Prismatic Grains Research Articles

AC electrical conduction mechanisms and dielectrical studies of DD3 kaolin sintered at high temperature

In this study, DD3 kaolin, as a raw material, was thermally treated at a temperature ranging from 1000 °C to 1600 °C. The X-ray diffraction (XRD) patterns show the appearance of cristobalite and mullite phases at sintered temperature less than 1300 °C. Moreover, they revealed the formation of mullite at a higher temperature (1600 °C). The surface of mullite sample (DD3 at 1600 °C) is showed roughly with prismatic grains having an average size in the range of 32–42 nm. The FTIR spectrum is dominated by the stretching Si-O modes and Al-O at the wave number ranging from 443 to 1257 cm−1. The dielectric and electrical properties were investigated in a wide frequency (0.1–10 MHz) and at a temperature ranging from 0 to 200 °C. A strong dispersion of permittivity constants was observed at a low frequency, which can be attributed to interfacial Maxwell–Wagner–Sillar (MWS) relaxation. It is found that the ac conductivity depends on frequency and temperature with the Joncher power law A(T) ωs. Temperature analysis showed that the electron conductivity is controlled by both the small polaron tunneling (SPT) and the hopping correlated barrier (CBH) models.

Three-dimensional topography modelling of regular prismatic grain coated abrasive discs

Regular precision-shaped abrasive grains are preferred to irregular grits due to their superior performance regarding uniform polishing. Three-dimensional modelling of abrasive disc’s topography is essential to understand the material removal rate and surface roughness estimations through finite element based numerical simulations. Topography modelling of one such precision (prism)-shaped grain abrasive disc is carried out in this work through stochastic studies. The abrasive discs are scanned using laser profilometer, and high-frequency noise is filtered out using spectral analysis. Spatial parameters such as autocorrelation length and texture aspect ratio are considered for the topology mapping. Based on the statistical information from the measured grit sizes #60 and #120, such as peak protrusion grain height, spatial distributions, the topography is simulated. The analysis reveals that the peak height and spatial distribution follow a normal distribution. Unlike irregularly shaped grains where the orientations of the grains are neglected, the height variations in the precision-shaped grains in coated abrasive discs are mainly caused by random orientations of the grains. So, iterations are carried out for orientation in the three (mutually perpendicular) axes of the grain till the required statistical parameters are achieved. Surface fitting is performed for the distributed grains and the 3D-surface parameters of the simulated coated abrasive topography match well with the actual discs.

Characterisation-Assisted Reduction Roasting of BHJ, West Singhbhum, Jharkhand, India

During the mining of high-grade iron ore, considerable amount of waste is generated. This poses a serious challenge to the environment as well as humans. But, with the depletion of high-grade ores, the Banded Haematite Jasper (BHJ) ore generated as a waste has attracted attention as an alternative source of iron. The purpose for this study is to develop the energy efficient process route for harnessing the Banded Hematite Jasper. Utilisation of the ore as a source of iron will also serve the additional purpose of getting rid of a major burden on the environment. Reduction roasting is a promising route for the beneficiation of the ore for the recovery of iron values profitably. However, no studies have been reported correlating the influence of the properties of the ore on the roasting process. This is crucial for developing a viable process route for reduction roasting. Preliminary characterization study of the sample indicated the presence of prismatic and specularity hematite grains embedded in fine grained siliceous matrix and vice versa. The ore contained 43.06% Fe, 36.54% SiO2 and 0.21% Al2O3. Davies Tube magnetic separator of the ore indicated the absence of highly magnetic materials in the ore. Magnetic separation of the roasted ore resulted in significant enrichment of the ore with respect to iron. The optimized parameters for reduction roasting were 13 mm particle size, 60 min roasting time and 600 °C temperature.

