Ceramics Grains Research Articles

Grain orientation and shape evolution of ferroelectric ceramic thick films simulated by phase-field method

The orientation and shape of ceramics grains was always neglected, resulting in a lot of information during sintering has not been excavated. In this study, a modified phase-field model in order to express the anisotropy of grain boundary energy is developed. The effects of the anisotropy of grain boundary energy on the grain orientation and shape evolution are investigated in detail. The ferroelectric ceramic thick films are prepared by tape casting. The comparison of experiment and simulation results shows that the anisotropy of grain boundary energy results in uneven grain orientation and bimodal grain size distribution. The quantitative analysis of grain microstructures helps to establish a relationship with the degree of anisotropy of grain boundary energy. Our findings provide a new way to judge the degree of anisotropy by calculating the relevant parameters in the SEM images of ceramics materials.

Features of ablation and modification of Al<sub>2</sub>O<sub>3</sub>‒TiC ceramic surface structure during two-coordinate pulsed laser treatment

The main regularities of influence of two-coordinate pulsed laser treatment parameters on processes of ablation and structural modification of Al2O3‒TiC ceramics were established. The ambiguous influence of laser treatment parameters mode on the intensity of the ablation process was revealed. It was shown that structural modification nature is determined by parallel processes of fragmentation, melting and fusion of original ceramics grains and defective surface layer with the formation of new elements was established. The influence of laser treatment parameters mode on nature of structural modification is manifested through the size and number of newly formed elements of surface structure.

Effect of substitution of (NiNb)4+ into B-sites on the phase formation, microstructure and electrical properties of Bi0.47Na0.47Ba0.06TiO3 ceramics

Bi0.47Na0.47Ba0.06Ti1-x(Ni1/3Nb2/3)xO3 ceramics (abbreviated as BNBT1-x(NN)x, x = 0, 0.01, 0.03 and 0.05) were synthesized by solid-state combustion. The effect of x on the phase formation, microstructure and electrical properties of BNBT1-x(NN)x ceramics was examined. The XRD pattern indicated the coexistence of rhombohedral and tetragonal phases in all the specimens. Moreover, Rietveld refinement confirmed that the tetragonal phase increased from 47 to 71% when x increased from 0 to 0.05. The morphology of BNBT1-x(NN)x ceramics was observed by SEM and the ceramics grains showed polygonal shapes and the grain growth tended to be anisotropic. With (NiNb)4+ substitution, the average grain sized decreased rapidly from 1.7 to 1.0 µm and the grain size distribution was narrower as the amount of (NiNb)4+ increased. The density, remnant polarization (Pr) and coercive field (Ec) rapidly decreased with increasing x. A significant decrease in the ferroelectric properties was caused by the increasing tetragonal phase.

Semiconductivity control of barium titanate ceramics produced by laser sintering

In this present work, we investigated the influence of the parameters used in laser sintering on the microstructures and electrical properties of the La-doped barium titanate. The structure and property of the ceramics were investigated by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Raman Spectroscopy (RS), Condutivite Atomic Force Microscopy (C-AFM) and Impedance Spectroscopy (IS). All samples have a Positive Temperature Coefficient Resistors (PTCR), as well as the IS data revealed a PTCR effect on the micro-region of BT:La ceramics grains. Electrical properties showed a strong dependence on heating and cooling rates, wich room-temperature resistivity (around 104 to 3.105 Ω.cm).

Cubic and Tetragonal Modifications in BaTiO3 Ceramic Samples: X-Ray Diffraction Analysis by the Rietveld Method

The structures of ground ceramic BaTiO3 samples, prepared by solid-phase synthesis at maximum sintering temperatures of 1350, 1400, 1410, and 1500°C, have been analyzed by the Rietveld method. It is shown that, along with the tetragonal phase, the samples contain the cubic BaTiO3 phase with an effective content of about 30 vol %. The cubic phase was observed as a thin layer on the surface of ceramics grains with tetragonal structure. The efficiency of preliminary analysis of specific ranges of X-ray diffraction patterns containing strong peaks is demonstrated.

