Effect of filler crystal habit and particle size of a nano-dispersed technological binder of the properties of a porous permeable ceramic material

The fundamental possibility is demonstrated of creating porous permeable ceramic materials with open porosity of not 40, but 60% made of fractionated powders, that makes it possible to increase markedly the productivity of filtering elements and filtration equipment for micro-and ultra-filtration based on them. Ceramic materials may be created from specially synthesized corundum powder whose crystals have the shape of tetragontrioctahedra. The open porosity and the ultimate strength in compression for ceramic materials made from these powders depends on the sizes of the powder particles used, the silica ash particles and forming method used. __________ Translated from Novye Ogneupory, No. 9, pp. 45–48, September 2007.     

Special characteristics of the technology of high-density technical ceramics. Chemical methods for obtaining the initial powders

Different methods for fabricating high-dispersity powders of oxides and their compounds are examined. These methods make it possible to obtain dense and strong ceramic materials with a regulatable microstructure. Chemical methods are most commonly used to obtain highly dispersed powders giving a uniform distribution of the components. The method of heterophase precipitation gives hydroxide powders which are x-ray amorphous, filter well, and consist of spherical aggregates 0.5 µm or less in size. On sintering the structure of the hydroxides is transferred to the oxides obtained, from which high-quality ceramic articles are manufactured.     

Preparation of bioactive TiO2–MgO composite ceramics with bone-promoting properties

In the field of hard tissue repair, titanium-based materials have excellent mechanical properties and magnesium-based materials have good bioactivity, but their shortcomings are that titanium-based materials do not have good bioactivity, while magnesium-based materials are limited in application due to their rapid degradation rate. In order to give full play to the advantages of these two materials, the TiO₂–MgO composite ceramic materials were prepared by combining the two elements and sintering at high temperature. By changing sintering temperature and MgO content, the structure composition and bioactivity of composite ceramic materials can be controlled. The surface morphology, mineralization ability in vitro, cytotoxicity and bone-promoting properties of composite ceramic materials were studied. The experimental results show that high MgO content composite ceramic materials will bring too strong alkalinity to the environment, which will accelerate the mineralization ability of materials, but is not conducive to the survival of cells. Composite ceramic materials with suitable sintering temperature and MgO content have good bioactivity and bone-promoting performance, while the porous structure produced by MgO degradation is beneficial to cell spreading and can form a good combination between the material and bone tissue at an early stage. Porous structure and Mg²⁺ can adjust the bone-promoting properties of materials together. Through the above experimental research, it is found that TiO₂–MgO composite ceramic material is a new type of material which is used in the field of hard tissue repair due to its good bioactivity.     

Aluminum oxide and ceramics based on it: 21st century materials

Some forms of corundum ceramic are considered, as corundum has so far been the most widely used material in various areas of engineering. Various materials are considered that have high strength, particularly in combination with partially stabilized zirconium dioxide. Information is given on additives to finely divided powder (<1 mm) in the form of nanopowders, and these are used for making strong and especially strong components. A possible sintering mechanism for finely divided powders with added nanopowders is considered in relation to the corundum ceramic Koral-2. __________ Translated from Novye Ogneupory, No. 3, pp. 155–160, March, 2008.     

The Use of Sphere Indentation Experiments to Characterize Ceramic Damage Models

Sphere impact experiments are used to calibrate and validate ceramic models that include statistical variability and/or scale effects in strength and toughness parameters. These dynamic experiments supplement traditional characterization experiments such as tension, triaxial compression, Brazilian, and plate impact, which are commonly used for ceramic model calibration. The fractured ceramic specimens are analyzed using sectioning, X-ray computed tomography, microscopy, and other techniques. These experimental observations indicate that a predictive material model must incorporate a standard deviation in strength that varies with the nature of the loading. Methods of using the spherical indentation data to calibrate a statistical damage model are presented in which it is assumed that variability in strength is tied to microscale stress concentrations associated with microscale heterogeneity.     

