Fabrication of textured cerium-doped lutetium oxyorthosilicate ceramics by slip casting in a strong magnetic field

Cerium-doped lutetium oxyorthosilicate (Lu₂SiO₅:Ce, LSO:Ce) slurry with 35 vol.% solid content and viscosity of 70 mPa $·$s (at a shear rate of 4 s^–1) was prepared via a 24-h ball milling for the powder calcined at 1200°C and adding ammonium polyacrylate of 0.2 wt.%. Textured LSO:Ce ceramics were fabricated via slip casting in a vertical magnetic field and subsequently pressureless sintering of 1650°C and hot isostatic pressing. The maximum Lotgering factor f and relative density of the textured ceramics were 0.48% and 98.8%, respectively. Effects of grain size, dispersant, viscosity of slurry, and magnetic flux density on the grain orientation of LSO:Ce ceramics were systematically investigated. Based on the spatial state of grains, we proposed the rotation rule that LSO:Ce grains with or without space constraints in the slurry align along a and c-axis under the induction of a strong magnetic field.     

Preparation of SiC porous ceramics by a novel gelcasting method assisted with surface modification

The SiC porous ceramics were successfully prepared by gelcasting process using Isobam as gelling agent. The pretreatment and surface modification of SiC was proposed to solve the problem that Isobam is difficult to crosslink and gelling with non-oxide ceramics. The SiC powder was firstly pre-oxidized, and then modified with a silane coupling agent to obtain a functional end group -NH₂ on the surface, so that it can be crosslinked with Isobam to obtain a green body with uniform structure. With low amount of organic matter, no additional debinding procedure was required before sintering. The suitable oxide thickness was investigated, which shows a linear relationship with the original SiC particle size. Moreover, the solid content of the slurry was adjustable between 40 vol% and 52 vol%, and the porosity of the products varied between 50% and 60% after sintering.     

叔丁醇基凝胶注模成型制备氧化铝多孔陶瓷

以微米级Al2O3粉料为原料,叔丁醇为溶剂,采用凝胶注模成型工艺制备了氧化铝多孔陶瓷,并研究了Al2O3浆料的固相体积分数(分别为8%、10%、13%和15%)对1 500℃保温2 h烧后氧化铝多孔陶瓷的气孔率、气孔孔径分布、耐压强度、热导率和显微结构的影响.结果表明:当Al2O3浆料的固相体积分数从8%增加到15%时,氧化铝多孔陶瓷烧结体的总气孔率从71.2%逐渐降低至61.2%,气孔平均孔径从1.0 μm逐渐减小至0.78 μm,耐压强度从16.0 MPa逐渐增大至45.6 MPa,而热导率从1.03 W·(m·K)-1逐渐增大至1.83W·(m·K)-1.     

Highly textured and strongly anisotropic TiB2 ceramics prepared using magnetic field alignment (9T)

Highly textured TiB₂ ceramics were prepared by slip casting an aqueous suspension in a magnetic field of 9 T, followed by sintering using Field Assisted Sintering Technology (FAST). Particle size refinement by ball milling improved both the degree of texturing and densification of the material (RD > 98 %). The sintered material exhibited a Lotgering orientation factor of 0.90, with the c-axis of TiB₂ oriented parallel to the magnetic field and FAST pressing direction. The texturing effect induced by the uniaxial pressing was negligible. The textured TiB₂ material exhibited a significant anisotropy in mechanical properties; the values of hardness and indentation elastic modulus measured along directions transverse to the c-axis of TiB₂ were 37 % and 13 % higher than the ones measured along the c-axis direction. Moreover, the specific wear rate of a surface of textured TiB₂ parallel to the field was one order of magnitude lower than a surface perpendicular to the field.     

