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Journal of Alloys and Compounds 688 (2016) 709e714
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Journal of Alloys and Compounds
journal homepage: http: / /www.elsevier .com/locate/ ja lcom
Glass-ceramics one-step crystallization accomplished by building Ca2þ
and Mg2þ fast diffusion layer around diopside crystal
Jian Yang a, Bo Liu a, *, Shengen Zhang a, **, Alex A. Volinsky b
a Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing, 100083, PR Chinab Department of Mechanical Engineering, University of South Florida, Tampa, FL, 33620, USA
a r t i c l e i n f o
Article history:Received 25 April 2016Received in revised form20 June 2016Accepted 3 July 2016Available online 5 July 2016
Keywords:Glass-ceramicsOne-step crystallizationFast diffusion layer
* Corresponding author.** Corresponding author.
E-mail addresses: [email protected] (B. Liu), zhan(S. Zhang).
http://dx.doi.org/10.1016/j.jallcom.2016.07.0270925-8388/© 2016 Elsevier B.V. All rights reserved.
a b s t r a c t
CaOeMgOeSiO2 (CMS) glass-ceramics preparation normally needs two-step heat treatment (nucleationand crystal growth). In this work, CMS glass-ceramics are successfully produced by one-step heattreatment by building Ca2þ and Mg2þ fast diffusion layer between the residual glass and diopside crystal.High-temperature X-ray diffraction (HTXRD) shows that diopside crystals formed during the crystalli-zation of CMS glass. At the glass layer between diopside crystal and residual glass, an enrichment of Naþ
is observed by energy dispersive spectroscopy line scanning (EDSLS), which increases the SieOeNabonds that promote Ca2þ and Mg2þ ions diffusion. This layer eventually promotes further growth orripening of the diopside crystals. Thus, nucleation and crystal growth of the CMS glass proceedcontinuously at the crystallization peak temperature (Tnc), detected by differential scanning calorimetry.Thereby, one-step crystallization of the CMS glass-ceramics becomes possible and practical.
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
Glass-ceramics are polycrystalline materials produced bymelting and subsequent heat treatment [1,2]. Glass-ceramics is oneof the most promising systems due to its excellent mechanicalproperties, high abrasive resistance and adequate chemical resis-tance. In recent years, glass-ceramics have been used as a sealantfor solid oxide fuel cells [2,3], biomedicine materials [4], optical[5,6] and building materials [7].
Usually, glass-ceramics is produced by either sintering or two-step crystallization due to its poor crystallization ability [3,8,9].However, both of them have nonnegligible drawbacks: in the sin-tering process, the quenching step wastes a lot of heat and gener-ates a large amount of wastewater, resulting in environmentalpollution; meanwhile, the two-step crystallization contains twoheat treatment steps (nucleation and crystal growth) [10e12],which is highly energy-consuming and carbon-emitting. Thus, highcosts of economic and environmental protection issues involving inglass-ceramics production largely restrict their industrializedpreparation and large-scale applications [13].
For many years, researchers have been working on newmethods to reduce the manufacturing cost and carbon emissions inglass-ceramics preparation. One very promising way is the one-step heat treatment that conducts the nucleation and crystalgrowth at the same temperature (Tnc) [14]. Small temperaturedifference (DT) between the optimal nucleation (Tn) and crystalgrowth temperature (Tc) is the necessary condition for one-stepcrystallization of glass-ceramics. Generally, the Tn and Tc of an ox-ide glass are sensitive to the chemical composition of parent glass[15,16]. Hence, optimizing composition of the parent glass isconsidered as a feasible way to reduce DT, with the improvement ofnucleation capacity or crystal growth capacity of parent glass[17e20].
