Ceramic Green Bodies Research Articles

Controlling the microstructure of ceramic particle stabilized foams: influence of contact angle and particle aggregation

Porous cellular alumina ceramic green bodies have been produced by combining the particle stabilized foam method with gelcasting. The suspension foams were stabilized by particles rendered weakly hydrophobic with short chain sulfonate surfactants. Poly vinyl alcohol (PVA) and 2,5-dimethoxy-2,5-dihydrofuran (DHF) were used as the gelcasting reagents. The microstructure (amount of porosity, average pore size, and morphology) of alumina (Al2O3) ceramic green body foams has been studied as a function of surfactant concentration and chain length. The morphology of gelled ceramic foams changes from closed cell (bubble like morphology) to open cell (granular morphology) as the surfactant concentration is increased beyond a critical level. The density (and porosity) of the gelled alumina green body foams changes as a function of the surfactant concentration in a non-linear manner. Measurement of the suspension viscosity, contact angle and aggregate size are used to explain the changes in density and morphology. The transition from the bubble like structure to the granular structure is due to an increase in the particle aggregate size rather than phase inversion induced by an increase in the contact angle. The same behavior is observed with three different chain length surfactants (ranging from 4 to 10 carbon atoms on the hydrophobic portion of the surfactant) but at a lower surfactant concentration as the chain length increases.

Sintering Behavior of Ceramic Green Body by Thermal Analysis Techniques

A series of physical and chemical changes, including the shrinkage and the phase transformation, will continuously take place during the sintering process of ceramic green body, which is important for designing a scientific and economic sintering schedule. The sintering behavior of two electrical porcelains and a structural tile were investigated using thermal analysis techniques. The dehydration, phase changes and densification of ceramic green body during sintering process were characterized by DSC-TG and thermal expansion curves. The sintering temperature range of ceramic green body was measured by thermal dilatometer with different heating rates. The sintering temperature Ts is linear to the logarithm of heating rate V. The results were compared with that measured by conventional refractoriness tester. The Ts is much more accurate to evaluate the sintering behavior of ceramic green body from the thermal analysis technique than that from the traditional method. The projected sintering schedule is very concordant to the experimental sintering results.

Polycrystalline ZnO and Mn-doped ZnO nanorod arrays with variable dopant content via a template based synthesis from Zn(II) and Mn(II) Schiff base type single source molecular precursors

The synthesis and full characterisation of pure and Mn-doped polycrystalline zinc oxide nanorods with tailored dopant content are obtained via a single source molecular precursor approach using two Schiff base type coordination compounds is reported. The infiltration of precursor solutions into the cylindrical pores of a polycarbonate template and their thermal conversion into a ceramic green body followed by dissolution of the template gives the desired ZnO and Mn-doped ZnO nanomaterial as compact rods. The ZnO nanorods have a mean diameter between 170 and 180 nm or 60–70 nm, depending on the template pore size employed, comprising a length of 5–6 μm. These nanorods are composed of individual sub-5 nm ZnO nanocrystals. Exact doping of these hierarchically structured ZnO nanorods was achieved by introducing Mn(II) into the ZnO host lattice with the precursor complex Diaquo-bis[2-(meth-oxyimino)-propanoato]manganese, which allows to tailor the exact Mn(II) doping content of the ZnO rods. Investigation of the Mn-doped ZnO samples by XRD, TEM, XPS, PL and EPR, reveals that manganese occurs exclusively in its oxidation state + II and is distributed within the volume as well as on the surface of the ZnO host.

