The influence of iron content on the promotion of the zircon structure and the optical properties of pink coral pigments

A sol–gel reaction starting from Si and Zr alkoxides, in water-ethanol mixtures, was employed to obtain iron doped zirconium silicate powders (zircon). The starting amount of the ferric salt in the sol–gel reacting mixture was varied in order to obtain Fe₂O₃/Zr molar ratios in the range 0.7–10%. The products of the sol–gel reaction were calcined in the range 800–1300 °C. X-ray diffractograms, EDX analyses and diffuse reflectance spectra were obtained and analysed for all the calcined powders; the colour of the pigments was characterised on the grounds of the CIE (Commission Internationale de l’Eclairage) standard procedure (CIE L¹a¹b¹ measurements). Results from the structural and spectral characterisations are examined and cross-compared to produce a consistent picture of the role played by iron on the promotion of the zircon lattice and on the optical properties of the reaction products.     

New spectroscopic and diffraction data to solve the vanadium-doped zircon pigment conundrum

The literature on V-doped zircon pigments offers a puzzling situation with unanswered questions about the origin of turquoise colour. In this study, new diffraction (XRPD) and spectroscopic (EPR, EAS) data were obtained on four industrial pigments, whose vanadium content is much higher than samples from previous investigations. Both EPR features and XRPD results indicate the occurrence of tetrahedrally-coordinated V⁴⁺ at the zircon interstitial site 16g. EPR spectra at 5?K exhibit a structured multiplet attributed to isolated V⁴⁺ ions and an exchange-narrowed line resulting by clustering of V⁴⁺ ions. The intensity of optical bands is stronger in samples affected by V⁴⁺ clusters. However, the turquoise colour stems from the window of transmitted green-bluish light, which depends on both the tail of the main optical band (²B???²A) and the onset of charge transfer phenomena. Small variations in these two bands readily turn into a chromatic shift from light blue to green.     

Recycling in ceramic glazes of zirconia overspray from thermal barrier coatings manufacturing

Glazed ceramic tiles are the most common building material for floor and wall covering. Glazes are produced from frits. The aim of this work is to make a total or partial replacement of a raw material, zircon, widely used in ceramic tiles manufacturing, with a waste material, in order to prepare ceramic frits. The waste material used in this work, is the overspray zirconia, which is produced during the deposition process by atmospheric plasma spraying (APS) of thermal barrier coatings (TBC) on turbine blades. In particular, a replacement of 100 wt%, 1 wt% and 0.2 wt% of zirconium silicate with zirconia has been studied. Ceramic glazes prepared mixing frits and other raw materials are applied on a single-fired tile. The glazes obtained were characterized with different analytical techniques. This study has revealed that the substitution of zircon with waste zirconia is possible in small percentages due to the presence of small amount of chromophore ions in the overspray zirconia, which tend to colour the glaze.     

Development of anorthite based white porcelain stoneware tile compositions

The scope of the present study is to develop alternative white porcelain stoneware tile compositions without using zircon (ZrSiO₄) which have shown better whiteness values than standard white porcelain stoneware tile composition containing zircon. These new compositions are designed to develop anorthite crystals in the microstructure. Technological properties of compositions such as water absorption (%), firing shrinkage (%) and colour (L^⁎, a^⁎, b^⁎) were measured and compared with standard white porcelain stoneware tile. Scanning electron microscopy (SEM), X-ray diffraction (XRD) analyses were also carried out. One of the new anorthite-based compositions was manufactured as white porcelain stoneware tile substitute of standard white porcelain stoneware containing zircon.     

Production of brown pigments for porcelain insulator applications

In this study, production of brown pigment for the ceramic insulator applications by using inexpensive natural raw materials or waste materials was undertaken. Different pigment compositions were designed, synthesised and examined. As a source of chromium, chromite, Cr₂O₃ and ferrochrome were used. Limonite, grinding waste, flotation waste and iron oxide scale were used as an iron source whereas manganese oxide and ferromanganese were used as a manganese source. The colour of glazed insulator bodies change from dark brown to light brown depending on the pigment composition. The pigments prepared with ferrochrome, manganese oxide, flotation waste (C6) or iron oxide scale (D4) and calcined at 1300 °C have a darker brown colour and possess suitable L*a*b* values as 30.1, 2.7, 1.6 for C6 and 30.9, 2.1, 0.3 for D4, which are closer to the L*a*b* values (30.1, 2.9, 0.1) of commercial MnFeCr pigments. The results indicate that waste materials containing iron can be used to produce brown pigments to be used in the insulator application in ceramic industry and as a result of this waste material can be converted into a value-added product. Also, ferrochrome can be used successfully as chromium source and help to reduce the cost of the pigment.     

