Defect-free Y2O3 transparent ceramics is an interesting material which is used for IR windows, nozzles or laser host, etc, due to its mechanical and optical properties, high thermal and chemical stability, and high melting point. To achieve full densification and the absence of pores, sintering aids are used. For yttria ceramics, the optimal choice is ZrO2. It has a significant influence not only on transmittance, but on the other characteri-stics of the material as well. For high transparency, ceramics has to remain single-phase, thus forming a solid solution of Y2O3 and ZrO2, but the solubility limit isn't studied in detail. Therefore, the presented work aims to investigate the solubility limit of ZrO2 in yttria ceramics and the dependence of properties on the concentration of sintering aid. Ceramics samples with a concentration of ZrO2 of 0-11 mol% were obtained by uniaxial and cold isostatic pressing (CIP) followed by vacuum sintering at 1735 °C for 22 h. The SEM and XRD analysis, refractive index and transmittance measurements were performed. No secondary phases were observed in analysed samples by SEM, which indicates the formation of a solid solution and full solubility of ZrO2 in the analysed range of concentration. Also, it was found that even 3 mol% of ZrO2 significantly improve the microstructure of samples. It allows for the elimination of residual porosity due to full densification during sintering. Also, the grain size decreased from 14 ?m for pure Y2O3 to 3.8-6.1 ?m for samples with 3-11 mol% of ZrO2. XRD analysis showed that the phase composition did not change between samples with 0 and 11 mol% of sintering aid. Decreasing of lattice parameters with increasing of ZrO2 amount was observed due to the replacement of Y3+ ions in the lattice by the smaller Zr4+. A steady increase of refractive index was observed with increasing Zr content. Obtained results allow us to conclude, that the solubility limit of ZrO2 in Y2O3 was not reached within the studied range and that the presence of ZrO2 improves ceramics properties. In the next step, optical transmittance was analysed. Among all obtained Y2O3:Zr4+ ceramics the highest transmittance (78.60% at 1100 nm at a thickness of 2.11 mm) was achieved for the sample with 7 mol.% of ZrO2.

Transparent ceramics stand as cutting-edge class of materials that benefit from the shaping possibilities of ceramic technology and from the crystalline structure that offers superior performance compared to glasses. Transparency may be obtained only when the material is free of defects that scatter light, viz. pores or secondary phases. Mostly, this means a requirement of a fully dense, single-phase, defect-free microstructure. However, the impact of small scatterers on transparency diminishes as their size decreases, allowing the development of multiphase materials, composites, that are transparent in the IR and even in the visible range for nanometric grain sizes. Conversely, another important topic in the field of transparent ceramics are macroscopic composites. The increasing optical quality of transparent ceramics in the past years has ignited a growing interest, especially in optics and photonics [1]. Transparent ceramics stand as counterparts to more traditionally used single crystals, which may have the same composition, but are obtained by different processes, mostly based on growth from melt. This process is time- and energy-consuming, and above all imposes significant limitations on the final shape of the components, which is obtained by machining. Transparent ceramics, in contrast, take advantage of the shaping flexibility of ceramic processing, in particular to produce composite or gradient structures without the need of post-processing and bonding [2]. Unlike the nanocomposites mentioned above, these composites are macroscopic, mostly with relatively small differences in chemical composition among the different parts. In the case of simple shapes and planar interfaces such structures may be obtained by diffusion bonding of polished single crystals, but the process is demanding and expensive. Ceramic processing allows us to shape such structures in the green state with a high degree of freedom, avoiding intermediate cutting, polishing and bonding steps. The aim of the presentation is to illustrate the possibilities and benefits of transparent ceramics with a particular emphasis on multimaterial components, spanning both nano- and macro-composites. The application potential will be illustrated on specific examples involving structures for lasers or protective domes and the shaping possibilities will be discussed. References [1] J. Hostasa, "Ceramics for laser technologies", pp. 110-124 in Encyclopedia of Materials: Technical Ceramics and Glasses - Vol. 3. Ed. M. Pomeroy, Elsevier, 2021. [2] F. Tian et al., J. Eur. Ceram. Soc., 42 (2022) 1833-1851.

