The Concurrent Sintering-Crystallization Behavior of Fluoride-Containing Wollastonite Glass-Ceramics

materials

Article The Concurrent Sintering-Crystallization Behavior of Fluoride-Containing Wollastonite Glass-Ceramics

Chuanhui Li 1, Peng Li 2,*, Jianliang Zhang 1, Fengjuan Pei 3, Xingchen Gong 4, Wei Zhao 2, Bingji Yan 2 and Hongwei Guo 2

1 School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China; [email protected] (C.L.); [email protected] (J.Z.) 2 School of Iron and Steel, Soochow University, Suzhou 215021, China; [email protected] (W.Z.); [email protected] (B.Y.); [email protected] (H.G.) 3 School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China; [email protected] 4 School of Metallurgical Engineering, Anhui University of Technology, Ma’anshan 243002, China; [email protected] * Correspondence: [email protected]

Abstract: The fabrication of well densified wollastonite with smooth appearance by direct sintering method is still a challenge due to the competitive behaviors between sintering and crystallization. In this study, the coarser glass frits with a size of 1–4 mm are subjected to heat treatment at different temperatures. An attempt of integrating differential thermal analyzer with a slag melting temperature characteristic tester was exploited to monitor the heat and geometry changes during the heating. The results showed that the addition of CaF can significantly promote the crystallization of wollastonite 2 ◦ at 940 C, while hindering the sintering ability. At higher temperature, the increase of CaF2 acts Citation: Li, C.; Li, P.; Zhang, J.; Pei, as flux and favors the formation of eutectics, leading to a decline in the precipitation amount of F.; Gong, X.; Zhao, W.; Yan, B.; Guo, wollastonite. The predominated liquid sintering brought fast shrinkage. It was found out that high H. The Concurrent Sintering- content of CaF2 narrows the dense sintering temperature range and results in uneven surfaces. In Crystallization Behavior of order to obtain wollastonite glass-ceramics with smooth appearance, the maximum content of CaF2 Fluoride-Containing Wollastonite in sintering glass-ceramics should be limited to 2 wt.%. Glass-Ceramics. Materials 2021, 14, 681. https://doi.org/10.3390/ Keywords: crystallization; fluoride; glass-ceramics; sintering ma14030681

Academic Editor: Dolores Eliche Quesada Received: 16 December 2020 1. Introduction

Accepted: 27 January 2021 Wollastonite is a calcium inosilicate mineral (CaSiO3), which belongs to the pyroxenoid Published: 2 February 2021 group of white inorganic materials with a ratio of Si:O = 1:3. The polymeric transition from para-wollastonite (β-CaSiO3) to pseudo-wollastonite (α-CaSiO3) occurs when temperature Publisher’s Note: MDPI stays neutral is above 1125 ◦C[1]. Wollastonite glass-ceramics possess outstanding characteristics, i.e., with regard to jurisdictional claims in high whiteness, low moisture absorption, low thermal expansion, low shrinkage, and published maps and institutional affil- low dielectric constant, which are suitable for wide applications in ceramics, chemicals, iations. electronic devices, dental implant, construction, and polymers [2–5]. The fluorides are often precipitated in CaO-SiO2 system owing to the antibacterial effect of F− in biomedicine application [6]. The low amount of fluorides could promote the nucleation and crystallization of wollastonite [7], whereas the high concentration of fluo- Copyright: © 2021 by the authors. rides leads to the precipitation of fluorosilicates (KMg3AlSi3O10F2, KNaCaMg5Si8O22F2), Licensee MDPI, Basel, Switzerland. fluorapatite (Ca5(PO4)3F), and CaF2, depending on its glass composition [4,6–8]. The This article is an open access article preparation of wollastonite-based glass-ceramics with well densified structure is still a chal- distributed under the terms and lenge via direct sintering process. Compared with the traditional melting process, direct conditions of the Creative Commons sintering is capable of initiating crystallization without adding nucleation agents, reducing Attribution (CC BY) license (https:// the processing temperature and energy consumption [8]. The raw materials are melted creativecommons.org/licenses/by/ and quenched into fine powders, then subjected to heat treatment for crystallization at 4.0/).

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relatively lower temperatures [9]. The crystallization and sintering mechanisms are usually in competition when direct sintering method is adopted for glass-ceramics’ preparation, which should be delicately balanced to produce densiﬁed structure with smooth surface. Theoretically, the key when fabricating dense glass-ceramics is to make the glass particles sinter to almost their full density by viscous ﬂow before crystallization. To understand what is occurring during the crystallization, one needs to monitor the microstructural change as a function of the heat treatment variables, such as time, temperature, and particle size [10,11]. This study attempted to provide a simple methodology that combines the techniques of differential thermal analyzer (DTA) and a slag melting temperature characteristic tester, with an aim of recording the exothermic reactions and the geometry changes during the heating. The variations in characteristic temperatures correlated to the thermal events of wollastonite-based glass-ceramic with respect to different CaF2 additions were analyzed. Hopefully, the essential knowledge gained in this work will be useful for engineering use.