Synthesis and characterization of PrxZr1-xSiO4 (x = 0–0.08) yellow pigments via non-hydrolytic sol-gel method

Yellow inorganic pigments Pr_xZr_1-xSiO₄ (x?=?0–0.08) have been prepared by a novel non-hydrolytic sol-gel (NHSG) method at 750?°C for 2?h. Replacing Pr⁴⁺ for Zr⁴⁺ in ZrSiO₄ increased the cell volume and changed the color from white to yellow gradually. The Si―O―Zr and Si―O―Pr bands were observed in the FT-IR spectra of xerogel, indicating it could reach homogeneous mixing at the atomic level. Therefore, it promoted the solid solution reaction between Pr and zircon at low temperature. The samples exhibit high doping limitation (x?=?0.08) and brilliant yellow hue (b^*?=?69.48) in contrast with the previously reported praseodymium zircon yellow pigments. The intense of yellow hue was increased with increasing the Pr doping content due to the increase of Pr⁴⁺/Pr³⁺ species. After applying on bisque ceramic tiles, the pigment exhibited excellent coloration, high thermal stability and low solubility in molten glazes, indicating its potential application in ceramic decoration.     

Colouring of opaque ceramic glaze with zircon pigments: Formulation with simplified Kubelka-Munk model

In this study a simplified Kubelka-Munk model is proposed for colour matching purposes. Opaque glazes were prepared to determine the absorption optical constants from the reflectance curves measured with a spectrophotometer. After the physical and chemical characterization of the glaze components (frit and pigments), to analyze the spectrophotometric results a simplification of the Kubelka-Munk model was suggested. To experimentally verify the model, two target green colour were reproduced in laboratory by adding in an opaque glaze a yellow praseodymium-doped zircon ((Zr,Pr)SiO₄) and blue vanadium-doped zircon ((Zr,V)SiO₄) pigments. The results were in good agreement with the experimental reflectance curves and the prediction of colour green glazes was possible with a reduced number of experiments.     

The effects of the NaF flux on the oxidation state and localisation of praseodymium in Pr-doped zircon pigments

The role that NaF plays in the preparation of Pr-doped zircon pigments was studied through the analysis of the nature and localisation of the Pr cations into the zircon matrix in samples prepared in the absence and in the presence of NaF. As previously observed, the addition of NaF caused a decrease of the minimum temperature required for zircon formation from 1400 to 1100°C, and an increase of the yellow colour intensity. In the absence of NaF, the Pr cations mainly presented a threefold oxidation state, being located out of the zircon lattice as Pr₂Zr₂O₇, whereas in the presence of this flux, most of the Pr cations showed a fourfold valence and formed a solid solution with the zircon lattice, which was then the main responsible for the stronger yellow colour observed in this case. After heating this pigment at 1400°C, we detected an exsolution of the Pr (IV) cations as Pr₈Si₆O₂₄ which was accompanied by a decrease of the yellow colour intensity. Therefore, it was concluded that the main role of NaF in the preparation of yellow Pr-zircon pigments is to decrease the temperature of zircon formation to the range in which the chromophore responsible for the bright yellow colour, i.e. the Pr (IV)-zircon solid solution, is stable.     

利用PDZ合成锆英石基陶瓷颜料

利用PDZ原料成功地合成出了锆钒蓝、锆镨黄、锆铁红陶瓷颜料,讨论了各工艺参数对颜料合成的影响,优化出了PDZ合成锆英石颜料的最佳工艺制度。     

Dense zircon (ZrSiO4) ceramics by high energy ball milling and spark plasma sintering

The addition of sintering additives has always been detrimental to the mechanical properties of sintered ceramics; therefore, methods to reduce or, as in this case, eliminate sintering additives are usually relevant. In this paper, dense zircon ceramics were obtained starting from mechanically activated powder compacted by spark plasma sintering without employing sintering additives.The high energy ball milling (HEBM) of starting powder was effective to enhance the sintering kinetics. The structural changes of the zircon powder introduced by the HEBM were evaluated. The phase composition and the microstructure of bulk zircon material were analyzed by SEM (EDAX) and XRD. The Vickers hardness and the fracture toughness were evaluated as well.Fully dense materials were obtained at 1400 °C with a heating rate of 100 °C/min, 10 min soaking time and 100 MPa uniaxial pressure. The zircon samples sintered at temperatures above 1400 °C were dissociated in monoclinic zirconia and amorphous silica. The dissociation was detrimental for the mechanical properties. Unlike conventional sintering methods (hot pressing, pressureless sintering) SPS permitted to overcome the dissociation of the zircon material and to obtain additive free, fully dense zircon ceramic with outstanding mechanical properties.     

