Last Glacial Maximum (LGM) climatic transition, a sharp decrease of the average grain-size is observed. to explain modifications to the microstructure with climatic change, we discuss the role of soluble and insoluble (microparticles) impurities in the grain-growth process of Antarctic ice, coupled with an analysis of the pinning of grain boundaries by microparticles. Our data indicate that high soluble impurity content does not necessarily imply a slowing-down of grain-growth kinetics, whereas the pinning of grain boundaries by dust particles located along the boundaries does explain modifications to the microstructure (small grain-sizes; change in grain-size distributions, etc.) observed in volcanic ash layers or dusty LGM ice.Moreover, classical mean-field models of grain-boundary pinning are in good quantitative agreement with the evolution of grain-size along the EPICA ice core. This suggests a major role for dust in the modification of shallow polar ice microstructure.

Processing of silicate dust grains in Herbig Ae/Be systems

We have analysed the 10 μm spectral region of a sample of Herbig Ae/Be (HAEBE) stars. The spectra are dominated by a broad emission feature caused by warm amorphous silicates, and by polycyclic aromatic hydrocarbons. In HD 163296 we find aliphatic carbonaceous dust, the first detection of this material in a HAEBE star. The silicate band shows a large variation in shape, due to variable contributions of three components: (i) a broad shoulder at 8.6 μm; (ii) a broad maximum at 9.8 μm; and (iii) a narrow feature with a broad underlying continuum at 11.3 μm. From detailed modeling these features can be identified with silica (SiO2), sub-micrometer sized amorphous olivine grains and micrometer sized amorphous olivine grains in combination with forsterite (Mg2SiO4), respectively. Typical mass fractions are 5 to 10 per cent of crystalline over amorphous olivine, and a few per cent of silica compared to the olivines. The detection of silica in emission implies that this material is heated by thermal contact with other solids that have a high absorptivity at optical to near-IR wavelengths. The observed change in peak position of the silicate band in HAEBE stars from 9.7 μm to 11.3 μm is dominated by an increase in average grain size, while changes in composition play only a minor rôle. The HAEBE stars, β Pic and the solar system comet Halley form a sequence of increasing crystallinity. We find that the abundance of SiO2 tends to increase with increasing crystallinity. This is consistent with the compositional changes expected from thermal annealing of amorphous grains in the inner regions of the disk. We confirm earlier studies that the timescale for crystallisation of silicates in disks is longer than that of coagulation. Our results indicate that the processes that governed grain processing in the proto-solar nebula, are also at work in HAEBE stars.

Effect of thermomechanical processing on grain boundary character distribution in  brass

The present paper reports new experimental work to evaluate two aspects of thermomechanical treatment of  brass: the effect on the grain boundary population of strain annealing iterations rather than single stage treatments, and the accompanying effect on mechanical properties, particularly ductility. Whereas the first stage of a two stage strain–annealing schedule improved ductility, during the second stage there was a further improvement which was more than expected from the rather small increase in average grain size. The ductility enhancements were related to changes in the coincidence site lattice (CSL) Σ3n populations, manifested by a combination of changes in the overall proportions of Σ3 boundaries from 42 to nearly 70%and the distribution of v/vm, where v/vm is a measure of the proximity to the exact CSL configuration. The data were interpreted in terms of the types of Σ3 boundaries generated, and the occurrence of multiple twinning and ‘back twinning’.

Comparison of Milankovitch periods between continental loess and deep sea records over the last 2.5 Ma

A high-resolution East Asian winter monsoon proxy record reconstructed from the Baoji loess section in China shows two major shifts in climate modes over the past 2.5Ma, one occurring at about 1.7–1.6Ma BP and the other at about 0.8–0.5 Ma BP. The 1.7–1.6Ma shift is characterized by a rather abrupt transition of winter monsoon variability from various periodicities to dominant 41-ka cycles, and accompanied by a substantial increase in intensity of winter monsoon winds as manifested by an increase in average loess grain size. The 0.8–0.5Ma event shows a relatively gradual transition from constant 41-ka cycles to predominant 100-ka climatic oscillations with a significant increase in amplitude. The 0.8–0.5Ma shift matches that registered in deep-sea δ18O records, whereas the 1.7–1.6Ma shift is absent in global ice volume changes. This comparison suggests that at about 1.6Ma BP, the ice sheets in the Northern Hemisphere may have reached a critical size, sufficient to modulate changes in the global climate system. The discrepancy of climate cyclicity between loess and deep-sea records over the 2.5–1.6Ma interval suggests that the older Matuyama climate evolution cannot be understood simply by a regular 41ka cycle model on a global scale. More long proxy records derived from continental deposits are needed.