Optical Y2O3 ceramics are actively researched as a multifunctional material and found wide practical application due to its mechanical and optical properties, chemical and thermal stability. This material should be possibly defect-free since the presence of pores deteriorates its optical and mechanical properties. Therefore, the choice of raw materials and the study of the effect of processing methods are fundamental. This work aims to study the influence of the initial powders and their milling conditions on the characteristics of Y2O3 transparent ceramics. The morphology and sintering behavior of four different commercial Y2O3 powders after milling under different conditions was investigated. 3 mol.% ZrO2 was used as a sintering aid. The samples were compacted by uniaxial pressing and CIP, and sintered in air at 1600°? for 4 h or in vacuum at 1735°C for 32 h. Firstly, the powders after milling at 80 rpm for 22 h were analysed. All studied powders were characterized by different values of specific surface area, particle size and agglomeration, which influenced densification. Among all samples obtained by sintering in air, only one provided a high density and uniform microstructure, and was thus used for further studies. Influence of milling conditions on powders parameters and properties of vacuum-sintered Y2O3 ceramics was determined. We found that 300 rpm for 65 min is optimal for obtaining powders with high sinterability. The specific surface area of this powder is 21.3 m2/g, and the average particle size is 480 nm. Decreasing of milling speed leads to an increase in the particles size. An increase in the milling time to 10 h is accompanied by an agglomeration of particles. Y2O3 vacuum-sintered ceramics were characterized by a relative density of 100% and transmittance of 78.1% (1100 nm). Acknowledgments: The authors are grateful to the JECS Trust for funding (Contract No. 2021293).

Five porcelain and porcelain stoneware bodies were investigated to compare sintering mechanisms and kinetics, phase and microstructure evolution, and high temperature stability. All batches were designed with the same raw materials and processing conditions, and characterized by optical dilatometry, XRF, XRPD-Rietveld, FEG-SEM and technological properties. Porcelain and porcelain stoneware behave distinctly during sintering, with the convolution of completely different phase evolution and melt composition/structure. The firing behavior of porcelain is essentially controlled by microstructural features. Changes in mullitization create conditions for a relatively fast densification rate at lower temperature (depolymerized melt, lower solid load) then to contrast deformations at high temperature (enhanced effective viscosity by increasing solid load, mullite aspect ratio, and melt polymerization). In porcelain stoneware, the sintering behavior is basically governed by physical and chemical properties of the melt, which depend on the stability of quartz and mullite at high temperature. A buffering effect ensures adequate effective viscosity to counteract deformation, either by preserving a sufficient skeleton or by increasing melt viscosity if quartz is melted. When a large amount of soda-lime glass is used, no buffering effect occurs with melting of feldspars, as both solid load and melt viscosity decrease. In this batch, the persistence of a feldspathic skeleton plays a key role to control pyroplasticity.

Transparent yttrium aluminum garnet (YAG) ceramics are mostly sintered under vacuum to favor pore closure. However, this may conceal the origin of microstructural defects, complicating process optimization. We describe a useful approach to understand the origin of defects in transparent YAG ceramics: reactive sintering was performed in air at a moderate temperature for a short time. The resulting microstructure allowed to understand the origin of defects in corresponding vacuum-sintered specimens. The porosity of air sintered samples could be related to the presence of aggregates of starting oxide particles, which eventually under vacuum react to form YAG, but leave behind pores