Colorimetry technique on BaTiO3 and KNbO3 ceramics to determine the phase transition

In this work, colorimetry was used in order to identify the phase transition of BaTiO₃ and KNbO₃ ceramic. The transition of the structure produces a change of color of the ceramic bulks because their optical properties change. The transition phase was detected with a color sensor and reflectance method on ceramic bulks. CIE L*a*b* and CIE L*u*v* color spaces were used to measure the color changes reported. Experimental results showed that the changes of color are suitable for detecting the phase transition in both perovskite structures. The phase transition in the proposed ferroelectric materials was also measured by means of the dielectric constant method and the results obtained were in agreement with the temperatures reported for phase transition changes on BaTiO₃ and KNbO₃. Therefore, the colorimetry technique proposed in this paper proved to be suitable for detecting structure changes as a relationship between color properties and temperature.     

Colour in ceramic glazes: Efficiency of the Kubelka–Munk model in glazes with a black pigment and opacifier

In this study the efficiency of the Kubelka–Munk model (already known and consolidated in other industrial sectors) was evaluated in the prediction of the colour of an opaque ceramic glaze obtained by a mixture of black pigment (spinel Ni–Fe–Cr) and zircon opacifier (ZrSiO₄). Glazes with different percentages of black pigment and opacifier were prepared to determine the absorption and scattering optical constants from the reflectance curves measured with a spectrophotometer. After the physical and chemical characterization of the glaze components (frit, pigment and opacifier), suggestions for the adaptation of the Kubelka–Munk model were made to facilitate the experimental procedure of analysis. The result obtained with the adapted Kubelka–Munk model was in good agreement with the experimental reflectance curves. The reproduction of the desired colour was possible with a reduced number of experiments and the model made it possible to correlate the colour with the added pigments concentration facilitating the formulation step.     

Compositional modifications to floor tile glazes opacified with zircon

Abstract

In the ceramic tile industry opaque glazes with considerable zircon content are widely used. However, the high cost of zircon limits its wide use in relevant glaze compositions. In the present study, a model opaque glaze was modified by changing the alumina/silica ratio, adding potassium oxide, or using a higher content of opaque frit while gradually eliminating the zircon content in the glaze batch. After glaze preparation, application, and single firing of glazed floor tiles under industrial conditions glazes were characterised by XRD, SEM, and EDX. The optimum alumina/silica ratio was found to be 0·26. Colour parameters L, a, and b of starting and final glazes showed that an increase in opaque frit content gives more positive improvement in opacity and in dry abrasion resistance than potassium oxide addition. Removal of zircon from the glazes resulted in a decrease of ～13–18% in glaze cost without detrimental effect on opacity.     

Examining correlations between composition,structure and properties in zircon-containing raw glazes

The effects of additions of zircon (5–17 wt%) to conventional single firing ceramic glazes is examined. The structural and morphological characteristics of the glazes were determined by Scanning Electron Microscopy, X-ray diffraction, infra-red spectroscopy, as well as ²⁷Al and ²⁹Si magic angle spinning (MAS) NMR. The presence of zircon microcrystals was revealed by X-ray diffraction (XRD) combined with scanning electron microscopy (SEM) and EDAX microanalysis. The chemical durability of the glasses is revealed to be acceptable for certain practical applications. The color characteristics of the glazes obtained were estimated in the HunterLab system (L*C*h*, emission spectra). The outcomes of the study established correlations between composition, structure and some of the physical properties.     

Ni-doped hibonite (CaAl12O19): A new turquoise blue ceramic pigment

A new structure for ceramic pigments was synthesized by a conventional solid state reaction process. It is based on Ni-doped hibonite, CaAl₁₂O₁₉, which assumes a turquoise-like blue colour similar to that of V-doped zircon. Hibonite is associated with anorthite, CaAl₂Si₂O₈, acting like a fluxing agent in order to lower the synthesis temperature, and with cassiterite, SnO₂, acting as a tin buffer to promote coupled Ni²⁺ + Sn⁴⁺ → Al³⁺ + Al³⁺ substitutions, in order to ensure the electric neutrality of the hibonite lattice. Since relatively low chromophore contents are required, this new system constitutes an interesting alternative to the common blue ceramic pigments based on cobalt aluminate spinel or vanadium-doped zircon, implying lower cost and environmental advantages. The pigments characterization was performed by X-ray powder diffraction, diffuse reflectance spectroscopy, CIELAB colorimetric analysis, and testing in ceramic glazes and bodies. The substitution of Al³⁺ by bigger ions, like Ni²⁺ and Sn⁴⁺, increases the cell volume compared to undoped hibonite and is responsible for the turquoise blue colour, as verified by UV–vis analysis. The chromatic mechanism is due to incorporation of Ni²⁺ in tetrahedral coordination, likely occurring at the site M3 of the hibonite lattice, where it partially substitutes the Al³⁺ ion. While this product shows a strong hue as a pigment, it is not stable after severe testing in glazes and attempts to improve its colouring performance are now under development.     