2. Materials and Methods The raw materials for wollastonite glass-ceramic were reagent-grade chemicals and the batch compositions were adopted from the Japanese company “Nippon Electric Glass”, which is named Neoparies [12]. As listed in Table1, samples F1–F4 were prepared by introducing fluorine in the form of CaF2 to 100 g of basic composition with verifying amounts of 1, 2, 3, 4 g, respectively. Figure1 shows the process flow diagram for the preparation of wollastonite glass- ceramic and subsequent characterizations. After mixing, the batch mixture was transferred into the platinum crucible and melted at 1500 ◦C for 2 h in a muffle furnace (GWL-1600, Luoyang Juxing Kiln Cos., Ltd., Luoyang, China) with air atmosphere. The melt was then quickly quenched in distilled water to prevent crystallization. The obtained glass frits were dried at 80 ◦C and crushed into coarse particles with a particle size of 1–4 mm, then heated at 940, 1000, 1060, 1120, and 1180 ◦C for 1 h to obtain glass-ceramics.

Table 1. Chemical compositions of the designed parent glasses.

Mass (GRAM) Sample 100 g No. CaF2 Al2O3 SiO2 CaO K2O Na2O BaO B2O3 ZnO Sb2O3 F1 7.04 58.84 17.03 2.10 2.94 4.10 1.01 6.50 0.44 1 F2 7.04 58.84 17.03 2.10 2.94 4.10 1.01 6.50 0.44 2 F3 7.04 58.84 17.03 2.10 2.94 4.10 1.01 6.50 0.44 3 F4 7.04 58.84 17.03 2.10 2.94 4.10 1.01 6.50 0.44 4

XRD analysis. The crystalline phases presented in the synthesized glass-ceramic samples were identiﬁed by an Ultima IV X-ray diffraction (Rigaku, Tokyo, Japan). The samples were subjected to grinding, then sieved for 5 min (mesh size of 74 µm). The obtained powder (≤74-µm particle size) was scanned between 2θ = 10–70◦ with a step size of 2θ = 0.02◦ and a scanning speed of 2◦/min. The phase composition of glass-ceramic samples (i.e., the amorphous and the crystalline phases’ amounts) was determined by the combined Rietveld-reference intensity ratio (RIR) methods using α-Al2O3 as an internal standard [13] and the calculated equation was as follows:

Ic Xc = (1) Ic + Ia

where Xc is the crystallinity of the test sample and Ic and Ia are the diffraction intensity of crystalline and amorphous phase in the test sample, respectively. MaterialsMaterials 2021,2021 14, x, 14FOR, 681 PEER REVIEW 3 of 113 of 11

Figure 1. The process flow diagram of the experiment. Figure 1. The process flow diagram of the experiment. SEM-EDS analysis. The fractured surfaces of the prepared glass-ceramics’ samples XRD analysis. The crystalline phases presented in the synthesized glass-ceramic sam- were polished and chemically etched for 3–5 s in 4 vol% hydrofluoric acid solution, followed ples were identified by an Ultima IV X-ray diffraction (Rigaku, Tokyo, Japan). The sam- by microstructure characterization by a field emission scanning electron microscope (SEM; plesSU5000, were subjected Hitachi, to Japan). grinding, Meanwhile, then sieved energy for 5 dispersive min (mesh spectroscopy size of 74 μm). (Oxford The obtained EDS X-MAX powder20, Oxfordshire, (≤74-μm particle UK) analysissize) was was scanned adopted between to identify 2θ = specific10–70° with elements. a step size of 2θ = 0.02° andDTA a scanning analysis. speed The of thermal 2°/min. stability The phase of glass composition powders of (particle glass-ceramic size ≤ samples74 µm) was (i.e.,characterized the amorphous by aand differential the crystalline thermal phases’ analyzer amounts) (DTA; Labsys was determined Evo Simultaneous by the com- Thermal binedAnalysis, Rietveld-reference SETARAM, intensity Caluire-et-Cuire, ratio (RIR) France) methods in using argon α-Al atmosphere.2O3 as an internal The sample stand- was ard [13]subjected and the to heatingcalculated from equation room temperature was as follows: to 1200 ◦C at a heating rate of 10 ◦C/min, and

