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Effectively promoting the crystallization of lithium disilicate glass-ceramics by free oxygen in the glass
Materials Chemistry and Physics 240 (2020) 122131
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Effectively promoting the crystallization of lithium disilicate glass-ceramics by free oxygen in the glass Zhengjie Shan a, Yanan Deng b, Jingxiao Liu a, *, Fei Shi a, **, Ming Liu a, Jing Zhou a, Haojie Zhang a, Chengtie Wu c, Tianshuang Liu d a
School of Textile and Materials Engineering, Dalian Polytechnic University, Dalian, 116034, China School of Information Science and Engineering, Dalian Polytechnic University, Dalian, 116034, China Shanghai Institute of Ceramics, Chinese Academy Sciences, Shanghai, 20050, China d Dalian Stomatological Hospital, Dalian, 116021, China b c
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Lithium disilicate glass-ceramics with high fracture strength were fabricated. � Introducing bigger Rb/Cs into the glassceramics increases the “free oxygen” content. � The “free oxygen” in the glass can pro mote the crystallization of lithium disilicate.
A R T I C L E I N F O
A B S T R A C T
Keywords: Fracture strength Lithium disilicate glass-ceramics Pressureless sintering Free oxygen
Lithium disilicate glass-ceramics were prepared with SiO2–Al2O3–Li2O–CeO2–P2O5-R2O as the base system through the traditional melting method and modified with K2O, Rb2O, Cs2O, and Tb4O7. The microstructure of the lithium disilicate glass-ceramics was characterized through differential thermal analysis, X-ray diffraction and scanning electron microscopy. The whiteness and fracture strength of the lithium disilicate glass-ceramics were investigated to validate their potential applications as dental materials. Results indicated that the mixed alkali effect exerted by the introduction of K2O, Rb2O, and Cs2O greatly improved the fracture strength of the modified lithium disilicate glass-ceramics. Furthermore, X-ray photoelectron spectroscopy and whiteness anal ysis proved that free oxygen increased through the introduction of larger alkali metal ions and terbium element in the glass-ceramics. The increase in free oxygen content through the introduction of Rb2O or Cs2O into the lithium disilicate glass-ceramics could effectively promote the formation of lithium disilicate crystals and the crystallization of lithium disilicate glass-ceramics. Research results indicated that lithium disilicate glassceramics with high fracture strength under pressureless sintering had been successfully fabricated.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Liu), [email protected] (F. Shi). https://doi.org/10.1016/j.matchemphys.2019.122131 Received 14 June 2019; Received in revised form 31 August 2019; Accepted 3 September 2019 Available online 4 September 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
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Materials Chemistry and Physics 240 (2020) 122131
1. Introduction
oxygen content could improve the crystallization and fracture strength of lithium disilicate glass-ceramics. This work and the research results are important for the development of high-strength lithium disilicate glass-ceramics.