Rapid prototyping technique for ceramic mini-devices containing internal channels with asymmetrical contour

The electrophoretic deposition (EPD) is used to serve as rapid prototyping technology (RPT) to manufacture ceramic mini-devices containing internal channels or asymmetrical contour patterns to overcome the technical obstacles by conventional ceramic forming techniques. The spacing and geometry of such internal channels, either liner, curvilinear, symmetrical or asymmetrical, can be considered at the preliminary design stage to prepare the fugitive inactive inserts to be involved in EPD operations. Such inserts are limited to any undesirable chemical reaction with the EPD suspension and of easy elimination from EPD-formed ceramic green body followed by pyrolysis or solvent dissolution to remove from embedment. In this study, ferroelectric ceramic mini-devices based on barium titanate with the pattern containing internal channels was deposited via EPD route unto electrically conducting or non-conducting inserts buried in the ceramic laminates. The inserts can be removed after drying step. Effects of EPD conditions on shape integrity of internal channel in the mini-devices were examined. This novel ceramic shaping technique enables us to produce the ceramic mini-devices in terms of formation of desirable array patterns for the unique prototype of ceramic components such as dielectric resonators, waveguide or mini-actuators.

Rapid casting of turbine blades with abnormal film cooling holes using integral ceramic casting molds

Film cooling is an important cooling method to decrease the turbine blade surface temperature, and its average cooling efficiency is mainly dependent on the cooling structures of internal passageways and the shapes of film cooling holes. Compared with standard cylindrical film cooling holes, abnormal film cooling holes have higher average cooling efficiency. But it is difficult to manufacture these holes using traditional machining methods. In this paper, a novel process was developed to fabricate turbine blades with abnormal film cooling holes by combining stereolithography (SL) technology with gelcasting technology. To decrease the drying shrinkage, the freeze-drying technique was applied to treat the wet ceramic casting mold green body surrounded by the SL mold, and the proper sintering process parameters were determined for lowering the sintered shrinkage. Finally, the integral ceramic casting mold was obtained, and a turbine blade with converging–diverging film cooling holes was rapidly cast to verify the feasibility of the proposed process.

Low‐ and High‐temperature Processes and Mechanisms in the Preparation of Porous Ceramics via Starch Consolidation Casting

AbstractStarch consolidation casting (SCC) is a versatile and well established process for shaping ceramic green bodies by casting starch‐containing suspensions into impermeable molds. The process is based on the ability of starch to swell and absorb water from aqueous ceramic suspensions after heating to approx. 80°C. The swelling factor is highest for potato starch (3.7) and significantly lower for the other starch types (down to approx. 2). A simple mathematical model is presented that allows the minimum consolidation time to be estimated on the basis of the swelling kinetics of starch granules as measured via laser diffraction. The critical temperature range for starch burnout before sintering the ceramic is 300–500°C, as determined by thermal analysis (thermogravimetry and differential thermal analysis). In this temperature range gaseous products may ignite, leading to strongly exothermic thermal effects. If the bodies are large and the starch content high this may lead to defects in the (still unsintered) ceramic green bodies. Therefore this temperature range has to be carefully controlled in the production of ceramics prepared by SCC. The residual pores (remaining after starch burnout) are too large to be closed during the firing step (insufficient driving force for sintering due to too low surface curvature). Therefore they do not contribute to shrinkage, and shrinkage is determined by the ceramic matrix alone. Porosities higher than 50% (corresponding to the maximum nominal starch content) may be obtained by combining SCC with partial sintering (resulting in porosities up to 70%).

Preparation of Gel-casting Slurry to Fabricate Large Scale Green Body

In this study, a gel-casting has been optimized to fabricate larger scale ceramic products. An aqueous alumina slurries with different powder mixing ratio of larger (18 μm) and smaller (0.4 μm) alumina powder, and the initiator which generates the common free radical (Ammonium persulfate) or thermal radical (2, 2' -Azobis [2-(2-imidazolin-2-yl)-propane] dihydrochloride) were prepared to investigate the effect of the gelation time on the structure of the resulting green body. Results showed that the common radical generator (Ammonium persulfate) took longer gelation time, resulting in the distorted green body due to the different shrinkage behavior in upper and lower part of green body. While thermal radical generator (2, 2'-Azobis [2-(2-imidazolin-2-yl)-propane] dihydrochloride) accelerated the polymerizing rate to exhibit uniform shrinkage behavior without cracking in the green bodies because of the uniform density especially for the case of large size powders. As a result, large-scale ceramic green body with uniform packing density was successfully prepared by optimizing the gel-casting conditions, showing the advantage of the gel-casting using starting reagent of thermal radical generator and the raw powders of the mixture of larger and smaller size powders.