During the various stages of ceramic tile production, sintering kinetics, phase transformations and variation of the main properties of non-crystalline matrix are considered the major parameters to be kept under control. Particularly, during the sintering process a complex evolution of both phase composition and chemistry of the liquid phase occurs, according to the dynamic equilibrium established between the residual minerals and the new crystalline phases formed during firing. This contribution aimed at comparing the evolution of phase composition and of non-crystalline matrix properties during the vitrification path of four representative industrial ceramic formulations (soft porcelain, vitreous china; two different batches of porcelain stoneware, including a glass-bearing one). These batches were designed and prepared at the laboratory scale, simulating the industrial ceramic process. The sintering kinetics of each sample was determined under isothermal conditions through an industrial-like firing schedule by optical thermo-dilatometric analysis. Samples were investigated between the temperature at which the viscous flow sintering starts (around 1000°C) up to the onset of deformation (up to 1400°C for porcelain), upon increasing dwell time. The phase composition was assessed by the Rietveld refinement and the chemical composition of the vitreous phase was obtained by subtracting the contribution of each mineralogical phase, considering its stoichiometric ideal formula. The melt properties were estimated by predictive models based on the chemical composition of the liquid phase. An increasingly faster sintering kinetics was observed in the order: soft porcelain, vitreous china, porcelain stoneware, glass-bearing stoneware. Different vitrification paths were observed with a correlation between the dissolution kinetics of feldspar and quartz. Remarkable differences were observed in those samples where mullite occurred as primary or secondary mullite. Those differences clearly reflected a distinctive evolution of chemical features and glass network connectivity parameters of the non-crystalline matrix. The porcelain stoneware sintered by fast cycles thanks to a sort of buffering effect played by quartz and primary mullite melting rates. In contrast, vitreous china and soft porcelain needed higher temperature and/or prolonged time to activate both the growth of secondary mullite and the contemporaneous quartz dissolution, and the variation of properties of the noncrystalline matrix.

Ceramic Yb:YAG is a suitable gain medium candidate for high-energy lasers. The use of a combination of TEOS and MgO sintering aids led to uniform microstructure independently on the increase of thickness. Samples were thoroughly characterised.

TiB2 is a promising material in several fields including impact resistant armor, seals, cutting tools, crucibles and wear resistant coatings given its physical, mechanical and chemical properties, in particular thanks to the combination of high hardness and exceptional wear resistance. It is however very difficult to sinter below 2000°C, also under mechanical pressure, and is limited by its low fracture toughness. By using sintering additives, it is possible to improve the sintering process and increase the mechanical properties since the additives react with oxidized layers to form secondary phases. In this study, we explored different preparation methods, various combinations of additives (B4C, Si3N4 and MoSi2), and sintering techniques (hot pressing and pressureless sintering). Thanks to the synergy between optimized process and tailored composition, an almost fully dense material was obtained at 1700°C with hardness of 24.4 ± 0.2 GPa and fracture toughness of 5.4 ± 0.2 MPa m0.5. However, the highest hardness value (30 ± 1 GPa) was obtained for samples sintered by pressureless sintering, featuring a core-shell grain structure.

Abstract: YAG-based transparent ceramics are conventionally prepared by vacuum sintering or by a double sintering process, viz. vacuum sintering followed by Hot Isostatic Pressing (HIP). The use of a pressure-assisted process on vacuum pre-sintered ceramics favours the closure of residual porosity, and is therefore suitable for the production of highly transparent ceramics, where pores would otherwise act as light scattering centres. On the other hand, these post-sintering treatments are effective with samples exhibiting a suitable microstructure after vacuum sintering, i.e. no secondary phases and only closed pores with a size smaller that the size of the grains. As an alternative to HIP, a fast post-sintering treatment with Spark Plasma Sintering (SPS) is proposed. In this poster we present a comparison of transparent YAG-based transparent ceramics obtained by vacuum sintering followed by post sintering with HIP and with SPS. Several combinations of vacuum sintering + HIP/SPS conditions were tested on YAG and Yb:YAG samples prepared by reactive sintering of single oxides in order to modify their microstructure, especially grain size and residual porosity. Magnesium oxide (M) or magnesium oxide with TEOS (M+T) were used as sintering aids. SEM and optical microscopy analyses were used to characterise the microstructure of the samples after vacuum sintering and after post-sintering, and to identify correlations between the microstructure and optical properties of transparent YAG ceramics. Acknowledgements: F. P. gratefully acknowledges the financial support of JECS TRUST.The authors from CNR ISTEC gratefully acknowledge the support from the Italian Ministry of Defence under PNRM Contract No. 8731 of 04/12/2019 (CeMiLAP²).