the crystallization peak temperature (Tp) for𝐼 each glass sample was measured. 𝑋 = (1) Raman analysis. Raman spectroscopy𝐼 (HR800,+𝐼 HORIBA JobinYvon, France) for the parent glass samples was performed to determine the inﬂuence of F− ions on the melt wherestructure Xc is the units. crystallinity of the test sample and Ic and Ia are the diffraction intensity of crystallineSintering and amorphous analysis. phase As shown in the test in Figure sample,2, arespectively. slag melting temperature characteris- ticSEM-EDS tester (CQKJ-II, analysis. Chongqing The fractured University surfaces of of Science the prepared & Technology, glass-ceramics’ Chongqing, samples China), wereequipped polished withand anchemically image recording etched for system 3–5 s andin 4 electricalvol% hydrofluoric furnace, was acid used solution, to record fol- the lowedgeometry by microstructure changes of thecharacterization glass samples by during a field the emission sintering. scanning The glass electron powder micro- (particle scopesize (SEM;≤ 74 SU5000,µm) was Hitachi, compressed Japan). intoMeanwhil a cylindere, energy (with dispersive a diameter spectroscopy of 3 mm and (Oxford height of EDS3 X-MAX mm) and 20, thenOxfordshire, heated to UK) 1300 analysis◦C in airwas with adopted a heating to identify rate of specific 10 ◦C/min. elements. The sample DTA analysis. The thermal stability of glass powders (particle size ≤ 74 μm) was char- images were analyzed using computer software to calculate the height HT and the area acterizedAT of by the a outline differential of the thermal sample. analyzer The characteristic (DTA; Labsys temperatures Evo Simultaneous in Table2 correlated Thermal to Analysis,sintering SETARAM, events were Caluire-et-Cuire, interpreted based France) on thein geometryargon atmosphere. changes. TheThe initialsample height was H0 ◦ ° ° subjectedand area to heating A0 were from measured room temperature at 500 C and to used 1200 asC a at reference. a heating The rate average of 10 C/min, value and of three the crystallizationreplicates of respective peak temperature samples were (Tp) for collected each glass to reduce sample measurement was measured. error. Raman analysis. Raman spectroscopy (HR800, HORIBA JobinYvon, France) for the parent glass samples was performed to determine the influence of F− ions on the melt structure units. Sintering analysis. As shown in Figure 2, a slag melting temperature characteristic tester (CQKJ-II, Chongqing University of Science & Technology, Chongqing, China), equipped with an image recording system and electrical furnace, was used to record the geometry changes of the glass samples during the sintering. The glass powder (particle

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size ≤ 74 μm) was compressed into a cylinder (with a diameter of 3 mm and height of 3 mm) and then heated to 1300 °C in air with a heating rate of 10 °C/min. The sample images were analyzed using computer software to calculate the height HT and the area AT of the outline of the sample. The characteristic temperatures in Table 2 correlated to sintering Materials 2021, 14, 681 events were interpreted based on the geometry changes. The initial height H0 and area4 of11 A0 were measured at 500 °C and used as a reference. The average value of three replicates of respective samples were collected to reduce measurement error.

FigureFigure 2. 2.Schematic Schematic representation representation of of a a slag slag melting melting temperaturetemperature characteristiccharacteristic testertester forfor sinteringsintering analysis.analysis.

Table 2. TheTable interpretations 2. The interpretations of the characteristic of the characteristic temperatures temperatures correlated to correlated sintering to events. sintering events.

Characteristic TemperaturesCharacteristic Temperatures Interpretations Interpretations ◦ 500 °C 500 C initial height initial H0 height, initial H0 ,area initial S area0 S0

The onset temperature of sintering,The onset temperature Tfs of sintering, where Tfs the heightwhere of the sample height of H samplefs becomes Hfs becomes 95–97% 95–97% of H of0 H0 the sample experiencesthe sample the maximum experiences the shrinkage maximum before shrinkage softening, before The temperature of maximumThe temperatureshrinkage, of T maximumms shrinkage, Tms wheresoftening, its height where stops its height stops decreasing and becomes constant decreasing and becomes constant where the sample begins to soften and the edges of the The onset temperature of softening, T where thed sample begins tosample soften becomes and the rounded edges [14 of] the sample The onset temperature of softening, Td becomes rounded [14] The softening temperature, Ts where the height of sample Hs becomes 75% of H0 The softening temperature, Ts where the height of sample Hs becomes 75% of H0 The hemispheric temperature, Thb where the height of sample Hhb becomes 50% of H0 The hemispheric temperature, Thb where the height of sample Hhb becomes 50% of H0 The ﬂowing temperature, Tf where the height of sample Hf becomes 25% of H0 The flowing temperature, Tf where the height of sample Hf becomes 25% of H0 3. Results 3. Results 3.1. The Wollastonite Formation at Different Temperatures 3.1. The Wollastonite Formation at Different Temperatures Figure3 shows the XRD results of the glass-ceramic samples after heat treatment at 940 ◦CFigure and 1180 3 shows◦C for the 60 XRD min. results Peaks correspondingof the glass-ceramic to para-wollastonite samples after heat (PDF treatment #10-0489) at were940 ° identiﬁedC and 1180 at ° 940C for◦C 60 as min. the onlyPeaks crystalline corresponding phase, to along para-wollastonite with an amorphous (PDF #10-0489) silicate humpwere locatedidentified at 20–35at 940◦ .° AC phaseas the transitiononly crystalline from para-wollastonite phase, along with to an pseudo-wollastonite amorphous silicate (PDFhump #74-0874) located wasat 20–35 observed°. A phase when transition temperature from reached para-wol 1180lastonite◦C. Figure to pseudo-wollastonite4 shows the extent of(PDF crystallization. #74-0874) was After observed heat treatment when temperature at 940 ◦C, thereached intensity 1180 of °C. para-wollastonite Figure 4 shows the peak ex- ° wastent greatly of crystallization. enhanced by After CaF 2heataddition. treatment The at corresponding 940 C, the intensity crystallinity of para-wollastonite was increased frompeak 19.71% was greatly to 35.18%. enhanced by CaF2 addition. The corresponding crystallinity was in- creasedAs shown from 19.71% in Figure to 535.18%., Raman spectra between 1200–800 cm −1 represent symmetric stretching vibrations of [Si-O] structure units and were deconvoluted to quantitatively estimate the relative fractions of Qn units (Qn: referring to symmetric stretching vibra- 0 4− 1 6− 2 4− tions of units with n numbers of bridging oxygen, Q -[SiO4] ,Q -[Si2O7] Q -[Si2O6] , 3 2− 4 Q -[Si2O5] ,Q -SiO2)[7,15–18]. According to Equations (2) and (3), the increment of Ca2+ and F− ions led to depolymerization of silicate glass. Due to the similar ion radius, the partial replacement of Si-O-Si bonds by F− will result in the formation of Si-F bonds. 2− As a consequence, [Si2O5] sheets (one non-bridging oxygen atom per silicon) would 2− transform into [SiO3] units (two non-bridging oxygen atoms per silicon) [17,18]. This Materials 2021, 14, x FOR PEER REVIEW 5 of 11