Milarite-group minerals from the NYF pegmatite Velká skála, Písek district, Czech Republic: sole carriers of Be from the magmatic to hydrothermal stage

Four distinct paragenetic, morphological and compositional types of milarite-group minerals were distinguished in the NYF pegmatite Velka skala, Pisek district, Czech Republic (Moldanubian Zone). The simply zoned (Kfs,Plg,Ab,Qz > Bt > Ap > Tur) pegmatite dike, ∼60 cm thick, is enclosed in durbachite (K,Mg-rich melasyenite). The milarite-group minerals include: yellowish, subhedral, prismatic grains of homogeneous milarite I (milarite s.s.); prismatic subparallel aggregates of white to grayish milarite II (zoned milarite s.s. to Sc-rich milarite, up to ∼20 mol% of the oftedalite component) from small pockets; colorless hexagonal prismatic crystals of milarite III (milarite s.s.) lining open fractures and microscopic fillings in recrystallized K-feldspar locally associated with REE-bearing minerals (allanite, aeschynite, titanite); very rare milarite IV (milarite s.s. to agakhanovite) as prismatic crystals and thin overgrowths with complex oscillatory zoning, ≤200 D:\Palash\HWNLM\07_06m in size, on crystals of milarite II and milarite III. Their mineral assemblages, textures and compositions suggest primary (magmatic) for milarite I and hydrothermal origin for milarite II, III, and IV. The simplified compositional evolution in these milarites is: Al + Ca → Be + Sc → Be + Y, HREE →Be+ LREE . Milarite-group minerals are the sole carriers of Be in the pegmatite, no relics or pseudomorphs after other Be-rich minerals were found; thus milarite is now established as an additional primary Be-rich mineral in granitic pegmatites. Mineral assemblages, chemical composition of minerals and close relation to the host durbachite suggest a NYF affinity of the Velka skala pegmatite as well as a close relation to euxenite-type pegmatites of the Třebic Pluton. Milarite-group minerals from Velka Skala exhibit very similar compositional evolution to those found in the Heftetjern pegmatite, Tordal, Norway.

Microstructure evolution, interfacial reaction and mechanical properties of lead-free solder bump prepared by induction heating method

The formation process and mechanism of the Sn3.5Ag/Cu solder bump during induction heating were investigated to elucidate the basic characteristics of the induction heating soldering process. During induction heating, solder balls quickly melted from the surface to the center, which could be utilized to prepare solder bumps of controlled height. Cu6Sn5 grains with a round scallop shape were generated at the interface as soon as the solder ball was wetted on the Cu substrate. With prolonged heating time, the uniform morphology of the interfacial Cu6Sn5 grains was transformed to a special morphology in which scallop-shaped grains occupy the central area of the interface, surrounded by prismatic grains. Numerous bulk Cu6Sn5 grains were precipitated and regularly distributed in the solder matrix, which affected the hardness and shear strength of the solder bumps.

Effects of post-reflow cooling rate and thermal aging on growth behavior of interfacial intermetallic compound between SAC305 solder and Cu substrate

The interfacial reactions between Cu and Sn3Ag0.5Cu (SAC305) solder reflowed under various cooling rates were investigated. It is found that the cooling rate is an important parameter in solder reflow process because it influences not only microstructure of solder alloy but also the morphology and growth of intermetallic compounds (IMCs) formed between solder and Cu substrate. The experimental results indicate that only scallop-like Cu6Sn5 IMC layer is observed between solder and Cu substrate in case of water cooling and air cooling, while bilayer composed of scallop-like Cu6Sn5 and thin layer-like Cu3Sn is detected under furnace cooling due to sufficient reaction time to form Cu3Sn between Cu6Sn5 IMC and Cu substrate which resulted from slow cooling rate. Samples with different reflow cooling rates were further thermal-aged at 423 K. And it is found that the thickness of IMC increases linearly with square root of aging time. The growth constants of interfacial IMC layer during aging were obtained and compared for different cooling rates, indicating that the IMC layer thickness increased faster in samples under low cooling rate than in the high cooling rate under the same aging condition. The long prismatic grains were formed on the existing interfacial Cu6Sn5 grains to extrude deeply into solder matrix with lower cooling rate and long-term aging, and the Cu6Sn5 grains coarsened linearly with cubic root of aging time.

