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Crystallization Behavior of Phosphate Glasses with Hydrophobic Coating Materials
We analyzed the effect of the addition of Li2O3, TiO2, and Fe2O3 on the crystallization behavior of P2O5–CaO–SiO2–K2O glasses and the effect of the crystallization behavior on the roughness and hydrophobicity of the coated surface. Exothermic behavior, including a strong exothermic peak in the 833–972 K temperature range when Fe2O3, TiO2, or Li2O3 was added, was confirmed by differential thermal analysis. The modified glass samples (PFTL1–3) showed diffraction peaks when heated at 1073 and 1123 K for 5 min; the crystallized phase corresponds to Fe3(PO4)2, that is, graftonite. We confirmed that the intensity of the diffraction peaks increases at high temperatures and with increasing Li2O3 content. In the case of the PFTL3 glass, a Li3Fe2(PO4)2 phase, that is, trilithium diiron(III) tris[phosphate(V)], was observed. Through scanning electron microscopy and the contact angles of the surfaces with water, we confirmed that the increase in surface roughness, correlated to the crystallization of the glass frit, increases hydrophobicity of the surface. The calculated values of the local activation energies for the growth of Fe3(PO4)2 on the PTFL1, PTFL2, and PFTL3 glass were 237–292 kJ mol−1, 182–258 kJ mol−1, and 180–235 kJ mol−1.
Hydrophobic and superhydrophobic solid surfaces have attracted considerable attention in scientific and industrial fields owing to their unique self-cleaning, antifogging, antisticking, and anticontamination properties [1–5]. It has been theoretically and experimentally confirmed that superhydrophobic surfaces can be obtained by the cooperative effects of topographically roughened surface structures . In accordance with this concept, researchers have prepared artificial superhydrophobic surfaces through hierarchical micro/nanostructures on specimen surfaces [6, 7] and have chemically modified these structures to reduce the surface energy . Various methods have been reported for creating superhydrophobic surfaces, such as sol-gel processing , plasma processing , and spray pyrolysis . However, these methods present limitations for industrial applications and are unsuitable for commercial products because they either are only applicable to small substrates or require time-consuming processing steps or expensive machinery . Recently, studies have been performed to overcome these problems. In particular, Reinosa et al. confirmed the influence of surface modification by crystallization on the hydrophobicity of surfaces, which occurs during the inorganic coating of glass frit by Fe2O3 and Cu microparticles . They confirmed that the surface was modified and the hydrophobicity increased through the crystallization of the inorganic coating material. However, they only analyzed the crystallization generated by the addition of Fe2O3 and Cu microparticles, neglecting crystallization by the composition of the glass frit itself. If hydrophobicity can be expressed just by the crystallization of glass frit, this can be used in various applications such as self-cleaning household ovens [13, 14] and outdoor glass insulators .
Phosphate glass exhibits attractive properties such as low glass transition and melting temperatures, high thermal expansion coefficients, biocompatibility, and high refractive indices. These specifications are suitable for many applications in photonics, fast ion conductors [16–19], glass-to-metal seals [20–22], low-temperature enamels , NH3 gas adsorption , and biomedical engineering . An important characteristic of phosphate glass is its advantageous crystallization behavior as compared to silicate glass [25, 26]. To further accelerate the crystallization of phosphate glass, nucleation agents such as Fe2O3 , Li2O3, and TiO2  are required; when the frit is coated with one of these agents, the glass can be crystallized using a short firing time .
In the present work, we investigated the effect of the addition of Li2O3, TiO2, and Fe2O3 on the crystallization behavior of a P2O5–CaO–SiO2–K2O glass system widely used in various fields [29, 30]. In addition, we confirmed the effect of the crystallization of the glass frit on the roughness and hydrophobicity of the final coated surface.
2. Materials and Methods
2.1. Glass Preparation
Glass samples with this composition (100–x–y)(0.74P2O5–0.14CaO–0.05SiO2–0.07K2O)–x(Fe2O3–TiO2)–y(Li2O3) were used. The glass samples were prepared using NH4H2PO4, CaCO3, SiO2, K2CO3, Fe2O3, TiO2, and Li2CO3, all of which had purities higher than 99.9%. This mixture was melted in an Al2O3 crucible in an electrically heated furnace under ordinary atmospheric conditions at a temperature of 850°C for approximately 30 min to evaporate ammonia, carbonate, and water and to minimize the tendency for subsequent phosphate volatilization. The temperature was then gradually increased to 1250°C and maintained for 30 min to homogenize the melt. The melt was quenched by pouring it onto a plate. X-ray diffraction (XRD) analysis using an Ultima IV system (Rigaku) was employed to confirm the amorphous state of the synthesized glasses and the results are shown in Figure 1 and Table 1.