Materials 2021, 14, x FOR PEER REVIEW 5 of 11

As shown in Figure 5, Raman spectra between 1200–800 cm−1 represent symmetric stretching vibrations of [Si-O] structure units and were deconvoluted to quantitatively As shown in Figure 5, Raman spectra between 1200–800 cm−1 represent symmetric estimate the relative fractions of Qn units (Qn: referring to symmetric stretching vibrations stretching vibrations of [Si-O] structure units and were deconvoluted to quantitatively Materials 2021, 14, 681 of units with n numbers of bridging oxygen, Q0-[SiO4]4−, Q1-[Si2O7]6−Q2-[Si2O6]4−, Q3-5 of 11 estimate the relative fractions of Qn units (Qn: referring to symmetric stretching vibrations [Si2O5]2−, Q4-SiO2) [7,15–18]. According to Equations (2) and (3), the increment of Ca2+ and of units with n numbers of bridging oxygen, Q0-[SiO4]4−, Q1-[Si2O7]6−Q2-[Si2O6]4−, Q3- F− ions led to depolymerization of silicate glass. Due to the similar ion radius, the partial [Si2O5]2−, Q4-SiO2) [7,15–18]. According to Equations (2) and (3), the increment of Ca2+ and replacement of Si-O-Si bonds by F− will result in the formation of Si-F bonds. As a conse- F− ions led to depolymerization of silicate glass. Due to the similar ion radius, the partial was strongly2− validated by the results of the deconvolution. It is clearly seen from Figure5F , quence, [Si2O5] sheets2 (one non-bridging− oxygen atom per silicon) would transform into3 replacementthe fraction of Si-O-Si Q units bonds increased by F withwill result the addition in the formation of CaF2 at of the Si-F expense bonds. As of Qa conse-units, [SiO3]2− units (two non-bridging oxygen atoms per silicon) [17,18]. This was strongly val- quence,whereas [Si the2O fractions5]2− sheets of (one the non-bridging Q0,Q1, and Q oxygen4 units doatom not per show silicon) much would variation. transform Moreover, into idated −by the2− results of the deconvolution. It is clearly seen from Figure 5F, the fraction of [SiOF ions3] units entered (two into non-bridging the silicate oxygen glass and atoms facilitated per silicon) the [17,18]. glass separation. This was strongly Both effects val- Q2 units increased with the addition of CaF2 at the expense of Q3 units, whereas the frac- idated by the results of the deconvolution. It is clearly seen from Figure 5F, the fraction of favored the0 precipitation1 4 of wollastonite. When the heat treatment temperature− reached tions of the◦ Q , Q , and Q units do not show much variation. Moreover, F ions entered Q10002 unitsC, increased the crystallinity with the had addition the highest of CaF value.2 at the However, expense of higher Q3 units, CaF whereasaddition the did frac- not into the silicate glass and facilitated the glass separation. Both effects favored2 the precipi- tionsbring of higher the Q crystallinity.0, Q1, and Q4 units On the do contrary, not show it much became variation. lesser, and Moreover, a much F steeper− ions entered decline tation of wollastonite. When the heat treatment temperature reached 1000 °C, the crystal- intowas the observed silicate when glass temperatureand facilitated reached the glass 1180 separation.◦C. Both effects favored the precipi- linity had the highest value. However, higher CaF2 addition did not bring higher crystal- tation of wollastonite. When the heat treatment temperature reached 1000 °C, the crystallinity. On the contrary, it became lesser,2− and− a much steeper2− decline2 −was observed when linity had the highest value.[Si2O However,5] + 2F higher→ [SiO CaF2F2 ]addition+ [SiO did3] not bring higher crystal-(2) temperature reached 1180 °C. linity. On the contrary, it became lesser, and a much steeper decline was observed when 2− 2+ temperature reached 1180 °C. [SiO3] + Ca → CaSiO3 (3)