Hydrothermally synthesized BaTiO3 textured in a strong magnetic field

Hydrothermally synthesized BaTiO₃ was colloidally consolidated by slip casting and by electrophoretic deposition in a high strength magnetic field of 17.4 T. Textured BaTiO₃ was successfully prepared by both processing techniques, with the {001} plane aligning normal to the direction of the magnetic field. The initial alignment in the green powder compact was preserved and enhanced after sintering from 1400 °C to 1500 °C. The sintering temperature hardly influenced the degree of texture in slip cast samples but did influence the grain size of all BaTiO₃ ceramics and the degree of texture in the electrophoretically deposited samples. The BaTiO₃ ceramics processed in the strong magnetic field showed crystallographic texture but not morphological, so the microstructure did not show any anisotropy. The highest Lotgering factor (0.85) was measured for the ceramics made by electrophoretic deposition and sintered at 1500 °C.     

Fabrication of c-axis oriented hydroxyapatite ceramics in a rotating high magnetic field using photopolymerization

C-axis oriented hydroxyapatite (HAp) was prepared by colloidal processing using a photopolymerization reaction in a rotating magnetic field with a UV-curable binder as the solvent. This technique achieved a short processing time of 150 s in a magnetic field as a result of rapid solidification induced by the photopolymerization. The slight difference in refractive index between the HAp material and UV curable resin allowed the fabrication of 1.6 mm thick green compacts. The oriented structure in the compact was maintained from the surface down to a depth of 700 μm, but was randomized at depths of 700 μm and beyond. The orientation degree was retained after sintering at 1250 °C, and the relative density of the compact was approximately 97 %. This technique can be effectively utilized for the manufacture of high-performance biomaterials.     

Effects of Ni addition on properties of vitrified bond CBN composites in strong magnetic field

Properties of vitrified bond with varying Ni doping amounts were extensively investigated. Effects of Ni addition on microstructures and properties of vitrified bond cubic boron nitride (CBN) composites prepared in strong magnetic field were investigated for applications in CBN grinding tools. Vitrified bond was characterized using three-point bending, scanning electron microscopy, X-ray diffraction and other methods. The refractoriness, fluidity, and bending strength of vitrified bond were evaluated. Bending strengths, microstructures, and phase compositions of vitrified bond CBN composites achieved using conventional and strong electromagnetic sintering techniques were compared. Results show that the addition of Ni to vitrified bond CBN composites improved the fluidity and bending strength of the vitrified bond. Strong electromagnetic sintering improved the mechanical strength and pore structure of vitrified bond CBN composites. Moreover, the introduction of the strong magnetic field facilitated Ni migration and aggregation in vitrified bond, rotated abrasives, and formed new substances, thus increasing the stability of vitrified bond CBN composite thermal material. Also, strong magnetic field inhibit grain growth of non-magnetic and ferromagnetic materials with fine-grain effect.     

Preparation and properties of mullite-bonded porous fibrous mullite ceramics by an epoxy resin gel-casting process

Porous fibrous mullite ceramics with a narrow range of pore size distribution have been successfully prepared utilizing a near net-shape epoxy resin gel-casting process by using mullite fibers, Al₂O₃ and SiC as raw materials. The effects of sintering temperatures, different amounts of fibers and Y₂O₃ additive on the phase compositions, linear shrinkage, apparent porosity, bulk density, microstructure, compressive strength and thermal conductivity were investigated. The results indicated that mullite-bonded among fibers were formed in the porous fibrous mullite ceramics with a bird nest pore structure. After determining the sintering temperatures and the amount of fibers, the tailored porous fibrous mullite ceramics had a low linear shrinkage (1.36–3.08%), a high apparent porosity (61.1–71.7%), a relatively high compressive strength (4.4–7.6 MPa), a low thermal conductivity (0.378–0.467 W/m K) and a narrow range of pore size distribution (around 5 µm). The excellent properties will enable the porous ceramics as a promising candidate for the applications of hot gas filters, thermal insulation materials at high temperatures.     