Recently, improving the nucleation capacity of the glass isbelieved to be significant to decrease DT [18,19]. However, to thebest of our knowledge, just a few papers reported success [14].During the crystallization of glass, glass network modifiers andnucleation agent ions are concentrated in the crystals. Then, theglass layer between the crystals and residual glass is rich in glassnetwork formers such as Al and Si. Thus, the glass layer becomes adiffusion barrier layer for ions of the crystal phase such as Ca2þ andMg2þ [11,21e23], as shown in Fig. 1. The diffusion barrier layerwould suppress crystal growth [24e26]. Thereby, higher temper-ature (Tc) for crystal growth is necessary to overcome the diffusionbarrier [27]. Therefore, avoiding the formation of diffusion barrier
Fig. 1. Schematic diagram of CMS glass one-step crystallization accomplished by building Ca2þ and Mg2þ fast diffusion layer around the diopside crystal.
J. Yang et al. / Journal of Alloys and Compounds 688 (2016) 709e714710
layer is necessary to improve the crystal growth capacity, decreaseDT and achieve one-step crystallization of glass-ceramics.
Generally, alkali metal ions are considered as glass networkmodifiers. Some researchers agree that some alkali metal ion willbond with the MO4 tetrahedron (M is a trivalent cations, such as Crand Fe) to balance the local charge in the crystal lattice [28]. And,the remaining alkali-metal ionwill escape the crystal phase and getinto the glass layer between the crystal particle and residual glass[7,29]. Then, alkali metal ions help to increase the SieOeNa bondcontent in the glass layer by transferring the SieOeSi bonds toSieOeNa bonds [7]. Consequently, the diffusion rate of Ca2þ andMg2þ in this glass layer increases, manifested as enhanced crystalgrowth ability of the glass [7,30]. Thus, alkali metal ions changes theCa2þ and Mg2þ diffusion barrier layer to a Ca2þ and Mg2þ fastdiffusion layer, as shown in Fig. 1. Further, the Ca2þ and Mg2þ fastdiffusion layer will promote the one-step crystallization of glass-ceramics.
To prove the feasibility of the above hypothesis, we tried toprepare CMS glass-ceramics by one-step crystallization. Na2O wasconsidered as the glass network modifier to build the Ca2þ andMg2þ fast diffusion layer. For comparison, CaF2 was used as thenucleation agent to improve the nucleation capacity of CMS glass.The crystallization kinetics index of the CMS glass was calculatedusing the Kissinger and Augis-Bennett equations. Experimentalevidence of the Ca2þ and Mg2þ fast diffusion layer are also pre-sented in this manuscript.
2. Experimental procedure
The raw materials were reagent grade CaO, MgO, SiO2, Al2O3,
Table 1The chemical composition of samples (g).
No. CaO MgO SiO2 Al2O3 Fe2O3 Cr2O3 CaF2 Na2Ob
GC-1a 22.15 14.53 51.33 4.84 5.81 1.33GC-2 22.15 14.53 51.33 4.84 5.81 1.33 7.00GC-3 22.15 14.53 51.33 4.84 5.81 1.33 7.00GC-4 22.15 14.53 51.33 4.84 5.81 1.33 7.00 7.00
a GC-1 is from Zhang Shengen’s paper [31].b Na2O is obtained by decomposition of Na2CO3.
Fe2O3, Cr2O3, Na2CO3 and CaF2. The compositions of 4 samples arelisted in Table 1. Homogeneous mixtures of the batches were ob-tained by dry mixing for 1 h in a ball mill. The mixtures were pre-heated at 900 �C for 2 h for Na2CO3 decomposition, then melted inalumina crucible at 1460 �C for 2 h in air. To obtain parent glass, themelts were cast into preheated steel molds and annealed at 600 �Cfor 30 min, close to the glass transition temperature, Tg. Afterannealing, the parent glass was cooled to room temperature in thefurnace.