Glass ceramics have a wide range of applications in the field of biomeidical materials, such as tissue regeneration [1,2] and dental restoration materials, etc. In restoration materials field, several com panies, including Ivoclar Vivadent AGm, have developed lithium dis ilicate glass-ceramics for use as dental restoration materials [3]. Commercial dental products based on lithium disilicate glass-ceramics, such as IPS Empress 2, have typical fracture strengths of 400 � 40 MPa. The fracture strength of these ceramics remains at approximately 400 MPa even after 17 years of use [4,5]. Previous works have shown that the fracture strength of lithium disilicate glass-ceramic prepared in the laboratory through pressureless sintering can reach 676–828 MPa, but they have not provided the fracture strengths of lithium disilicate glass-ceramics measured through the conventional three-point bending method [6,7]. Glass-ceramics fabricated through pressureless sintering have three-point fracture strengths of only 224–307 MPa [8–10]. Other techniques have been applied to increase the fracture strength of lithium disilicate glass-ceramics. For example, hot pressing sintering could improve the compactness and increase the fracture strength of glass-ceramics to 308–400 MPa [4,11,12]. Lithium disilicate glass-ceramics with fracture strength of 350 MPa can be pre pared through the reaction sintering method [13]. Additionally, ce ramics with fracture strength of 340 MPa could be fabricated through zirconia toughening technology [14]. However, these techniques only enhance the fracture strength of lithium disilicate glass-ceramics externally. Therefore, how to improve the fracture strength of glass-ceramics through pressureless sintering needs to be focused on. At present, the fracture strength of lithium disilicate glass-ceramics can only be increased by adjusting their composition. For example, the addition of P2O5 as a nucleator can promote the phase separation and reduce the crystallization temperature of lithium disilicate. The maximum fracture strength of lithium disilicate glass-ceramics with 1 mol% P2O5 content could reach 310 MPa [15,16]. The introduction of Al2O3 could cause lithium silicate to crystallize during the initial heat treatment stage and then change into lithium disilicate at increased temperatures [17,18]. K2O is an alkali metal oxide that is commonly used to fabricate lithium disilicate glass-ceramics. The preparation of nearly all lithium disilicate glass-ceramics requires K2O, which can reduce the viscosity and softening point of glass and can effectively promote the formation of lithium silicate [19,20]. Alkali metal ions mainly break Si–O bonds in glass-ceramics. The reduction in field strength and oxygen-binding ability with the increment in alkali metal ion content increases free oxygen generation. The equation for lithium disilicate production and previous works show that lithium disilicate formation will consume free SiO2 in the glass [10,21]. In this work, the effects of alkali metal oxides, such as K2O, Rb2O, and Cs2O on the crystallization behavior, microstructure, and fracture strength of lithium disilicate glass-ceramics were investigated by using the SiO2–Li2O–Al2O3–P2O5-R2O glass system as the base glass. The presence of free oxygen in the glass samples was confirmed, and the increment in the free oxygen content of glasses upon the introduction of alkali metal ions that are larger than Kþ ions verified that increasing free
2. Experimental The lithium disilicate glass-ceramics based on SiO2–Li2O–Al2O3R2O–P2O5–CeO2 system were prepared according to the composition listed in Table 1, in which R2O represents alkali metal oxides such as K2O, Rb2O or Cs2O. The analytic grade SiO2, Li2CO3, R2CO3 (R ¼ K, Rb, Cs), Al(OH)3, NH4H2PO4, TiO2, ZrO2, Tb4O7 and CeO2 were chosen as the raw materials. After the batches were melted in a platinum crucible in an electric furnace at 1600 � C for 1 h in the air, the melt was poured into a preheated 30 mm � 100 mm graphite mould to get glass speci mens. The temperature measurement error of the melting furnace is �10 � C. Then the casting glass specimens were annealed immediately at 500 � C for 1.5 h with natural cooling to room temperature so as to relieve internal stress. Finally, the two-stage crystallization heattreatment was performed according to the DTA analysis results. In the experiment, the glass samples were first nucleated at 530 � C for 1 h, and then were heat-treated with two-stage crystallization at 700 � C and 700–850 � C for 1 h, respectively. The temperature error of the muffle furnace is assumed as � 5 � C. The crystallization temperatures of the glass-ceramics were deter mined by differential thermal analyzer (DTA, WCR-2D, China). The parent glass was milled and sieved to obtain the particles less than 300 μm. Then the parent glass powders were heated from room tem perature to 1100 � C with a heating rate of 10 � C min 1, 15 � C min 1 and 20 � C min 1 in the air, respectively. The temperature measurement error of the DTA is assumed as � 5 � C. The phase composition of the glassceramics was determined by X-ray diffractometer (XRD, D/max-38, Japan) using the Cu Kα (λ ¼ 0.154059 nm) at 40 kV and 30 mA with the scanning from 10� to 70� at a scanning speed of 5� min 1. The morphology of the glass-ceramics was observed by a scanning electron microscopy (SEM, JEOL JSM-7800F, Japan), using the broken surface of the specimens which were etched with 2 vol% HF solution for 30 s. The surface composition of the sample and the binding energies of Tb in the sample were determined by X-ray photoelectron spectroscopy (XPS, ESCALAB 250). The fracture strength tests were conducted for 5 times on a universal strength tester (Three-point bending tests, DZS-1, China) with a crosshead speed of 1 mm min 1. Five glass-ceramic samples with 2 � 10 � 25 mm were used for the three-point bending tests at room temperature, and then the corresponding results were used for calcu lating the average fracture strength and the standard deviation. The measurement error is assumed as � 5 N. The whiteness of the glassceramics was determined by whiteness tests (whiteness tests, HY-BDY, China).