助烧剂和造孔剂对真空烧结SiC多孔陶瓷性能的影响

以粒度≤0.063mm的SiC为主要原料,分别加入30%(质量分数)的Al2O3-Y2O3与10%的Al2O3-高岭土复合助烧剂,并外加不同量(分别为12.8%、26.3%、30.0%和36.4%)的造孔剂羧甲基纤维素钠(CMC),制样后首先在空气炉中经过300℃2h或1100℃4h的预烧,然后在真空炉中于1550℃4h真空烧结而制备成SiC多孔陶瓷,并研究了助烧剂种类以及造孔剂CMC外加量对SiC多孔陶瓷显微组织、显气孔率及抗折强度的影响。结果显示:采用Al2O3-Y2O3作为助烧剂的SiC多孔陶瓷比Al2O3-高岭土作助烧剂的具有较高的抗折强度,显气孔率稍有减小;随着羧甲基纤维素钠量的增加,加入两种助烧剂的SiC多孔陶瓷均表现为显气孔率增加,抗折强度降低。     

Effect of wet-jet milling on the c-axis orientation of hydroxyapatite ceramics fabricated using a strong magnetic field

Controlling the crystal orientation of hydroxyapatite, an inorganic material that is a major component of human hard tissues is important to fabricate better artificial bones and artificial tooth roots. To obtain highly oriented hydroxyapatite ceramics under a strong magnetic field, a good dispersion of the raw materials in the slurry must be obtained. This study investigates the effect of wet-jet milling of a slurry on the orientation of hydroxyapatite ceramics fabricated using a strong magnetic field. Although the prolonged ball milling with ZrO₂ balls of the raw powder fractures the primary particles of hydroxyapatite, wet-jet milling is used to successfully pulverize agglomerated hydroxyapatite raw powder without changing the morphology of the primary particles. Evidently, ceramics with a highly oriented c-axis of hydroxyapatite are obtained by molding the wet-jet milled slurry in a strong rotating magnetic field. They exhibit anisotropy in fracture toughness, and the fracture toughness calculated from the crack length perpendicular to the rotating axis is higher than that calculated from the crack length parallel to it. Furthermore, these values are higher than those of randomly oriented hydroxyapatite ceramics, which may be owing to the crystal orientation dependence of the fracture toughness of the hydroxyapatite grains and grain boundaries.     

Preparation of 1 1 1-textured BaTiO3 ceramics by templated grain growth method using novel template particles

Dense BaTiO₃ ceramics with 1 1 1-texture were prepared by the TGG process. Platelike BaTiO₃ particles with their 1 1 1 direction perpendicular to the plate face were prepared by the reaction of platelike Ba₆Ti₁₇O₄₀ particles with BaCO₃ particles in molten NaCl. A green compact was composed of the aligned, platelike BaTiO₃ template particles dispersed in the matrix of small, equiaxed BaTiO₃ particles. Sintering caused densification and also the growth of template particles at the expense of matrix particles, resulting in texture development. Densification prior to grain growth was found to be necessary to obtain highly textured ceramics, and the effect of pre-sintering conditions on texture development was examined.     

Fabrication of textured B4C ceramics with oriented tubal pores by strong magnetic field-assisted colloidal processing

Highly structure-controlled B₄C ceramics were prepared via strong magnetic field-assisted slip casting of a slurry, containing B₄C base particles and pore-forming agents with a fiber shape. To achieve gas release at a lower porosity for maintaining its mechanical strength, these B₄C ceramics had a structure in which a large number of oriented tubal pores were dispersed in a crystallographically-aligned and dense B₄C matrix phase. The B₄C microstructure, such as structuration and orientation degree distributions of the B₄C grains and tubal pores, was characterized by SEM observation, EBSD analysis, and X-ray CT. Among the investigations, it was found that the oxidic impurities, as an inhibitor of sintering, which existed on the B₄C surface, can be removed by ethylation and azeotropy due to an ethanol treatment followed by vacuum drying. Thus, an ethanol treatment of a green compact before sintering was significantly effective for the fabrication of the B₄C ceramics, including the microstructure that coexisted with a dense matrix phase with tubal pores. The resultant ceramic specimens showed the remarkable three-point bending strength of 459?554 MPa, which is two times higher when compared to conventional B₄C pellets with a similar porosity.     