Influence of CaO and SiO2 additives on the sintering behavior of Cr,Ca:YAG ceramics prepared by solid-state reaction sintering

Yttrium aluminum garnet optical ceramics (Y3−xCaxAl4.95Cr0.05O12) doped with 0.1 at.% of chromium ions and 0.5, 0.8 and 1.2 at.% of calcium ions were prepared by reactive sintering in vacuum at 1750°C. The influence of SiO2 and CaO sintering aids on optical properties and microstructure of Cr,Ca:YAG ceramics was investigated. A decrease in CaO concentration led to a reduction of the Cr,Ca:YAG ceramics transparency from 81 to 0% at 1064 nm, moreover, samples with a lower concentration of CaO had abnormal grains and high-level porosity. The obtained features were attributed to the interactions between silica and calcium oxide during vacuum sintering of Cr,Ca:YAG ceramics. We propose and discuss two possible ways in which CaO and SiO2 influence the formation of Ca,Cr:YAG ceramics. The first one involves the appearance of a liquid phase thanks to the CaO–SiO2 interaction during the heating stage of ceramics sintering which results in abnormal growth of ceramics grains. The second mechanism consists on the mutual consumption of Si4+ and Ca2+ ions during the isothermal stage of ceramic sintering, which leads to a decrease in Cr4+ concentration. Both mechanisms have a negative effect on the optical and laser properties of the ceramics.

Fracture mechanisms of polycrystalline advanced ceramics

Polycrystalline advanced ceramics are synthetic products produced by sintering together selected ceramics grains in a metal matrix serving as a binder. In order to be able to propose their optimisation, achieving high performance cutting and leading to reduced operating costs and improved working environment, relevant fracture mechanisms involved in their failure need to be determined. In this work, experimental results of plane strain fracture toughness obtained earlier on single-edge-V-notched-beam specimens were supported with microscopy analysis. These findings establish a clear connection between the fracture toughness results and the fracture mechanisms visible on and beneath the fracture surfaces, revealing adiabatic conditions that occur at the crack tip during fracture.

Fracture mechanisms of polycrystalline advanced ceramics

Polycrystalline advanced ceramics are synthetic products produced by sintering together selected ceramics grains in a metal matrix serving as a binder. In order to be able to propose their optimisation, achieving high performance cutting and leading to reduced operating costs and improved working environment, relevant fracture mechanisms involved in their failure need to be determined. In this work, experimental results of plane strain fracture toughness obtained earlier on single-edge-V-notched-beam specimens were supported with microscopy analysis. These findings establish a clear connection between the fracture toughness results and the fracture mechanisms visible on and beneath the fracture surfaces, revealing adiabatic conditions that occur at the crack tip during fracture.

Electronic ceramics fractal microstructure analysis - Minkowski Hull and grain boundaries

Consolidation parameters have influence and can be used to control structure of BaTiO3 based materials. Microstructure and dielectric properties of Yb2O3 doped BaTiO3-ceramics, sintered from 1320 °C to 1380 °C have been investigated. The correlation between microstructure, capacity and dielectric properties of doped BaTiO3-ceramics, based on fractal geometry and micro-contact surfaces, has been developed. Using the fractal descriptors of the grains contact surface, the microstructure reconstruction constituents, as grains and pores shapes or intergranular contacts, has been successfully done. Obtained results indicated that fractal analysis contact surfaces descriptors of different shapes are very important for the prognosis of BaTiO3-ceramics microstructure and capacity and dielectric properties. The morphology of ceramics grains pointed out the validity of developing new structure analytical methods, based on different grains’ shape geometries. The grains contact structure based on Minkowski hull is presented as a new tool for BaTiO3-ceramics materials structure research. The materials properties prognosis are determined according to the correlations synthesis–structure–property, within Minkowski hull fractal frame.