HT-FTIR micro-spectroscopy of cordierite: the CO2 absorbance from in situ and quench experiments

In this work, we address the intensity evolution of the CO2 FTIR bands of cordierite as a function of increasing T, by comparing data obtained from in situ versus quenching measurements. A natural well-characterized cordierite from Kragero (Arendal region, Norway) was studied up to 1200 °C using a heating stage fitted on a FTIR microscope. Two different oriented sections (001) and (010), respectively, were examined in order to check for the effect of the channel orientation on the CO2 release from the matrix. Spectra collected in situ show that increasing temperatures induces an increase in peak width for all CO2-related bands. The effects on the integrated absorbance A i are different for the different modes. Most notably, the integrated intensity A i of the anti-symmetric stretching mode (ν3) increases significantly up to 800 °C and then progressively decreases to 1000/1200 °C, depending on the sample orientation. Data obtained on quenched samples reveal that there is no variation in the band intensity for T < 900 °C, and thus, the absorbance increase observed for in situ measurements may be related to an increase in the molar absorption coefficient e. Combination of in situ with quenched data reveals that the CO2 loss from the cordierite matrix starts around 800 °C and is strongly dependent on the thickness and shape of the examined sample: It is favored for small and tabular-shaped grains, while being significantly reduced for large and prismatic grains. Fracturing along direction normal to [001] multiplies the diffusion interfaces, thus enhancing the CO2 loss from the matrix.

Surface micromorphology of dental composites [CE-TZP]-[Al2O3] with Ca(+2) modifier.

The objective of this study was to characterize the three-dimensional (3D) surface micromorphology of the ceramics produced from nanoparticles of alumina and tetragonal zirconia (t-ZrO2) with addition of Ca(+2) for sintering improvement. The 3D surface roughness of samples was studied by atomic force microscopy (AFM), fractal analysis of the 3D AFM-images, and statistical analysis of surface roughness parameters. Cube counting method, based on the linear interpolation type, applied for AFM data was used for fractal analysis. The morphology of non-modified ceramic sample was characterized by the rather big (1-2 μm) grains of α-Al2O3 phase with a habit close to hexagonal drowned in solid solution of t-ZrO2 with smooth surface. The pattern surfaces of modified composite content a little amount of elongated prismatic grains with composition close to the phase of СаСеAl3О7 as well as hexahedral α-Al2O3-grains. Fractal dimension, D, as well as height values distribution have been determined for the surfaces of the samples with and without modifying. It can be concluded that the smoothest surface is of the modified samples with Ca(+2) modifier but the most regular one is of the non-modified samples. A connection was observed between the surface morphology and the physical properties as assessed in previous works.

Yusupovite, Na2Zr(Si6O15)(H2O)3, a new mineral species from the Darai-Pioz alkaline massif and its implications as a new microporous filter for large ions