Table 1 The glass compositions of the lithium disilicate glass-ceramics (mol%). Samples name
SiO2
Li2O
K2O
Rb2O
Cs2O
Al2O3
P2O5
CeO2
TiO2
La2O3
ZrO2
Tb4O7
SiO2/Li2O
K-2.4 R-2.4 C-2.4 RC-2.4 KRC-2.4 K-2.36 R-2.36 Tb–K Tb-RC
67.49 67.49 67.49 67.49 67.49 66.37 66.37 66.37 66.37
28.09 28.09 28.09 28.09 28.09 28.10 28.10 28.65 28.65
1.90 – – – 0.63 1.88 – 2.00
– 1.90 – 0.95 0.63 – 1.88
– – 1.90 0.95 0.63 – –
1.00
1.00
1.01 1.01 1.01 1.01 1.01 1.28 1.28 1.50 1.50
1.10 1.10 1.10 1.10 1.10 1.10 1.10 1.10 1.10
0.41 0.41 0.41 0.41 0.41 0.33 0.33 0.33 0.33
– – – – – 0.36 0.36 – –
– – – – – 0.07 0.07 – –
– – – – – 0.46 0.46 – –
– – – – – 0.05 0.05 0.05 0.05
2.40 2.40 2.40 2.40 2.40 2.36 2.36 2.31 2.31
2
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3. Results and discussion
introduced in the glass-ceramics, which may be owing to the in teractions of various alkali ions in the glasses. It is reported that the mixed-alkali effect has important influences on the conductivity and diffusivity of glasses [23,24]. As for the C-2.4 sample, the crystallization activation energy of lithium disilicate is 220.0 � 55.6 kJ mol 1, which is slightly lower than the research result of 230 � 8 kJ mol 1 reported by Thieme et al. [25,26].
3.1. DTA analysis The DTA results for the different base glass powders heated at 10 � C min 1 in air are shown in Fig. 1. In the DTA curves, Tg is the glass transition temperature; Tp1, Tp2, and Tp3 represent the crystallization temperatures of lithium silicate, lithium disilicate, and quartz, respec tively; and Tm is the melting temperature. The measurement error is assumed to be �5 � C. As shown in Fig. 1, the crystallization tempera tures of lithium silicate and lithium disilicate gradually increased as the radii of the introduced alkali metal ions increased (Csþ > Rbþ > Kþ). However, the crystallization temperature of the RC-2.4 sample was be tween that of the R-2.4 sample and C-2.4 sample, and the crystallization temperature of the KRC-2.4 sample was close to that of the R-2.4 sample. These results illustrate that the radii of alkali metal ions will directly affect the crystallization temperature of the lithium disilicate glassceramics. The absence of Tp3 from the DTA test curve of KRC-2.4 and K-2.4 but not from those of other samples indicates that the presence of Kþ ions will weaken the quartz crystal peak. The Tm of the glassceramics indicates that the introduction of different alkali metal ions will negligibly change the melting temperature of the glass, and the silicon oxide network structure of the glass is similar. The activation energies that corresponded to each DTA crystalliza tion event (Ek) were calculated in accordance with the Kissinger model given by Eq. (1) [22]: ln
β ¼ T 2p
Ek þ ln A RTp
3.2. XRD analysis The XRD patterns of different samples after heat treatment at different temperatures are shown in Fig. 2. The XRD results for K-2.4 samples are provided in Fig. 2a. Lithium disilicate began to crystallize at 700 � C. The number of lithium disilicate crystals gradually increased and that of lithium silicate crystals gradually decreased with the incre ment in temperature. Lithium disilicate crystals had almost completely crystallized when the temperature was increased to 800 � C and did not show any further changes even when the temperature reached 850 � C. The K-2.4 sample had lithium disilicate as the main crystalline phase and contained a small amount of quartz crystals. By contrast, lithium dis ilicate in the R-2.4 samples (Fig. 2b) began to crystallize at 750 � C. A small amount of lithium silicate remained when the temperature was increased to 800 � C. Lithium silicate completely transformed into lithium disilicate when the temperature was increased to 850 � C. The final crystal phase of the R-2.4 sample was the same as that of the K-2.4 