A parameter for the evaluation of sintering of ceramic materials

A method for analyzing the sintering process in ceramic materials in an isothermal regime using a multifactor evaluation parametern, which can be one of the main control parameters of the sintered system, is suggested. The results of experimental investigations of sintering processes in clays and mixtures used in the production of various ceramic pieces are presented. The method is simple and can be used in laboratory and industrial conditions.     

Is direct microwave heating well suited for sintering ceramics?

This paper presents a thorough analysis of direct microwave heating as a sintering process of ceramic materials. It questions why susceptor-assisted microwave heating is used in most experimental works, although direct microwave coupling is preferable for taking advantage of the possible beneficial effects of the microwave field on the sintering phenomena. This issue was investigated by conducting both experiments and numerical simulations. The experiments consisted of sintering alumina and yttria doped zirconia powder samples in a 2.45 GHz resonant cavity with automatic thermal monitoring, whereas the numerical simulations coupled electromagnetics, thermal transfer and sample deformation. Alumina and yttria doped zirconia are widely used materials and they exhibit different microwave field behaviours (transparent and absorbent, respectively), which are representative of most ceramic materials. The influence of the insulating material was discussed by considering different sintering cell designs. The very low coupling capacity of alumina made its direct heating very difficult. It was therefore necessary to apply a strong electric field to heat it. This situation promoted the absorption of microwave energy by other elements such as the insulating material, leading to heating instabilities and degradation of the insulation cell. In the case of zirconia, its coupling properties change abruptly with increasing temperature. It is poorly absorbent at low temperature, highly absorbent at intermediate temperature and it finally becomes reflective at the end of the sintering process. The consequences of this behaviour are (i) a very difficult control of direct heating (ii) a propensity to form damaging hot spots and (iii) the impossibility to reach high temperatures without forming plasma. Therefore, this experimental and numerical study showed that direct microwave heating is not suitable for conducting reliable and homogeneous sintering of classical ceramics. This explains why susceptor-assisted heating is most of time preferred.     

On the localized impedance spectroscopic characterization of grain boundaries: General aspects and experiments on undoped SrTiO3

Impedance measurements using microelectrodes on adjacent grains of polycrystals are a valuable tool for a detailed analysis of grain boundary properties of ceramic materials. In order to support proper interpretation of such experiments two fundamental aspects are discussed in the theoretical part of this paper: first, it is shown that “localized” experiments using contact pads are often not reasonable and can lead to severe misinterpretation. Second, results from finite element calculations illustrate as to how far conclusions on the grain boundary resistivity distribution can be drawn from a measured grain boundary resistance statistics. In the experimental part, microelectrode impedance measurements and orientation imaging data on undoped polycrystalline SrTiO₃ are presented and interpreted in terms of the homogeneity of the space charge potentials at grain boundaries.     

高温耐磨抗氧化陶瓷材料的研究进展

介绍了陶瓷材料的氧化及磨损机理等方面的近期研究成果,总结了陶瓷材料高温耐磨性、抗氧化性的研究进展,最后提出了改善陶瓷材料耐磨性及抗氧化性的可能途径和方法.     

混料设计在电极涂液配方中的应用

混料设计是解决配方问题非常有用的工具.本文介绍了混料设计结合Minitab统计软件,应用于氯碱行业的电极涂液配方设计的方法.T涂层是最新研发的产品.采用传统的试错法一直无法解决多响应变量的配方设计问题.该产品的响应变量为电位、寿命和酸电解耐受时间3个指标,而原材料主要包含U、R、T、I 4种材料.混料设计结果表明U、R...     

