Zbigniew Pędzich

Influence of surface-modified montmorillonites on properties of silicone rubber-based ceramizable composites

Ceramizable (ceramifiable) silicone rubber-based composites are modern elastomeric materials for fire protection application. The most important sector of economy using such materials is cable industry because there are special types of electric circuits that have to keep working in the case of fire. These kinds of composites can create ceramic phase protecting copper wire from melting under high temperature. When temperature increases, polymer matrix degrades (creating silica residue as one of the products) and mineral particles dispersed in silicone rubber matrix stick together creating stiff, durable, insulating and porous ceramic skin. In this paper, the influence of surface modification of montmorillonite with quaternary ammonium salts on ceramization of their silicone rubber composites is presented. Filler modification was studied, determining changes to its surface energy and thermal stability. Mechanical properties, flammability and thermal stability of composites were determined. Ceramization of the composites was discussed from the point of view of their mechanical properties and structure of ceramic phase after heat treatment, determined by compression stress tests, porosimetry and scanning electron microscopy adequately. Results show that type of modifier applied strongly affects properties of silicone rubber-based ceramizable composites before and after ceramization. Samples containing surface-modified montmorillonite produce significantly less heat during their thermal decomposition than composite filled with unmodified montmorillonite. Moreover, incorporation of montmorillonite modified with ammonium salt of linear organic chain causes the creation of nano-porous structure after ceramization. On the one hand, it facilitates heat insulation, but on the other hand, high total volume of pores adversely affects mechanical endurance of the ceramic phase.

The TZP–Chromium Oxide and Chromium Carbide Composites

Microstructure and mechanical properties of the 2.9(Y) TZP matrix composites with chromium carbide and chromium oxide inclusions were investigated. Two chromium carbides were used, Cr 3 C 2 and Cr 7 C 3 . The TZP matrix powder was prepared by the coprecipitation-calcination technique. Attrition mixing allowed to homogenise the matrix and the additive powders. Coprecipitation was additionally applied in the case of the TZP-Cr 2 O 3 system. Hot pressing at 1400°C for 40min resulted in the higher densification than the pressureless sintering at 1500°C for 120 min. The TZP--carbide systems showed better properties and higher densification in the case of the Cr 7 C 3 additive. It was related to the reaction between the matrix and Cr 3 C 2 . Microscopic observations revealed quite different crack propagation path in the system with the oxide and carbide additives. It allowed to suggest mechanisms of the toughness increase over the value of the pure matrix.

Effect of carbon fibers on thermal properties and mechanical strength of ceramizable composites based on silicone rubber

Ceramizable (ceramifiable) silicone composites are one of the most important elastic technical materials produced industrially. These composites are commonly used to increase flame retardancy of electrical cables and to ensure integrity of electricity network during fire by their ability to create a continuous ceramic structure. In this paper, ceramizable silicone composites filled with different contents of carbon fibers were tested. The research was focused on the characterization of ceramic structure created during heat treatment of the composites and thermal properties of the composites. For this purpose, morphology (SEM) and compression strength of the ceramic structures were studied. To describe process of ceramic structures creation, TG/DSC analysis was done. These tests have demonstrated that, the increase in carbon fibers amount improves the mechanical properties of ceramic structure regardless of heat treatment conditions.

The Abrasive Wear of Alumina Matrix Particulate Composites at Different Environments of Work

The paper presents the results of investigation on the wear of alumina-based composites containing submicrometre particles of other phases in different conditions of work. Three types of phases were used as additives: tungsten carbide, yttria stabilized zirconia and metallic tungsten. The above mentioned phases were used separately or simultaneously as mixes. Two types of tests were performed - the Dry Sand Test and the Miller Test in wet pulp. Coarse silicon carbide grains were used as an abrasive medium in both cases. The results indicate that the optimal wear susceptibility in given work environment could be achieved by the addition of different type of additives.

Thermal Stability and Flammability of Styrene-Butadiene Rubber-Based (SBR) Ceramifiable Composites

Ceramifiable styrene-butadiene (SBR)-based composites containing low-softening-point-temperature glassy frit promoting ceramification, precipitated silica, one of four thermally stable refractory fillers (halloysite, calcined kaolin, mica or wollastonite) and a sulfur-based curing system were prepared. Kinetics of vulcanization and basic mechanical properties were analyzed and added as Supplementary Materials. Combustibility of the composites was measured by means of cone calorimetry. Their thermal properties were analyzed by means of thermogravimetry and specific heat capacity determination. Activation energy of thermal decomposition was calculated using the Flynn-Wall-Ozawa method. Finally, compression strength of the composites after ceramification was measured and their micromorphology was studied by scanning electron microscopy. The addition of a ceramification-facilitating system resulted in the lowering of combustibility and significant improvement of the thermal stability of the composites. Moreover, the compression strength of the mineral structure formed after ceramification is considerably high. The most promising refractory fillers for SBR-based ceramifiable composites are mica and halloysite.