sample. The XRD test results for the C-2.4 samples (Fig. 2c) show that the main crystal phase was lithium metasilicate when the heat treatment temperature was 700 � C. Lithium disilicate crystals began to crystallize at 800 � C. Comparing the XRD patterns of K-2.4, R-2.4, and C-2.4 sam ples revealed that the crystallization temperature of lithium disilicate increased with the increase in the radii of the alkali metal ions. This tendency is consistent with the DTA results. The DTA test indicates that the temperature of lithium disilicate crystallization for RC-2.4 was between that for R-2.4 and C-2.4 and that for KRC-2.4 was close to that for R-2.4. According to the above rules, the temperature of lithium disilicate crystal formation for RC-2.4 should be between 750 � C and 800 � C, whereas that for KRC-2.4 was almost close to 750 � C. However, the XRD results contradict these inferences. Spe cifically, lithium disilicate crystals began to crystallize at 700 � C and remained in RC-2.4 after the final 850 � C heat treatment (Fig. 2d). KRC2.4 underwent the same phenomenon as RC-2.4 (Fig. 2e). The almost complete formation of lithium disilicate crystals in KRC-2.4 at 700 � C indicates that crystallization temperature was considerably lower than 700 � C. Lithium silicate crystals were present as secondary crystal pha ses in RC-2.4 and KRC-2.4 along with quartz crystals after heat treat ment at 850 � C.
(1)
where β is the heating rate, Tp is the peak temperature, Ek is the acti vation energy, R is the molar gas constant (R ¼ 8.314 J mol 1 K 1), and A is the pre-exponential factor. The values of crystallization activation energy (Ek1, Ek2, and Ek3) and the first, second, and third exothermic peak (TP1, TP2, and TP3 respectively) are listed in Table 2. Table 2 shows that the crystallization activation energy of lithium disilicate gradually decreased as the radii of the introduced alkali metal ions increased when only one type of alkali metal ion was present in the glass-ceramics. These effects facilitated the generation of lithium dis ilicate. The introduction of Csþ ions will promote the formation of lithium silicate. By contrast, the introduction of alkali metal ions with a larger radius than Kþ ions can inhibit the formation of quartz. Inhibiting quartz formation is advantageous for improving the fracture strength of lithium disilicate glass-ceramics because the secondary quartz crystals cannot form pin-like interlocking structures with lithium disilicate crystals. However, the crystallization activation energy of lithium dis ilicate and quartz crystals increased when various alkali metal ions were
3.3. SEM analysis Fig. 3a–c shows the SEM images of K-2.4, R-2.4, and C-2.4 after heat treatment at 850 � C. The images show that K-2.4 had crystal diameters of between 0.5 and 0.9 μm and a crystal length of approximately 3 μm. The K-2.4 sample had small grain sizes and a broad size distribution range. The lithium disilicate crystal diameter of R-2.4 was larger than that of K-2.4. Sample R-2.4 had crystals with diameter of approximately 0.7–1.0 μm and length of approximately 3 μm. The crystals in sample C2.4 had a width of approximately 1 μm and a length of approximately 3 μm. The partially enlarged image of the sample (Fig. 3d) reveals the presence of a large number of coral-like crystals on the surfaces of lithium disilicate crystals. The above mentioned XRD analysis indicates that this coral-like crystal is quartz or lithium silicate crystals. The SEM images of RC-2.4 and KRC-2.4 are provided in Fig. 4 and those of RC-2.4 are given in Fig. 4a and b. Rod-like lithium disilicate crystals were almost absent from the SEM images. Instead, irregular spherical crystals were present in the images. The magnified SEM images show that the crystal diameter fell in the range of 100–300 nm.
