Platelet Grains Research Articles

Preparation of excellent performance ZTA ceramics and complex shaped components using digital light processing 3D printing technology

Complex cellular structured Al2O3 ceramics is difficult to fabricate due to its poor toughness. To overcome this limitation, La2O3 and ZrO2 were introduced into Al2O3 ceramics in digital light processing (DLP) 3D printing. The toughening of ZrO2 and in-situ formation of platelet grains by La2O3 addition dramatically increased the performance of printed ceramics (flexural strength of 556.6 MPa, fracture toughness of 6.94 MPa·m1/2, Vickers hardness of 18.88 GPa, and an apparent porosity of 0.09%), which exceeds most of those fabricated by traditional process. Furthermore, four different cellular structures were successfully fabricated with a Diamond-structure being the best (compressive strength of 40.62 MPa, Young’s modulus of 2.41 GPa, energy absorption of 0.45 MJ/m3, cellular density of 1.70 g/cm3, and specific strength of 23.83 MPa/(g/cm3)). This study presents a novel strategy for using DLP 3D printing technology in the fabrication of lightweight, high-strength and complex shaped ceramic components. It aims to expedite the rapid application of DLP 3D printing technology in various fields such as aerospace and medical.

Hot rolling deformation behavior of nanocrystalline Nd-Fe-B magnets: One-step powder forming at large size

Anisotropic nanocrystalline Nd-Fe-B magnets were successfully prepared by hot rolling technique. A systematic study of the microstructure, texture, and magnetic properties of hot-rolled (HR) Nd-Fe-B magnets was conducted to gain an insight into the deformation process. The results indicate that: the hot-rolled magnets have good uniformity after one-step rolling of the cold pressed precursors. As the deformation increases, the Br and (BH)max increase from 7.1 kG to 11.9 kG and from 10.9 MGOe to 30.1 MGOe, respectively. Platelet grains were gradually formed and arranged in the fine grain region, resulting in a c-axis texture. During the deformation process, fine grain region of the rare-earth-rich phase was rapidly squeezed into the coarse grain region, which deteriorated the magnetic properties. The microstructural evolution and deformation processes of hot-rolled Nd-Fe-B magnets were analyzed. The conclusion was that the process of deformation involved melt-spun ribbon densification, dissolution-precipitation, anisotropic grain growth, and grain alignment. Additionally, the preparation process and development prospects of hot-rolled magnets were also discussed.

Recrystallization sintering and characterization of composite powders composed of two types of SiC with dissimilar particle sizes

AbstractPure silicon carbide (SiC) ceramics were prepared through recrystallization sintering by using two types of SiC powder, with different particle sizes, as the raw materials. The effects of the fine powder content on the bulk density, porosity, flexural strength, and grain morphology were investigated. In the synthesis process, silicon nitride (Si3N4) was used as the sintering additive that decomposed and transformed into SiC to promote the growth of SiC grains. The added fine powder was exploited in the evaporation and condensation process and grain amalgamation caused by the movement of the grain boundaries. Thus, a dissimilar fine powder content modulated the microstructure and mechanical strength of the SiC ceramics. The results indicate that the bulk density and flexural strength increase to a maximum of 2.12 g/cm3, 44.2 MPa, respectively, when the fine powder content is 40 wt.%. Three kinds of grain morphologies, that is, uniform equiaxed grains, round, equiaxed grains, and hexagonal platelet grains, and the maximum average pore size (3.62 μm) are obtained when the fine powder content is between 0 wt.% and 60 wt.%. In addition, the main crystal phase 6H‐SiC is partially converted to 4H‐SiC when the fine powder content is up to 60 wt.%.

