Size control of C@ZrSiO₄ pigments via soft mechano-chemistry assisted non-aqueous sol-gel method and their application in ceramic glaze

Abstract

The zircon-based encapsulation carbon black (C@ZrSiO₄) inorganic pigments with controllable size were successfully prepared by non-aqueous sol-gel method with the assistance of soft mechano-chemistry (SMC) activation precursors. The XRD and FT-IR results showed that the SMC activation enhanced the reactivity of precursors and facilitated the formation of ZrSiO₄ phase with the calcination temperature at 900–1050 °C, indicating the successfully synthesis of pure ZrSiO₄ phase. The average size of C@ZrSiO₄ inorganic pigments significantly declined from 45.20 to 4.50 μm with the increase of SMC activation time of precursors, however, the L* values of pigments increased from 24.11 to 46.35. It was mainly because of the decreased amounts of protected carbon in ZrSiO₄ matrix and the enhanced light reflectance, which lead to the slight color change from deep black to gray black tone. The colored glaze samples presented a black hue after applied in the ceramic glaze at 1200 °C. Moreover, the black hue intensity of glazed samples was higher than that of the corresponding C@ZrSiO₄ inorganic pigments, indicating the prepared pigments have excellent tinctorial strength and hiding power. These results ensure that the obtained C@ZrSiO₄ inorganic pigments possess the outstanding chromatic performance, excellent high-temperature stability, high chemical stability and suitable particle size, and exhibit great potential in the application of the industrial ceramic decoration.

Introduction

The zircon-based encapsulation carbon black (C@ZrSiO₄) inorganic pigments were firstly prepared by D. Speer and J. Horst [1,2] at 1990s, which attracted extensive attention and investigation due to their friendly environment, excellent high temperature stability, strong tinctorial strength and high-hiding power since then [[3], [4], [5]]. Enormous studies have shown that many kinds of preparation methods such as the sol-gel method [6], sol-gel-spraying method [7], layer-by-layer self-assembly method [8] and non-hydrolytic sol-gel method [9] as well as several carbon sources including the commercial carbon black [10,11], phenolic resin [9], spherical carbon [12] and plant derived carbon [3,4] have been used to synthesize the C@ZrSiO₄ pigments. Although the chromatic properties of C@ZrSiO₄ ceramic pigments have been considerably improved, their limitations and challenges for applying in industrial ceramic decoration still exist, such as the larger particle size and impure crystalline phase [13]. Consequently, the C@ZrSiO₄ inorganic pigments with small particle size, excellent chromatic properties and a stable dense structure are highly desirable to meet the requirement for industrial ceramic decoration.

As we know, favorable properties of inorganic pigments play an important role for their application in glazes, ceramic bodies and ceramic inks [[14], [15], [16]], which include chromatic performance, high-temperature stability, chemical stability and particle size (usually expressed in average particle size) [[17], [18], [19]]. Particularly, the particle size of pigments has significant influence on the optical properties and decorative effects. It is common that the ceramic pigments with an average particle size about 1–20 μm can be used in glazes. The optimum particle size distribution is approximately 90% particles in smaller range of 5–20 μm [20,21]. Usually, the large-sized pigment particles will be directly crushed and milled into fine-sized particles when used in ceramic decorative field [22]. Unfortunately, the zircon based encapsulation pigments can be easily cracked into smaller-sized particles along with the separating interface during micronization, which may break encapsulation structure of pigments and affect their chromatic performance [21].

Soft mechano-chemistry (SMC) method is a mechanical assisted synthesis way via milder mechanical grinding of starting materials or precursors to enhance reactant reactivity and reduce the particle size of final ceramic product [[23], [24], [25], [26]], which is usually combined with a chemical method such as the solid-state method [26,27], precipitation method [28] or hydrolytic sol-gel method [24,29]. This technique has been successfully used in various ceramic pigments synthesis, including Fe₂O₃@ZrSiO₄ inclusion pigments [23], (Ni, Sb)-co-doped rutile ceramic pigments [26] and Nd₂Si₂O₇ ceramic pigments [30], etc. More importantly, the SMC activation precursors can not only avoid breaking encapsulation structure of pigments, but also increase the inter-particle contact of precursors [26]. In previous study, we successfully synthesized the high color rendering C@ZrSiO₄ encapsulation pigments by simple non-aqueous sol-gel method [31]. However, the particle size of those pigments needs further optimization for better application. Therefore, in this paper, the previously reported non-aqueous sol-gel method has been improved by introducing a soft mechano-chemistry treatment aiming to synthesize high-qualitiy C@ZrSiO₄ black pigments with smaller and controllable size. The present study was focused to explore the effects of different activation time and calcination temperature on the synthesis, particle size distribution, micro-morphologies, microstructures and chromatic properties of pigments. Meanwhile, the relationship between the particle size and chromaticity performance of C@ZrSiO₄ inorganic pigments was revealed. Moreover, the color performance of C@ZrSiO₄ inorganic pigments within ceramic glaze was also evaluated in this work.