Differential scanning calorimetry (DSC) scans were obtainedwith a NETZSCH STA 409 C/CD thermal analyzer in air. The refer-ence material was a-Al2O3 powder, and the parent glass powdersamples were heated from room temperature to 1000 �C atdifferent heating rates of 10 �C min�1, 15 �C min�1, 20 �C min�1 and30 �C min�1. The crystalline phases in the glass-ceramics sampleswere identified from X-ray diffraction (XRD) patterns, which wereobtained using an X-ray diffractometer (Philips APD-10, mono-chromatic Cu Ka radiation), at a scan rate of 9� min�1. The dynamiccrystallization process of the glasses was examined by in-situ high-temperature X-ray diffraction (HTXRD) using a 21 kW powerdiffractometer (model M21XVHF22, Mac Science, Yokohama,Japan). For the high-temperature measurements, the scan rate ofthe 2q angle was 9� min�1, and the heating rate was 10 �C min�1.The microstructure of the glass-ceramics bulk samples was inves-tigated on fresh fracture surfaces using Carl Zeiss EVO 18 scanningelectron microscope (SEM), working in the secondary electrondetectormode at 10 kV acceleration voltage. The composition of thecrystalline and glassy phases was estimated using energy disper-sive X-ray spectroscopy (EDXS) on a fracture surface.
3. Results and discussion
3.1. Crystallization kinetics parameters of CMS glasses
As seen in Fig. 2(a), no crystalline phases were detected by XRDanalysis. This indicated that the nucleation and crystal growthdidn’t occur during casting and annealing processes. The parentglass is completely amorphous. Hence, investigating one-stepcrystallization behavior of the parent glass is reasonable andreliable.
Excellent crystallization capacity and three-dimensional
Fig. 2. (a) XRD patterns of the parent glasses; (b) DSC curves at 10 K/min of the glasses, (c) Variation of ln(Tp2/a) vs. 1000/Tp for different CMAS glasses.
J. Yang et al. / Journal of Alloys and Compounds 688 (2016) 709e714 711
crystallizationmechanism are the two basic conditions of the glass-ceramics one-step crystallization. The crystallization activationenergy (E) is an important index for evaluating the crystallizationcapacity of parent glass [32]. The Avrami index (n) is directly relatedto the crystallization mechanism of parent glass [20]. Thus, DSCexperiments were used to investigate the crystallization kineticsindexes (E and n) of the parent glasses. Fig. 2(b) shows the DSCresults obtained from the parent glass powders. There is just oneexothermic peak located in the DSC curves. It is noted that theunique exothermic peak may be associated with the formation ofthe main crystallization phase. The effects of Na2O and CaF2 on thecrystallization kinetics of the CMS glass were further analyzed bythe non-isothermal DSC method. E and nwere calculated using theArrhenius, Kissinger and Augis-Bennett equations as follows[33e36]:
k ¼ n exp�� ERT
�(1)
ln
T2pa
!¼ E
RTpþ ln
ETp
� ln n (2)
n ¼ 2:5DT1
� RT2pE
(3)
Here, k is the reaction rate constant, n is the frequency factor, E isthe crystallization activation energy, R is the gas constant, T is theabsolute temperature; Tp is the crystallization peak temperature inthe DSC curve and a is the heating rate of DSC; n is the Avramiindex; DT1 is the width at half maximum of the DSC exothermicpeak. E can be calculated from the slope of ln(Tp2/a)�1000/Tp. Low Evalue indicates high crystallization ability [20]. The value of n isrelated to the crystallization mechanism of the glass, and n ¼ 1indicates surface crystallization, while n ¼ 3 implies three-dimensional crystallization [36,37].
Fig. 2(c) shows the relationship between ln(Tp2/a) and 1000/Tp.The activation energy was calculated from the slope of the obliquelines, as listed in Table 2. Compared with the sample GC-1, the Evalue of GC-2 increases slightly with the CaF2 addition. However,
Table 2Crystallization activation energy (E) and Avrami index (n) of the CMS glass samplescalculated from the Kissinger and Augis-Bennett equations.
Sample GC-1 GC-2 GC-3 GC-4
E (KJ/mol) 301.2 337.3 195.4 216.6n 1.2 3.4 2.8 2.9
the E values of the samples GC-3 (195.4 kJ/mol) and GC-4 (216.6 kJ/mol) obviously decrease with Na2O addition, compared with thesamples GC-1 (301.2 kJ/mol) and GC-2 (337.3 kJ/mol). This impliesthat Na2O improves the crystallization ability of the CMS glass.