Microstructure and texture formation mechanism of nanocrystalline anisotropic Nd-Fe-B magnets by novel hot forward-extrusion

In this paper, the microstructure and texture formation mechanism of nanocrystalline anisotropic Nd-Fe-B magnets prepared by a one-step heating forward-extrusion method were systematically studied. The hot-extruded (HE) Nd-Fe-B magnet showed the same heterogeneous microstructure characteristics as the traditional die-upsetting magnet. With the decrease of extrusion width (w1), the c-axis texture gradually enhanced, accompanied by an increasing proportion of fine-grain region and the improvement of the ordered arrangement of platelet grains. In addition, strong c-axis texture was formed by the ordered arrangement of platelet grains benefiting from the difference of stress distribution caused by the difference of side length of the extrusion port. By adjusting the dimension of the rectangle extrusion port, the magnetic properties of HE Nd-Fe-B magnets were optimized. The optimized magnetic properties with (BH)max of 241.3 kJ/m3, Hci of 1025.3 kA/m, Mr of 0.94 MA/m can be acquired for the hot extrution (HE) Nd-Fe-B magnet with extrusion width (w1) of 2.5 mm. In this method, the demoulding process can be avoided successfully by using cold pressing instead of hot pressing, which is of great significance for continuous and high-volume production.

Effects of grain boundary diffusion process on magnetic properties enhancement and microstructure evolution of hot-deformed Nd-Fe-B magnets

Grain boundary diffusion (GBD) process is an important approach for producing Nd-Fe-B magnets with high coercivity and high thermal stability. The GBD for hot-deformed Nd-Fe-B magnets with nanocrystalline microstructure is more complicated compared to sintered magnets. Here, we investigated the effects of different GBD methods, i.e., intergranular addition (in-situ GBD 1#), in-situ GBD from magnet surface during hot pressing and hot deformation (in-situ GBD 2#), and conventional GBD, on the magnetic properties and microstructure of hot deformed magnets. After the treatment by these three GBD approaches using 2 wt% Pr40Tb30Cu30 diffusion source, the coercivity of the hot-deformed magnet increases from 1281 to 1567, 1412 and 2022 kA/m, respectively. The coercivity enhancement is attributed to the formation of local (Nd,Tb)2Fe14B phase with strong magnetic anisotropy. Reduced grain orientation is found in both in-situ GBD 1# and conventional GBD treated samples mainly due to the local stress state variation and the rotation of platelet grains. Interestingly, the in-situ GBD 2# processed sample has a high orientation at diffusion surface, which may be caused by the modified surface state of the magnet by the diffusion source. Compared with the in-situ GBD processes, the conventional GBD exhibits a higher utilization efficiency of Tb. Since the in-situ GBD is effective to treat thick hot-deformed magnets, further effort should be aimed at enhancing its diffusion efficiency.

Effects of initial α‐phase content on properties of pressureless solid‐state sintered SiC ceramics

Abstractα‐ and β‐SiC starting powders of similar particle sizes were used to investigate the effect of initial α‐phase content on the electrical, thermal, and mechanical properties of pressureless solid‐state sintered (PSS) SiC ceramics with B4C and C additives. For β‐SiC starting powders, a coarse‐grained microstructure with elongated platelet grains was formed by the transformation of 3C to 6H and finally to 4H‐SiC phase. In contrast, materials prepared from α‐SiC powders exhibited a fine‐grained microstructure with platelet grains. This study revealed the beneficial effect of α‐SiC starting powders in achieving low electrical resistivity and high thermal conductivity in PSS SiC ceramics, which was attributable to their higher sinterability, lower impurity content, and lower 6H to 4H‐SiC phase transformation rate compared with β‐SiC powders. The electrical resistivity decreased by an order of magnitude and the thermal conductivity increased by 32% with an increase in initial α‐phase content from 0 to 100%. The flexural strength increased by approximately 16% with increasing initial α‐phase content due to a decreased flaw size with decreasing grain size. However, the fracture toughness and hardness were insensitive to the change in initial α‐phase content.