Fig. 1. DTA curves of basic glasses modified with different alkali metal ions. 3
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Table 2 Crystallization temperature and crystallization activation energies of different samples of glass-ceramics. Samples name
Tg (� C)
Tp1 (� C)
Ep1 (kJ mol
K-2.4 R-2.4 C-2.4 RC-2.4 KRC-2.4
475 481 493 491 481
657 676 700 683 667
203.4 � 23.9 214.5 � 12.1 150.8 � 8.0 234.6 � 16 175.2 � 4.3
1
)
Tp2 (� C)
Ep2 (kJ mol
804 839 895 849 835
381.2 � 23.5 314.9 � 48.3 220.0 � 55.6 434.7 � 9.9 451.9 � 0.6
1
)
Tp3 (� C)
Ep3 (kJ mol 1)
Tm (� C)
– 895 914 882 –
– 392.9 � 27.3 288.8 � 49.9 669.9 � 41.6 –
984 988 987 981 989
Note: The data were obtained with heating rate β ¼ 10 � C min 1; Tg–glass transition temperature; Tp1 and Ek1–crystallization temperature and activation energy of Li2SiO3; Tp2 and Ek2–crystallization temperature and activation energy of Li2Si2O5; Tp3 and Ek3–crystallization temperature and activation energy of SiO2. The temperature measurement error of the DTA is assumed as � 5 � C.
Fig. 2. XRD patterns of the glass-ceramics modified with different alkali metal ions after heat treatment at different temperatures (Li2Si2O5, Lithium disilicate, JCPDS 40–0376; Li2SiO3, Lithium silicate, JCPDS30-0766; SiO2, Cristobalite, JCPDS 27–0605 and Quartz, JCPDS 14–0654).
Combined XRD analysis reveals that RC-2.4 samples contained high numbers of lithium silicate and quartz crystals after heat treatment. Therefore, the irregular spherical crystals were considered to be lithium silicate and quartz crystals. Only a large number of spherical crystals
were observed through SEM because the lithium disilicate crystals were occluded by numerous lithium silicate and quartz crystals. Fig. 4c and d presents the SEM images of KRC-2.4. The sizes of the lithium disilicate crystals exceeded 1.0 μm. Some of the lithium 4
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Materials Chemistry and Physics 240 (2020) 122131
Fig. 3. SEM images of different lithium disilicate glass-ceramics after heat treatment at 850 � C (a: K-2.4, b: R-2.4, c and d: C-2.4).
Fig. 4. SEM images of RC-2.4 and KRC-2.4 samples after heat treatment at 850 � C (a and b are the RC-2.4 sample, and c and d are the KRC-2.4 sample).