Synthesis of new vanadium cordierite pigment by sol–gel processing

Abstract

A new ceramic pigment with the cordierite high temperature structure was synthesised by sol–gel processing. A small region of a solid solution phase was detected containing <5 wt-%V₂O₅ . The crystallisation behaviour and thermal stability of the pigment was studied. Unit cell parameters, crystallite size, and the phase constitution of calcined powders were determined by X-ray diffraction. The xerogel powders were studied by thermal analysis and infrared spectroscopy. The compositions of fired samples were obtained by inductively coupled plasma analysis and the L ^*a^*b^*colour parameters were measured.     

Environmental optimisation of blue vanadium zircon ceramic pigment

Thermal treatment and the addition of V₂O₅ and of different mineralising agents (NaF, NaCl and borax) have been optimised in terms of colour yield (L*a*b*) and of environmental considerations (atmospheric emissions and vanadium leachates). The formation of different reaction intermediates depending on mineralising agent (ZrV₂O₇ for no mineraliser, NaVO₃ and/or sodium–vanadium bronzes for added sodium halides, and a borosilicate vitreous phase for added borax) proves be important for the pigment synthesis, leading to different zircon yields and to different colour performance in the fired pigments. The composition (ZrSiO₄)(V₂O₅)_0·19(NaF)_0·05(NaCl)_0·10 has been found to be the optimal environmentally (firing loss = 5·8%, vanadium leached = 284 ppm and b* = −16·4), though the non-mineralised composition (with 0·19 mol of V₂O₅ per zircon formula weight) also performs well. ©     

Effects of starch addition on the characteristics of CoAl2O4 nano pigments

Nanocrystalline cobalt aluminate spinel, CoAl₂O₄, was prepared via a microwave‐assisted solution combustion process applying various mixtures of urea, glycine, and starch as a novel mixed fuel. The effects of starch addition (0, 10, 20, and 30 wt%) on the physical characteristics (e.g. crystallite size and colour) of the blue nano pigments were also investigated. The resultant powders were characterised by means of X‐ray diffraction, scanning electron microscopy, electron dispersive X‐ray analysis, and CIE L*a*b* colour measurements. The presence of a CoAl₂O₄ spinel lattice after calcination of precursors at 600 °C was confirmed by X‐ray diffraction patterns, and the crystallite sizes were ca. 10–39 nm. Colorimetric data pointed to the formation of bright‐blue pigments at low levels of starch addition. Scanning electron microscope images showed that starch enrichment reduced the agglomeration and size of synthesised nanoparticles.     

A fluidized bed kinetic model for the fluorination of dissociated zircon

A reaction kinetic model for the fluorination of plasma dissociated zircon (PDZ, or ZrO₂.SiO₂) with hydrogen fluoride was developed. The model uses reaction as rate‐limiting step, with a shrinking core of PDZ in a porous matrix of zirconia (ZrO₂). This model was used to develop models for a multi‐stage fluidized bed reactor. These models facilitated the determination and design of an optimally configured multi‐stage fluidized bed reactor for the fluorination of PDZ. It was shown that a combination cross‐/countercurrent multi‐stage fluidized bed reactor could yield significant improvement over the conventional countercur‐rent multi‐stage fluidized bed reactor for certain reaction kinetic conditions.     

Eu3+ -Nd2O3 blue pigmented solid solutions

Abstract

Usually, Nd₂O₃ shows a polymorphism based on the hexagonal Ln₂O₃ A type and cubic Ln₂O₃ C type structures, which have coordination numbers of 7 and 6 respectively. The Nd³⁺ (4f²) ion in an octahedral environment produces a reddish shade. In the present study, Eu_xNd_2-xO₃ compositions were prepared by ceramic and coprecipitation (from chloride solution) methods using several flux agents. Using the ceramic route, Nd₂O₃ C type structure was detected and the samples were colourless, but using coprecipitation, NdOCl crystallised from an amorphous coprecipitate at low temperature, developing the unusual monoclinic EuNd₂O₃ B type doped polymorph which is blue in colour. When enamelled using a conventional industrial tile glaze, these blue coprecipitated samples gave L*a*b* = 75/1/-11 in the CIE colour convention. Samples were characterised by X RD, U V-V-NIR spectroscopy, and SEM-EDX techniques.