Weathering products at the historic Qulaan gold prospect, Eastern Desert, Egypt: Implication on the mobility and distribution of arsenic, gold and silver

Gold mineralization of the historic Qulaan prospect is hosted in hydrothermally altered rhyolite. A variety of supergene minerals that includes arsenic, gold and silver phases were formed as a consequence of superimposed weathering on primary mineralization hosted in the hydrothermally altered rhyolite at the prospect. The arsenic-bearing phases are represented mainly by arsenic-rich ferric oxyhydroxides and Ca–Fe arsenates (arseniosiderite). Gold occurs in the form of native platelet and irregular grains that are commonly associated with ferric oxyhydroxide, while the Ag-phase, represented by iodargyrite, occurs in sericite-rich cavities. The formation of these minerals is related to oxidation of primary arsenian pyrite during the weathering process. Although pyrite oxidation results in an acidic condition, the presence of arseniosiderite as well as iodargyrite and the association between ferric oxyhydroxide and gold indicate an increase in the pH during the weathering process. Relict feldspar as well as hydrothermal chlorite and calcite in the host rocks are responsible for neutralizing the earlier acidic condition during the weathering of rhyolite.

Analysis on deformation and texture formation mechanism of hot-deformed Nd-Fe-B magnets based on heterogeneous structure evolution

In this paper, microstructure, micromagnetic structure, texture, together with magnetic properties of the hot-deformed (HD) Nd-Fe-B magnets were systematically studied to understand the deformation process and the formation mechanism of c-axis texture. The results show that the platelet grains are formed in the fine-grain regions at the initial stage of the deformation. As the amount of deformation increases, the proportion of platelet grains increases and arranges gradually, causing the formation of c-axis texture, till the grain merging occurres when the deformation is excessive. It should be noted that the rare earth-rich phase in the fine-grained region slowly diffuses to the coarse-grained region where only grain growth can be observed during deformation. The deformation mechanism and formation of c-axis texture in HD Nd-Fe-B magnets can be deduced to be accomplished by the processes of dissolution-precipitation diffusion, grain rotation and grain arrangement, based on the characterization of microstructure and texture evolution. Also, approaches to optimize the preparation process and magnetic properties of the hot-deformed Nd-Fe-B magnets were discussed.

Controlled Deposition of Lead Iodide and Lead Chloride Thin Films by Low-Pressure Chemical Vapor Deposition

Lead halide thin films, such as lead iodide (PbI2) and lead chloride (PbCl2), are used as precursor films for perovskite preparation, which is frequently achieved by vacuum thermal evaporation but rarely by the low-pressure chemical vapor deposition (CVD) method. Here, we report on the deposition of PbI2 and PbCl2 thin films on glass substrates by employing the low-pressure CVD method. The effect of the substrate temperature on the structure and morphology of the lead halide films is investigated. Crystalline films were realized for both lead halides, with PbI2 films showing high texture compared to the reduced texture of the PbCl2 films. Large lateral grain sizes were observed for the PbI2 films with a flat platelet grain morphology and an average grain size up to 734.2 ± 144.8 nm. PbCl2 films have columnar grains with an average grain size up to 386.7 ± 119.5 nm. The PbI2 films showed a band gap of about 2.4 eV, confirming its semiconducting properties, and the PbCl2 had a wide band gap of 4.3 eV, which shows the insulating properties of this material.

WC-0.5Co-xB4C materials prepared by spark plasma sintering: Formation of WC platelet grains

In this study, WC-0.5 Co cemented carbides containing WC platelet are prepared by spark plasma sintering (SPS) with the certain B4C content. The effect of B4C additive on formation mechanism of WC platelet grains is mainly investigated. According to XRD and EPMA analyses, the excess (101¯1) peak is present only when adding 1 wt% B4C, and B2CoW2 and carbon are generated around WC platelet grains. TEM studies indicate that some planar defects and facet-roughening surfaces (steps/kinks) exist in platelet grains. It illustrates that the growth of WC platelet crystal is through the defect-assisted nucleation. WC platelet may cause the reducing of hardness, but it can increase fracture toughness.