Yusupovite, ideally Na 2 Zr(Si 6 O 15 )(H 2 O) 3 , is a new silicate mineral from the Darai-Pioz alkaline massif in the upper reaches of the Darai-Pioz river, area of the joint Turkestansky, Zeravshansky, and Alaisky ridges, Tajikistan. Yusupovite was found in a pegmatite composed mainly of reedmergnerite, aegirine, microcline, and polylithionite. It occurs as prismatic grains about 2 mm in size embedded in reedmergnerite; associated minerals are quartz, pectolite, zeravshanite, mendeleevite-(Ce), fluorite, leucosphenite, a pyrochlore-group mineral, neptunite, telyushenkoite, moskvinite-(Y), and shibkovite. Yusupovite is colorless, transparent with a white streak, has a vitreous luster, and does not fluoresce under ultraviolet light. Cleavage is perfect on {110}, parting was not observed. Mohs hardness is 5. Yusupovite is brittle with a splintery fracture. The measured and calculated densities are 2.69(2) and 2.713 g/cm 3 , respectively. Yusupovite is optically biaxial (+) with refractive indices (λ = 589 nm) α = 1.563(2), β = 1.565(2), γ = 1.577(2); 2 V meas = 42(3)°, 2 V calc = 45°, strong dispersion: r > v. Yusupovite is monoclinic, C 2/ m , a = 14.5975(4), b = 14.1100(4), c = 14.4394(4) A, β = 90.0399(4)°, V = 2974.1(3) A 3 . The six strongest reflections in the X-ray powder diffraction data [ d (A), I, ( hkl )] are 7.05, 100, (020); 3.24, 96, (420); 3.10, 69, (241, 241); 5.13, 53, (202, 202); 6.51, 42, (201, 201); 3.17, 34, (042). The chemical composition (electron microprobe) is: Nb 2 O 5 0.39, SiO 2 58.84, ZrO 2 16.55, HfO 2 0.30, FeO 0.01, Y 2 O 3 3.05, Cs 2 O 2.58, K 2 O 0.95, Na 2 O 8.91, H 2 O calc 7.40, total 98.98 wt%, with H 2 O calculated from structure refinement. The empirical formula (based on 17.5 O apfu) is (Na 1.76 K 0.12 Cs 0.11 ) ∑1.99 (Zr 0.82 Y 0.17 Nb 0.02 Hf 0.01 ) ∑1.02 (Si 6.01 O 14.98 )(H 2 O) 2.52 , Z = 8. The crystal structure of yusupovite was refined to R 1 = 3.46% based on 4428 observed reflections. In the crystal structure, there are six Si sites occupied by Si, two M sites occupied mainly by Zr with minor Y and Hf. Si tetrahedra form an epididymite Si 6 O 15 ribbon along [010]. Epididymite ribbons and Zr-dominant M octahedra share common vertices to form a heteropolyhedral Si-Zr-O framework. There are six interstitial sites partly occupied by alkali cations Na, K, and Cs. The three [7]-coordinated Na sites are occupied by Na at 95, 84, and 78%. The three A sites are occupied by K and Cs at 12, 18, and 16%. There are 10 W sites occupied by H 2 O groups at 18–84%. Due to (K,Cs), Na and H 2 O disorder, the symmetry of yusupovite decreases from orthorhombic, space group Pbcm (elpidite), to monoclinic, space group C 2/ m , and the b unit-cell parameter of yusupovite is doubled compared to the corresponding cell parameter in elpidite, b yus = 2 a elp . Yusupovite, ideally Na 2 Zr(Si 6 O 15 )(H 2 O) 3 , is a dimorph of elpidite, Na 2 Zr(Si 6 O 15 )(H 2 O) 3 .

Syn-collisional lower continental crust anatexis in the Neoproterozoic Socorro-Guaxupé Nappe System, southern Brasília Orogen, Brazil: Constraints from zircon U–Pb dating, Sr–Nd–Hf signatures and whole-rock geochemistry

The southern Brasília Orogen occurs along the southernmost border of the São Francisco Craton. This Orogen is structured as a pile of Ediacaran syn-metamorphic thicked-skinned nappes that diachronically migrate toward the cratonic margin. A magmatic arc environment is represented by the Socorro-Guaxupé Nappe (SGN), which is a thick segment of partially molten lower to middle continental crust. The SGN comprises three main units: a lower granulitic, an intermediate diatexitic and an upper metatexitic. High-temperature metamorphic peak was reached at ∼625Ma, when large volumes of high-K diatexitic rocks were formed. Charnockitic and mangeritic magmatic rocks (São José do Rio Pardo suite) and in situ leucosomes of mafic composition are inferred to have formed under hornblende-dehydration melting conditions (i.e. T≥850°C). Granitic diatexites (Pinhal-type) were also formed, and zircon thermometry indicates T≥850°C. Elemental and isotopic geochemistry indicate that these diatexites and in situ leucosomes derived from melting of lower to middle continental crust. TIMS zircon U–Pb ages are presented for the lower and middle units. U–Pb dating coupled to Lu–Hf analysis (LA-MC-ICP-MS) was performed on charno-enderbitic in situ leucosomes from metatexitic rocks that also resulted from hornblende-dehydration melting. Two different zircon typologies were recognized. The first comprises bipyramidal-prismatic grains with oscillatory zoning and high luminescence, sometimes preserving low-luminescence 670Ma cores. The high-luminescence grains show 19 concordant ages at 621±16Ma. The second typology comprises isometric or soccer-ball-type grains. These grains show sector zoning and a concordia age of 608±4Ma. Th/U ratios from soccer-ball grains vary from 1.253 to 2.107, whereas prismatic grains vary from 0.118 to 1.774. Initial ɛHf signatures between −13 and −21 in both typologies provide evidence of a reworked crustal reservoir. The older prismatic grains may be related to the onset of metamorphic peak (∼625Ma), whereas the isometric ones provide evidence of long-lived high-T conditions until ∼610Ma.

