Assessment of humidity self-regulation functionality for ceramic tiles

The development of ceramic tiles with humidity regulating capacity is a topic of great interest for the comfort of interior spaces. The humidity regulating capacity of fired specimens from five compositions used for porous ceramic products has been assessed and their microstructure has been characterised. The main novelty of the work consists of obtaining pieces with regulating functionality from common ceramic raw materials used in industrial practice, establishing a kinetic model and confirming the role of porosity and microstructure.The results have shown that moisture adsorption and desorption curves fit very well to a pseudo-second order model. Additionally, it has been found that the humidity regulating capacity is not related to the total porosity, but it depends on the presence of mesopores in the structure of the pieces. Concerning the tested compositions, those containing gibbsite present higher regulating capacity as a consequence of their greater amount of mesopores.     

陶瓷砖吸水率测试方法的探讨

就陶瓷砖国际标准测试方法ISO10545—3:1995与中国标准试验方法GB/T 3810.3—2006中采用真空法测试陶瓷砖吸水率存在的差异进行探讨,通过比对试验结果并将试验数据进行分析,提出了对现行中国陶瓷砖标准吸水率测试方法修改的建议。     

Cathode-Ray Tube panel glass replaces frit in transparent glazes for ceramic tiles

The disposal of Cathode Ray Tubes (CRT) from end-of-life personal computers and TV screens represents a serious problem in electronic-waste management. As an assembly of different materials, finding a use for each of a monitor's parts is a critical step forward a solution. However, the CRT panel is a silicate glass with a relatively high proportion of alkaline and alkaline-earth oxides, for which recycling is a natural task, and the replacement of frit in ceramic glazes arises as an interesting alternative. In this context, we investigated the effect of CRT panel glass in glazes for ceramic tiles based on a comparative analysis. We replaced up to 40?wt% of commercial transparent frit with CRT panel glass in the formulation of one reference slurry. Chemical analyses were conducted by X-ray fluorescence (XRF) and inductively coupled plasma optical emission (ICP-OES) spectrometry. The thermal expansion coefficient and the glass transition and dilatometric softening temperatures were characterized by dilatometry and compared to such properties calculated as a function of composition, using the SciGlass software and database. 20- and 30-min firing cycles were applied in a fast-firing roller kiln, replicating industrial conditions. The samples transparency was measured by spectrophotometry and compared to the colorimetric parameters of a standard glaze. The maximum content of panel glass possible to add in the transparent glaze formulation without affecting the expected properties was 20?wt%, above which transparency decreased due to heterogeneities. The reformulation of a ceramic glaze with waste CRT panel glass was successful, thus suggesting an interesting approach for disposal of other electronic wastes.     

Photocatalytic Nb2O5-doped TiO2 nanoparticles for glazed ceramic tiles

TiO₂ nanoparticles are currently used as coating for self-cleaning building products. In order to achieve high self-cleaning efficiency for outdoor applications, it is important that titania is present as anatase phase. Moreover, it is desirable that the particle sizes are in nano-range, so that a large enough surface area is available for enhanced catalytic performance. In this work, TiO₂ nanoparticles doped with 0–5 mol% Nb₂O₅ were synthesized by co-precipitation. Nb₂O₅ postponed the anatase to rutile transformation of TiO₂ by about 200 °C, such that after calcination at 700 °C, no rutile was detected for 5 mol% Nb₂O₅-doped TiO₂, while undoped TiO₂ presented 90 wt% of the rutile phase. A systematic decreasing on crystallite size and increasing on specific surface area of TiO₂ were observed with higher concentration of Nb₂O₅ dopant. Photocatalytic activity of anatase polymorph was measured by the decomposition rate of methylene blue under ultraviolet and daylight illumination and compared to commercial standard catalyst (P25). The results showed enhanced catalysis under UV and visible light for Nb₂O₅-doped TiO₂ as compared to pure TiO₂. In addition, 5 mol% Nb₂O₅-doped TiO₂ presented higher photocatalytic activity than P25 under visible light. The enhanced performance was attributed to surface chemistry change associated with a slight shift in the band gap.     