Preferred orientations and microstructures of lanthanum phosphate films prepared via laser chemical vapor deposition

In this study, the vapor-phase growth of lanthanum phosphate (LaPO4 monazite) films having preferred orientations on polycrystalline alumina substrates was demonstrated via laser chemical vapor deposition using metal-organic precursors. The effects of the laser power densities and oxygen (O2) partial pressure on the crystal orientations, microstructures, and textures of the LaPO4 films were investigated. Monoclinic LaPO4 monazite was grown at laser power densities of 62–96 W cm−2 (deposition temperatures of 1075–1120 K) and an O2 partial pressure of 257 Pa. The (111¯)-oriented LaPO4 films with small pyramidal facets of less than 1 μm in sizes and large platelets were grown at laser power densities lower than 85 W cm−2. The preferred orientation of the monoclinic LaPO4 changed from (111¯) to (101¯) as the laser power density was increased to 95 W cm−2, at which large platelets inclined from the substrate surface were formed. Deposition with a reduced O2 partial pressure of 200 Pa induced the growth of highly (111¯)-oriented LaPO4 films with small pyramidal facets, which comprised (001), (100), and (010) faceted planes. At a low O2 partial pressure of 133 Pa, (100)-oriented LaPO4 films with platelet grains vertically grown from the substrate were formed. The deposition rates of the LaPO4 films were as high as 58.6 μm h−1.

Hexagonal barium ferrite films on a Pt(1 1 1)/Si(0 0 1) substrate and their local magnetic properties

Abstract Hexagonal barium ferrite films were deposited on Pt(1 1 1)/Si(0 0 1) substrates by pulsed laser deposition. This paper presents an investigation of the crystal structure, morphology and local magnetic properties of the deposited films. The characterization results demonstrated orientation and morphological dependences on the Pt thickness and annealing temperature. A film prepared with a 200-nm-thick Pt buffer layer and an annealing temperature of 900 °C exhibited oriented platelet grains with a mean diameter of ∼400 nm and excellent magnetic properties with a saturation magnetization of ∼311 emu/cm3, a uniaxial anisotropy field of ∼15.5 kOe and a remanent squareness in the out-of-plane direction of ∼68.6%. Furthermore, a first-order reversal curve (FORC) diagram showed that the film had a positive mean interaction field (Hu) between particles and broad distributions of coercivity (Hc) centred at Hu = 600 Oe and Hc = 2500 Oe.

Microstructure and mechanical behaviour of SrO doped Al2O3 ceramics

Microstructure and mechanical behaviour of the SrO doped Al2O3 ceramics have been studied with a dopant concentration levels of, 0 ppm (AS0), 1000 ppm (AS1), 3000 ppm (AS3), and 5000 ppm (AS5), respectively. High density (~99.1%) and finer grain size (~3.4 µm) have been obtained for the Al2O3 ceramics at an optimum dopant concentration of 5000 ppm SrO (AS5) attributed to the important effects of in-situ formed strontium hexaluminates (SrAl12O19). Controlled grain growth behaviour in SrO doped Al2O3 (AS5) may be ascribed to the zener pinning stresses and solute drag effect actuated by the grain boundary phase SrAl12O19 with platelet morphology. Improved densification behaviour in SrO doped Al2O3 (AS5), presumably because of the lower interfacial energy of newly formed Al2O3/SrAl12O19 interfaces and key benefits of secondary phase (SrAl12O19) inhibited grain growth mechanism. The activation energy for densification in SrO doped Al2O3 (AS5) is found to be 550 ± 30 kJ/mol, which is higher compared to undoped Al2O3 and the estimated activation energies have been phenomenonlogically correlated to the lower energy of Al2O3/SrAl12O19 interfaces. Improved mechanical properties in terms of fracture toughness (~3.7 MPa. m1/2), strength (~890 MPa) and hardness (~13.8 GPa) in optimum SrO doped Al2O3 (AS5) ceramics are triggered by a combination of toughening mechanisms primarily including SrAl12O19 platelet grain bridging, crack deflection, grain pullout, and crack tip healing.