2.2. Measurements and Analysis
The glass frits were coated by the following procedure. The glass was ground and screened with a 44 μm mesh and then sprayed onto the surface of a low-carbon steel substrate (SPP steel), producing coatings with thicknesses of 200–220 μm. The coatings were fast-fired in an air atmosphere using an electrically heated furnace. The coating thickness of each sample is shown in Figure 2. The heating temperatures of 1073 K and 1123 K were applied for 5 min, which is a typical enamel sintering cycle. The phase analysis and crystallization of the heated samples were examined by XRD (Rigaku-Ultima IV). To determine the surfaces’ hydrophilicity or hydrophobicity, a contact angle measurement system (Krüss DSA 100 Easy Drop) was used to find the contact angle of water with the coated surfaces. The roughness and nano- and microstructures were studied by field emission scanning electron microscopy (FE-SEM, Supra 25, Carl Zeiss).
Crystallization analysis was performed by a nonisothermal method that is easier and faster than the isothermal method [31, 32]. In order to determine the activation energy for crystallization of the samples, differential thermal analysis (DTA) measurements were performed using approximately 30 mg of the heat-treated glass powders in an air atmosphere at heating rates of 5, 10, 15, and 20 K/min up to 1273 K. The DTA results were further analyzed to obtain the activation energy values for the crystallization of each sample using the Ozawa  method. The slope of each graph was determined by the method of least squares.
3. Results and Discussion
3.1. Crystallization Behavior and Phase Analysis
The DTA curves for the glass powders obtained at different heating rates are presented in Figure 3. The DTA curves show the exothermic effect of heating, followed by a strong exothermic peak in the 833–972 K temperature range when Fe2O3, TiO2, or Li2O3 is added. Furthermore, the DTA curves of the PFTL3 glass show an initial exothermic effect followed by a strong exothermic peak; Figure 3(c) also shows partial overlapping of the two peaks. The characteristic temperatures of the glass (, , and ) increased with increasing heating rate, as shown in Table 2. From Table 2, it is confirmed that decreases with the addition of Fe2O3 or TiO2. This means that Fe2O3 and TiO2 act as network modifiers in phosphate glass and that they weaken the glass structure. In addition, the DTA data reveal that the addition of Li2O3 shifts the endothermic peaks to lower temperatures; that is, a lower temperature is needed to initiate crystallization .
To confirm the nucleation results of the DTA curves, XRD analysis was performed. Figure 4 presents the XRD patterns for glass samples heated for 5 min at both 1073 K and 1123 K. The PFTL1–3 glass samples all show diffraction peaks; the crystalline phase corresponds to the compound Fe3(PO4)2, that is, graftonite. The data confirms that the intensity of the diffraction peaks increases at higher heating temperatures and with increased Li2O3 content. In addition, in the case of the PFTL3 glass, a Li3Fe2(PO4)2 phase is observed. This indicates that further crystallization occurs at higher temperature and with increased Li2O3 content. Through XRD analysis, we confirmed that the two exothermic peaks in the DTA curves are the result of crystal phases of Fe3(PO4)2 (peak 1 of PFTL1, PFTL2, and PFTL3) and Li3Fe2(PO4)2 (peak 2 of PFTL3).
3.2. Hydrophobicity and Surface Characteristics
As seen in the figures, the noncrystalline PFTL0 glass, that is, a typical phosphate glass, has very strong hydrophilicity; this is expected from the chemical composition of phosphate glass. The phosphate group is negatively charged, making the head polar. The phosphate heads are thus attracted to the water molecules in their environment. For these reasons, phosphate glasses have strong hydrophilicity . As the glass crystallized, the contact angle increased up to a maximum of approximately 87°, obtained with PFTL1. In addition, the PFTL2 and PFTL3 glasses, which were more thoroughly crystallized, had lower hydrophobicity than the PFTL1 glass; all glass samples heated at 850°C had lower contact angles than the glass samples heated at 800°C. These results were analyzed through surface characteristics.
For solids, the term “surface energy” reflects the affinity of the surface to other materials; the higher the surface energy of the solid, the more the energy gained upon bringing this surface into contact with other materials. Therefore, the surface energy describes the adhesive properties of the material, which can be “activated” by different surface treatments or by changing the material chemistry.
According to Abou Neel et al., the total surface free energy of phosphate glass is very high, more than 75 mN m−1 [36, 37]. This is because of the polar characteristics of phosphate glasses, as mentioned above.
When Fe2O3 or TiO2 is added to the phosphate glass, the surface energy of the glass is reduced [36, 37]. The reason for this is associated with the formation of hydration-resistant Ti–O–P and Fe–O–P bonding in place of hydration-susceptible P–O–P bonding. The hydrophobicity of the coated glass surfaces was further analyzed by surface morphology.