Molecular based, chimie douce approach to 0D and 1D indium oxide nanostructures. Evaluation of their sensing properties towards CO and H2

The synthesis of a new molecular In(III) precursor complex, the generation of nanoscaled In2O3 (bixybyte structure) in 0D and 1D morphology, and the sensoric behavior of the 0D and 1D In2O3 nanostructures towards reducing gas atmospheres is studied. The indium precursor complex can be converted to a ceramic green body under mild reaction temperatures (160 °C), followed by conversion into crystalline indiumoxide at higher temperatures (350 °C). Geometric confinement by endotemplating of the molecular In precursor in track-etched polycarbonate templates yields polycrystalline indium oxide nanotubes in high yields. Controlled conversion of the molecular precursor without geometric confinement gives nanoparticulate crystalline In2O3. Both morphologically different In2O3 nanomaterials are sensitive to reducing gas conditions, however they show distinct differences towards reducing H2 and CO gas atmospheres.

Thermogelling behaviour of starches to be used in ceramic consolidation processes

Starch is one of the agents used as both consolidator/binder of the ceramic suspension and pore former at temperature for porous ceramic processing. In this work, the thermogelling behaviour of aqueous suspensions of different starches (potato, cassava, and corn) was studied by dynamic rheological testing and optical microscopy in order to optimize the thermal consolidation of ceramic green bodies prepared by the starch consolidation method. Viscoelastic properties ( G ′, G″, tan δ) as a function of the temperature (30–95 °C) were determined by non-isothermal rheological measurements (temperature sweep tests). In these tests, the influence of experimental variables on viscoelastic properties, such as the amount of Dolapix CE-64 as dispersant (0, 3, 4.5, and 6 wt%), the heating rate (1, 2, 5, and 10 °C/min) and the starch concentration (14 and 40 vol%) was analyzed. Swelling capacity and the analysis of volume-weighted granule size distributions (median volume granule diameters, distribution widths and arithmetic mean diameters) in aqueous suspension, as a function of the temperature, was also evaluated. In addition, the degree of disintegration of starch granules and the rheological stability of the gels were also studied by dynamic time sweep test. The obtained results are useful in order to establish the experimental variables to be used in the thermal consolidation step for the production of porous ceramics with controlled microstructures.

Diametral compression test: Analysing the H/ D ratio influence on the mechanical resistance of UO 2-green pellets

Fired ceramic components and green compacted bodies from ceramic powder are both fragile structures. Their particles have weak mechanical links or are just agglomerated by a binder. Due to the compaction process failure can occur during unloading and ejection stages and also after a certain level of densification or powder cohesion has been achieved. Ceramic materials normally have worse standard deviation in the splitting tensile strength values when subjected to static or dynamic impact tests than metals. The height-to-diameter ( H/ D) ratio has been studied for several materials showing that by having different geometrical correlations the mechanical resistance of the tested specimens is highly affected. Cylindrical UO 2-green pellet samples, having approximately the same density with three different diameters and four different heights, were pressed and experimentally studied by means of the diametral compression (Brazilian) test. By combining the diameters and the heights different H/ D ratios could be tested. Results, which were analysed using Weibull statistics, showed that the cylinder size has a great influence on the Weibull module ( m), whereas for the Weibull tensile strength no conclusive tendency could be observed except if we keep the height fixed and increase the pellet diameter. Pellets having the same height showed an increased tendency for the m value if their diameter is increased. The largest volume by each diameter has the highest Weibull module values.

Effect of lamination conditions for green ceramic tapes on adhesion strength, gas permeability, and yield during binder removal

The trade-off between adhesion strength and gas permeability of green ceramic tapes was examined for tapes laminated at times of 120–600 s, pressures of 1·8–7 MPa, and temperatures of 50–85°C. The dielectric in the green tapes was barium titanate, and the main components in the binder were poly(vinyl butyral) and dioctyl phthalate. The adhesion strength increased with increasing lamination time, pressure and temperature whereas the permeability decreased. The trade-off between permeability and adhesion strength with lamination conditions also influenced the component yield and cycle time for the thermal removal of binder from multilayer green ceramic bodies.