Abstract: Fluoride sintering additives are frequently utilized for the densification of various ceramics, but the current comprehension of the mechanism by which they affect densification is lacking behind empirical experience. A prominent example is LiF, which is commonly used in the preparation of transparent ceramics (MgAl2O4, Y2O3, YAG, MgO). It is generally accepted that LiF melt allows the rearrangement of particles, enhances densification and later in the process escapes from the system due to its high vapor pressure, so ideally no secondary phase is present in the final product. However, the second - and the most essential - step of enhanced densification is a source of scientific dispute. It is not clear if oxygen vacancies are responsible for this enhancement and if so, under what circumstances are they created. The present work tries to shed more light on fluoride additives and how they work throughout the whole process of preparation. The results suggest that the mutual dissolution of sintering additive and the base ceramic is the key aspect. Acknowledgements: This work was supported from the grant of Specific university research - grant No. A1_FCHT_2022_002

Ultra-High Temperature Ceramic Matrix Composites (UHTCMCs) are a new class of materials designed to merge the best features of ceramic matrix composites (CMCs), like damage tolerance, with those of ultra-high temperature ceramics (UHTCs), like ablation resistance. Applications that will benefit from these outperforming materials are found in aviation, defense, nuclear and space, in particular components that will have to bear high thermo-mechanical loads in highly corrosive environments above 2000°C and to resist several launches and re-entries undergoing near-zero erosion. The most recent manufacturing technologies for the production of UHTCMCs and thermo-mechanical and ablation properties are reviewed here.

We present here an investigation aimed at exploring the role of the microstructure on the magnetic properties of nanostructured cobalt ferrite. Bulk, almost fully dense, nanograined ferrites have been obtained starting from nanopowders prepared by a simple, inexpensive, water-based, modified Pechini method. This synthesis yielded largely aggregated, pure single-phase cobalt ferrite nanoparticles of ca. 35 nm average size, which have been then densified by high-pressure field-assisted sintering. Different sintering conditions (pressure up to 650 MPa and temperature up to 800 °C) and procedures have been used on both as-prepared and milled nanopowders in order to obtain materials with a spectrum of complex microstructures. In all cases, the sintering process did not produce any change in the phase composition. At the same time, using a high uniaxial pressure in combination with relatively low sintering temperatures and times, allowed for obtaining a high degree of densification while preserving the nanometric size of the crystallites. Moreover, we observed that in the densified materials the best magnetic properties are not necessarily associated with a more uniform microstructure, but rather arise from a delicate balance between moderate aggregation, grain size and high density.