PVD hard coatings on ceramic tiles for aesthetic applications: surface characterisation and corrosion properties

The development of innovative ceramic tiles looks for materials with improved mechanical and tribological properties as well as a higher corrosion resistance (high relative humidity, daily watering, household chemical cleaners). In addition, a greater durability leads to lower environmental impact. Along with their improved functionality and recyclability, ceramic tiles should also provide aesthetic properties. Ceramic tiles can be treated to modify the physico-chemical properties of the surface by metal coatings or metallic compounds, also providing an attractive metallic sheen appearance. In the present paper, titanium nitride (TiN) and zirconium nitride (ZrN) coatings were deposited on glazed porcelain stoneware by an industrial PVD multicathode arc deposition system under a reactive nitrogen atmosphere. After the process, the tiles showed a gold-like colour, a smooth surface and a coating thickness between 0.7 and 1.6 μm. The coating composition, scratch resistance and corrosion behaviour have been evaluated. It can be concluded that both coatings are suitable for use in domestic environments due to their stability and resistance to aggressive conditions. Few references have been found regarding these coatings on ceramic tiles for domestic and industrial applications, but it has been proved that they bring added value to traditional ceramics, giving new functional properties of ceramics both decorative and highly corrosion and mechanical resistance.     

A new empirical viscosity model for ceramic suspensions

This paper presents a new predictive viscosity model for ceramic suspensions. Starting from Einstein's model (1906), various theoretical, empirical, and phenomenological models have been proposed for different suspension systems. However, there is still a lack of reliable model for ceramic suspensions used in colloidal ceramic shape-forming methods. Here, the rheological properties of ceramic suspensions comprising NiO/YSZ (nickel oxide/yttria stabilized zirconia) as the ceramic powder, and furfuryl alcohol as the suspending media were measured over a range of shear-rates (between 1 and 1000 s⁻¹) and different solid volume fractions from 0 to 0.4010. An empirical equation was then developed for the ceramic suspensions using the mobility parameter (?/(?_m−?)), which links Einstein's model with the more recent relative viscosity models. The proposed model was used to predict the relative viscosity data, showing excellent agreement to the experimental data from this study and with reported data in literature for other ceramic systems. The model was also used to estimate the maximum solid volume fraction for the ceramic suspensions (?_m=0.571), with better accuracy than those estimated by existing models.     

Elastoplastic coupling to model cold ceramic powder compaction

The simulation of industrial processes involving cold compaction of powders allows for the optimization of the production of both traditional and advanced ceramics. The capabilities of a constitutive model previously proposed by the authors are explored to simulate simple forming processes, both in the small and in the large strain formulation. The model is based on the concept of elastoplastic coupling – providing a relation between density changes and variation of elastic properties – and has been tailored to describe the transition between a granular ceramic powder and a dense green body. Finite element simulations have been compared with experiments on an alumina ready-to-press powder and an aluminum silicate spray-dried granulate. The simulations show that it is possible to take into account friction at the die wall and to predict the state of residual stress, density distribution and elastic properties in the green body at the end of the forming process.     

Extended analytical model for interfacial stresses of double-lap joints under harmonic loads

In this work, a modified analytical model with closed-form solution is proposed by following the existing framework in the prior works, to analyze the dynamic responses of interfacial shear and peel stresses in adhesively bonded double-lap joints subjected to harmonic axial load. By applying the dynamic equilibrium along the through-thickness direction, the differential equation governing adhesive peel stress is first contained in this model. The dynamic responses of interfacial stresses obtained by finite element simulations under four different loading conditions are used to validate this analytical model. The parametric study based on the proposed model is also implemented to assess the effect of some geometrical parameters on the dynamic response of adhesive interfacial stresses.     