Effect of Al2O3 addition on texturing in a rotating strong magnetic field and densification of B4C

The properties of ceramics can be improved by controlling the microstructure through texturing ceramics in a strong magnetic field. Fabricating dense boron carbide (B₄C) requires high temperature sintering, therefore sintering additives are often used in order to densify B₄C ceramics at lower temperatures. However, combined effect of texturing and sintering additives on densification of B₄C has not been made clear yet. Here we report the effect of alumina (Al₂O₃) sintering additive on texturing in a strong magnetic field and densification of B₄C. Texturing was performed by rotating superconducting magnet at 12 T during slip casting process. Electron backscatter diffraction (EBSD) was used to observed the texturing projection. {0001} plane is clearly oriented in the plane parallel to rotating magnetic field. In addition, Lotgering factor was also calculated as quantitatively evaluation of texturing degree. Results on densification showed that addition of Al₂O₃ successfully increased density of B₄C sintered by spark plasma sintering (SPS) at 1800^oC to 97.8%. Formation of aluminum borate (Al₅BO₉) as secondary phase was detected by X-Ray diffraction (XRD). It is considered that the generation of Al₅BO₉ assisted finer densification of B₄C ceramic. Textured B₄C sintered at 1700^oC by SPS without alumina addition exhibited the highest orientation of c-axis. Addition of alumina caused decrease in degree of orientation of c-axis.     

Influence of in-situ magnetic field pressing on the structural and multiferroic behaviour of BiFeO3 ceramics

Polycrystalline single phase BiFeO₃ (BFO) ceramic samples have been prepared by conventional solid state sintering and also by in-situ magnetic field pressing followed by solid state sintering. The influence of in-situ magnetic field pressing on the structural, magnetic, ferroelectric and thermal properties has been investigated in this work. X-ray diffraction analysis and Reitveld refinement shows the single phase characteristics of BFO samples. Further texture formation and the development of compressive lattice strain have been observed in the magnetic field pressed samples. A change in Fe-O-Fe bond angle and suppression of spiral spin structure results in the enhanced magnetization value M_s = 136 memu/g at 2 T. Similarly spontaneous polarization has also improved with a P_max value of 1.3 μC/cm². DSC plot shows a significant variation in heat flow and enthalpy at the Neel transition (T_N = 372 °C) and ferro to paraelectric transition (T_C = 820 °C) for the magnetic field pressed BFO samples.     

Texturing of hydrothermally synthesized BaTiO3 in a strong magnetic field by slip casting

Barium titanate powder was processed by slip casting in a rotating strong magnetic field of 9.4 T. The orientation factor of the sintered compact was analyzed by the X-ray diffraction technique and the microstructure (grain-size) was analyzed by scanning electron microscope. The hydrothermally prepared barium titanate was used as matrix material and the molten-salt synthesized barium titanate, with a larger particle-size, was used as template for the templated grain-growth process. Addition of large template particles was observed to increase the orientation factor of the sintered cast (5 vol% loading). Template particles acted as starting grains for the abnormal grain-growth process and the average grain-size was increased after sintering. Increasing the solid loading (15 vol%) resulted in a similar orientation factor with a decrease of the average grain-size by more than half. However, addition of templates to the 15 vol% cast had a negative effect on the orientation factor. The impingement of growing particles was stated as the primary cause of particle misorientation resulting in a low orientation factor after sintering. Different heating conditions were tested and it was determined that a slow heating rate gave the highest orientation factor, the smallest average grain-size and the highest relative density.     

Enhanced power factor of textured Al‐doped‐ZnO ceramics by field‐assisted deforming

Field‐assisted deforming method has been used to prepare c‐axis textured Al‐doped‐ZnO Ceramics (AZO) ceramics. In such cases, AZO ceramics with different degree of texture can be controlled efficiently. As a consequence, the electrical conductivity has been significantly enhanced for AZO ceramic with high degree of texture. The electrical conductivity for highly textured AZO is as high as 29.5 S·m^?1, 6 times higher than random orientation AZO ceramics. The enhanced electrical conductivity leads to a higher power factor of 5.3×10^?4 W·m^?1·K^?2 at 750 K, a 60.6% improvement over the random orientation AZO ceramic.     