Growth Behavior of WC Grains in Ni&lt;sub&gt;3&lt;/sub&gt;Al Matrix during Liquid Phase Sintering

Growth behavior of WC grains with two different sizes of 10 μm and 4 μm in the same liquid matrix has been studied in the WC–40vol.%Ni3Al system. The samples were sintered under a carbon saturated condition. Surfaces of the coarse WC grains are strongly micro-faceted and stepped. With the increase of sintering time, the average size of the fine WC grain increases continuously and very large plate-like triangular prismatic WC grains appear. The present investigation thus shows that the growth behavior of fine WC grain is affected by lower solubility of W in Ni3Al matrix and the abnormal growth of WC grains is suppressed. However, the growth behavior of coarse WC grain is governed only by the difference in energy between the facets.

Geochemical heterogeneity and reconstruction of the source of Upper Jurassic sandstone in the western part of West Siberia

The Shaim region located in the western part of the West Siberian Megabasin is the most wellstudied in this large and very important area. The crystalline basement is relatively shallow (1.5-2.2 km) there. A complex geological and geophysical study of this area was performed (1), and a geological map (1 : 200 000) of the preJurassic basement was compiled; a fragment of it is shown in Fig. 1. Two large structures are mapped there: the Danilov graben mostly composed of cenotypal basalt and rarely rhyolite of the Triassic age and the Shaim-Kusnetsov megaanticlinorium represented by a chain of Late Carboniferous-Early Permian monzodiorite-granosyenite massifs and their metamorphic schist frame (also known as a "granite-schist axis"). The preJurassic basement is overlain by the Upper Jurassic Vogulkinskaya Forma� tion mostly composed of sandstone. Within the Danilov graben, this formation is revealed by holes in the Tal'nikovaya, Symor'yakhskaya, Danilovskaya, and SeveroDanilovskaya areas, where it occurs on the Triassic rhyolite-basalt complexes. The same sedi� ments are observed on the territory of the "granite- schist axis" within the Tolumskaya, Teterevskaya, Mortym'yaTeterevskaya, and Trekhozernaya areas. In this study, we report a set of the chemical and geochemical analyses only from the Danilovskaya, Tal'nikovaya, Teterevskaya, and Trekhozernaya areas. The Vogulkinskaya Formation in the western part of West Siberia is mostly composed of sandstone (including glauconiterich), as well as gritstone, con� glomerate, rarely argillite, calcic sandstone, and orga� nogenic detrital limestone, which are almost not observed in other formations of the sedimentary cover in West Siberia (2). The thickness of the Vogulkinskaya Formation reaches 70 m. It is oilbearing and replaced by argillite of the Abalakskaya Formation. Sandstones from the Vogulkinskaya Formation are visually indistinguishable from each other; they have a lightgray up to brownish color and are almost always finegranular with a clay-carbonate cement. The mineralogy of sandstones of the Vogulkinskaya For� mation is considered for the area of the "granite- schist axis." They are composed of quartz and plagio� clase fragments; cement is clay-carbonate. Carbonate prevails, and its composition corresponds to calcite with admixtures of MgO (up to 1.07 wt %) and FeO (up to 0.79 wt %); sometimes the portion of carbonate reaches 50-60%, which allows us to attribute these rocks to sandy limestone. Glauconite, mica, plagio� clase, rutile, pyrite, and tourmaline are registered among the minor and accessory minerals. Quartz forms isometric grains with a size up to 1 mm. Albite is represented by tabular individuals with a size of 0. n mm. Mica often forms individual laths and aggregates, usually deformed. According to the composition, it corresponds to the muscovite-aluminoseladonite series (most analyses plot in the muscovite field) and contains up to 6% of the paragonite endmember. Tourmaline is characterized by individual wellformed prismatic grains with a size up to 0.5 mm; its chemical composition corresponds to that of dravite (48-50% of endmember) with a high concentration of foitite and magnesiofoitite (in total, up to 30-35%), olenite (up to 12%), and uvite (up to 10%). Rutile does not contain impurities and sometimes is replaced by titan� ite from the margins.