Fabricating densified hydroxyapatite ceramics from a precipitated precursor

A fine hydroxyapatite precursor was prepared by reacting calcium hydroxide and an orthophosphoric acid solution. To fabricate sintered hydroxyapatite ceramic, the resulting precursor was not calcined in powder form. Instead, it was compacted in a uniaxial die, followed by thermal treatment of the powder pellets at various temperatures, prior to being sintered at a high temperature. A sintered density of 98.15% theoretical was obtained at a sintering temperature as low as 1000°C. Prolonged dwelling at 1200°C, up to more than 20 h, did not affect the phases present in the sintered hydroxyapatite, although it resulted in a slight fall in sintered density due to the grain coarsening. The increase in average grain size leads to a noticeable improvement in fracture toughness of the sintered hydroxyapatite.

The effect of excess neodymia on the grain growth of Nd1+xBa2−xCu3Oy solid solutions

The grain growth of dense, fine-grained Nd1+xBa2−xCu3Oy (x = 0.1−0.4) specimens has been examined in pure O2(g) at 938 °C and 967 °C. No detectable change in average grain size was observed for Nd1.4Ba1.6Cu3Oy within 72 h at 967 °C; however, a significant increase in average grain size developed between 18 and 24 h at 967 °C for Nd1.3Ba1.7Cu3Oy, and within 8−12 h at ≤967 °C for Nd1.2Ba1.8Cu3Oy and Nd1.1Ba1.9Cu3Oy. Microstructural analyses revealed that sudden changes in average grain size coincided with the formation of relatively large (abnormal) grains. A broadening of the grain size distribution was also observed. TEM analyses revealed that grain boundaries were free of second phases. The possible role of anisotropy in grain boundary energy and/or mobility on grain growth is discussed.

Changes in the tensile strength and fracture mode with thickness of electrodeposited copper foil

The tensile strength and fracture mode changed with thickness of copper foil obtained from an acid copper sulfate bath under a same electrodeposition condition. The tensile strength decreased with increasing foil thickness due to an increase in average grain size of the foil. The tensile fracture mode changed from fractures normal to tensile axis to inclined fractures with increasing film thickness. The phenomena have been discussed based on a hardness distribution along the thickness and the local necking and fracture strains.

The study of 57Fe Mössbauer spectra for La0.7Sr0.3FeO3−δ with different structure

AbstractSamples of La0.7Sr0.3FeO3−δ were obtained by sintering the precursor prepared by the Polyethylene Glycol (PEG) method. The measurements of XRD and 57Fe Mössbauer spectra showed that the heat treatment of the precursor at different temperatures leads to phase transformations from the orthorhombic to the cubic and finally to the rhombohedral phase, to an increase of average grain size, and to changes of the electronic states of the Fe ions. It was thought that in the perovskite oxides the conditions of synthesis or treatment may have an important influence on structure and properties by adjusting the valence state or/and introducing oxygen vacancies.

Nanocrystalline Solutions as Precursors to The Spray Deposition of Cdte Thin Films

ABSTRACTIn this article we report the first nanoparticle-derived route to smooth, dense, phase-pure CdTe thin films. Capped CdTe nanoparticles were prepared by injection of a mixture of Cd(CH3)2, (n-C8H17)3 PTe and (n-C8H17)3P into (n-C8H17)3PO at elevated temperatures. The resultant nanoparticles 32-45 Å in diameter were characterized by x-ray diffraction, UV-Vis spectroscopy, transmission electron microscopy, thermogravimetric analysis and energy dispersive x-ray spectroscopy. CdTe thin film deposition was accomplished by dissolving CdTe nanoparticles in butanol and then spraying the solution onto SnO2-coated glass substrates at variable susceptor temperatures. Smooth and dense CdTe thin films were obtained using growth temperatures approximately 200 °C less than conventional spray pyrolysis approaches. CdTe films were characterized by x-ray diffraction, UV-Vis spectroscopy, atomic force microscopy, and Auger electron spectroscopy. An increase in crystallinity and average grain size as determined by x-ray diffraction was noted as growth temperature was increased from 240 to 300 °C. This temperature dependence of film grain size was further confirmed by atomic force microscopy with no remnant nanocrystalline morphological features detected. UV-Vis characterization of the CdTe thin films revealed a gradual decrease of the band gap (i.e., elimination of nanocrystalline CdTe phase) as the growth temperature was increased with bulk CdTe optical properties observed for films grown at 300 °C.