A new eco-friendly mass formulation based on industrial mining residues for the manufacture of ceramic tiles

New eco-friendly mass formulations based on the scheelite and kaolin residues were developed to manufacture ceramic tiles. The start raw materials (scheelite residue, kaolin residue, feldspar and plastic clay) were characterized as to their chemical composition, main mineralogical phases, and particle size distribution. Three ceramic masses with 37 wt% of kaolin residues and different contents of the scheelite residues (2 wt%, 5 wt%, and 10 wt%) were formulated. The mass formulations were uniaxially pressed (19.6 MPa) to obtain samples with dimensions of 60 mm × 40 mm x 7 mm, which were dried at 110 °C/24 h, and sintered at different temperatures (1150 °C, 1200 °C, and 1250 °C). Dilatometric experiments measured thermal expansion coefficients. The results are in agreement with the literature, i.e., 6.0 μm/m°C^?1, 6.1 μm/m°C^?1 and 6.4 μm/m°C^?1 to samples with 2 wt%, 5 wt%, and 10 wt% of scheelite residues, respectively. The potential of the mass formulations studied was evaluated by linear shrinkage, water absorption, apparent density, apparent porosity, flexural strength, and mineralogical phase identification. The results were compared with the literature experimental data and International Technical Standards. It was concluded that the samples investigated have suitable properties for use as ceramic and porcelain tiles. Also, the pseudowollastonite and mullite phases were identified in the sample with the lowest concentration of scheelite residue. These phases are responsible for increasing flexural strength.     

Effects of roughness parameters on slip resistance for different methods used to determine the coefficient of friction for ceramic floor tiles

This work attempts to evaluate the correlation between the roughness parameters and different standardized methodologies for determining the coefficient of friction of ceramic floor tiles. Eight different types of ceramic tiles were evaluated, and each one was characterized for their coefficient of friction/slip resistance in accordance with the standards NBR 13818, ANSI A137.1, and AS 4586. The surfaces were also characterized in relation to surface roughness by means of contact profilometry. The measured friction and roughness parameters were correlated by means of analysis of variance. The results showed a correlation tendency according to a third-degree equation for tests performed in wet conditions. The study results showed that the roughness parameters influence the coefficient of friction with a confidence level of 95%. Considering the safe values indicated for the respective standards for the evaluated methods, a Rz value of 25 μm ensures that a tested surface can be considered safe in a wet-condition test, regardless of the method used to determine the coefficient of friction.     

FE coupled to SPH numerical model for the simulation of high-velocity impact on ceramic based ballistic shields

Predictive models are an important tool in the design and optimization of ballistic shields. Indeed, several authors in the literature have developed numerical models for simulating high-velocity impact on ceramic-based ballistic shields which are based on the finite element method. Element erosion is usually implemented in finite element models simulating impact to remove excessively distorted elements but, it leads to energy loss, which in turns may lead to the production of incorrect results. Due to the absence of a fixed mesh, the smoothed particle hydrodynamics method is well suited for large deformation problems, overcoming the limitations of the finite element method. On the other hand, the smoothed particle hydrodynamics method is computationally more expensive than the finite element method. Thus, a numerical model combining the lower computational cost of finite elements and the capability of smoothed particle hydrodynamics of dealing with crack formation and fracturing would be an interesting solution for the simulation of high-velocity impact on ceramics. The aim of this work is therefore to develop a finite element coupled to smoothed particle hydrodynamics numerical model for the simulation of high-velocity impact on ceramic-based ballistic shields. High-velocity impact tests were performed on Al₂O₃ tiles and the experimental results were used for the calibration of the numerical models; furthermore, high-velocity impact test were performed on multilayer targets with Al₂O₃ front layer and AA6061-T6 backing layer for the validation of the numerical models. This study proved that this approach is more appropriate for the simulation of the response of ceramic materials rather the common finite element model.     

A progressive damage model for ceramic matrix mini-composites with thick interphases

Toughness enhancement in ceramic matrix composites (CMCs) with brittle matrix and fiber phases is often accomplished by introducing a weak finite-thickness interphase between the fiber and matrix. The current work presents a progressive damage model to predict the tensile response of single tow CMCs (mini-composite) representative of a unidirectional composite at the microscale. Implementation of a 3-phase shear-lag model for a geometrically accurate representation of the underlying microstructure in CMCs with finite thickness interphase has been highlighted. A probabilistic progressive modeling approach has been adopted, accounting for multiple matrix cracking, interfacial debonding, and fiber failure in 3-phase mini-composites. The predicted tensile response of CMCs from the progressive damage modeling approach agrees with experimental results obtained for C/BN/SiC mini-composites validating the approach.     