Structure, morphology and optical-luminescence investigations of spinel ZnGa2O4 ceramics co-doped with Mn2+ and Eu3+ ions

The polycrystalline zinc gallate ZnGa2O4: Mn2+ and ZnGa2O4: Mn2+, Eu3+ samples have been synthesized via high-temperature solid-state reaction ceramic technique. The obtained ceramics have been characterized employing the methods of X-ray diffraction analysis, transmission electron microscopy, energy-dispersive X-ray spectroscopy, positron annihilation lifetime spectroscopy and optical-luminescent spectroscopy. The XRD analysis testified in favor of successful formation of spinel structure in the prepared samples with small amount of additional phase observed in the ZnGa2O4: Mn2+, Eu3+ ceramics. The grains of irregular shape with a homogeneous distribution of Eu3+ ions in a volume were identified with TEM technique. The band gap of ZnGa2O4: Mn2+ spinel was estimated from optical absorption spectra in UV–Vis range. The characteristic bands related to electronic transitions of Mn2+ and Eu3+ ions were found in optical absorption and excitation spectra. The photoluminescence emission spectra exhibited matrix luminescence along with emission band of Mn2+ ions and narrow lines of Eu3+ ions in blue, green and red spectral region, respectively. The intensity ratio of Eu3+ emission lines confirms the high asymmetry around Eu3+ ions. These findings correlate well with results of positron annihilation lifetime spectroscopy showing intense reduction of positron trapping rate deeply in ceramics grains due to Eu3+ ions penetration.

Modeling of the Ceramics Structure for the Finite Element Analysis

Ceramic materials are extensively used for impact protection systems. The finite element method is generally used for strength calculation of such materials. As ceramics is a heterogeneous material, its characteristics depend significantly both on characteristics of its components, and on the material structure as a whole. Therein, micro-modeling of ceramics structures is the object of a large number of studies. The publications review shows that modeling of pores in ceramic materials is realized as removal of groups of finite elements. Pores are commonly modeled as spheres. In addition, ceramics grains are modeled as spherical elements. It is clear that such approaches do not enable obtaining a description of the real structure of ceramics. In this regard, author tries to describe the ceramics structure as a set of one or several phases and a variety of pores in a homogeneous material of the remaining part of the ceramic plate. For the purpose of universality ceramics grains and pores are modeled as six-sided subareas. Sides of these subareas may have curvilinear forms, so intersections and modifications of curved surfaces provide a way to get subareas with fewer sides than initial subareas. For the initial version, an equation of sphere is chosen to describe these surfaces. Scaling of subareas and their location within the ceramic plate is executed using random variables. The obtained mathematical model of a ceramic plate is used for finite element meshing.Preliminary calculation with ANSYS/LS-DYNA has shown that quality of 8-node finite element mesh generated by the considered method is sufficient to such FEA programs.

Bioactive composites fabricated by freezing-thawing method for bone regeneration applications

Hydrogels are widely used for controlled delivery of therapeutic agents. However, hydrogels lack bioactivity to encourage bone formation and mechanical integrity. Moreover, chemically crosslinked hydrogels exhibit cytotoxic effect. To overcome these limitations poly-vinyl alcohol (PVA) and poly-acrylic acid (PAA) blends were combined with ceramic materials based on β tricalcium phosphate, wollastonite, and magnesium silicate with different pore size distributions. The final 3D matrix was physically crosslinked using various freeze thawing (F/T) cycles. FTIR and SEM analysis showed that ceramics were dispersed within the polymer matrix and formed hydrogen bonds. Swelling studies in buffer solution pH 7.4 showed an increase in polymer swelling when ceramic was added. Furthermore, rheological testing demonstrated that incorporation of ceramics caused an increase in mechanical properties which varies with different pore size distributions of ceramics grains added. DSC thermograms showed increased Tg values for samples containing ceramics. Antimicrobial activity containing ciprofloxacin was tested against a pathogen associated with osteomyelitis and presented positive results with ciprofloxacin. The combination of increased strength and ability to encapsulate a clinically relevant antimicrobial agent indicates that the composite tested in this study has potential for the treatment of osteomyelitis. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 761–773

Microhardness of Ceramics Obtained by Different Methods from Nanopowders of Different Oxides