Figure 3. XRD results of wollastonite glass-ceramics obtained by heat treatment at different temperatures (a) 940 °C, (b) 1180 °C. FigureFigure 3. 3. XRDXRD results results of of wollastonite wollastonite glass-ceramics obtainedobtained byby heatheat treatmenttreatmentat at different different tempera- tem- peraturestures (a) 940(a) ◦940C, (°bC,) 1180(b) 1180◦C. °C.

Figure 4. The crystallinity of wollastonite glass-ceramics calculated by Rietveld reﬁnement from the XRD patterns.

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Figure 4. The crystallinity of wollastonite glass-ceramics calculated by Rietveld refinement from the XRD patterns.

SiO +2F → SiOF + SiO (2) Materials 2021, 14, 681 6 of 11 SiO +Ca →CaSiO (3)

FigureFigure 5. ( a5.) Raman(a) Raman spectra spectra of the of quenched the quenched glass meltsglass F1–F4,melts F1–F4, (b–e) deconvolution (b–e) deconvolution of the Raman of the spectra Raman for the glass n n melts,spectra (f) relative for the fraction glass melts, of the Q (f) unitsrelative for thefraction glass meltsof the as Q a functionunits for of the the glass CaF2 content.melts as a function of the CaF2 content. 3.2. The Microstructure Characterization of Glass-Ceramics

3.2. The Microstructure CharacFigureterization6 shows SEMof Glass-Ceramics images of the glass-ceramics with different CaF 2 contents after heat treatment at 1120 ◦C for 60 min. Dense body was found in all glass-ceramic specimens. Figure 6 shows SEM images of the glass-ceramics with different CaF2 contents after µ heat treatment at 1120White °C fibrousfor 60 andmin. dendritic Dense body grains was with found a length in of all 10 glass-ceramicm were identified speci- as wollastonite phase based on EDS analysis (Table3) and were the only crystalline phase observed on the mens. White fibrous and dendritic grains with a length of 10 μm were identified as wol- etched fracture surface. The high concentration of fluorine element was derived from HF lastonite phase basedacid on etching. EDS analysis It was hard (Table to distinguish3) and were whether the only the fluoridecrystalline was phase actually ob- dissolved into served on the etchedthe fracture wollastonite surface. structure The hi togh form concentration a solid solution. of fluorine Salman element et al. suggested was de- wollastonite rived from HF acid etching.could acquire It was considerable hard to distinguish amounts of whether Na+,K+ ,the Fe2+ fluoride, and Mg was2+, as actually well as F−, since the dissolved into the wollastoniteradius of last structure (1.36 Å) is to close form to thata solid of O solution.2−(1.40 Å) Salman and could et al. replace suggested O2− easily [4]. With wollastonite could acquirethe increment considerable of CaF2 amountscontent, theof Na precipitation+, K+, Fe2+, amountand Mg of2+, wollastoniteas well as F grains−, became since the radius of lastfewer (1.3 but6 Å) in is much close larger to that size of (see O2−(1.40 Figure Å)6d). and Due could to its replace bone-breaking O2− easily properties, the CaF2 addition to parent glass can lower the viscosity, with a consequence of the reduction [4]. With the increment of CaF2 content, the precipitation amount of wollastonite grains of the melting temperature and inter-particle free surface [19–21]. Since the nucleation of became fewer but in much larger size (see Figure 6d). Due to its bone-breaking properties, wollastonite is governed by surface nucleation by direct sintering method, the number the CaF2 addition to ofparent heterogeneous glass can nucleationlower the vi sitesscosity, of wollastonite with a consequence would decrease. of the Meanwhile, reduc- the mass tion of the melting temperaturetransfer for wollastoniteand inter-partic growthle free could surface be enhanced [19–21]. by Since lower the viscosity, nucleation thereby forming of wollastonite is governedlarger wollastonite by surface grains. nucleation by direct sintering method, the number of heterogeneous nucleation sites of wollastonite would decrease. Meanwhile, the mass transfer for wollastoniteTable growth 3. Semiquantitative could be energy enhanced spectrum by analysislower viscosity, of the selected thereby phases shownforming in Figure 6. larger wollastonite grains. Element (wt.%) Spot No. Phase O Si Ca Na Al F Zn Ba K G1 Glass matrix 25.53 21.01 5.87 1.43 4.71 27.31 7.37 5.35 1.43 W1 6.81 4.56 24.50 0.78 0.73 62.61 --- W2 11.49 4.73 17.62 1.29 0.84 62.75 --- wollastonite W3 10.58 4.58 22.16 1.36 1.13 60.20 --- W4 6.74 5.65 23.41 1.09 0.81 62.29 ---

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FigureFigure 6. 6.TheThe microstructure microstructure images images of wollastonite of wollastonite glass-ceramics glass-ceramics with different with different CaF2 addition CaF2 addition ° obtainedobtained by by heat heat treatment treatment at at1120 1120 C.◦ (C.a): ( asample): sample F1; ( F1;b): (sampleb): sample F2; (c F2;): sample (c): sample F3; (d F3;): sample (d): sample F4. F4.

