Colloidal Processing Route Research Articles

Enhanced properties of ZrSiO4/ZrO2 composites produced by colloidal processing and spark plasma sintering

Zirconium oxide and silicate are well-known reference materials in ceramics. However, the performance of zircon/zirconia composites has not received the required attention considering the interesting properties and capabilities. Zircon and zircon/zirconia suspensions were optimised by colloidal processing routes. These suspensions were either slip cast to obtain pieces that were conventionally sintered or freeze-dried to obtain homogeneous powder mixtures for consolidation by non-conventional Spark Plasma Sintering (SPS). Pure zircon materials with densities up to 97.0 % and a hardness of 10.1 GPa were obtained by conventional sintering (1550 ºC/120 min) whereas densities higher than 99.5 % and hardness up to 12.7 GPa were achieved by SPS (1500 ºC/5 min). By introducing 20 vol.% zirconia as a sintering additive, densities up to 99.3 % and a hardness of 11.1 GPa were achieved by conventional sintering (1500 ºC/120 min) whereas densities of 99.9 % and a hardness of 13.5 GPa were obtained by SPS (1400 ºC/5 min).

Direct ink writing of ATZ composites based on inks prepared by colloidal or hydrogel route: Linking inks rheology with mechanical properties

The Direct Ink Writing (DIW) technique stands out due to its fast processing, low amounts of additives demanded for the ink’s preparation and, consequently, the relatively quickly thermal post-processing cycles. On this behalf, the development of inks for DIW is usually based on colloidal or hydrogel-based processing routes, which derive from interparticle interactions or gel-particle interactions, respectively. The aim of this work was to evaluate the impact of colloidal or hydrogel-based ceramic inks processing routes in the properties of Alumina Toughened Zirconia (ATZ) composites prepared by DIW. It was found that the alumina grains were homogeneously dispersed into the zirconia matrix on samples derived from the colloidal ink, while big agglomerates could be found on those derived from the hydrogel ink. The good homogeneity observed on the colloidal route is related with the particle-particle (electrosteric) stabilization mechanism that induced an efficient dispersion of the starting powders, in contrast with the hydrogel route where the steric stabilization mechanism is dominant. Given the high interest of these composites as biomaterials, the achievements here attained open future opportunities for the near-net-shape processing of geometrically complex and customized dense and tough prosthesis and implants by DIW with mechanical performance comparable with conventional processing routes.

Thermomechanical behaviour of WC-W2C composites at first wall in fusion conditions

Metallic tungsten and tungsten carbide are some of the more relevant candidates proposed because of their high melting point, shielding capabilities and resistance to neutron damage. In this work, the microstructural and high temperature mechanical characterization of WC matrices with additions of 10, 20 and 50 vol% of W, dispersed by colloidal processing routes and sintered by Spark Plasma Sintering (SPS), are presented. Flexural elastic modulus, fracture strength and SENB toughness has been determined by the three-point bending tests at 600, 800, 1000 and 1200 °C.Microstructural analysis shows that after SPS all the metallic tungsten has reacted to form W2C, and sintering process is able to stabilize phases corresponding to temperatures higher than achieved as a consequence of the additional energy provided by the method. Mechanical results indicated that resistance of all materials increases as the operating temperature increases. Furthermore, remarkably great results are obtained for the composition with 10 and 20 vol% of W. This macroscopic behaviour has been related to the microstructural decomposition of the metastable W2C, to WC and W, as temperature for the bending tests approach to the equilibrium temperature (1250 °C). It was found that the presence of metallic tungsten in the final microstructure could be the responsible of the plastic behaviour of the tested pieces at 1200 °C and the doubled value of the toughness.

Influence of roughness on initial in vitro response of cells to Al2O3/Ce-TZP nanocomposite

ABSTRACT Al2O3/Ce-tetragonal zirconia polycrystal (TZP) nanocomposite was synthesized by a colloidal processing route and sintered in air atmosphere. Sandblasting treatment was made to alumina toughened zirconia (ATZ) nanocomposite in order to evaluate the influence of surface roughness on the osteogenic differentiation performing in vitro growing a human osteoblast-like cell line, SaOs-2, and human adipose-derived mesenchymal stem cells (hADMSC) osteogenic differentiated. Smooth roughness values around Ra = 0.5 µm were obtained when the abrasive material was below 90 µm increasing the expression of BGLAP and IBSP genes and Ra = 1.5 µm was found with particles of sizes between 90 and 250 µm upregulating SPARC gene. The non-cytotoxicity and haemocompatibility of ATZ nanocomposite were proved. Alumina-ceria-stabilized zirconia nanocomposite presented in this work exhibits a high potential for application in the fabrication of dental implants due to their biological behavior and very promising mechanical properties.