Characterization of Mechanical Loss in Piezoelectric Materials Using Temperature and Vibration Measurements

In this research, a unique approach for characterizing mechanical loss in piezoelectric materials is developed. Instead of characterizing mechanical loss through electrical measurements, thermal and mechanical measurements are used. A comprehensive heat transfer model has been developed to describe the temperature rise due to heat generation in piezoelectric materials in resonance conditions. Using this model, along with experimental temperature and vibration measurements, the mechanical quality factor can be calculated. Advantages of this method over electrical impedance methods include high sensitivity to change in losses due to precise temperature and vibration measurements and evaluation of losses from a single frequency (versus a frequency sweep). This method has been applied to a hard PZT ceramic at several frequencies near its fundamental resonance at a tip vibration velocity of 300 mm/s RMS. In these high power conditions, the standard deviation of the mechanical quality factor resulting from the impedance data was 15%, while those resulting from the new method are less than 2.5%. The results from the impedance measurements using a constant vibration velocity sweep agree with the magnitude of the quality factor calculated from new method, giving evidence to its validity.     

Anwendung von modernen Methoden der statistischen Versuchsplanung bei der Entwicklung von Lackrohstoffen

Application of Modern Methods for the Statistical Planing of Experiments in the Development of Raw Materials for Paints Generally, the action of several factors should be investigated in the development of raw materials for paints. For this purpose, complete or fractional factorial plans of experiments are useful. Such plans should be oriented towards the ultimate aim of the experiments. The author has found during the development of low-profile hard polyester resins and filling resins that from factorial planing of experiments with 2–5 variables at two niveaus, the essential variables, their effects of first and second order as well as the direction of continuation of the experiments can be determined. In an example the Plackett-Burman-procedure is discussed which is especially suited for preliminary testing in the case of 5 or more variables. The optimization of a process requires the application of experimental plans in which the variables are varied at least at 3 niveaus. When more than 3 variables are investigated, it is advantageous to use factorial plans that are complemented by incomplete blocks. As an example the optimization of an epoxy primer using the Box-Behnken design is discussed.     

Methodology for measuring fracture toughness of ceramic materials by instrumented indentation test with vickers indenter

Virtual crack closure technique and elastoplastic finite element method were employed to calculate the stress intensity factors (SIF) of ceramic materials on the tip of both half‐penny crack (HPC) and radial crack (RC) induced by Vickers indenter and the value of fracture toughness (K_IC) was extracted by the design of equi‐SIF contour of HPC and RC crack front. Through dimensional theorem and regressive analysis, a functional relationship between instrumented indentation parameters, crack length of Vickers impression and fracture toughness of ceramic materials was established, thus a novel methodology has been presented for measuring fracture toughness of ceramic materials by instrumented Vickers indentation. Both numerical analysis and experiments have indicated that this methodology enjoys higher measurement precision compared with other available indentation methods. The methodology is universally suitable for HPC, RC as well as transition cracks and capable of determining fracture toughness and elastic modulus in a single indentation test. In addition, it saves the effort of measuring the diagonal length of Vickers impression in case that the impression remains unclear.     

Creation of new porous permeable ceramic materials and technology for making articles from them — a real way for a technological breakthrough in the main branches of industry

The creation of new porous permeable ceramic materials with a different microstructure and a controlled pore size from nano- to microsize that have properties markedly surpassing those of traditional materials used in industry, and organization of the production of various objects made from them will make it possible to obtain a considerable economic and ecological effect. Specific examples are provided for the use of objects made of porous permeable ceramics developed in ZAO NTTs Bakor in various branches of industry. Translated from Novye Ogneupory, No. 11, pp. 37–41, November 2008.     

A statistical study of poly(ε‐caprolactone) crystallinity in poly(ε‐caprolactone)–silica sol–gel materials and their in vitro calcium phosphate‐forming ability