Table 3. Semiquantitative energy spectrum analysis of the selected phases shown in Figure 6. 3.3. Influence of CaF2 Content on Concurrent Sintering and Crystallization Behavior During the fabrication of glass-ceramics, theElement controlling (wt.%) of concurrent events, i.e., Spot No. Phase crystallization and sintering, isO essentialSi toCa ensure Na theAl final productF Zn quality. Ba ForK better understandingG1 Glass the relationship matrix 25.53 between 21.01 these5.87 two1.43 events, 4.71 shrinkage27.31 7.37 and 5.35 DTA 1.43 curves of samplesW1 F1–F4 are shown in Figure6.81 7. 4.56 The correlated24.50 0.78 characteristic 0.73 62.61 temperatures - - for - glass samplesW2 were extracted from the 11.49 curves. 4.73 According 17.62 1.29 to Table 0.844 , with 62.75 increasing - - CaF 2 - content, wollastonite ◦ ◦ the initialW3 sintering temperature 10.58 Tfs decreased 4.58 22.16 from 1.36 775 1.13C to 60.20760 C, and - the - temperature - ◦ ◦ of maximumW4 shrinkage Tms decreased 6.74 5.65 from 23.41 895 1.09C to 870 0.81 C. 62.29 Since diffusion - - and - viscous flow are responsible for sintering, it is clear that the presence of CaF2 will greatly enhance

3.3.the Influence sintering of kinetics.CaF2 Content In thison Concurrent case, CaF Sintering2 in glass and acted Crystallization as a powerful Behavior network disrupter, weakeningDuring the structurefabrication of of silica glass-ceramics, network and the improving controlling the of mass concurrent migration events, [7,8 ].i.e., At this crystallizationstage, the sintering and sintering, necks between is essential adjacent to ensure particles the final were product developed quality. and For the better cavities un- were derstandingfilled by viscous the relationship flow. between these two events, shrinkage and DTA curves of samples F1–F4 are shown in Figure 7. The correlated characteristic temperatures for glass sam- Table 4. Characteristic temperatures for samples F1–F4. ples were extracted from the curves. According to Table 4, with increasing CaF2 content, the initial sintering temperature Tfs decreased from 775 °C to 760 °C, and the temperature Sample No. of maximum shrinkage Tms decreasedItems from 895 °C to 870 °C. Since diffusion and viscous F1 F2 F3 F4 flow are responsible for sintering, it is clear that the presence of CaF2 will greatly enhance ◦ the sinteringTheonset kinetics. temperature In this case, of crystallization, CaF2 in glass T xacted/ C as a powerful 815 network 808 disrupter, 804 805 ◦ weakening theCrystallization structure of peak silica temperature, network and Tp improving/ C the mass 858 migration 841 [7,8]. 840 At this 840 ◦ stage, the sinteringThe onset necks temperature between of adjacent sintering, pa Trticlesfs/ C were developed 775 and 765the cavities 764 were 760 ◦ The temperature of maximum shrinkage, Tms/ C 895 883 874 870 filled by viscous flow. ◦ The ability of crystallization vs. sintering, ∆T1= Tms − Tx/ C 37 42 34 30 ◦ The onset temperature of softening, Td/ C 1075 1113 1134 1150 ◦ The softening temperature, Ts/ C 1085 1118 1139 1155 ◦ The hemispheric temperature, Thb/ C 1145 1148 1159 1170 ◦ The flowing temperature, Tf/ C 1215 1213 1204 1200 Dense sintering temperature range, ◦ 70 35 25 20 ∆T2= Thb − Td/ C

