Colloidal processing of ceramics is reviewed with an emphasis on interparticle forces, suspension rheology, consolidation techniques, and drying behavior. Particular attention is given to the scientific concepts that underpin the fabrication of particulate-derived ceramic components. The complex interplay between suspension stability and its structural evolution during colloidal processing is highlighted.

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Colloidal Processing of Ceramics

This review article focuses on the sol-gel preparation of high temperature superconducting oxides wherein different classes of gel technologies were utilized. These involve: 1) the sol-gel route based upon hydrolysis-condensation of metal-alkoxides, 2) the gelation route based upon concentration of aqueous solutions involving metal-chelates, often called as “chelate gel” or “amorphous chelate” route, and 3) the organic polymeric gel route. This paper reviews the current status of these sol-gel processes, and illustrates the underlying chemistry involved in each sol-gel technology. It is demonstrated that the chemical homogeneity of the gel is often disturbed by the differences in the chemistries of the cations. Prior to gelation the starting precursor solution containing various forms of metal-complexes must be chemically modified to overcome this problem. Illustration of a variety of strategies for success in obtaining a homogeneous multicomponent gel with no precipitation is focal point of this review article.

Invited review “sol-gel” preparation of high temperature superconducting oxides

When thin films of colloidal fluids are dried, a range of transitions are observed and the final film profile is found to depend on the processes that occur during the drying step. This article describes the drying process, initially concentrating on the various transitions. Particles are seen to initially consolidate at the edge of a drying droplet, the so-called coffee-ring effect. Flow is seen to be from the centre of the drop towards the edge and a front of close-packed particles passes horizontally across the film. Just behind the particle front the now solid film often displays cracks and finally the film is observed to de-wet. These various transitions are explained, with particular reference to the capillary pressure which forms in the solidified region of the film. The reasons for cracking in thin films is explored as well as various methods to minimize its effect. Methods to obtain stratified coatings through a single application are considered for a one-dimensional drying problem and this is then extended to two-dimensional films. Different evaporative models are described, including the physical reason for enhanced evaporation at the edge of droplets. The various scenarios when evaporation is found to be uniform across a drying film are then explained. Finally different experimental techniques for examining the drying step are mentioned and the article ends with suggested areas that warrant further study.

https://iopscience.iop.org/article/10.1088/0034-4885/76/4/046603/pdf

Drying of thin colloidal films.

Epitaxial thin films of Ba2YCu3O7−x (BYC) were prepared on (001) LaAlO3 single‐crystal substrates by metalorganic deposition of metal trifluoroacetate precursors. This is an ex situ process that requires high‐temperature annealing in a humid atmosphere to produce stoichiometric BYC thin films. The chemically derived superconducting films were found to have high critical temperatures and high current densities when crystallized under low‐oxygen partial pressures. Superconducting films of 70 nm thickness with zero‐field critical current densities greater than 5×106 A/cm2 at 77 K and zero resistance at 92 K were prepared by annealing at 780 and 830 °C in 2.5 × 10−4–1 × 10−3 atm oxygen furnace atmospheres. As the film thickness was increased, the superconducting properties and surface smoothness of the films tended to degrade. This behavior was consistent with a microstructural model in which the films are composed of a dense slab of c‐axis normal BYC near the film/substrate interface with the overlying mater...

Effect of growth conditions on the properties and morphology of chemically derived epitaxial thin films of Ba2YCu3O7−x on (001) LaAlO3

Large-area, uniform, high critical current density (Jc) YBa2Cu3O7−x (YBCO) superconductor films are now routinely obtained by metalorganic deposition using trifluoroacetates (TFA-MOD) This method does not require any expensive vacuum apparatus at any time during the whole process Thus, TFA-MOD is regarded as one of the most suitable candidates for fabricating a YBCO tape for many high-power applications This method originated from an electron beam process using BaF2 developed by Mankiewich et al Afterwards, Gupta et al reported using TFA-MOD to prepare a similar precursor film These two ex situ processes used fluorides instead of BaCO3 to avoid the fatal deterioration in Jc, which is caused in the resulting films through metal carboxylic groups Fluorides not only avoid such deterioration but also lead to perfectly c-axis-oriented epitaxial crystal growth In conventional metalorganic deposition, nucleation in the precursor film causes random orientation in the resulting film However, in TFA-MOD, nanocrystallites in the precursor film never cause such disorder Furthermore, during the firing process of TFA-MOD, water and HF gas diffuse quickly between the film surface and growth front of the YBCO layer This diffusion never limits the growth rate of YBCO What distinguishes TFA-MOD from conventional metalorganic deposition? What happens during heat treatment? In this paper, we discuss all the TFA-MOD processes and the peculiar growth scheme of the YBCO layer in TFA-MOD using the model of a quasi-liquid network In addition, we review the history of TFA-MOD and recent results and discuss the prospects of future applications

https://iopscience.iop.org/article/10.1088/0953-2048/16/11/R01/pdf

Review of a chemical approach to YBa2Cu3O7−x-coated superconductors—metalorganic deposition using trifluoroacetates