The structure of nanocrystalline silicon formed by laser-induced explosive crystallization

Abstract The structure of nanocrystalline silicon formed by pulsed laser induced explosive crystallization of amorphous silicon has been studied using Raman spectroscopy. It is found that the properties of nanocrystalline silicon are independent of the laser energy density used to initiate explosive crystallization. From the spectrum in the low-wavenumber region it is infrared that the grains in nanocrystalline silicon are highly distorted. The average grain size, as determined from the position of the main peak in the spectrum, is 5–7 nm. Annealing at 500–600°C results in a decrease of the distortion and an increase in average grain size by 1–2 nm.

RESTRAINTS ON THIN SECTION ANALYSIS OF GRAIN GROWTH IN UNSTRAINED POLYCRYSTALLINE ICE

Tests were performed at -1°C to evaluate the effects of a free surface and the thickness dimensions of thin sections on the growth of grains in fine-grained, pore-rich, strain-free polycrystalline ice. Results show that negligible growth of grains occurs when the mean size of grains is more than 1.5 to 2 times the section thickness. Grain growth in thicker sections was significant for the fact that grain boundary migration, leading to 3-4 fold increases in average grain size, was virtually unaffected by the presence of large numbers of bubbles in the ice. Nor was there any evidence to indicate any concentrating of bubbles along migrating boundaries. Grain boundary grooving was a characteristic feature of most sections undergoing grain growth. This implies actual migration of grooves during grain growth. The fact that the total length of grooves decreased with increasing grain size also implies some process of groove consumption during grain growth. Three-dimensional grain growth measurements in bulk samples compared favorably with those obtained from sections two to three times thicker than the mean grain diameter.

Grain growth in three dimensions: A lattice model

Grain growth is the term used to describe the increase in average grain size R which occurs upon annealing a polycrystalline aggregate after primary recrystallization is complete. In the long time limit, R increases with time t raised to a value n which is the growth exponent. Most existing grain growth theories implicitly assume that grains can be described as spherical and that growth occurs in an average environment. This, however, ignores the fact that adjacent grains share common boundaries, resulting in a microstructure that is topologically connected. These theories predict long term growth kinetics of the order n=1/2. Since experimental studies of grain growth yield an exponent less than 1/2, the discrepancy may be a consequence of neglecting topological constraints and the detailed environment. To examine this possibility a lattice model for 2-dimensional grain growth was developed in which topology and local environment are included. While this 2-dimensional model accurately simulates grain growth in thin encapsulated films, the validity of its predictions for 3-dimensional grain growth remains to be established. This paper reports the first results of our extension of the lattice grain growth model to 3 dimensions.

Coarsening of cobalt grains dispersed in liquid copper matrix

Coarsening of spherical Co-rich grains dispersed in Cu-rich liquid matrix is investigated. The specimen compositions are 50 pct Co-50 pct Cu, 40 pct Co-60 pct Cu, and 30 pct Co-70 pct Cu by weight. The annealing temperatures have been varied between 1150 and 1300 °C. The specimens have been prepared by usual powder metallurgy technique from fine Co and Cu powders. Due to the equal density of Co and Cu, the Co-rich grains remain uniformly dispersed in the liquid matrix. The increase of average grain size with annealing time,t, follows closely thet 1/3 law predicted for diffusion controlled mechanism by Lifshitz, Slyozov, Wagner (LSW) and Ardell. The observed increase of the growth rate with increasing solid grain fraction is a clear evidence for diffusion controlled mechanism. The linear intercept distribution of the grains agrees closely with the predictions for reaction controlled growth in LSW theory, but the result is also consistent with Ardell’s prediction that when the grains grow with small inter-grain distance under diffusion control, the grain size distribution is almost identical to that of the reaction controlled growth in LSW theory. The estimated values of the diffusion constant and its activation energy agree in order of magnitude with the typical values for diffusion in liquid metal.