Studies of a mixing process induced by a transverse rotating magnetic field

This study reports on research results in the field of a mixing process under the action of a transverse rotating magnetic field (TRMF). The main objective of this paper is to present the effect of this type of a magnetic field on the mixing time. The proposed dimensional analysis of Navier–Stokes equation including the Lorentz force allows describing the analyzed process by means of the relationships basing on the dimensionless numbers (the mixing time number, the magnetic Taylor number, and the rotational Reynolds number). The possibility of using the magnetic particles (Fe₃O₄) as active micro-stirrers under the influence of a TRMF for active enhancement of a mixing process was considered. Moreover, the effect of a particle content on homogenization efficiency by applying a TRMF was also investigated. The obtained experimental results suggest that the mixing time under the TRMF and MDF conditions may be worked out by using the relation between the mentioned mixing time number and the modified Reynolds number (particle Reynolds number). Important conclusions referring to the discussion of experimental studies of a mixing process are also specified.     

Preparation and properties of glass–ceramics derived from blast-furnace slag by a ceramic-sintering process

Glass–ceramics were synthesized using ground blast-furnace slag and potash feldspar additives by a conventional ceramic-sintering route. The results show 5 wt% potash feldspar can enhance the sintering properties of blast-furnace slag glass and the results glass–ceramics have desirable mechanical properties. The main crystalline phase of the obtained glass–ceramic is gehlenite (2CaO·Al₂O₃·SiO₂). A high microhardness of 5.2 GPa and a bending strength higher than 85 MPa as well as a water absorption lower than 0.14% were obtained.     

Enhanced ionic conductivity of aluminum tungstate by crystallographic orientation in a strong magnetic field

The ionic conduction of multiply charged ions, rather than singly charged ions, is beneficial for energy storage and sensor applications. The low mobility of multiply charged ions is one important obstacle to the implementation of these applications. Chemical methods, such as doping and solid solution formation, have been used to improve ionic conductivity. However, the apparent performance of ceramic electrolytes can be improved by the crystallographic alignment of anisotropic grains. In this study, crystal-oriented aluminum tungstate ceramics were processed by slip casting in a strong magnetic field. The b-axis- and c-axis-oriented aluminum tungstate ceramics can be produced by this technique. The orientation of grains along the b-axis could enhance ionic conductivity by at least 1.77 times compared to that of a randomly oriented sample and 2.13 times compared to that of the c-axis-oriented sample. The results of this study suggest that this method can improve the ionic conductivity of an anisotropic material using polycrystalline processing instead of difficult single-crystal synthesis techniques.     

Fabrication and electrical properties of textured (Na,K)0.5Bi0.5TiO3 ceramics by reactive-templated grain growth

Textured (Na_0.85K_0.15)_0.5Bi_0.5TiO₃ (NKBT) ceramics with a relative density of >94% were fabricated by reactive-templated grain growth. Plated-like Bi₄Ti₃O₁₂ template particles synthesized by the NaCl–KCl molten salt process were aligned by tape casting in a mixture of original oxide powders. The effect of sintering temperature on the grain orientation and electrical properties of textured NKBT ceramics were investigated. The results show that the textured ceramics have a microstructure with plated-like grains aligning in the direction parallel to the casting plane. The degree of grain orientation increased at increasing sintering temperature. The textured ceramics show anisotropic electrical properties in the directions parallel and perpendicular to the casting plane. The dielectric constant parallel to {h 0 0} plane is three times higher than that of the perpendicular direction in textured NKBT ceramics. The optimized sintering temperature is 1150 °C where the maximum dielectric constant is 2041, the remnant polarization is 68.7 μC/cm², the electromechanical coupling factor (k₃₁) and the piezoelectric constant (d₃₃) amount to 0.31 and 134 pC/N, respectively.