The n value of the sample GC-1 is 1.2, indicating surface crys-tallization. Compared with GC-1, the n value increases with therespective addition of Na2O or CaF2. The n values of the GC-2, GC-3and GC-4 samples are 3.4, 2.8 and 2.9, respectively, close to 3,indicating three-dimensional crystallization. This means that bothNa2O and CaF2 promote three-dimensional crystallization.
3.2. Crystalline phases and microstructure of the CMS glass-ceramics
The above crystallization kinetics analysis implied the one-stepcrystallization potential of the CMS glasses. However, the Arrhe-nius, Kissinger and Augis-Bennett equations are based on a series offundamental assumptions. Hence, it is necessary to verify the one-step crystallization capacity of the CMS glass using experimentalresults.
Fig. 3(a) shows XRD patterns of the glass samples crystallized atTp for 1 h. The Tp is considered as Tnc for one-step crystallization ofCMS glass. The XRD pattern of GC-1 was obtained from the surfacecrystallized parts of the sample, since internal part of GC-1 was stillglass. The four crystallized glasses have the same main crystallinephase (diopside, CaMgSi2O6). This indicates that the addition ofNa2O or CaF2 has no effect on the main crystalline phase.
The SEM micrographs of the one-step crystallized glasses arepresented in Fig. 3. For the GC-1 sample, the diopside crystals grewfrom the surface to the interior of the parent glass, resulting in atypical surface crystallization microstructure, as seen in Fig. 3(b). Inthe GC-2 sample, just a few crystal nuclei grew up during the one-step heat treatment, forming mixed microstructure of the bulkdiopside phase and themassive glass phase, as seen in Fig. 3(c). Thismay be the result of high crystallization activation energy of theGC-2 sample, as discussed above.
In Fig. 3(d) and (e), a large number of granular diopside crystalswere uniformly located in the CMS glass-ceramics and formedtypical three-dimensional crystallization microstructure. This sug-gests that themajority of crystalline nuclei grew uniformlywith theaddition of Na2O. The crystal particles in Fig. 3(e) are smaller andmore homogeneous than in Fig. 3(d). Thus, it is reasonable to saythat the addition of CaF2 contributes to grain refinement and im-proves the microstructure of Na-containing CMS glass-ceramics byincreasing the crystal nucleus density.
In summary, the one-step crystallization of CMS glass was suc-cessfully achieved by adding Na2O into the parent glass. Although
Fig. 3. (a) XRD patterns of the glass-ceramics crystallized at Tp for 1 h (GC-1 with no addition, GC-2 with CaF2, GC-3 with Na2O, GC-4 with Na2O and CaF2); SEM micrographs of theglass-ceramics crystallized at Tp for 1 h: (b) GC-1 with no addition; (c) GC-2 with CaF2; (d) GC-3 with Na2O and (e) GC-4 with Na2O and CaF2.
J. Yang et al. / Journal of Alloys and Compounds 688 (2016) 709e714712
CaF2 does not have the ability to promote one-step crystallization,CaF2 contributes to refine grain and improve microstructure ofNa2O-containing CMS glass-ceramics.
3.3. The effects of Na2O and CaF2 on one-step crystallization of CMSglass
The parent glasses are metastable [38]. During the one-step heattreatment, amorphous phase separation, nucleation and crystalgrowth proceed continuously in the CMS glass at Tnc. Generally, theamorphous glass will separate into phase-separated regions andresidual glass phase during the phase separation stage. The phaseseparated regions are rich in certain elements, bringing thecomposition closer to that of the resulting crystal nucleus [23,39].Therefore, it is easier for the formation of crystal nucleus, since theatomic species required for the crystal nucleus growth are suffi-cient. Besides, the interface between the two regions can act asheterogeneous nucleation sites, which further reduces the nucle-ation barrier [9,39]. As a result, the nucleation activation energywill be decreased.