The method of spark plasma sintering (SPS) is used for the creation of fine-grained ceramics on the basis of various nanodimensional powders of alumina and other oxides. A comparison of the microhardness of ceramic samples obtained from 11 alumina nanopowders and 2 of their composites was held. The microhardness of ceramics obtained both by SPS, and by the traditional method (at successive pressing and sintering) is compared. The dependence of ceramics microhardness on the phase composition of the initial nanopowder and the average size of its particles were investigated. Besides alumina (Al2O3) nanopowders, a comparison was made of the microhardness of ceramics from nanopowders of 13 other oxides (SiO2, ZnO, Fe3O4, CuO, Cr2O3, Gd2O3, CoO, WO3, MgO, Nb2O5, TiO2, Y2O3, ZrO2) obtained both by SPS, and by the traditional method. It is found that the microhardness of the ceramics created by the method of spark plasma sintering, is significantly higher than the microhardness of the ceramics obtained by the traditional method; at the SPS method the average size of ceramics grains decreases (to 1 micron and less).

Fractal corrections of BaTiO3-ceramic sintering parameters

Morphology of ceramics grains and pores as well as Brownian character of particle dynamics inside ceramics specimen contributes to better understanding of the sintering process. BaTiO3-ceramics, studied in this paper, has light fractal form and it is emanated in three aspects. First, the surface of grains, even in starting green body as well as distribution of grains shows fractal behavior. Second, existence of pores and their distribution follow the rules of fractal geometry. Third, movement of particles inside viscous flow underlies the rule of Brownian motion, which is essentially a fractal category. These three elements, each in its domain influence sintering dynamics, and can be described by dimensionless quantitative factors, ?s ?p and ?m, being normalized to the interval [0,1]. Following sintering process, the associate formulae of Frenkel, Scherer and Mackenzie-Shuttleworth are shown from the angle of view of ceramics fractal dimension changing that approaches to 3. Also, it is shown that the energy balance is not violated after applying fractal correction to quasi equilibrium of the energy emanating from surface area reduction ES and energy adopted by viscous flow Ef .

The contribution of fractal nature to BaTiO 3-ceramics microstructure analysis

The structure of BaTiO 3 based materials can be controlled by using different technological parameters and different additives. In this paper, microstructure and dielectrical properties of Er 2O 3 doped BaTiO 3-ceramics, sintered from 1320 °C to 1380 °C have been investigated. Microstructural investigations were carried out using scanning electron microscope equipped with energy dispersive spectrometer. Grain size distribution was determined by quantitative metallography method. The new correlation between microstructure and dielectric properties of doped BaTiO 3-ceramics, based on fractal geometry and contact surface probability, has been developed. Using the fractals and statistics of the grains contact surface, a reconstruction of microstructure configurations, as grains shapes or intergranular contacts, has been successfully done. Obtained results indicated that fractal analysis and statistics model for contact surfaces of different shapes were very important for the prognosis of BaTiO 3-ceramics microstructure and dielectric properties. The morphology of ceramics grains pointed out the validity of developing new structure analytical methods, based on different grains’ shape geometries. The grains contact model based on ellipsoidal geometry is presented as new modeling tool for structure research of BaTiO 3-ceramics materials. The directions of possible materials properties prognosis are determined according to the correlations synthesis–structure–property.The statistical approach to the investigation of BaTiO 3-ceramic grains concerning the relationship between the temperature and the area of the contact surface is also introduced.

Properties of a thermal lens in laser ceramics

A model of thermal optical effects in laser ceramics was constructed, which takes into account random orientations of crystallographic axes in ceramics grains. Analytic expressions for the thermally induced phase, its average value and dispersion were derived. The effect of the beam-phase modulation with the characteristic transverse size of the order of the grain size was predicted. It was shown that deterioration of the parameters of the beam quality caused by this effect is inversely proportional to the ratio of the length of the ceramic element to the grain size.