Phase Composition and Microstructure of Binderless WC-ZrC Cemented Carbides Fabricated by Spark Plasma Sintering

Binderless WC-based cemented carbides with different fractions (0-9 wt.%) of ZrC nanopowder were fabricated through spark plasma sintering at 1600 °C under a uniaxial pressure of 50 MPa. The addition effect of ZrC nanopowder on the phase composition and microstructure of the fabricated materials were explored with the help of X-ray diffraction and scanning electron microscope. The results indicated that W2C phase was detected in the samples with 0-3 wt.% ZrC nanopowder, but with further increase in ZrC added fraction, ZrO2 phase instead of W2C phase was detected. The apparent density decreased gradually with the increase in added fraction of ZrC nanopowder, while the relative density increased initially and then decreased, reaching its maximum of about 98.2% when the added fraction of ZrC nanopowder was about 3 wt.%, indicating that appropriate added fraction of ZrC nanopowder can improve the densification of binderless WC cemented carbides. Without ZrC nanopowder, the coarsening and abnormal growth of WC grains were serious, resulting in many large prismatic WC grains in the samples. However, Such phenomena could be suppressed by adding ZrC nanopowder, resulting in much finer and more homogenous microstructure after 1-3 wt.% ZrC nanopowder was added. When the added fraction of ZrC nanopowder was higher than 3 wt.%, the agglomeration of ZrC nanopowder became more and more serious.

Metamorphic zircon formation at the transition from gabbro to eclogite in Trollheimen–Surnadalen, Norwegian Caledonides

Abstract A transition from gabbro to eclogite has been investigated at Vinddøldalen in south-central Norway, with the aim to link reaction textures to metamorphic zircon growth and to obtain a direct U–Pb zircon age of the metamorphic process. In the different rocks of the transition zone zircon occurs as (I) igneous prismatic grains, (II) metamorphic polycrystalline rims and pseudomorphs after baddeleyite, and (III) as tiny (&lt;10 µm) bead-like zircon grains. Textural relations suggest that type II zircon formed by breakdown of baddeleyite in the presence of silica, whereas Fe–Ti oxides were the main Zr source for the type III zircon. Subsolidus liberation of Zr and formation of bead zircon took place by oxyexsolution of titanomagnetite during fluid-assisted metamorphism, and by resorption of Fe–Ti oxide in rock domains that were completely recrystallized to eclogite. SIMS (secondary ion mass spectrometry) and TIMS (thermal ionization mass spectrometry) dating provides comparable U–Pb ages of magmatic zircon and baddeleyite. Baddeleyite (TIMS) yielded an age of 1457±11 Ma for the gabbro emplacement. Bead-type metamorphic zircon from eclogite gave 425±10 Ma (TIMS) dating the metamorphic transition from gabbro to eclogite in the upper basement of the Lower Allochthon in the south-central Scandinavian Caledonides.

In-Situ Preparation and thermal shock resistance of mullite-cordierite heat tube material for solar thermal power

In order to improve the thermal shock resistance of solar thermal heat transfer tube material, the mullite-cordierite composite ceramic as solar thermal heat transfer tube material were fabricated by pressureless sintering using α-Al2O3, Suzhou kaolin, talc, and feldspar as starting materials. The important parameter for solar thermal transfer tube such as water absorption (Wa), bulk density (Db), and the mechanical properties were investigated. The phase composition and microstructure of the composite ceramics were analyzed by XRD and SEM. The experimental results show that the B3 sintered at 1 300 °C and holding for 3 h has an optimum thermal shock resistance. The bending strength loss rate of B3 is only 2% at 1 100°C by air quenching-strength test and the sample can endure 30 times thermal shock cycling, and the water absorption the bulk density and bending strength are 0.32%, 2.58 g·cm−3, and 125.59 MPa respectively. The XRD analysis indicated that the phase compositions of the sample were mullite, cordierite, corundum, and spinel. The SEM images illustrate that the cordierite is prismatic grain and the mullite is nano rod, showing a good thermal shock resistance for composite ceramics as potential solar thermal power material.