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Figure 7. Figure 7. Shrinkage and DTA curves of samples F1–F4 (Note: T Txx—onset—onset temperature temperature of of crystalliza- crystalliza- tion;tion; TTpp—crystallization peakpeak temperature;temperature; T Tfsfs—onset—onset temperaturetemperature of of sintering; sintering; T msTms—temperature—temperature of maximumof maximum shrinkage shrinkage before before softening; softening; Td—onset Td—onset temperature temperature of softening; of softening; Ts—softening Ts—softening temperature; tem- Tperature;hb—hemispheric Thb—hemispheric temperature; temperature; Tf—flowing Tf—flowing temperature; temperature; AT,HT are A theT, H areaT are andthe area height and of height sample imagesof sample at temperatureimages at temper T, respectively).ature T, respectively). (a) sample F1; (a) ( bsample) sample F1; F2; (b ()c )sample sample F2; F3; (c (d) )sample sample F3; F4. (d) sample F4. Based on the DTA curves, an exothermic peak corresponding to crystallization reaction appeared during the sintering process. When CaF2 content increased from 1 to 4 wt.%, the Table 4. Characteristic temperatures for samples F1–F4. ◦ ◦ initial crystallization temperature Tx decreased from 815 C to 805 C and the crystallization peak temperature T decreased from 858 ◦C to 840 ◦C. TheseSample results No. suggest that the Itemsp addition of CaF2 could lower the glass stabilityF1 and promoteF2 crystallization,F3 whichF4 is inThe accordance onset temperature with the XRDof crystallization, results. It is worth noting that the Tx of all samples was 815 808 804 805 lower than their TTmsx/,°C indicating the crystallization occurred before they reached their maximum shrinkage values before softening.° The ability of crystallization vs. sintering, Crystallization peak temperature, Tp/ C 858 841 840 ◦ 840 ∆T1 = Tms − Tx, was evaluated to justify the interactions. The larger value of 42 C obtained The onset temperature of sintering, Tfs/°C 775 765 764 760 by F2 sample indicated a larger final density could be achieved with 2 wt.% addition of The temperature of maximum shrinkage, CaF2. After the crystalline phase was precipitated,895 consequently,883 the slope874 of shrinkage870 Tms/°C curves was dramatically decreased. Figure7 shows that, when the sintering temperature roseThe toability T , of the crystallization height of samples vs. sintering, F1–F4 became 73.8% (F1), 73.5% (F2), 75.2% (F3), and ms ° 37 42 34 30 74.7% (F4) ofΔ itsT1= initial Tms − T heightx/ C H . It implied that the crystallization that occurred on 0 ° theThe surfaces onset temperature of the glass of particles softening, hindered Td/ C the1075 sintering process1113 and reduced1134 the1150 sample ° shrinkage.The softening temperature, Ts/ C 1085 1118 1139 1155 TheAs hemispheric the temperature temperature, further increased, Thb/°C the1145 densification 1148 process was1159 suspended, 1170 as indicatedThe byflowing a plateau temperature, stage, and T lastedf/°C until the 1215 cylinder sample1213 became1204 softened. 1200 At this period,Dense the sintering continuous temperature crystallization range, increased the viscosity of the residual glass and froze 70 35 25 20 the sintering. ItΔT might2= Thb be − T thed/°C potential reason why the samples with higher addition of CaF2 had the lower Tms. When the temperature approached the onset temperature of softening Td, aBased dramatic on the decline DTA trend curves, in an height exothermic was observed. peak corresponding The residual to glass, crystallization the viscosity reac- of whichtion appeared viscosity during was greatly the sintering reduced process. at high When temperatures, CaF2 content accelerated increased thefrom liquid 1 to 4 phase wt.%, sinteringthe initial ofcrystallization powder compacts. temperature The samples Tx decreased began from to soften 815 ° andC to shrink 805 °C faster. and the Based crystal- on Tablelization4, the peak softening temperature temperatures Tp decreased T d and fromTs as858 well °C asto the840 hemispheric°C. These results temperature suggest that Thb forthe samplesaddition F1–F4 of CaF increased2 could lower with thethe additionglass stability of CaF and2. However, promote thecrystallization, flowing temperature which is

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exhibited an opposite trend. The former can be ascribed to the enhanced precipitation of wollastonite phase with high melting point, which led to an increment in Td, Ts, and Thb. The latter was due to the fluxing effect of CaF2, which led to degraded refractoriness of glass-ceramics. Deng et al. found out that the addition of a moderate amount of CaF2 in CaO-Al2O3-MgO-SiO2 glass led to a decrease in the melting temperature and viscosity [22]. In this study, the dense sintering temperature range was proposed as an important index for evaluating sintering ability of glass-ceramics and was defined as ∆T2 = Thb − Td. High- quality glass-ceramics with smooth surfaces require a broad, dense sintering temperature range, which is of great benefit to practical production. It is evident from Table4, increasing ◦ ◦ CaF2 amount decreased ∆T2 from 70 C to 20 C. Based on the results from Table4 and Figure8, the dense sintering temperature ∆T2 of glass-ceramics should be greater than ◦ Materials 2021, 14, x FOR PEER REVIEW30 C in order to obtain a smooth surface. Therefore, the content of CaF2 for10 wollastonite of 11

glass-ceramics was limited in 2 wt.%.