Usually, CaF2 is considered as the nucleation agent, which con-tributes to promoting phase separation and nucleation [18,19]. It isa surprising result that the Ea value of the GC-2 glass with CaF2 ishigher than GC-1 with no addition, as mentioned above. The au-thors hoped that the Ea value would decrease with adding CaF2.This concurs with the findings of other authors, where Ea increasedwith the addition of CaF2.
A possible explanation for this phenomenon is, with the addi-tion of CaF2, that there is a large crystal nucleation density duringthe isothermal hold [39]. The crystal nucleus grows to a smallextent because of the open glass network. This could lead to asituation where there is less F present in the residual glass sur-rounding the nuclei. When the nucleation ends, a fluorine-deficientlayer forms around the nuclei. Then the crystal nuclei growth issuppressed because of the closed network and lower ions diffusionmobility, which is manifested by the increase of Ea. After all, Eameasures mainly the easiness of glass crystallization rather thanthat of glass nucleation [40]. The mixed microstructure and corre-sponding explanation of crystallized glass GC-3 (as shown inFig. 3(c)) also prove the speculation above.
The GC-3 sample with Na2O has Ea values as low as 195.4 kJ/mol.The low E value indicates that the parent glass easily nucleates andcrystallizes [41]. The n value of GC-3 is close to 3, implying thatthere are enough nuclei that grew up during the crystal growth
stage [42]. All these results are confirmed by the high degree ofcrystallization (as seen in Fig. 3(a)) and the uniform crystal particlesin Fig. 3(d). In brief, it can be concluded that there are enoughnuclei formed and grew up in the GC-3 sample during the one-stepheat treatment.
The sample GC-3 completed nucleation and crystal growthcontinuously at Tnc. According to the above hypothesis, the one-step crystallization of CMS glass is accomplished by the formingof Ca2þ and Mg2þ fast diffusion layer. Thus, two experimentalphenomena should be observed in the sample GC-3: 1) Naþ willprefer to diffuse into the glass layer around the crystals; 2) theresidual glass will eventually transform into Na-containing crystalphase in midst of crystallization, with the existence of Ca2þ andMg2þ fast diffusion layer.
Fig. 4(a) shows the SEM image of the GC-3 glass heat treated at700 �C for 25 min and EDXS line scan along the trace of the straightline indicated in the SEM image. The Naþ content in the crystalparticle is obviously lower than in glass layer around the crystals, asseen in Fig. 4(a). This result indicated that Naþ diffused into theglass layer around the crystal. Thus, it is the direct evidence of thefirst predicted experimental phenomena. The reason for this phe-nomenon maybe that the chemical potential of Naþ in the glassphase is lower than in the crystal phase. As a result, the Naþ
changes the SieOeSi bond to SieOeNa bond and increases theSieOeNa bond content in the glass layer. Thus, the silica network ofthe glass layer will be more “open”. Because the SieOeNa bondscontribute to Ca2þ and Mg2þ diffusion, the glass layer turns into aCa2þ and Mg2þ fast diffusion layer. This layer will help the crystalsto grow, which is proved by the low Ea value and three-dimensioncrystallization mechanism (n ¼ 2.8) of the GC-3 glass.
To prove the second experimental speculated phenomenon, theisothermal phase formation sequence of the GC-3 glass wascollected using HTXRD. To obtain more phase evolution details,700 �C was chosen, at which the phase evolution rate is relativelyslow and convenient for detection. Fig. 4(b) displays the HTXRDpatterns recorded at 700 �C with elapsed time of 40 min. As seen inFig. 4(b), the Na-containing phases (sodium aluminum silicate andsodium silicate) appeared when the diopside crystallization pro-cess was finished. This indicates that some Naþ escaped from thediopside crystals and entered in the glass phase. At the late stage ofcrystallization, the residual glass is rich in Naþ. In the end, theseNaþ ions were bound with silicon and aluminum ions to form thesodium silicate and sodium aluminum silicate phases. This furtherconfirmed the existence of the Ca2þ and Mg2þ fast diffusion layer.