Structure of prismatic halloysite

The crystal structure of halloysite, despite being one of the most commonly occurring clay minerals on the Earth’s surface, remains elusive. This paper reports on a multi-methodological study to shed more light on the atomic arrangement of halloysite from Olkhon Island, Lake Baikal, Russia, which has a prismatic morphology. It has been investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), selected-area electron diffraction (SAED) and, in particular, high-resolution transmission electron microscopy (HRTEM), to reveal its atomic structure and formation process. XRD analysis indicated a basal spacing of ca. 7.2 A and two characteristic peaks with d = 4.28 and 4.03 A on the tail of the 02,11 band. The halloysite grain cross sections displayed various prismatic forms, ranging from a rectangle to a regular 18-sectored polygon in SEM and TEM. The SAED pattern from a sector of the polygonal prisms with the incident beam parallel to the prism axes showed a regular one-layer oblique reciprocal lattice, similar to that of kaolinite along Y i -directions. HRTEM imaging performed with the new computer-assisted minimal-dose system and an incident beam perpendicular to the prism axis showed stacking of most dioctahedral 1:1 layers with their pseudo-mirror plane perpendicular to the prism axis and an almost random, or rather, alternating lateral stagger to the right or left from the preceding layer, which corresponds to interlayer displacements of τ + and τ − in Zvyagin symbols, or layer displacements of t 1 and t 2 used to describe the stacking in kaolinite. This stacking feature explains the SAED pattern from the side of the prismatic grains and the two characteristic peaks on the tail of the 02,11 band in the XRD pattern. Based on these results, it is proposed that tubular halloysite initially forms as a hydrated one with the pseudo-mirror plane of the kaolinite layers perpendicular to the tube axis, then dehydrates with, possibly, partially hydrogen-bonded interlayers, and finally transforms to a prismatic one consisting of sectored flat layers with the complete hydrogen-bonded interlayers. During this transformation, stacking with comparable ratio and frequent alternation of τ + and τ − is formed, to minimize morphological change of the tubes.

Preparation and microwave dielectric properties of Bi2Ti4O11 ceramics

Single phase of Bi2Ti4O11 ceramics, which belong to meta-stable phase compounds, were synthesized by controlling the reaction time through conventional solid-state method. The effects of annealing time on phase composition of Bi2Ti4O11 ceramic powders and sintered ceramics were studied by XRD analysis. Second phase Bi2Ti2O7 appeared when the annealing time shorter than 4 h. However, pure phase of Bi2Ti4O11 powders can be formed by prolonging the annealing time to 6 h at 1,000 °C. The sintering temperatures on microstructure and microwave dielectric properties of Bi2Ti4O11 ceramics were investigated. The results show that ceramics sintered at 1,075–1,175 °C are single phase of Bi2Ti4O11 and present two different sizes of prismatic shape grains. Smaller size crystals grow into larger ones with increasing sintering temperature. The ceramics sintered at 1,125 °C reach a maximum density and have a microwave dielectric properties of er = 51.2, Q × f = 3,050 GHz and τf = −297 ppm/°C.

Effect of Aluminum Content on Mechanical Properties and Thermal Conductivities of Sintered Reaction‐Bonded Silicon Nitride