The PhD activity was divided into three parts: - The first part was focused on the development and characterization of light weight bulk material for the fabrication of ballistic protection systems. Sintering additives or others agents (?-SiC, nano-SiC, Si3N4, TiO2, WC) are used to improve the densification with conventional methods, avoiding if possible the application of mechanical pressure. In particular B4C-TiB2 composites were contaminated with WC to study the effect on densification, microstructure and properties. WC was introduced through a mild or a high energy milling with WC-6 wt% Co spheres or directly as sintering aid to 50 v% B4C / 50 v% TiB2 mixtures. High energy milling was very effective in improving the densification thanks to the synergistic action of WC impurities, acting as sintering aid, and size reduction of the starting B4C-TiB2 powders. As a result, the sintering temperature necessary for full densification decreased to 1860 °C and both strength and hardness benefited from the microstructure refinement, 860 ± 40 MPa and 28.5 ± 1.4 GPa respectively. High energy milling was then adopted for producing mixture of B4C-TiB2 that spanning from 100 v% B4C and 100 v% TiB2 studying the effect of different sintering technique such as hot pressing, pressureless sintering and gas pressure sintering. The B4C-rich composition showed the highest hardness and strength value in all sintering technique ~30 GPa and ~800 MPa respectively whilst the TiB2-rich composition showed the highest value of toughness, ~5 MPa?m0.5. - The second part of this thesis was focused on fabrication of ultra-high temperature ceramic composites (UHT-CMCs) observing the influence of different coatings on carbon fibers through electrophoretic deposition technique. Different configurations of continuous carbon fiber-reinforced ultrahigh temperature ceramics (UHTCs), by combining coatings and matrix, were produced via electrophoretic deposition (EPD) and slurry infiltration. The toughening of non-periodic fiber distribution induced by the EPD process was investigated through work of fracture analysis. The results show that a non-periodic fiber distribution results in toughness increase from 8 MPa?m0.5 to 11 MPa?m0.5 with respect to a periodic fiber distribution. This toughness improvement does not strongly affect the flexural strength, which is mainly related to the fiber volumetric amount. It is shown that the assembling of carbon fibers into bundles (i.e. by dispersing the fibers with a non-periodic distribution) increases the crack propagation energy dissipated on the crack-wake from 0.5 kJ/m2 to 1 kJ/m2, which can be mainly ascribed to the fiber/bundle pull-out. On the other hand, the energy dissipated on the crack-tip (as fiber/matrix debonding) is fiber distribution-independent and increases from 0.3 kJ/m2 to 0.4 kJ/m2 with increasing the fiber amount from 33 vol% to 40 vol%. Finally, work of fracture (WoF) analysis is proposed as test to evaluate pull-out toughening instead of push-in and push-out tests. - The third part of this thesis was focused on the production of High entropy metal diboride (HEMB). This novel class of ceramic materials represent a radically new approach to extend the chemical composition window of ultra-high temperature ceramics (UHTCs). In this work, arc-melting was used to produce dense HEMBs starting from UHTC powders. In order to understand the influence of each individual diboride within the quinary system (HfB2, ZrB2, TiB2, TaB2 and CrB2), we investigated five quaternary equimolar solid solutions e.g. Hf-Zr-Ti-Ta, Hf-Zr-Ti-Cr, Hf-Zr-Ta-Cr, Hf-Ti-Ta-Cr, Zr-Ti-Ta-Cr and the overall quinary equimolar combination. Arc-melting allowed a rapid screening of favorable and unfavorable combinations. The produced HEMBs were free from undesired oxides and characterized by linear variation of lattice parameters typical of diborides and binary solid solutions. Because of evaporation during arc melting, CrB2 was hardly found in the solid solution, suggesting that vapor pressure should be taken into account when designing HEMB compositions especially for operating temperatures exceeding 2000 °C. Finally, Vickers microhardness ranged between the typical values of starting diborides.

Sintering is a crucial step for structural ceramics as it leads to the elimination of porosity and hence to the achievement of high thermo-mechanical performances. The class of materials known as ultra-high temperature ceramics (UHTCs) requires a combination of severe consolidation process, such high temperature above 2000°C and the application of pressure, which leads to coarse microstructure with poor mechanical behavior. UHTCs include borides and carbides of transition metals and are characterized by melting point above 3000°C and physic-chemical properties suitable for application in the aero-space field. Efforts have been concentrated on the mitigation of the sintering conditions and the addition of secondary phases forming eutectics or transient liquid phase is the most successful approach. Drawbacks of multiphase system can be related to the precipitation of reaction phases at the triple points or formation of grain boundary glass that may hamper the mechanical behavior at high temperature. Transmission electron microscopy is the most suitable method to investigate the microstructure at nano-scale length and obtain a set of information not achievable with other analytical techniques. A series of UHTC systems is presented and the process-microstructure-properties relationships are discussed.