Kinetic Studies of Gold Recovery from Diluted Chloride Aqueous Solutions Using Activated Carbon Organosorb 10 CO

The kinetic studies of gold(iii) chloride complex ions recovery from acidic solution using activated carbon (AC) were carried out using spectrophotometry. AC samples were characterized in terms of surface area, porosity, and zeta potential. The surface functional groups were also identified. It was found that adsorption of AuCl4– onto AC was followed by reduction of the ions to the metallic form. The process obeyed the first order reaction model, but the reaction was controlled by diffusion. Arrhenius and Eyring–Polanyi equations were used for determination of the activation parameters. Distribution of gold across the AC pellets was also determined and discussed according to the porous material theory.

Microstructural Characterization of Yttria-Stabilized Zirconia Sintered at Different Temperatures Using 3D EBSD, 2D EBSD and Stereological Calculations

A set of yttria-stabilized zirconia samples sintered at increasing temperatures was investigated using two-dimensional (2D) and three-dimensional (3D) electron backscatter diffraction (EBSD) techniques to calculate grain size distributions and grain boundary densities. The obtained results were compared to the results of stereological calculations and revealed that mean intercept length, a commonly used stereological parameter, is ca. 20% lower than an average grain diameter derived from 2D and 3D EBSD data. Moreover, the results based on 2D and 3D EBSD analyses were similar to each other in grain boundary density, while the values obtained from the stereological approach were noticeably lower.

Effect of Milling Parameters on Microstructure and Selected Mechanical Properties of Si3N4 – Graphene Composites

In the presented work the effect of graphene platelets (GPLs) grade and mixture homogenization parameters on the microstructure and selected properties of Si3N4 – graphene composites have been investigated. Three different commercial grades of the multilayer graphene platelets were used as a filler for ceramic matrix. The mixtures of silicon nitride powder with 2wt% of GPLs were prepared using planetary mill at 200 – 400 rpm for 1 – 16 h. The water, acetone and isopropanol have been tested as a wetting medium. The composites were sintered using Spark Plasma Sintering – SPS at 1650 °C under 35 MPa of uniaxial pressure during the whole cycle. Microstructure, density, Young modulus, hardness and indentation fracture toughness of the sintered samples have been investigated. The microstructure homogeneity of the composites depends mainly on the milling duration. Milling process should last for about 4 – 8 h at 200 rpm for optimum results. Mechanical properties of the composites are slightly varied depending on the mixture preparation parameters. The homogenization time has a little stronger influence on the Youngs modulus and hardness in the case of composites containing larger graphene sheets, in connection with the fragmentation of its microstructure. The best properties have composites reinforced by the GPLs characterised both by the smallest thickness and lateral size of nanosheets.

Ceramizable Composites for Fire Resistant Applications

The paper concerns composite materials made of silicone rubber matrix and ceramic fillers used as flame resistant coverings for electrical cables. Under fire, such materials must be able to form, relatively quickly, compact and stiff protecting coating, strong enough to maintain integrity of electrical circuit, even up to melting temperature of metal core. The residue of fired silicone rubber or silica filled elastomer exhibit a form of white powder. There is no evidence of solidification of silica particles, even after heating at 1100°C. However, the addition of some ceramic phases results in reaction with silica matrix (starting at about 900°C) producing a liquid phase, what facilitates particle binding. At lower firing temperatures (600°C) the problem of binding between the product of pyrolysis (silica) and filler is also present, what results in formation of fragile surface shield. The problem can be overcome by the addition of certain inorganic materials to the silicone rubber matrix. The paper discusses their influence on ability of silicone rubber composites, additionally containing glassy phase, wollastonite, mica, aluminium hydroxide, montmorillonite or calcined caoline, to ceramization.

Fracture of Oxide Matrix Composites with Different Phase Arrangement

The paper presents the analysis of the fracture path in composites with the alumina or zirconia matrix containing inclusions of another oxide or tungsten carbide. The description concerns the fracture of composite systems with 10 vol.% of inclusions (particulate composites), other three-systems with 33 vol.% of inclusions of each phase and the two phase composite with the equal phase content (50 : 50 vol.%). The different phase proportion and diversified content of inclusions cause significant differences in the microstructure of composites. The main difference between particulate composites and composites with continuous phases depends on a possibility of occurrence of toughening mechanisms connected with crack – inclusions interactions. In connection with a different interphase boundary strength, this phenomenon leads to significant differences in the way of composite fracture. The presented results of mechanical properties of composites were analyzed in the aspect of their way of fracture.