Fig. 4. (a) SEM image of the sample GC-3 heat treated at 700 �C for 25 min and EDXS line scanning along the trace of the straight line indicated in the SEM image; (b) Real-timeHTXRD of glass GC-3 containing Na2O treated at 700 �C with elapsed time of 40 min; (c) SEM image of the GC-4 sample containing Na2O and CaF2 heat treated at 730 �C for 30 minand EDXS line scanning along the trace of the straight line indicated in the SEM image; (d) Real-time HTXRD of glass GC-4 treated at 730 �C with the elapsed time of 60 min.
J. Yang et al. / Journal of Alloys and Compounds 688 (2016) 709e714 713
The sample GC-4 contains both Na2O and CaF2. The two typicalexperimental phenomena of the Ca2þ and Mg2þ fast diffusion layerwere also observed during the GC-4 glass one-step crystallization,as shown in Fig. 4(c) and (d). Unlike the Naþ distribution trend,there is no distinct F content fluctuation between the crystal phaseand glass phases, as shown in Fig. 4(c). Besides, the F-containingphase (cuspidine) and Na-containing phase (sodium aluminumsilicate, SAS) were detected near the end of crystallization stage ofthe GC-4 glass, as seen in Fig. 4(d). These experimental phenomenacan be explained by the following reasons (or conjecture). First,CaF2 induced the formation of large density crystal nucleus. How-ever, the F content fluctuation between the crystal nucleus and theglass layer is too small to be detected. Because the glass layeraround these crystal nucleus was Na-rich, the growth ability ofthese crystal nucleus was increased. With the development ofcrystallization, the F content fluctuation disappeared gradually.Finally, the residual glass crystallized into the SAS and cuspidinephases. Due to the high nuclei density and excellent crystal growthability, the GC-4 glass-ceramics has uniform compact microstruc-ture, as seen in Fig. 3(e).
All these results along with the above discussion confirm thatthe one-step crystallization of CMS glass-ceramics can be achievedby building Ca2þ and Mg2þ fast diffusion layer surrounding thecrystals. The aim of building Ca2þ and Mg2þ fast diffusion layer is toimprove the “open degree” of the glass network around the crystalsand then increase the Ca2þ and Mg2þ ions diffusion ability withinthe glass layer around crystals. Besides, based on the results and
discussion above, it is suggested that building a fast diffusion layeraround the crystals to achieve one-step crystallization may be alsoeffective in other glass-ceramics systems.
4. Conclusions
The one-step crystallization of CMS glass-ceramics was achievedby building Ca2þ and Mg2þ fast diffusion layer between the diop-side crystal and the residual glass, as evidenced by SEM-EDXS andHTXRD. The probable explanation for this phenomenon is proposedas follows. During the crystallization, diopside crystals phase outfrom the homogeneous parent glass, concomitantly forming a glasslayer between diopside crystal and residual glass. As experimen-tally observed, Naþ ions are rich in this layer, which promotes theformation of SieOeNa bond. The SieOeNa bonds further promoteCa2þ andMg2þ ions diffusion in the glass layer. This layer eventuallypromotes further growth or ripening of the diopside crystals,leading to the nucleation and crystal growth of the CMS glassproceed continuously at Tnc. Thus, one-step crystallization of theCMS glass-ceramics becomes possible and practical.
Author contributions
S. G. Zhang, J. Yang and B. Liu conceived and performed theresearch. J. Yang wrote the manuscript. Alex A. Volinsky polishedthe manuscript.
J. Yang et al. / Journal of Alloys and Compounds 688 (2016) 709e714714
Competing financial interests
The authors declare no competing financial interests.
Acknowledgments
The work was supported by the National Natural ScienceFoundation of China (U1360202, 51472030 and 51502014); Na-tional Key Project of the Scientific and Technical Support Programof China (2012BAC02B01); Fundamental Research Funds for theCentral Universities (FRF-TP-15-050A2); China Postdoctoral ScienceFoundation Funded Project (2014M560885).
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