Effect of Sintering Temperature on Dielectric Properties of 72 wt%(Al2O3):28 wt%(SiO2) Ceramics

The various sintered samples comprising of 72 wt%(Al2O3) : 28 wt%(SiO2) based ceramics were fabricated using a colloidal processing route. The phase analysis of the ceramics was performed using an X-ray diffractometer (XRD) at room temperature confirming the presence of Al2O5Si and Al5.33Si0.67O9.33. The surface morphology of the fracture surface of the different sintered samples having different sizes of grain distribution. The resistive and capacitive properties of the three different sintered samples at frequency sweep (1 kHz to 1 MHz). The contribution of grain and the non-Debye relaxation process is seen for various sintered samples in the Nyquist plot. The ferroelectric loop of the various sintered sample shows a slim shape giving rise to low remnant polarization. The excitation performance of the sample at a constant electric signal has been examined utilizing a designed electrical circuit. The above result suggests that the prepared lead-free ceramic can act as a base for designing of dielectric capacitors or resonators.

Novel Strategy To Prepare Hierarchically Porous Ceramic Microspheres via a Self-Assembly Method on Tunable Superamphiphobic Surfaces.

Porous ceramic microspheres have been widely used in many fields such as drug delivery, chemical catalysis, environmental protection, noise reduction by absorption, and separation and purification. However, the methodology to prepare porous ceramic microspheres based on traditional colloidal processing routes faces the problems of precise control of the diameter, degree of sphericity, uniformity of the microstructure (size, porosity, shape, etc.), and so forth. Herein, we propose a new methodology to prepare hierarchically porous ceramic microspheres with a mechanism based on the superwettability strategy without the requirement of any special equipment or complicated procedures. In such an approach, a ceramic emulsion with an extremely low viscosity was prepared by an emulsion-assisted self-assembly method, which would be repelled on the superamphiphobic surface to form a submillimeter-sized Al2O3 microsphere. Compared with the traditional colloidal processing approaches to prepare ceramic microspheres, the homogeneity and precision of the ceramic microspheres prepared via our approach are much finer, while our approach is quite simple, highly efficient, and cost-saving. Moreover, the diameter of the microspheres and the microstructure of the pores (size, density, porosity, etc.) in the ceramic microspheres could be flexibly manipulated. Our methodology has solved the key problems in the preparation of ceramic microspheres which have not been solved in the past decades and provided the solution for engineering through the delicate scientific design. We anticipate that this example of the combination of superwettability science with traditional structural ceramics could provide an important application direction of advanced techniques for fabricating ceramics.

Room temperature electrical properties and excitation performance of Al2O3 doped fused silica ceramics

Al2O3 doped fused silica ceramic samples were synthesized by colloidal processing route. X-ray diffraction technique helped to give insight upon the phase formation of the synthesized samples. The dense growth of grains was depicted from surface micrographs. The frequency-dependent electrical properties of both sintered and quenched samples were examined from 100 Hz till 1 MHz. The frequency-dependent AC conductivity followed Jonscher’s power law. The dielectric relaxation and Nyquist plot confirm the non-Debye nature in both sintered as well as quenched samples. The ferroelectric properties were depicted in the prepared samples. The sintered and quenched Al2O3 doped fused silica samples reveals that the samples get excited on the application of electric field and further can be utilized in filter application.

Studies of structural, dielectric and impedance spectroscopy of fused silica ceramics fabricated through colloidal processing

In the present work, the fused silica was fabricated using colloidal processing route. The fabricated samples were quenched (five time thermal cycling) and the results of the structural and electrical properties of the sintered sample and quenched sample have been studied and compared. The structural analysis confirmed the formation of the fabricated ceramics. The morphology of the sample suggested that there is close packing of the grains. The relaxation and conduction mechanism were analyzed using impedance and modulus spectroscopy. The dielectric parameters were inspected on varying temperature and frequency range. The impedance study showed the dependence of electrical properties on both frequencies and temperatures having corresponded with the morphology.

Preparation and co-dispersion of TiO2-Y2O3 suspensions through the study of their rheological and electrokinetic properties

The development of Generation IV sodium-cooled fast reactors (SFR) is currently studied by several countries, France in particular. To manufacture the UO2-PuO2 fuels for these new types of reactors, new innovative wet colloidal processing routes are investigated. Among these wet colloidal processes, some involve at first the preparation of high solid content water-based suspensions. This key step needs to be investigated in order to obtain highly and easily processable suspensions, featuring optimal viscosity and dispersion state. The structures and properties for all intermediate and final products involved in such ceramic manufacturing processes are heavily affected by these suspension characteristics. Therefore, they are critical to ensure a compliant final product (i.e. fuel pellets) with the required density, homogeneity, mechanical strength and absence of defects. In this scope, preparation process of such suspensions was developed by the use of UO2 and PuO2 surrogating (i.e. mimicking) powders, TiO2 and Y2O3 respectively.