Barium Titanate-Polymer Composites Produced via Directional Freezing

In this study, we use a freeze casting technique to construct ceramic-polymer composites in which the 2 phases are arranged in an electrically parallel configuration. By doing so, the composites exhibit dielectric constant (K) up to 2 orders of magnitude higher than that of composites with ceramic particles randomly dispersed in a polymer matrix. In this technique, an aqueous ceramic slurry was frozen unidirectionally to form ice platelets and ceramic aggregates that were aligned in the temperature gradient direction. Upon freeze-drying, the ice platelets sublimed and left the lamellar ceramic structure intact. The green ceramic body was fired to retain the microstructure, and then the space between ceramic lamellae was infiltrated with a polymer material. The finished composites exhibit the high dielectric constant (1000) of ferroelectric ceramics while maintaining the unique properties of polymer materials such as graceful failure, low dielectric loss, and high dielectric breakdown.

Rapid fabrication of alumina-based ceramic cores for gas turbine blades by stereolithography and gelcasting

Injection moulding is accepted as one of the most important methods for shaping complex ceramic cores, which are used to form intricate internal cooling passages of gas turbine blades. But the relatively long lead time and high costs involved in the fabrication of hard tooling render it uneconomical for new products development and low-volume production. In the study, a rapid prototyping process is developed to fabricate complex-shaped alumina-based ceramic core by combining stereolithography (SL) with gelcasting. SL is utilized to fabricate an integral sacrificial resin mold, and gelcasting is utilized to form a wet ceramic core green body through polymerization of aqueous ceramic slurry. The freeze-drying process is adopted to treat the wet green body surrounded by the resin mold, the drying shrinkage is decreased, and the generation of crack can be prevented. The sintering shrinkage of ceramic core is controlled by adding magnesium oxide power and developing a novel sintering process. After the resin mold is burnt out, the complex-shaped alumina-based ceramic core is obtained.

Pressure Distribution and Defect Formation in Green Ceramic Bodies During Supercritical Extraction of Binder

Supercritical extraction (SCE) with carbon dioxide at 10–40 MPa and 55°–90°C has been used to remove binder from multilayer green ceramic bodies. Defects such as cracking and delamination were occasionally observed in the green bodies following the extraction process, and these defects were attributed to pressure gradients that arise during isothermal depressurization from conditions of SCE. A model based on flow in porous media was developed to describe the temporal and spatial distribution of pressure within the green body during depressurization. The model incorporates both nonideal pressure–volume–temperature behavior and nonconstant viscosity of the supercritical fluid. The effects of the body size and gas‐phase permeability on the pressure within the green body were examined.

Effect of monomer content on physical properties of silicon nitride ceramic green body prepared by gelcasting

The gelcasting technique was employed to prepare Si 3N 4 green body. The monomers used in the research were acrylamide (AM) and N, N′-methylenebisacrylamide (MBAM). The influences of the monomer content (AM and MBAM) and the ratio of monomers (AM/MBAM) on the warpage rate, shrinkage rate, and the flexural strength of Si 3N 4 ceramics green body were investigated. Both warpage rate and shrinkage rate of green body were found to decrease with the increase of monomer content, and monotonically increase with the ratio of monomers after drying. The variation of warpage rate with the ratio of monomers is evident when monomer content is 20 wt.%, but the variations are not evident when monomer contents are 40 and 55 wt.%. The flexural strength of the green body is highest at an optimum value of the monomers ratio, and increases with increasing monomer content, reaching 50–90 MPa when monomer contents are 40 and 55 wt.%.

A Novel Temperature Induced Gelation Forming Method

We describe the production of complex shaped ceramic green bodies with high strength and reliability using a novel forming method: temperature-induced gelation. Gelation is performed by moderately decreasing the temperature of the suspension, which induces in situ gelation and forms a network to bridge the suspended particles, leading to a stiff green body. The gelation mechanism is based on the separation of dispersant KD1 from solvent or the collapse of adsorbed layer on particle surface, which depends on the stability of starting suspensions.

