Pressureless sintering of β-SiAlON ceramic compositions using fluorine and oxide additive system

SiAlONs are silicon aluminium oxynitride ceramic materials with a range of technically important applications, from cutting tools to specialised refractories and the properties of SiAlONs can be tailored for specific purposes. In this study, different β-SiAlON compositions were prepared using fluoride (MgF₂ as fluorine source plus Y₂O₃) and oxide (MgO plus Y₂O₃). These compositions were pressureless sintered under nitrogen atmosphere in the range of 1450–1750 °C for 0.5–2.5 h for comparison of densification behaviour and mechanical properties. Densities of samples were measured and analyses of result products were carried out using SEM and XRD. The F-doped sintered β-SiAlON ceramics showed better densities and less pore micrographs especially at lower temperatures compared with the fluorine free samples. Full densifications were achieved for β-SiAlON ceramics with fluoride addition at 1700 °C for 60 min. Consequently, fluorine addition to additive system has a good effect on mechanical properties and densification behaviour.     

Questioning the validity of present models for estimating the performances of zeolite-polymer mixed matrix membranes

An investigation was carried out to determine the applicability of the conventional models to the mixed matrix permeability data reported in the literature. The series, parallel, Maxwell, effective medium theory (EMT), geometrical mean, and te Hennepe models were employed to estimate the zeolite permeabilities pertaining to various gases. The variations of the zeolite permeabilities with respect to the model, polymer type, and zeolite loading used were taken as indicators of the suitability of the models to be applied to permeability data. Data reported for various zeolites and polymers were utilized in the calculations. The results obtained showed that in many cases, taking into consideration the zeolite and polymer permeabilities and the zeolite loading is not sufficient to describe the performances of the zeolite-polymer mixed matrix membranes in an accurate manner. It is demonstrated that the properties of an additional phase, the interphase, should also be taken into consideration for a better prediction.     

Low-methoxyl pectin–zeolite hydrogels controlling drug release promote in vitro wound healing

This study presents the design of novel hydrogel films, based on low-methoxyl (LM) pectin and NaA- or ZnA-zeolite particles, to serve as wound dressing materials with controlled drug delivery properties. We studied the effects of the preparation method of hydrogels, the amounts of crosslinker, drug and zeolite, and the type of cation in zeolites on the drug release mechanisms from the hydrogels. Ionic strengths of both film and external medium dictated the drug release behavior of the films, while the other parameters also played essential roles. NaA-zeolite hydrogels prepared using membrane diffusion controlled system, could reach a drug release ratio of 86% within 5 h. The drug-free hydrogels displayed no cytotoxicity while supporting cell proliferation and migration. Our cost-effective LM pectin–zeolite hydrogels promise to be effective wound dressing materials with controlled drug delivery ability, transparency, good swelling properties, ability to hold fluids, good oxygen transmission rate, and biocompatibility. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47640.     

In vitro evaluation of the use of zeolites as biomaterials: effects on simulated body fluid and two types of cells

Various zeolites were kept in simulated body fluid (SBF) for different periods of time. Possible changes that may occur in the crystalline structures of zeolites and the chemical composition of SBF were determined by various analysis techniques after this treatment. The possible effects of two different zeolites on the morphology and viability of chronic myelogeneous leukemia and swiss albino fibroblast culture cells were also investigated. It was determined that when different types of zeolites were kept in the SBF for up to 14 days, their crystal structures were not affected. Observable amounts of Si were detected in the SBF samples after their treatment with all the zeolites investigated. Another variation in the chemical composition of SBF, worth to mention, was the increase of about 10% in its K content after the treatment carried out by using clinoptilolite. The zeolites KA and silicalite, which allowed the lowest and highest amount of silicon transfer into the SBF, respectively, were observed not to have any significant biological effect on the two different cell generations investigated under the conditions used in this study.     