disilicate crystals even have diameters of 1.5 μm as inferred from an enlarged image of KRC-2.4. This characteristic indicates that lithium disilicate in the KRC sample grew rapidly, and thick lithium disilicate crystals were produced at the same heat treatment temperature and time. These results indicate that the rapid growth of lithium disilicate in KRC-2.4 accounts for the larger sizes of the lithium disilicate crystals in KRC-2.4 than those in other samples at the same heat treatment tem perature and time. It is possible that the cointroduction of various alkali metal ions (Kþ/Rbþ/Csþ ions or Rbþ/Csþ ions) into the glass-ceramics can effectively promote the crystallization of lithium disilicate
crystals. However, the cause of this phenomenon needs to be further investigated. 3.4. Fracture strength analysis Fig. 5 shows the fracture strength of different samples after heat treatment at 700 � C-900 � C. The lithium disilicate glass-ceramics that have been heat-treated at 850 � C presented the highest fracture strength among all samples. The maximum fracture strengths of RC-2.4 and KRC2.4 were 262 � 12 MPa and 271 � 14 MPa, respectively. However, the 5
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excessively high heat treatment temperatures. In fact, 900 � C is close to the glass melting point. 3.5. XPS analysis and whiteness analysis Fig. 6a shows the full XPS spectra of Tb–K and Tb–RC samples before and after heat treatment at 850 � C. The spectra were calibrated by using the C1s spectrum obtained at 284.6 eV. The signal peaks of C1s, Si2s, Si2p, O1s, Tb3d5, and Tb3d3 were the major peaks. The O1s signal peak was attributed to a large number of O elements, and the Tb3d3, Tb3d5, Si2s, and Si2p signal peaks were attributed to Tb or Si elements in the glass. The signal peaks of Cs3d3 and Cs3d5 in the XPS test spectrum of the Tb–RC sample were attributed to the introduction of elemental Cs. However, the peaks of other elements, such as P, Al, and Ce, from the full XPS spectrum were poorly resolved due to their lower content. Fig. 6b–e shows the narrow spectrum of elemental Tb in the Tb–K and Tb–RC samples before and after heat treatment. The narrow spectra of the Tb element in the Tb–K sample before and after heat treatment (Fig. 6bc) and in the Tb–RC (Fig. 6d) sample before heat treatment show almost the same Tb3d5 peak, which shows the superposition of two small peaks. Fitting the Tb3d5 peaks reveals that the binding energy of the two peaks were 1246.66 and 1240.37 eV. The Tb3d5 peak of the heat-treated Tb–RC sample (Fig. 6e) was different from that of the other three samples. The binding energies of
Fig. 5. Fracture strength of lithium disilicate glass-ceramic samples subjected to different heat treatment temperatures.
fracture strength of K-2.4 modified with only Kþ ions was only 175 � 9 MPa. Therefore, these results show that the introduction of Rbþ and Csþ can effectively improve the fracture strength of lithium dis ilicate glass-ceramics, particularly when Kþ, Rbþ, and Csþ were cointroduced. Fracture strength decreased when the heat treatment tem perature reached 900 � C because lithium disilicate crystals dissolve at
Fig. 6. (a–e) XPS test results for Tb–K and Tb–RC before and after heat treatment at 850 � C and (f) images of Tb–K and Tb-RC after heat treatment at 850 � C. 6
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the Tb–RC sample (Fig. 6e), were 1247.97 and 1241.97 eV. The shift in the Tb3d5 binding energy of the Tb–RC sample from the low state before heat treatment to the high state after crystallization indicates that the valence of the Tb element in the Tb–RC sample may have changed from Tb (III) to Tb (IV) [27]. Fig. 6f shows the photographs of Tb–K and Tb–RC after heat treatment. Tb–K and Tb–RC were white and yellowish in color, respectively. Fig. 7 shows the whiteness test results for different samples after heat treatment at 850 � C. The whiteness values of samples Tb–K and Tb–RC were lower than those of lithium disilicate glass-ceramic glasses without Tb. The yellowish colors of Tb–K and Tb–RC cannot be ascribed to their different Li2O content and instead can be attributed to the introduction of Tb and the increase in Al content. The color and whiteness of Tb–K were different from those of Tb–RC. Notably, these samples were modified with different alkali metal elements. Nevertheless, the white color of K-2.4 and RC-2.4 shows that the color of the glass-ceramics is unaffected by the type of alkali metal oxide used as a modifier. Thus, only the interaction between different alkali metal elements and Tb el ements results in color change. Therefore, as inferred from XPS results, color change may be attributed to the valence transition of Tb from Tb (III) to Tb (IV) due to oxidation during heat treatment.