Figure 8. The influence of CaF2 addition on the appearance of the glass-ceramic samples at differ- Figure 8. The influence of CaF addition on the appearance of the glass-ceramic samples at different ent temperatures. 2 temperatures. It is well known that to obtain well-densified glass-ceramics, crystallization has to be Based on the above results, three stages of sintering process were identified. At the suppressed until most of the shrinkage has occurred. Our work demonstrated a compro- firstmised stage, balance sintering between necks sintering formed and between crystallization the adjacent could glassbe reached, particles, in order and theto yield interparticle contactdense glass-ceramics. area increased In this by regard, neck growth. two potential The routes viscous were flow proposed led to by stronger implementing inter-particle bondingtwo stages and of theheat elimination treatment at of the pores. expense The of second the crystallinity. stage had Duri a largeng that, temperature the sequence range, from ◦ approximatelyof sintering and 900 crystallization to 1100 C, can where be properly the densification adjusted: (1) was The frozen precipitation due to theof crystal- crystallization. Asline shown phases in took Figure place8, thefirst, rough then was surface followed of the by glass-ceramic liquid sintering samples at high wastemperature, observed. or Clearly, compared(2) conducting with liquid sintering, sintering the to crystallization ensure a flat surface, wasmore then subjected dominant to forlow samplestemperature containing highercrystallization. CaF2 content. The third stage was characterized by fast shrinkage, which derived from the liquid formation4. Conclusions as the temperature approached the softening temperature. At this stage, the To produce well-densified, wollastonite-based glass-ceramics with smooth appear- liquid sintering became the dominant process. CaF2 acted as flux agent to form low- temperatureance, it was suggested eutectics. the An content increasing of fluoride amount should of melt be limited would in be 2 transferred wt.%. The influences from the particle of CaF2 on the sintering and crystallization behavior can be drawn as follows: surface or bulk into the neck area via surface or volume diffusion and result in enhanced sintering(1) As the kinetics. amount In of addition, CaF2 increased, the deformation both the onset and crystallization rearrangement temperature of the particles Tx and favored flatteningtemperature the sample of maximum surfaces. shrinkage Along with Tms thefor glass-ceramics sintering, as confirmed decreased.in The XRD depoly- analysis, the merization effect of fluoride led to the enhancement in crystallization. The accompa- precipitation amount of wollastonite in equilibrium with the melt decreased. For samples nying grain boundary creation and coarsening reduced the shrinkage rate and resulted in uneven appearance. (2) At high temperature, the dominant liquid sintering brought severe shrinkage and a smooth appearance for the product. Accordingly, the precipitation amount of wollastonite decreased with the increment of CaF2, due to its characteristic fluxing effect. The dense sintering temperature range (ΔT2 = Thb − Td) was inversely related to the fluoride content, implying that high CaF2 addition was detrimental to final properties of product. In this regard, the evaluations of sintering and crystallization kinetics will be imple- mented with mathematical models in future work. More microstructural information,

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containing lower CaF2, most of the sintering necks disappeared, resulting in a flat glass- ceramic appearance while maintaining considerable crystallinity (F1-1120 ◦C, F2-1180 ◦C). F3 and F4 samples, owing to the lower flowing temperature, had a low crystallinity and exhibited over-burnt surface. It is well known that to obtain well-densified glass-ceramics, crystallization has to be suppressed until most of the shrinkage has occurred. Our work demonstrated a compro- mised balance between sintering and crystallization could be reached, in order to yield dense glass-ceramics. In this regard, two potential routes were proposed by implementing two stages of heat treatment at the expense of the crystallinity. During that, the sequence of sintering and crystallization can be properly adjusted: (1) The precipitation of crystalline phases took place first, then was followed by liquid sintering at high temperature, or (2) conducting liquid sintering to ensure a flat surface, then subjected to low temperature crystallization.

4. Conclusions To produce well-densified, wollastonite-based glass-ceramics with smooth appearance, it was suggested the content of fluoride should be limited in 2 wt.%. The influences of CaF2 on the sintering and crystallization behavior can be drawn as follows:

(1) As the amount of CaF2 increased, both the onset crystallization temperature Tx and temperature of maximum shrinkage Tms for glass-ceramics decreased. The depolymerization effect of fluoride led to the enhancement in crystallization. The accompanying grain boundary creation and coarsening reduced the shrinkage rate and resulted in uneven appearance. (2) At high temperature, the dominant liquid sintering brought severe shrinkage and a smooth appearance for the product. Accordingly, the precipitation amount of wollastonite decreased with the increment of CaF2, due to its characteristic fluxing effect. The dense sintering temperature range (∆T2 = Thb − Td) was inversely related to the fluoride content, implying that high CaF2 addition was detrimental to final properties of product. In this regard, the evaluations of sintering and crystallization kinetics will be imple- mented with mathematical models in future work. More microstructural information, such as grain coarsening and pore development, will be of great use for a better understanding of the concurrent sintering-crystallization behavior.

Author Contributions: Conceptualization, P.L.; data curation, F.P.; Funding acquisition, H.G.; inves- tigation, C.L. and X.G.; methodology, P.L. and B.Y.; project administration, J.Z. and H.G.; resources, W.Z.; validation, F.P.; writing—original draft, C.L.; writing—review and editing, P.L. and W.Z. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Project of Transformation of Scientific and Technological Achievements of Inner Mongolia Autonomous Region (2019CG073) and National Natural Science Foundation of China (51574169). Data Availability Statement: The data presented in this study are available in this article. Acknowledgments: The authors thank Soochow University and Inner Mongolia University of Science and Technology for their support with the analysis. Conflicts of Interest: The authors declare no conflict of interest.

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