In recent years, oxide materials based on garnet structure are being investigated as very promising candidates in the field of scintillation because of their high density, good chemical stability, optical transparency, and the possibility to easily incorporate luminescent rare-earth ions. Several studies demonstrated that garnet crystals show high light yield and advantageous timing performances, which make them of interest for applications in medical imaging and high energy physics detectors [1]. Among synthetic garnets, Ce-doped gadolinium gallium aluminum garnet (GGAG:Ce) is a relatively new and interesting material. It is a mixed garnet that has displayed very good scintillation and luminescence properties: its high density enhances the interaction with ionizing radiation, and the presence of Gd provides a high cross section for thermal neutron capture [2]. GGAG:Ce preserves a crystalline cubic structure, which allows to produce it in the form of transparent polycrystalline ceramic [3] with favorable characteristics for optical applications such as lasers, LEDs, and scintillators. In this work, ceramic samples were produced by reaction sintering from commercial oxide powders: the mixed powders were pressed into pellets and sintered by a combined process of air sintering and hot isostatic pressing. The sintering process was carefully selected and the use of sintering additives was optimized to eliminate porosity, which is crucial to achieve a good optical transparency. Optical properties were studied by means of optical absorption spectroscopy, steady-state and time resolved photo- and radio- luminescence, and correlated to the fabrication process parameters. Moreover, trapping phenomena caused by the presence of point defects were investigated by wavelength resolved thermally stimulated luminescence in a wide temperature range (10 - 800 K); a significant persistent luminescence signal was also singled out and investigated as a function of temperature. The presence of point defects was also evidenced by the occurrence of a sensitization of the radio-luminescence signal as a function of increasing cumulated X-ray dose, related to a competitive process between traps and Ce recombination centers in free carrier capture. Finally, preliminary results on recently developed layered Y3Al5O12:Pr/Gd3(Ga,Al)5O12:Ce (YAG:Pr/GGAG:Ce) ceramics for particle detection and discrimination will be also reported. This work has been supported by H2020 European Institute for Innovation and Technology (EIT) SPARK project (16290) and H2020 Rise project INTELUM (644260). [1] M. T. Lucchini et al., Nucl. Instrum. Methods Phys. Res. A 816 (2016) 176-183 [2] J. Dumazert et al., Nucl. Inst. Methods Phys. Res. A 882 (2018) 53 [3] Y. Ye et al., Opt. Mater. 71 (2017) 23

UHTCMCs represent a novel class of materials, which can potentially couple the high oxidation resistance of UHTCs to the damage tolerance of CMCs, provided that a suitable matrix/fiber interface is tailored. Their specific application is in hypersonics and propulsion. Current technologies for UHTCMC manufacturing originate from consolidated technologies for CMCs, such as chemical vapor infiltration (CVI), polymer infiltration and pyrolysis (PIP), reactive metal infiltration (RMI). The adaptation of these techniques to obtain a UHTC matrix is not easy, for several reasons. For instance, there are no commercially available polymeric precursors of UHTC phases, to be used for PIP. Infiltration with reactive alloys containing borides poses major problems of wettability and residues of metal in the final composite. In this talk, we explore the manufacturing of UHT-Composites via slurry infiltration followed by hot pressing. Several kinds of composites have been produced using different preforms, continuous and discontinuous fibres, different textures and architectures. As for the sintering of bulk ceramics, we show that an appropriate choice of doping elements is of fundamental importance for a successful densification. Even more, the choice of sintering aids affects not only the nature of fiber/matrix interface, but also the high temperature behavior of the materials. Recently, hot pressed UHTCMCs have been tested in relevant environment showing an excellent ablation and erosion resistance up to temperatures of 2500-3000°C.

Densi ceramici trasparenti a base Cr:YAG sono stati preparati tramite sinterizzazione in vuoto (a 1735°C o 1750 °C per 16 ore). Sono stati testati vari parametri del processo di produzione: composizione chimica (additivi di sinterizzazione), omogeneizzazione, essicamento, sinterizzazione e annealing. Si presenta un metodo che ha portato alla trasmittanza di 73.75 % a 1400 nm su uno spessore del campione di 1.134 mm e densità di 4.55 g/cm3. I campioni preparati sono stati caratterizzati tramite microscopia elettronica a scansione (SEM), la loro densità è stata misurata con il metodo di Archimede. Sono stati misurati gli spettri di trasmittanza dei campioni trasparenti. Si mostra l'effetto del processo di trattamento delle polveri (omogeneizzazione, essicamento) alla trasparenza dei ceramici ed è presentata una combinazione di questi parametri idonea per la produzione di materiale trasparente. Un tentativo è stato fatto per l'aumento della conducibilità termica del Cr:YAG tramite l'aggiunta di particelle metalliche (tungsteno, molibdeno) oppure di nitruro di alluminio prima della sinterizzazione. Le particelle metalliche non hanno resistito all'ossidazione durante la fase di annealing, mentre il nitruro di alluminio non ha resistito al processo di sinterizzazione. Tuttavia, l'approccio presentato può essere uttilizzato per un tipo di ceramici YAG che non richiede la fase di annealing dopo la sinterizzazione.