Self-reinforced high-alumina refractory castables

Among the available routes to enhance the mechanical behavior and toughening degree of ceramics, in situ formation of acicular or platelet grains in the resulting microstructure has gained special attention. Various studies focused on investigating the self-reinforced role of these elongated phases (acting as bridging sites in the wake of a crack), as well as the use of very reactive raw materials to favor faster and more effective sintering/densification, have been carried out for compositions comprised mainly by fine raw materials. Nevertheless, there is a lack of studies considering more complex ceramics, such as refractory castables, which commonly present heterogeneous microstructures with coarse and fine components. Based on that, this work addresses the evaluation of self-reinforced high-alumina refractories containing reactive aluminas (AloxX spheres and/or submicron alumina) and calcium aluminate cement as binders. Boron carbide and microsilica were also added to some of the prepared compositions to favor the in situ formation of Al18B4O33 or CA6 phases with needle-like morphology. Various experimental evaluations (cold and hot mechanical strength, hot elastic modulus, SEM, XRD, thermal shock resistance, refractoriness under load, etc.) were carried out to identify the influence of the new phases’ formation in the overall performance of such materials. According to the attained results, both mineralizing agents (B4C and SiO2) induced the formation of Al18B4O33 or CA6 grains in the designed compositions. However, a remaining liquid content was still present in these samples at a high temperature, resulting in some drawbacks concerning their hot properties. Additionally, submicron alumina proved to be an interesting binder as it induced improved sintering and led to a refractory with high stiffness (~200GPa), high thermal shock resistance and mechanical strength.

Process-tolerant pressureless-sintered silicon carbide ceramics with alumina-yttria-calcia-strontia

Process-tolerant SiC ceramics were prepared by pressureless sintering at 1850–1950°C for 2h in an argon atmosphere with a new quaternary additive (Al2O3-Y2O3-CaO-SrO). The SiC ceramics can be sintered to a>94% theoretical density at 1800–1950°C by pressureless sintering. Toughened microstructures consisting of relatively large platelet grains and small equiaxed grains were obtained when SiC ceramics were sintered at 1850–1950°C. The presently fabricated SiC ceramics showed little variability of the microstructure and mechanical properties with sintering within the temperature range of 1850–1950°C, demonstrating process-tolerant behavior. The thermal conductivity of the SiC ceramics increased with increasing sintering temperature from 1800°C to 1900°C due to decreases of the lattice oxygen content of the SiC grains and residual porosity. The flexural strength, fracture toughness, and thermal conductivity of the SiC ceramics sintered at 1850–1950°C were in the ranges of 444–457MPa, 4.9–5.0MPam1/2, and 76–82Wm−1K−1, respectively.

Influence of synthesis temperatures on the crystalline grain growth and morphology of lanthanum magnesium hexaaluminate

Lanthanum magnesium hexaaluminate (LaMgAl11O19, LMA) was prepared at different temperatures by solid-state reaction. Phase compositions and crystal morphologies of specimens synthesized at different temperatures were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM). It was observed that the crystalline grain size of LMA was not only dependent on the preparation temperature but also on its powder morphology. In the temperature range of 1300–1550 °C, LMA showed platelet grain and the average crystalline grain size increases with the increase in temperature. At 1600 °C, if the powder was sintered for two times, the equiaxed grain could be found with the decrease in grain space, resulting in the reduction of the crystalline grain size. Styles of specimens (powder or disk) might have no obvious influence on morphologies and sizes of LMA crystalline grains which were synthesized with the well-dispersed raw material mixtures. The synthesis temperature played a key role in influencing the free space for the formation and growth of crystalline grains.