Fabrication of Si3N4 preforms from Si3N4 produced via CRN technique

Ceramic preforms with randomly distributed particles as reticulated porous structure which are generally used for metal infiltration as reinforcement, membranes, catalyst supports etc. Preforms are characterized by open porosity making possible their infiltration by liquid metal alloys. In this work, quartz powders using carbon black as a reducing agent were used for alpha Si₃N₄ powders synthesis through a carbothermal reduction and nitridation (CRN) process. The CRN process was carried out under nitrogen flow at 1,450 °C for 4 h. At high temperatures, carbon as reducing agent reacts with the oxygen of SiO₂, and the resulting metallic silicon compounds with nitrogen gas to obtain silicon nitride powder. The reacted powders were used to obtain reticulated ceramic by replica method. The powders containing various bentonite ratios were mixed in water to prepare slurry. The slurry was infiltrated into a polyurethane sponge. A high porous ceramic foam (preform) structure was achieved after burn out of the sponge. All ceramic preforms were sintered to increase stiffness (in the temperature range 900–1,350 °C). The sintered ceramic foams were subjected to compressive tests. The scanning electron microscopy was used to examine the reticulated ceramic foam structure, and X-ray diffraction analysis was performed to determine phases.     

Coatings of Na-aluminosilicate zeolites prepared using predictions from an artificial neural network method

Predictions made by the Generalized Regression Neural Networks (GRNN) method were used to relate the initial compositions of various reaction mixtures to the types of Na-aluminosilicate zeolites that may be obtained from these compositions. In the light of the predictions made, coatings were prepared on stainless steel plates, which were characterized by X-ray diffraction and scanning electron microscopy prior to and after syntheses. Coatings of zeolites P, X, A, analcime as well as their mixtures could be obtained from a variety of previously unknown clear solution compositions, generally in good accordance with the predictions made by the GRNN method. Different textural properties were obtained for the coatings of the same zeolite, such as P and X, which could be prepared from a relatively wide range of compositions.     

Desorption kinetics of thick zeolite coatings prepared by induction heating for adsorption heat pump applications

Journal of Porous Materials - Zeolite coatings were directly crystallized on stainless steel substrates, which included double layer wire mesh cylinders and tubes, by induction heating. X-ray...     

Artificial neural network methods for the prediction of framework crystal structures of zeolites from XRD data

Extracting information about the structures of zeolites and other crystalline materials from X-ray diffraction (XRD) data simply by using statistical methods may provide an impetus for the discovery and identification of unknown materials. In this study, the possibility of using artificial neural network methods for relating framework crystal structures to XRD data reported in literature was investigated. Generalized Regression Neural Networks and Radial Basis Function-Based Neural Networks were utilized in the investigations. The results obtained by neural networks, using fivefold cross validation technique, were compared to the actual values as well as to those determined by multilinear regression. The predictions made by these neural network methods were, in general, more reliable than those performed by regression. The best predictions were achieved for the estimation of the framework densities of zeolites, which provided quite small deviations from the actual values.     

Adsorption kinetics of zeolite coatings directly crystallized on metal supports for heat pump applications (adsorption kinetics of zeolite coatings)

A series of theoretical studies had previously indicated that the utilization of zeolite coatings directly crystallized on metal surfaces might improve the performance of adsorption heat pumps significantly. In this study, for the first time, kinetic measurements were performed to determine the rate of adsorption of zeolite coatings, which is the relevant parameter for increasing the power density of heat pump applications. The zeolite coatings investigated were prepared by using the substrate heating method. A scale-up of the substrate heating system was made in order to be able to grow zeolite coatings on 5 × 5 cm² stainless steel plates, instead of the 1 × 1 cm² plates, conventionally used in synthesis experiments. Two different reactors were constructed in which zeolite A and X coatings, with mass equivalent thicknesses of about 38 μm and 230 μm, respectively, could be obtained. The zeolite X coating, which had a quite open nature, exhibited quite favorable overall adsorption kinetics when compared to a reference sample, a paper-like polymer structure containing zeolite laminated on a metal layer.