Porous materials from oppositely charged nanoparticle gel emulsions

Sol–gel processing is an important colloidal processing route for the synthesis of macroporous materials of tunable microstructure. More recently, methods that use particle stabilized foams or emulsions have been developed. However, such methods generally involve the use of an external surface active additives that bind the particles together during processing. In this work, we present a simple and scalable additive-free method for the fabrication of porous materials that are both macroporous and microporous in nature. In particular, we show that silica-alumina porous materials can be fabricated by processing oppositely charged nanoparticle gel emulsions which are a dispersion of oil droplets in a space spanning network of colloidal particles. The nanoparticle gel emulsions obtained are initially characterized by optical microscopy and oscillatory rheology. The water and oil in these emulsion gels are removed by drying and it is finally sintered at 1000 °C to obtain porous material. The pore size and their distribution in the three-dimensional porous material is characterized using scanning electron microscopy (SEM) and X-ray tomography. Microscopy observations revealed a homogeneous distribution of macro-pores of 20–40 μm in diameter along the width, breadth and thickness of the porous material with an average porosity of 40% measured from 2D X-ray computed tomography images. One of the important features of the porous materials thus fabricated is the presence of nanosized pores due to the inherent porosity of the nanoparticle gel as well as micron sized pores due to the oil droplets.

Mechanical characterization of Ti(C,N)-based cermets fabricated through different colloidal processing routes

In this work the influence of different colloidal processing routes on the mechanical behavior of several Ti(C,N)-based cermets has been studied. The materials were designed with a Ti(C,N) content ranging between 80 and 85 vol %, in a Fe-Ni alloy matrix (with and without carbon addition). Three processing techniques were investigated: (i) Slip Casting (SC), (ii) Slip Casting + Cold Isostatic Pressing (SC+CIP), and (iii) Spray-Dry + Uniaxial Pressing (SDP). Biaxial strength distributions were determined in all samples using the ball-on-three-balls (B3B) method on disc-shaped specimens. Results were interpreted in the framework of Weibull theory. The characteristic strength ranged between 1090 MPa and 1870 MPa. A fractographic analysis performed on selected specimens showed different critical flaws, depending on the processing route, related to the composition and porosity level in each case. Single Edge V-Notched Beam (SEVNB) specimens fabricated with SDP were tested under 4-point-bending, in order to identify the effect of Fe-Ni content and carbon addition on their fracture resistance. Detailed fractographic analyses revealed slightly different mechanisms of fracture associated with the Fe-Ni and carbon content. Fracture toughness values ranging between ∼12 and ∼14 MPa m1/2 were measured. Based upon these findings the SDP route is proposed to improve both the strength and toughness of Ti(C,N) cermets, which can be enhanced by the addition of carbon during the colloidal processing.

Toward greener electrochemical synthesis of composition-tunable luminescent CdX-based (X = Te, Se, S) quantum dots for bioimaging cancer cells

A novel versatile and clean colloidal processing route was developed for synthesizing CdX (X=Te, Se or S) quantum dots (QDs) based on the simultaneous production of the respective precursors via cadmium sacrificial anode electrochemical methodology. The CdX QDs stabilized by 3-mercaptopropionic acid (MPA) were synthesized in one-pot electrochemical process using aqueous medium at room temperature with full reaction time of approximately 16min. The CdX quantum dots conjugates were extensively characterized by UV–vis, PL, FTIR, TEM-EDS, XRD, DLS, XPS, and Zeta potential, evidencing that very stable colloidal quantum dots were produced with uniform narrow-size distributions and average nanoparticle diameter of approximately 3.0nm and quantum yield (QY) of approximately 10%. The cell viability response of HeLa cells toward the CdX-MPA conjugates clearly demonstrated that they are non-toxic based on the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) mitochondrial metabolic activity assay for 24h of incubation with HeLa cervical cancer cells. Moreover, they showed effective fluorescent activity with time-dependent intensities for evaluating continuous endocytosis of cancer cells, thus, offering innumerous possibilities to be used as fluorescent nanoprobes for bioimaging and biosensing of cancer cells.