Micromagnetic simulations on the grain shape effect in Nd-Fe-B magnets

Micromagnetic simulations were performed to study the effect of grain shape and defect layer in Nd-Fe-B magnets. It was found that the coercivity can be increased by a factor of ∼2 by changing the grain shape from the triangular prism to the spheroid. Both the anisotropy field contribution and the shape contribution to the coercivity, and thus also the final coercivity, were found to decrease in the order: spheroid > circular prism > hexagonal prism > square prism > triangular prism. Sputtered columnar grains and hot-deformed platelet grains with a constant volume were also considered. Results show that the coercivity initially increases with the aspect ratio and then nearly saturates above the ratio of ∼4. Simulations of multigrain ensembles showed that depending on the grain shape, compared to the case of single grain, a further decrease of ∼10%–45% in the coercivity is induced by magnetostatic coupling.

Wear behaviour of doped WC–NI based hardmetals tested by four methods

Cemented tungsten carbide systems have been widely used in machine tools, ammunition rounds, and sporting equipment, as they possess the necessary combination of hardness and toughness to endure wear environments. In most applications, the abrasion phenomena has been identified as one of the sources of surface damage and material degradation. In this work, WC–Ni hardmetal is doped with zirconium carbide or titanium carbide to improve its abrasive and erosive wear resistance. TiC has been known to produce platelet grain structure meant for strengthening leading to enhanced abrasion resistance. ZrC can suppress grain coarsening and abnormal growth of WC grains while increasing hardness. Tribological characteristics are obtained from slurry and dry erosion, as well as high and low stress abrasion tests. The composite of WC–8Ni–TiC works best overall for various wear conditions with exception of high stress abrasion. WC–8Ni–ZrC does not provide any benefit of wear resistance in this study. Wear rate and mechanisms of wear were analysed by and discussed based on scanning electron microscopy studies.

Synthesis of sodium beta alumina films by heat treatment of sodium aluminum oxides

Sodium beta alumina (Na-β-alumina) films were synthesized by heat treatment of NaAl6O9.5 and γ-NaAlO2 films at temperatures of 1 373-1 573 K. Single-phase γ-NaAlO2 and NaAl6O9.5 films were prepared by laser chemical vapor deposition at the deposition temperatures of 976 and 1 100 K, respectively. Subsequent heat treatment of the films resulted in the formation of Na-β-alumina with α-Al2O3 at temperatures above 1 373 K for NaAl6O9.5 and 1 473 K for γ-NaAlO2. On heat treatment at temperatures of 1 473-1 573 K, the faceted morphology with terraces of the as-deposited (110)-oriented γ-NaAlO2 films transformed to a porous morphology with platelet grains comprising Na-β-alumina and α-Al2O3. On heat treatment at temperatures of 1 373-1 473 K, the pyramidal, faceted grains of as-deposited NaAl6O9.5 films transformed to planer, shapeanisotropic morphology in the film of mixed Na-β-alumina and α-Al2O3. A dense morphology was observed in both the as-deposited and heat-treated NaAl6O9.5 films.

Improvement in mechanical/physical properties of TiC-based ceramics sintered at 1500 °C for inert matrix fuels

Dense TiC-based ceramics were obtained at 1500°C by hot pressing, benefiting from Ti and Si additives in the TiC matrix. The in-situ formed Ti3SiC2 platelet grains refined TiC grains and reinforced the obtained ceramics. The TiC–20vol.% Ti3SiC2 ceramic exhibits a much higher flexural strength of 1003MPa, compared with 450MPa of the pure TiC ceramic. Owing to the formation of continuous heat-conducting paths by the bridged Ti3SiC2 platelet, a high thermal conductivity over 33W/m·k was measured in TiC–40vol.% Ti3SiC2 at 500°C, which is nearly 1.5 times than that in the pure TiC ceramics with a similar grain size.