SEM micrographs of the PFTL glass samples heated at 800 and 850°C for 5 min are shown in Figure 7. We can determine the causes of changes of hydrophobicity of the PFTL glasses from these micrographs. The typical phosphate sample, PFTL0, shows a flat surface very similar to those usually found on commercial enamels. PFTL1–3 heated at 800°C for 5 min have pseudocellular structures (hereafter, cellular regions) resembling those found on the surface of hydrophobic leaves [12, 38]. Thus, hydrophobicity increases when the glass crystallizes. We think that PFTL2 and PFTL3 exhibited lower hydrophobicity than PFTL1 owing to the increase in the size of the nanostructures and the corresponding reduced fraction of intergranular surface voids under the water drop. As previously demonstrated, a higher fraction of empty space below the water drop increases the contact angle, making the surface more hydrophobic . In addition, PFTL1–3 heated at 850°C for 5 min presented rough surfaces completely covered in crystalline phases owing to the high crystallization of these glasses. It is thought that these rough surfaces induced low hydrophobicity. We confirmed that the increase in surface roughness occurring by the crystallization of the glass frit increased the hydrophobicity of the surface through the analysis of hydrophobicity and surface characterization. However, the contact angle with water never exceeded 90°, which is the general standard for hydrophobic materials. This is thought to be because of the strong hydrophilic nature of phosphate glasses, as previously described.
3.3. Crystallization Kinetics
We analyzed the activation energy for the crystallization of the glass samples in order to verify the impact of surface crystalline phase on the hydrophobicity. The activation energy was calculated only for the Fe3(PO4)2 phase of the PTFL1, PTFL2, and PFTL3 glasses.
The Johnson-Mehl-Avrami (JMA) model, which is the most general method of calculating activation energies, implies that the activation energy, , should be constant during the transformation process . However, some authors have shown that values are not necessarily constant; instead, they vary during the transformation in nonisothermal methods . The activation energy for different crystallization volume fractions is not constant during the whole transformation owing to changes in the nucleation and growth behaviors during crystallization . The variation of can be expressed by the local activation energy . The value of can be determined from nonisothermal DTA results using the method proposed by Ozawa according to the following relationship :where is the gas constant and and are the temperature and the heating rate, respectively, corresponding to the value of the crystallized fraction, .
The value of at a specific temperature can be determined from the DTA curves  by the ratio . Here, is the total area of the crystallization peak between temperature (where crystallization begins) and temperature (where the crystallization is completed) and is the area between and , as shown schematically in Figure 8. The graphical representation of as a function of shows typical sigmoid curves for different heating rates, as presented in Figure 9, indicating that the formation of the crystalline phase proceeds by a combination of nucleation and growth processes.
Using the experimental data presented in Figure 9, the plot of ln() versus for various values of (0.1 < < 0.9) was obtained, and the results are shown in Figure 10. The value of was calculated from the slope.
Figure 11 shows the variation of versus for the studied glass. It is observed that for the exothermal peak of all samples, corresponding to Fe3(PO4)2, is high during the initiation of the crystallization process and then it decreases. In addition, it is confirmed that the activation energies of PFTL2 and PFTL3 are lower than that of PFTL1. This means that the crystal phases of PFTL2 and PFTL3 are more easily formed than that of PFTL1, and these results support the SEM and hydrophobicity results given above. By calculating the activation energy for crystallization, we think that PFTL2 and PFTL3 had lower hydrophobicity than PFTL1 owing to the increase in the size of their surface crystal phase structures and the corresponding reduced fraction of intergranular surface voids because of their lower activation energies.
This study analyzed the effect of the addition of Li2O3, TiO2, and Fe2O3 on the crystallization behavior of a P2O5–CaO–SiO2–K2O glass system and the effect of the crystallization of glass frit on the roughness and hydrophobicity of the coated glass surface.
We confirmed through XRD and DTA results that crystallization of P2O5–CaO–SiO2–K2O glass occurs with the addition of Li2O3, TiO2, or Fe2O3. In addition, through DTA analysis, it was confirmed that decreases with the addition of Fe2O3 or TiO2, which indicates that Fe2O3 and TiO2 act as network modifiers in phosphate glasses and weaken the glass structure. The surfaces of PFTL1–3 heated at 800°C for 5 min presented a cell-like microstructure in which we observed nanostructures owing to the crystallization of graftonite and trilithium diiron(III) tris[phosphate(V)]. This hierarchical structure is similar to those found on the surfaces of some hydrophobic leaves. For these two reasons (adding Fe2O3 and TiO2, which act as network modifiers, and broken P–O–P bonding, which increases the surface morphology), the hydrophobicity increases in accordance with the degree of crystallization of the -, -, and Fe2O3-doped P2O5–CaO–SiO2–K2O glasses. However, the contact angle of the surfaces of these glasses with water never exceeded 90°, which is the general standard of hydrophobicity. We think this is owing to the particular characteristics of the phosphate groups, which are strongly hydrophilic. Through this study, we confirmed the possibility of developing inorganic and hydrophobic coating materials using the crystallization of glass. In addition, we established the need for additional research to overcome the compositional limitations of phosphate glasses to express hydrophobicity with existing hydrophobic coating materials.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
This research was financially supported by the Ministry of Education, Science and Technology (MEST) and National Research Foundation of Korea (NRF) through the Human Resource Training Project for Regional Innovation.
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