Effect of graphene addition on the mechanical and electrical properties of Al2O3-SiCw ceramics

This paper presents a study on graphene-reinforced Al2O3-SiCw ceramic composites and the relationship between graphene oxide (GO) loading and the resulting mechanical and electrical properties. Well-dispersed ceramic-GO powders were fabricated using a colloidal processing route. Dense composites were obtained via spark plasma sintering, a technique that has the ability to reduce GO to graphene in situ during the sintering process. The mechanical properties of the sintered composites were investigated. The composite with only a small amount of graphene (0.5 vol.%) showed the highest flexural strength (904±56MPa), fracture toughness (10.6±0.3MPa·m1/2) and hardness (22±0.8GPa) with an extremely good dispersion of graphene within the ceramic matrix. In addition to these exceptional mechanical properties, the sintered composites also showed high electrical conductivity, which allows the compacts to be machined using electrical discharge machining and thus facilitates the fabrication of ceramic components with sophisticated shapes while reducing machining costs.

Highly Porous Thermoelectric Nanocomposites with Low Thermal Conductivity and High Figure of Merit from Large-Scale Solution-Synthesized Bi2 Te2.5 Se0.5 Hollow Nanostructures.

To enhance the performance of thermoelectric materials and enable access to their widespread applications, it is beneficial yet challenging to synthesize hollow nanostructures in large quantities, with high porosity, low thermal conductivity (κ) and excellent figure of merit (z T). Herein we report a scalable (ca. 11.0 g per batch) and low-temperature colloidal processing route for Bi2 Te2.5 Se0.5 hollow nanostructures. They are sintered into porous, bulk nanocomposites (phi 10 mm×h 10 mm) with low κ (0.48 W m-1 K-1 ) and the highest z T (1.18) among state-of-the-art Bi2 Te3-x Sex materilas. Additional benefits of the unprecedented low relative density (68-77 %) are the large demand reduction of raw materials and the improved portability. This method can be adopted to fabricate other porous phase-transition and thermoelectric chalcogenide materials and will pave the way for the implementation of hollow nanostructures in other fields.

Highly Porous Thermoelectric Nanocomposites with Low Thermal Conductivity and High Figure of Merit from Large‐Scale Solution‐Synthesized Bi2Te2.5Se0.5 Hollow Nanostructures

AbstractTo enhance the performance of thermoelectric materials and enable access to their widespread applications, it is beneficial yet challenging to synthesize hollow nanostructures in large quantities, with high porosity, low thermal conductivity (κ) and excellent figure of merit (z T). Herein we report a scalable (ca. 11.0 g per batch) and low‐temperature colloidal processing route for Bi2Te2.5Se0.5 hollow nanostructures. They are sintered into porous, bulk nanocomposites (phi 10 mm×h 10 mm) with low κ (0.48 W m−1 K−1) and the highest z T (1.18) among state‐of‐the‐art Bi2Te3−xSex materilas. Additional benefits of the unprecedented low relative density (68–77 %) are the large demand reduction of raw materials and the improved portability. This method can be adopted to fabricate other porous phase‐transition and thermoelectric chalcogenide materials and will pave the way for the implementation of hollow nanostructures in other fields.

Effect of drying methods of Al2O3-GO powder mixture on the properties and microstructure of sintered composites obtained by spark plasma sintering

Well-dispersed alumina ceramic-graphene oxide (GO) powder mixtures were fabricated using a colloidal processing route and three drying techniques were applied: in vacuum, spray and freeze dryers. Dense composites were obtained via spark plasma sintering, the technique reducing GO to graphene in situ during the sintering process. The mechanical properties of the sintered composites were investigated. The composites obtained by spray drying have shown the fracture toughness of 4.5 ± 0.4 MPa·m 1/2 , and hardness of 18.4 ± 0.8 GPa with an extremely high dispersion of graphene within the ceramic matrix.

High temperature performance of mullite whisker-reinforced ZTA

In this work, an oxide-based ceramic matrix composite (CMC) consisting of a zirconia toughened alumina (ZTA) matrix and reinforced with mullite whiskers is produced with the purpose of providing more oxidation resistance and cost-effective alternative to covalent discontinuously reinforced ceramics. ZTA has enhanced toughness, strength and creep resistance over single-phase alumina or zirconia. ZTA can further be strengthened by the inclusion of SiC whiskers; however, these whiskers are prone to oxidation at temperatures above 1000℃ leading to loss of properties. In this work, mullite whiskers are used as the reinforcement due to its stability in oxidizing atmospheres are high temperatures. Mullite whiskers are grown through the molten salt method and incorporated into the ZTA matrix using a colloidal processing route. The microstructure and room temperature properties have been reported in an earlier paper. Whisker additions have been shown to improve the flexural strength of ZTA at 1200℃ by 59.31%. There is some concern over the stability of large diameter whiskers at high temperatures, especially in environments with excessive moisture content or residual alkali contamination. Further work will be carried out to address these concerns as well as to develop a statistical analysis of the results presented.