The progressive depletion of the main feldspathic flux deposits in the World is forcing the ceramic industry to search for suitable substitutes. The aim of this study is to assess the potential of some feldspar sources in the Egyptian Eastern Desert, particularly syenites from Abu Khruq, in the manufacture of ceramic tiles. Beneficiated samples obtained by lab-scale mineral processing were tested into porcelain stoneware batches (from 10% to 30% wt in replacement of feldspars) and compared with a reference body through a laboratory reproduction of the industrial processing. The technological behavior of unfired tiles does not suffer any significant alteration due to the use of syenites. On the other hand, syenite-bearing bodies exhibit some changes in the phase composition and the chemistry of the melt, which turns richer in alkali, especially K2O. The consequently increased sintering rate depends mainly on the viscosity of the liquid phase formed during firing. In conclusion, syenites can be used without technological hindrances to manufacture porcelain stoneware tiles. The firing behavior of syenite-bearing batches can be reasonably adjusted by setting key parameters (e.g., the feldspar amount and the Na/K ratio), but the color of fired bodies requires to furtherly lower the iron oxide.

Porcelain stoneware is sintered by partial vitrification, through viscous flow of an abundant liquid phase formed at high temperature. The present contribution will overview the evolution of phase composition of porcelain stoneware during firing at different temperatures and soaking times. The firings were conducted in two distinct ways: dynamic (i.e. with a ramp simulating the industrial heating cycle in a roller oven) and static, by inserting the sample into the chamber furnace directly at the maximum temperature. Each mixture was characterized from the chemical point of view and, once fired, its phase composition was determined by quantitative XRPD (Rietveld method). The transformations affecting the minerals of the starting mixture determine a continuous variation of the phase composition during the heating treatment: feldspars melt quickly (K-feldspar>plagioclase) - largely melted at 1100°C - while quartz is only partially dissolved at the highest temperature. The liquid phase changes its chemical composition according to the dynamic equilibrium established with both the residual minerals (quartz, feldspar) and the new crystalline phases formed during the firing (mullite). Variations of the chemical composition of the liquid phase reflected on its physical properties, particularly on viscosity and surface tension, which define the densification kinetics in the sintering process.

The viscosity of a porcelain stoneware at high temperatures is crucial to understand the vitrification path, the viscous flow sintering kinetics, and the pyroplastic deformation of this material. The final viscosity of a porcelain stoneware has to be determined considering both the viscosity of the liquid phase formed by the melting of feldspars - and other minerals - and the viscosity of the body made up of a suspension of crystals dispersed in the melt. A fundamental theoretical background along with semi-empirical constitutive laws on the viscous flow sintering, the glass densification, as well as on the high viscosity of liquids and melts already exists. Different approaches are needed to measure/estimate the two viscosities and the parameterization depends on both chemical composition of the liquid phase and persistence of crystal phases in the melt. In this work, a first attempt to predict the viscosity of a porcelain stoneware liquid phase is proposed by means of a detailed overview of preexisting models for high temperature viscosities of glasses and melts. Albeit models developed for glasses take into account a large number of oxides and they can be applied to melts characterized by a wide compositional range, the maximum concentration of alumina expected by these models is too low compared with that of the systems here investigated. On the other hand, the models proposed for granitic melts, although based on a lower number of oxides, take into account alumina levels closer to those of the systems of interest. In this contribution is demonstrated that the latter models can be used to predict the viscosity at high temperature of porcelain-like bodies. Comparative examples are provided for porcelain stoneware tiles, vitreous china, and porcelain bodies.