into lithium disilicate glass-ceramics. For example, activation energy decreased from 381.2 kJ mol 1 to 314.6 kJ mol 1 owing to the intro duction of Kþ ions and to 220 kJ mol 1 owing to the introduction of Rbþ and Csþ ions. The amount of generated free oxygen increased with the increase in the radii of the alkali metal ions. This effect favored the formation of lithium disilicate crystals. The crystallization temperature of lithium disilicate also gradually increased as the radius of the intro duced alkali metal ions increased. Nevertheless, excessively high crys tallization temperatures will cause glass-ceramics to melt. Experimental results show that lithium disilicate glass-ceramics will melt at the heat treatment temperature of 900 � C. Therefore, although the introduction of Csþ ions could produce additional free oxygen, the heat treatment temperature of glass must be excessively high or even close to the melting point so as to enable lithium disilicate crystallization. This requirement is disadvantageous for the preparation of lithium disilicate glass-ceramics through the CAD method. However, the crystallization activation energy of lithium disilicate rapidly increased when two or more alkali ions were simultaneously introduced into the glass samples, such as RC-2.4 and KRC-2.4 samples. By contrast, the crystallinities of glasses modified with two or more alkali ions were higher than those of glasses modified with Rbþ or Csþ only. The reason for this phenomenon remains unclear. Nevertheless, in the glass-ceramics modified with two or more alkali ions, lithium disilicate has high crystallinity and the glassceramics exhibit high fracture strength. The ability of free oxygen to promote lithium disilicate crystalliza tion can also be proved through SEM and XRD analyses. The SEM images show that K-2.4 had the smallest lithium disilicate crystal sizes of only 0.5–0.9 8. The lithium disilicate crystal sizes of Rb-2.4 were between 0.7 and 1.0 μm and those of C-2.4 were approximately 1.0 μm. Crystal size increased with the increase in alkali metal ion radius. However, the presence of numerous small point-like crystals on the surfaces of the lithium disilicate crystals in Cs-2.4 may be attributed to the increment in the crystallization temperature of lithium disilicate crystal upon the introduction of Csþ and the incomplete generation of lithium disilicate at 850 � C [28]. The whiteness of lithium disilicate glass-ceramics modified with different alkali metal ions and terbium element can be explained through XPS, which indicated that the glass-ceramics contain free oxy gen and terbium element. The tendency of Tb (III) undergoing conver sion to Tb (IV) causes the color of the glass samples modified with Tb/ Rbþ or Tb/Csþ to change during heat treatment. Modifying lithium disilicate glass-ceramics with only Kþ ions did not change the valence of Tb and the color of the glass-ceramic under the same experimental conditions. Thus, free oxygen production can be increased by intro ducing Rbþ and Csþ into lithium disilicate glass-ceramics. Lithium disilicate glass-ceramics are usually prepared from a com plex glass composition system composed of multiple elements [3]. In order to further prove that the introduction of large radius alkali metal ions can effectively improve the fracture strength of lithium disilicate glass-ceramics, we designed four new lithium disilicate glass-ceramic systems (e.g. K-2.36, R-2.36, Tb–K and Tb-RC samples in Table 1) based on previous laboratory studies. In the new glass system more el ements are introduced, such as La2O3 as a toughening agent, ZrO2 and TiO2 as second nucleating agents, and Tb4O7 as a coloring agent. Table 3 shows the comparison of the fracture strengths of previously reported glass-ceramics with those obtained in the present study. The fracture strength of Kþ-modified K-2.36 in our work was 270 � 21 MPa, which was lower than the fracture strength of 310 MPa reported by Wang et al. However, the introduction of Rbþ ions increased fracture strength to 351 � 32 MPa, which exceeded the fracture strengths of lithium dis ilicate glass-ceramics prepared through pressureless sintering and even the fracture strengths of hot-pressed sintered lithium disilicate glass-ceramics. It can be expected that the fracture strength can be effectively increased through the production of free oxygen as long as Rbþ or Csþ ions are added to other lithium disilicate glass-ceramics with other compositions.