An analytical solution of different configurations of the linear viscoelastic normal and frictional-elastic tangential contact model

In the current study an analytical solution describing the impact of a spherical particle on a rigid wall is derived. The contact is linear viscoelastic in normal and frictional-elastic in tangential direction. Due to its simplicity, the model combination considered is one of the most common in the framework of the Discrete Element Method especially for large-scale simulations. The linear viscoelastic normal model is realized according to Zhang and Whiten [1996. The calculation of contact forces between particles using spring and damping models. Powder Technology 88, 59-64.] assuming a contact to be ceased when the normal force attains a value of zero. In literature the frictional elastic tangential model is employed in three different configurations following Cundall and Strack [1979. A discrete numerical model for granular assemblies. Geotechnique 29, 47-65], Di Maio and Di Renzo [2004. Analytical solution for the problem of frictional-elastic collisions of spherical particles using the linear model. Chemical Engineering Science 59 (16), 3461-3475] and Brendel and Dippel [1998. Lasting contacts in molecular dynamics simulations. In: Herrmann, H.J., Hovi, J.-P., Luding, S. (Eds.), Physics of Dry Granular Media. Kluwer Academic Publishers, Dordrecht, 1998, p. 313]. The differences among these configurations are briefly explained, whereas the focus is set on the two most accurate model formulations given in the first two papers. All important final collision properties as positions and velocities are derived in an analytical form. Based on these results a comparison with experimental data of particle/wall and particle/particle collisions by Foerster et al. [1994. Measurements of the collision properties of small spheres. Physics of Fluids 6(3), 1108-1115], Lorenz et al. [1997. Measurements of impact properties of small, nearly spherical particles. Experimental Mechanics 37(3), 292-298] and Gorham and Kharaz [2000. The measurement of particle rebound characteristics. Powder Technology 112(3), 193-202] is performed, showing very good agreement especially for the model configuration of Di Maio and Di Renzo. The analytical solution as derived here helps understanding the complex collision process and can be applied for the evaluation of integration methods or in the context of an event-driven discrete element method.     

An overview of ceramic molds for investment casting of nickel superalloys

Accelerating advancements in technological systems have demonstrated a need for alloys with drastically improved thermomechanical and chemical properties, called superalloys. Ceramic molds are typically used in near-net shape investment casting processes of superalloy components due to their chemical inertness and high-temperature capabilities. Ceramic molds, however, often suffer from shortcomings in vital properties including flexural strength, thermal shock resistance, permeability, dimensional stability, corrosion resistance, and leachability, which have restricted their ability to adequately process modern alloy castings. This study analyses these limitations and illustrates how to address them, particularly regarding ceramic mold and slurry design, processing of shells and cores, material selection, and testing and characterization. By utilizing advanced processing methods including additive manufacturing and gel-casting, more dimensionally accurate and preferentially built molds can be formed. Additionally, by varying the mold composition to achieve more chemically inert structures, reactions with the mold can be mitigated to reduce chemically induced defects.     

Unified tensile model for unidirectional ceramic matrix composites with degraded fibers and interface

In order to overcome the roughness of the previously proposed micromechanical model [Acta Mech. Sin. (2011) 382], an enhanced multiscale analytical model was thus developed based on the rule of mixture, shear-lag theory and statistical approach to forecast the load carrying capacity of the prestressed ceramic matrix composites (CMCs) subjected to high-temperature oxidation. For comprehensive characterization of the mechanical degradation mechanisms, the oxidation induced fiber necking (or embrittlement) and fiber-matrix interface weakening were both taken into account. The suggested model was then applied to 2D-C/SiC composites. The influences of interface friction resistance, interface recession length, fiber necking factor and oxidation duration upon the residual mechanical property were investigated. Parametric analysis demonstrates that the modified formulations are much more reasonable than the previous model. The predicted residual tensile modulus and strength for the 2D-C/SiC composite agree well with the experimental data and furthermore the microscopic damage mechanisms were correlated properly with the macroscopic fracture morphologies.     