Composite poly(ε‐caprolactone) (PCL)–silica materials for potential use in orthopaedic applications have been prepared by a sol–gel method using an experimental design approach to investigate the effect of synthesis variables, separately and together, on the physical form of the organic polymer. A combination of differential scanning calorimetry, X‐ray diffraction and Fourier‐transform infrared methods were used to obtain information on the arrangement of the organic polymer in the hybrid material. As our studies investigated the effect of synthesis variables simultaneously, it was possible to establish that the increase of tetraethyl orthosilicate (TEOS)/PCL and HCl/TEOS molar ratios decreased the poly(ε‐caprolactone) crystallinity and provided for a better mixing of the two phases. At a mechanistic level it was possible to show that increase in catalyst content affected the condensation of silicon containing species. In vitro calcium phosphate‐forming ability tests using the static biomimetic method have been carried out on selected PCL–silica sol–gels. In vitro bioactivity was only observed for PCL–silica sol–gel composites with high silica content (30% weight). Changes in catalyst levels had a smaller but still significant effect. Calcium phosphate formation on largely non‐porous surfaces is proposed to occur via the formation of a silica sol–gel layer, and is influenced by the topography and the chemistry of the materials surface. Copyright © 2003 Society of Chemical Industry     

A numerical investigation of the flame stability in porous burners employing various ceramic sponge-like structures

In order to optimize the porous burner for the application as a pilot burner of stationary gas turbines aiming to reduce NO_x emissions a fundamental study investigating the influence of the thermo-physical properties of the porous structure on the flame stabilization in a porous burner was conducted. This work presents a numerical study of the stability of one-dimensional laminar premixed flame in porous inert ceramic sponge structure. A set of steady computations are considered, using a numerical model that takes into account solid and gas energy equations as well as detailed chemistry. The model considers additionally the enhancement of both, thermal and species diffusivity by the flow dispersion, whereas the dispersion coefficients of the investigated structures have been determined from three-dimensional flow simulations using MRI (magnet resonance imaging) and CT (computer tomography) data to regenerate the real sponge structures. Hence, it was possible to calculate a thickened flame front as it was detected in experiments, too. The computations were conducted for different operational conditions and different burner configurations in respect to geometrical and material properties of the porous inert media. The numerical predictions showed very good agreement with the corresponding experimental stability data. The obtained numerical results were used for the formulation of a simple stability model based on the Pe number that enables a prediction of the lean blow-off limits in the combustion systems employing porous burner concept.     

Study on force relaxation of ceramic paste

Abstract

A theory is given for the relationship between the applied force and time for ceramic pastes during relaxation. The theory provides a simple expression based on the fundamental parameters of the system. Experimental work has been carried by means of squeeze film flow tests. In this investigation, it is found that a three element model can be used to describe the relaxation response of the ceramic paste used in the present work. This model indicates that ceramic pastes are viscoelastic fluids with highly viscous properties. The particle packing in the material may affect the relaxation response. The addition of fibre can enhance the elastic property of ceramic paste. The experimental data have demonstrated that using fibre to replace powder can make the pastes more solid-like. The surface condition can influence the relaxation parameters, which is evident when the material is highly elastic.     

Fracture Toughness of Advanced Structural Ceramics: Applying ASTM C1421

The three methods of determining the quasi‐static Mode I fracture toughness (K_Ic) (surface crack in flexure—SC, single‐edge precracked beam—PB, and chevron‐notched beam—VB) found in ASTM C1421 were applied to a variety of advanced ceramic materials. All three methods produced valid and comparable K_Ic values for the Al₂O₃, SiC, Si₃N₄, and SiAlON ceramics examined. However, not all methods could successfully be applied to B₄C, ZrO₂, and WC ceramics due to a variety of material factors. The coarse‐grained microstructure of one B₄C hindered the ability to observe and measure the precracks generated in the SC and PB methods while the transformation toughening in the ZrO₂ prevented the formation of the SC and PB precracks and thus made it impossible to use either method on this ceramic. The high strength and elastic modulus of the WC made it impossible to achieve stable crack growth using the VB method because the specimen stored a tremendous amount of energy prior to fracture. Even though these methods have passed the rigors of the standardization process there are still some issues to be resolved when the methods are applied to certain classes of ceramics. It is recommended that, when appropriate, at least two of these methods be employed to determine the K_Ic, especially when a new or unfamiliar ceramic is being evaluated.