Microstructures and mechanical properties of dense particle gels: Microstructural characterisation

The macroscopic mechanical properties of wet ceramic green bodies and densely packed coagulated colloidal particle gels strongly depend on the local arrangement of the powder particles on length scales of a few particle diameters. Heterogeneous microstructures exhibit up to one order of magnitude higher elastic properties and yield strengths than their homogeneous counterparts. The microstructures of these gels are analysed by the “straight path” method quantifying quasi-linear arrangements of particles. They show similar characteristics than force chains bearing the mechanical load in granular material. Applying this concept to gels revealed that heterogeneous colloidal microstructures show a significantly higher straight path density and exhibit longer straight paths than their homogeneous counterparts.

Green ceramic machining: A top-down approach for the rapid fabrication of complex-shaped ceramics

Green ceramic machining (GCM) has been investigated as an alternative method for the mould-free fabrication of complex-shaped ceramics, where ceramics in ‘unsintered’ green state are machined to complex-shaped via CAD/CAM technology. Recent advancement of CNC machining technology makes GCM a potential alternative for the rapid fabrication of ceramics, especially for some one-of-a-kind products. In this work, we investigated two processing routes to make ceramic green bodies which were suitable for green machining. The effects of processing methods on the green ceramic strength and toughness, machinability and surface finishing of the final ceramics were discussed. Examples of applications of the GCM in the fabrication of ceramic lattice scaffolds for bone tissue engineering and dental restorations have been demonstrated. The results show that gelformed ceramics have better mechanical properties and machinability than protein coagulation cast ceramics. But the debinding for gelformed thick wall components was found more difficult. It is therefore more suitable for the fabrication of ceramic microcomponent or thin sheet structures such as ceramic lattice scaffolds. Protein coagulation cast ceramics have good machinability and surface finishing for thick wall ceramic components. However, inhomogeneous and anisotropic shrinkage of complex-shaped ceramics were observed during sintering stage. Possible causes and solutions have been discussed.

Microstructure and material properties of double-network type fibrous (Al 2O 3–m-ZrO 2)/t-ZrO 2 composites

Al 2O 3–(m-ZrO 2)/t-ZrO 2 composites with a novel double-network microstructure were fabricated by multi-pass extrusion process using polymer bound ceramic green body. A simultaneous micro- and macro-level microstructure tailoring was made where unidirectionally aligned two-phase Al 2O 3–(m-ZrO 2) fibers were enclosed in a t-ZrO 2 phase connected in a network formation and this network was further enclosed in a thicker t-ZrO 2 phase. Composite green rods were hot extruded and reassembled in parallel for extrusion and after several passes of extrusion very fine microstructure with dimension of a few micrometers was obtained. Material properties such as hardness, bending strength and fracture toughness were measured for the composites. Microstructure characterization was carried out by SEM technique. In the 4th passed Al 2O 3–(m-ZrO 2)/t-ZrO 2 composites, the outer network of t-ZrO 2 was 15 μm, inner network 0.8 μm and the inner Al 2O 3–(m-ZrO 2) core was 2.5 μm. The highest hardness, fracture strength and toughness values were about 1452 Hv, 1006 MPa and 8.6 MPa m 1/2, respectively, in the sample sintered at 1500 °C.

Compaction and Pressureless Sintering of Zirconia Nanoparticles

Four nanometer‐sized zirconia powders stabilized by 3 mol% Y2O3 were used for the preparation of dense bulk ceramics. Ceramic green bodies were prepared by cold isostatic pressing at pressures of 300–1000 MPa. The size of the pores in ceramic green bodies and their evolution during sintering were correlated with the characteristics of individual nanopowders and with the sintering behavior of powder compacts. Only homogeneous green bodies with pores of <10 nm could be sintered into dense bodies (>99% t.d.) at a sufficiently low temperature to keep the grain sizes in the range <100 nm. Powders with uniform particles 10 nm in size yielded green bodies of required microstructure. These nanoparticle compacts were sintered without pressure to give bodies (diameter 20 mm, thickness 4 mm) with a relative density higher than 99% and a grain size of about 85 nm (as determined by the linear intercept method).