An analytical model for real gas flow in shale nanopores with non‐circular cross‐section

An analytical model for gas transport in shale media is proposed on the basis of the linear superposition of convective flow and Knudsen diffusion, which is free of tangential momentum accommodation coefficient. The present model takes into the effect of pore shape and real gas, and is successfully validated against experimental data and Lattice–Boltzmann simulation results. Gas flow in noncircular nanopores can be accounted by a dimensionless geometry correction factor. In continuum‐flow regime, pore shape has a relatively minor impact on gas transport capacity; the effect of pore shape on gas transport capacity enhances significantly with increasing rarefaction. Additionally, gas transport capacity is strongly dependent of average pore size and streamline tortuosity. We also show that the present model without using weighted factor can describe the variable contribution of convective flow and Knudsen diffusion to the total flow. As pressure and pore radius decrease, the number of molecule‐wall collisions gradually predominates over the number of intermolecule collisions, and thus Knudsen diffusion contributes more to the total flow. The parameters in the present model can be determined from independent laboratory experiments. We have the confidence that the present model can provide some theoretical support in numerical simulation of shale gas production. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2893–2901, 2016     

An approximate theory for stresses in dust cake of ceramic candle filters

A simplified theory is developed that yields the approximate stresses within a dust cake on the outer surface of ceramic candle filter and gives the critical factors which ensure complete detachment of cake. The radial and tangential stresses in cake during filtration have been derived by consideration of active state of stress in the dust cake. Back pulses of cleaning gas expand dust cake on the condition, that their power is just adequate to properties of cake, such as angle of internal friction and cohesion. Expansion of the dust cake induces tension cracks and disintegration of the cake. Back pulses of higher values hamper expansion of cake and cause “patchy” cleaning.     

Generalized analytical solution for compressive forces in adhesively-bonded-joint assembling

Normal forces exerted by the adhesive to the substrate during the squeeze flow occurring in compaction of bonded joints are analyzed using theoretical, numerical and experimental techniques. An analytical solution, derived from the squeeze-flow theory of a viscoplastic material, is generalized to be valid for any initial shape of the adhesive cord. The rheology of the material is modeled according to the Herschel–Bulkley model and is fitted with experimental data available from the characterization of an epoxy-based adhesive. The analytical law is compared with a numerical model, where the two-phase problem for the squeeze-flow test is solved by finite-volume methods using a commercial CFD solver. The results obtained with these two approaches show excellent agreement with experimental forces obtained for a wedge-shaped specimen. The proposed methodology can therefore be useful for the optimization of the bond lines in assembling processes.     

A micromechanical damage model for oxide/oxide ceramic matrix composites with hierarchical porosity under thermomechanical loading

This work presents a micromechanical damage model to describe the microstructural damage behaviors of ceramic matrix composites with hierarchical porosity during thermomechanical loading. The microstructure evolution may cause the nonlinear constitutive behavior, and a hierarchical porosity-based elasto-plastic constitutive model was developed. Damage mechanisms of matrix-crack, hierarchical pore nucleation and fiber-breaking are incorporated into the formulation of the damage model to describe various micromechanical damage modes of ceramic matrix composites accurately. Two damage variables are proposed for the damage evolution of matrix and fiber bundles. The main damage mechanisms in the matrix are matrix-cracking, and fibers breaking in the fiber bundles. The performance of the proposed damage model is verified by comparing with the existing experimental data. The proposed damage model outperforms the existing counterparts by capturing the microstructural damage mechanism and integrated into the damage model, and the contribution of different damage mechanisms can be quantified. The present work will provide a robust tool for describing the damage behaviors of matrix and fiber bundles in the ceramic matrix composites under thermomechanical loading, as well as allow a more accurate characterization of microstructural damage for a large extent of ceramic matrix composites.     

An analytical model for spherulitic growth in fiber-reinforced polymers

An analytical model for the isothermal crystallization of fiber reinforced polymers is presented. The model is based on approximate expressions for the volume of intersection between a sphere and cylinder. These expressions are used to account for the effect of the fibers on the overall crystallization process. Expressions for the average volume of spherulites truncated by the fibers are computed. The crystallization process is divided into time frames during which specific types of fiber truncations are encountered. Three different time sequences for the occurrence of the truncations are also derived according to the fiber volume fraction. The depressing effect of the fibers on the overall crystallization process is demonstrated with simple examples. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 70: 1677–1687, 1998