A systematic study was conducted of the effect of Al impurities on the mechanical and thermal properties of sintered reaction‐bonded silicon nitrides (SRBSNs). 2 mol.% Y2O3 and 5 mol.% MgSiN2 were added as sintering aids to high‐purity raw Si powder, followed by respective additions of 0, 0.01, 0.1, 0.2, and 0.4 mass% of Al relative to Si. The prepared powder mixes were press formed and nitrided in a 0.1 MPa nitrogen atmosphere at 1400°C for 8 h. Postsintering of the nitrides was conducted under a pressurized nitrogen atmosphere of 0.9 MPa at 1900°C for 6 h. The SRBSN specimens exhibited hardly any difference in their four‐point bending strength, which ranged from 800 to 850 MPa. However, their fracture toughness clearly decreased with increasing additive Al content. This is thought to be due to a reduction in the proportion of coarse prismatic grains improving the toughness with increasing additive Al content. A significant reduction in the thermal conductivity was also observed with increasing additive Al content. This is conjectured to be due to the formation of SiAlON through a substituted solid solution of aluminum (Al) and oxygen (O) in β‐Si3N4 crystals. An inductively coupled plasma and hot gas extraction method were employed to directly analyze Al and O in β‐Si3N4 particles. Approximately half of the additive Al was confirmed to be in solid solution in β‐Si3N4 particles along with the solid solution of O in a chemical formula that nearly corresponds to β‐Si6–zAlzOzN8–z. The chemical analysis results give a Z value range of 0.0006–0.024.

Geochemistry of zircons from ultrametapmorphic granitoids in junction zone of Aldan Shield and Dzhugdzhur-Stanovoi Fold Region

The geochemistry of zircons from autochthonous granite gneiss (Lc1) anatectic (Lc3–4) and injection (Lc5) leucosomes has been studied. Neoarchean prismatic zircon grains with cores that reveal oscillatory zoning and are overgrown by a couple of rims have been seen to occur in Lc3–4. The prismatic grains are occasionally modified into isometric grains with block structure by Paleoproterozoic secondary alteration, which is accompanied by the depletion in HREE, Y, Nb, U; enrichment in Ti, Li, LREE; increasing Th: U ratio and Ce anomaly; and decreasing Eu anomaly. The Paleoproterozoic alteration is related to the low-temperature amphibolite-facies metamorphism followed by partial melting. The Neoarchean prismatic zircons were formed under the conditions of high-temperature amphibolite-facies ultrametamorphism at a temperature of ∼700°C. Judging by the higher Ce/Ce* ratio, the metamorphic rounded zircons were formed at a higher oxygen fugacity as compared with ultrametamorphic zircons from Lc1 and Lc3–4. Specific variation trends of trace element concentrations in prismatic L1 and L3–4 zircons, occasionally with opposite directions, emphasize their different origin. The former are products of metasomatic granitization completed by selective melting with appearance of dispersed melt drops, while the latter are products of anatexis in the open system and by lit-par-lit migmatization. Prismatic zircons L5 are characterized by rhythmic zoning in the core surrounded by rims. The concordant U-Pb age of rims is 129 Ma; the 206Pb/238U age of cores varies from 2213 to 147 Ma. The appreciable enrichment (by a factor of 2–13) of zircons in all minor elements from the core to the rims is caused by the effect of residual postmagmatic fluid, which not only altered zircons, but also facilitated the recrystallization of granite into a pegmatoid variety.

Palaeoproterozoic episodic magmatism and high‐grade metamorphism in the North China Craton: evidence from SHRIMP zircon dating of magmatic suites in the Daqingshan area

The Daqingshan area in the North China Craton (NCC) incorporates several suites of magmatic intrusives including gabbros, granodiorites, charnockites and syenogranites, which were overprinted by subsequent metamorphic events. In this study, we report detailed morphological and textural characteristics as well as sensitive high resolution ion microprobe (SHRIMP) U–Pb ages of zircons from ten representative samples of the magmatic suites including syenogranites, charnockites and granodiorites. The zircons in these rocks are characterized by typical magmatic crystallization features including elongate prismatic grains with angular to sub‐angular faces which are overprinted by a variety of reworked textures indicating strong recrystallization. These structures are terminated by thin and variable metamorphic overgrowths. U–Pb analyses of the zircons reveal multiple tectono‐thermal events including magmatic crystallization at 2.30–2.00, 1.96–1.94 and 1.85–1.82 Ga and metamorphic overprint at 1.97–1.94 and 1.87–1.82 Ga. The episodic magmatism and regional metamorphism recorded in our study show a complex sequence of tectonic processes during the early to late Palaeoproterozoic associated with the amalgamation of crustal blocks within the NCC, with the youngest episode of the syenogranites representing regional extension at the end of the final cratonization of the NCC. Copyright © 2012 John Wiley &amp; Sons, Ltd.