Dielectric permittivity and Cr 3+ impurity ion probe luminescence in SrTiO 3 sol–gel ceramics

In order to inspect bulk properties of the SrTiO 3 ceramics grains, dielectric permittivity and temperature shift of a zero-phonon R-emission line of the Cr 3+ impurity-ion probes were studied in sol–gel SrTiO 3:Cr (3 × 10 −2%) ceramics of the average grain size ∼24 μm. We found that non-dispersive permittivity (100 Hz to 1 MHz) increases on cooling less steeply than in single crystals reaching ∼5000 at 30 K. At lower temperatures the dispersion appears and the permittivity saturates at ∼8 K ( ɛ′ ≈ 7000, 400 Hz). However, the temperature shift of the R-line in ceramics remains practically identical to that for SrTiO 3 single crystals, in which R-line shifts with temperature is Δ ν ∼ 1/ ɛ′ ∼ ω TO 1 2 . This indicates that the bulk of the grains in ceramics exhibits properties inherent to SrTiO 3 quantum paraelectric, which agrees with the “generalized brick-wall” approach developed recently to describe polarization response of granular high permittivity materials.

A Method for the Digital Image Analysis of Ceramic Grains Based on Shape Factor Segmentation

A very simple and robust method for ceramics grains quantitative image analysis is presented. Based on the use of optimal imaging conditions for reflective light microscopy of bulk samples, a digital image processing routine was developed for shading correction, noise suppressing and contours enhancement. Image analysis was done for grains selected according to their concavities, evaluated by perimeter ratio shape factor, to avoid consider the effects of breakouts and ghost boundaries due to ceramographic preparation limitations. As an example, the method was applied for two ceramics, to compare grain size and morphology distributions. In this case, most of artefacts introduced by ceramographic preparation could be discarded due to the use of perimeter ratio exclusion range.

Evaluation of Fracture Toughness of Porous Ceramics

The indentation fracture (IF) method, the single-edge precracked beam (SEPB) method, and the single-edge V-notched beam (SEVNB) method were applied to evaluate the fracture toughness of four kinds of porous ceramics of SiC, Al2O3 and Mg2Al4Si5O18 with porosity ranging from 37 to 43%. The microstructures of these materials were composed of ceramics grains, glassy grain boundaries and pores. Each grain was joined together with the glassy grain boundary phase. The IF and SEPB methods were not applicable because both precracks and indenter traces were not visible. On the other hand, the SEVNB method was applicable because the V-notch could be easily machined by grinding. In the case of the SEVNB method, the applied load versus back-face strain plots under four-point bending showed nonlinearity prior to the maximum load. The R-curve behavior was estimated from the compliance change of specimens. The fracture toughness of porous ceramics was smaller than that of dense ceramics, and increased with increasing crack extension. Since the stable crack predominantly propagated along glassy grain boundaries, the R-curve behavior depended on the loading rate and matrix grain size. The increment of the R-curve by grain bridging became larger for coarser-grain sized ceramics.

Evaluation of Fracture Toughness of Porous Ceramics.

The indentation fracture (IF) method, the single-edge precracked beam (SEPB) method, and the single-edge V-notched beam (SEVNB) method were applied to evaluate the fracture toughness of four kind of porous ceramics of SiC, Al2O3 and Mg2Al4Si5O18 with porosity ranging from 37 to 43%. The microstructures of these materials were composed of ceramics grains, glassy grain boundaries and pores. Each grain was joined together with the glassy grain boundary phase. The IF and SEPB methods were not applicable because both precracks and indenter traces were not visible. On the other hand, the SEVNB method was applicable because the V-notch could be easily machined by grinding. In the case of the SEVNB method, the applied load versus back-face strain plots under four point bending showed nonlinearity prior to the maximum load. The R-curve behavior was estimated from the compliance change of specimens. The fracture toughness of porous ceramics was smaller than that of dense ceramics, and increased with increasing crack extension. Since the stable crack predominantly propagated along glassy grain boundaries, the R-curve behavior depended on the loading rate and matrix grain size. The increment of R-curve by grain briding got bigger for coarser-grain sized ceramics.