Improved microstructure and fracture properties of short carbon fiber-toughened ZrB2-based UHTC composites via colloidal process

Ultra-high temperature ceramics are potential materials for a variety of high temperature applications because of excellent thermo-mechanical properties and oxidation resistance. To further improve their fracture properties, a novel colloidal process was proposed to fabricate the short carbon fiber-toughened ZrB2–ZrSi2 composites. Microstructure analysis found that the colloidal processing route could avoid the fibers' agglomeration and alleviate the fibers' damage, which minimizes the structural defects and retains the fibers' strength. The relative density of composites achieves 98.35% and the distribution of fibers in matrix is homogeneous. Mechanical tests indicate that the flexural strength is 458MPa and the fracture toughness is 6.9MPa·m1/2. In comparison to the composite obtained by conventional processing route, the fracture toughness increases by 47%. The main mechanisms for improved fracture properties could be attributed to the crack deflection, fiber sliding and fiber bridging.

Glycol chitosan/nanohydroxyapatite biocomposites for potential bone tissue engineering and regenerative medicine

In the last few decades, research on biocomposite nanomaterials has grown exponentially due to the global demand for novel solutions in bone tissue engineering and repair. In the present study, it is reported the design and synthesis of biocomposites based on glycol chitosan (GLY-CHI) matrices incorporated with nano-hydroxyapatite particles (nHA) produced via an eco-friendly chemical colloidal process in water media followed by solvent casting and evaporation methods at room temperature. The structure, morphology, and crystallinity of the components and biocomposites were extensively characterized by light microscopy (LM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), wavelength dispersive X-ray fluorescence spectroscopy (WD-XRF), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and X-ray micro-computed tomography analysis (μCT). Furthermore, cytotoxicity and cell viability tests were performed on three cell lines using a 3-(4,5-dimethylthiazol-2yl) 2,5-diphenyl tetrazolium bromide (MTT) assay, an alkaline phosphatase (ALP) activity test, and LIVE/DEAD® assays. The results demonstrated that the GLY-CHI ligand played a major role in the nucleation, growth and colloidal stabilization of calcium phosphate particles at nanoscale dimensions with a narrow distribution and average size of 74±15nm. The FTIR spectroscopy associated with the XRD results indicated that nanosized hydroxyapatite (nHA) was the predominant calcium phosphate phase produced in the colloidal processing route. In addition, the X-ray micro-CT analysis of the nanocomposite membranes showed that nHA particles were homogenously dispersed in the glycol-chitosan polymeric matrix. Moreover, according to the in vitro bioassays, the biocomposites showed an adequate cell viability response and non-cytotoxic behavior toward osteoblastic-like (SAOS) and embryonic cell lines (HEK293T). Finally, the results of osteogenic differentiation tests demonstrated that the nHA/GLY-CHI composites are osteoinductive for human bone marrow mesenchymal stem cells (HBMS), which can be envisioned for prospective use in tissue engineering (e.g., bone, cartilage and periodontal) applications.

Spark Plasma Sintered Si3N4/TiN Nanocomposites Obtained by a Colloidal Processing Route

Ceramic Si3N4/TiN (22 vol%) nanocomposites have been obtained by Spark Plasma Sintering (SPS). Our colloidal processing route allows obtaining dispersed nanoparticles of TiN smaller than 50 nm avoiding the presence of agglomerates. The nanostructured starting powders were obtained by using a colloidal method where commercial Si3N4submicrometer particles were coated with anatase TiO2nanocrystals. A later nitridation process led to the formation of TiN nanoparticles on the surface of Si3N4. A second set of powders was prepared by doping the above defined powders with yttrium and aluminium precursors using also a colloidal method as sources of alumina and yttria. After thermal nitridation and SPS treatment, it has been found that the addition of oxides dopants improves the mechanical performance (KIC,σf) but increases the electrical resistivity and significantly reduces the hardness. This is due to the formation of a continuous insulating glassy phase that totally envelops the conductive TiN nanoparticles, avoiding the percolative contact between them. The combination of colloidal processing route and SPS allows the designing of tailor-made free glassy phase Si3N4/TiN nanocomposites with controlled microstructure. The microstructural features and the thermoelectrical and mechanical properties of both kinds of dense SPSed compacts are discussed in this work.