Drying of binder-free granular ceramic films was studied to identify processing variables which affect their cracking behavior. Films were prepared from electrostatically stabilized suspensions of α-alumina in water. A critical cracking thickness (CCT) was determined, above which films would spontaneously crack during drying. The effects of particle size, liquid surface tension, drying rate, dispersion stability, and sedimentation time were evaluated by a statistical design methodology. The CCT for films prepared on glass substrates was used as a measure of the effect of each variable on cracking. The statistically significant variables were particle size, dispersion stability, and sedimentation time. The effect of substrate constraint was also studied by producing films on a Teflon substrate and a pool of liquid Hg. The observations were consistent with a capillary formed tensile stress acting on the entire film rather than differential stress generated by a moisture gradient over the film thickness.

Drying of Granular Ceramic Films: I, Effect of Processing Variables on Cracking Behavior

Films composed of ceramic particles were observed during drying. The films were prepared from 20 vol% aqueous dispersions of α-alumina and α-quartz and were free of any organic binder. Conditions for uniform film saturation during drying were established by consideration of a liquid transport model and by direct observation of the drying films. Drying stresses were measured in situ by a substrate deflection method based on an optical interference technique. Simultaneous stress and weight measurements were used to determine stress as a function of saturation. The maximum stress occurred near 100% saturation and was approximately 2 and 1.1 MPa for films produced from 0.35-and 0.68-μm particles, respectively. The maximum stress decreased from 2 to 0.9 MPa for films produced from the 0.35-μm particles when 0.005 wt% surfactant was added to the slurry. The surfactant decreased the liquid surface tension from 72 to 32 dyn/cm. These observations are direct evidence of the effects of capillary tension on the state of stress in a ceramic body. Mechanical properties of the green ceramic films were estimated by use of a linear elastic fracture model. Knowledge of the critical cracking thickness and maximum stress in the film was used to estimate the fracture resistance of the granular film. The fracture resistance values are approximately 0.02 and 0.007 MPa·m1/2 for films produced from alumina and silica, respectively. The difference in mechanical behavior of the silica and alumina films is similar to that expected by the difference in Hamaker constants between the two materials.

Drying of Granular Ceramic Films: II, Drying Stress and Saturation Uniformity

The effect of heat treatment on microstructure development in Ba2YCu3O7−x films prepared from metal trifluoroacetate precursors is investigated. The growth of textured Ba2YCu3O7−x on BaZrO3 substrates is shown to be strongly influenced by the furnace atmosphere used during calcination. Differential thermal analysis was used to show that conditions that promote c-axis texture are also those which cause partial melting of Ba–Y–Cu–O compositions that are depleted in barium relative to Ba2YCu3O7−x.

Texture development in Ba2YCu3O7−x films from trifluoroacetate precursors

Barium yttrium cuprate powders were prepared by a sol-gel emulsion process in which an aqueous nitrate solution containing Ba, Y, and Cu in the correct stoichiometry was emulsified in heptane. The resulting micrometer-sized droplets were gelled by adding a high-molecular-weight primary amine that extracts nitric acid from the aqueous phase and raises the pH of the droplets. Thermal gravimetric analysis revealed that the correct overall stoichiometry was obtained at temperatures lower than 400 °C during calcination, but X-ray diffraction showed that calcination at temperatures greater than 800° may be required to produce the correct phase. One possible advantage of this technique is the control of powder morphology on a micrometer scale so that powders can be prepared for advanced ceramics processing methods. Also, segregation of components, if any, will be restricted to distances less than one micrometer.

Ba2YCu3O6.9 Powder Preparation by Sol-Gel Emulsion Techniques

Decomposition of metal-organic precursors to Ba2Ycu3O7-X films is difficult because of the high reaction temperature required to decompose the BaCO3 intermediate. The recently proposed use of metal trifluoroacetate (TFA) solutions offers an alternative path to barium-containing superconducting films. The TFA salts decompose to the metal fluorides forming BaF2, eliminating BaCO3 from the system. Ultimate conversion to BYC, however, is shown not only to depend on hydrolysis of the BaF2 at high temperatures, but also hydrolysis of copper trifluoracetate at low temperatures to prevent the volatilization of Cu(TFA)2. These processes result in unique microstructural behavior which can be characterized by electon microscopy and Auger spectroscopy. Effects due to substrate interactions have been eliminated by use of a chemically inert substrate material, BaZrO3.

Raymond C. Chiu

Papers

Drying of Granular Ceramic Films: I, Effect of Processing Variables on Cracking Behavior

Drying of Granular Ceramic Films: II, Drying Stress and Saturation Uniformity

Texture development in Ba2YCu3O7−x films from trifluoroacetate precursors

Ba2YCu3O6.9 Powder Preparation by Sol-Gel Emulsion Techniques

Metal‐Organic Decomposition and Microstructure Development in Ba2ycu3o7‐X Films from Metal Trifluoroacetate Precursors