3.6. Free oxygen theory Fig. 8 shows the underlying mechanism of the generated free oxygen to promote the formation of lithium disilicate crystals upon the intro duction of alkali metal ions with large radii into the lithium disilicate glass-ceramics. Usually, lithium disilicate glass-ceramics are prepared as follows: the base glass is first fabricated and then heat-treated for transformation into lithium disilicate glass-ceramics. As shown in Fig. 8a and b, alkali metal elements mainly break Si–O bonds in the glass. When the alkali metal breaks the Si–O bonds in the glass, some small groups containing Si and O elements, which can be named as “dissociated SiO2 groups”, will be produced, and the oxygen in the “dissociated SiO2 groups” can be called as “free oxygen”. As can be seen from Equations (2) and (3), the dissociated SiO2 groups are the “raw material” for the formation of lithium silicate and lithium disilicate crystals. However, the field strength of alkali metal ions decreases with the increase of their radius, which leads to the weakening of their binding ability with free oxygen. Therefore, alkali metal ions with bigger radius will enhance the ability of glasses to generate free oxygen (see Fig. 8b and c). Thus, the amount of free oxygen has a significant effect on the crystallization of lithium disilicate glass-ceramics (see Fig. 8c and d). SiO2(liquid)þLi2O(liquid)—Li2SiO3(Crystal)
(2)
Li2SiO3(Crystal)þ SiO2(liquid)—Li2Si2O5(Crystal)
(3)
The results of DTA and the crystallization activation energy of lithium disilicate show that the activation energy of lithium disilicate has decreased after the introduction of alkali metal ions with large radii
Fig. 7. Whiteness test of different samples after crystallization at 850 � C. 7
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Fig. 8. The underlying mechanism of the free oxygen formation and promoting the growth of lithium disilicate crystals. Table 3 Comparison of three-point fracture strengths obtained in previous works and the current study. Authors
Heat treatment
Samples composition
Alkali metal ions
Fracture strength (MPa)
Our K-2.36 Our R-2.36 Ting Zhao et al. [9] Fernandes et al. [19] Fernandes et al. [29] Wen et al. [12] Yuan et al. [9] Denry et al. [30] Drummond et al. [31] H€ oland et al. [4] Wang et al. [15]
Pressureless at 850 � C Pressureless at 850 � C Pressureless at 910 � C Pressureless at 900 � C Pressureless at 900 � C Hot pressing at 820 � C Hot pressing at 900 � C Hot pressing Hot pressing at 930 � C Hot pressing Pressureless at 830 � C
SiO2–Li2O–ZrO2–P2O5–Al2O3–K2O–CeO–Tb4O7–TiO2–La2O3 SiO2–Li2O–ZrO2–P2O5–Al2O3–Rb2O–CeO–Tb4O7–TiO2–La2O3 SiO2–Li2O–ZrO2–P2O5–Al2O3–K2O–CeO SiO2–Li2O–Al2O3–K2O SiO2–Li2O–Al2O3–K2O SiO2–Li2O–ZrO2–P2O5–Al2O3–K2O SiO2–Li2O–ZrO2–P2O5–Al2O3–K2O-others IPS Empress 2 LD GCs IPS Empress 2 SiO2–Li2O–ZrO2–P2O5–Al2O3–K2O–CeO–MgO–CaO
Kþ Rbþ Kþ Kþ Kþ Kþ Kþ Unknown Unknown Unknown Kþ
270 � 21 351 � 32 282 � 14 224 � 4 201 � 14 290 � 10 308 � 25 280 � 53 205 400 � 40 310
4. Conclusion [3]
In this work, lithium disilicate glass-ceramic containing various al kali metal ions were prepared. It has been clarified that free oxygen exists in the lithium disilicate glass-ceramics and the amount of free oxygen will increase with the increase of alkali metal ions radii. More over, free oxygen can increase the crystallinity and crystal size of lithium disilicate and thus is favorable for improving the fracture strength of lithium disilicate glass-ceramics. Particularly, the as-prepared lithium disilicate glass-ceramics with introducing Rb element shows higher fracture strength than other current studies. The experimental results indicate that lithium disilicate glass-ceramics with a fracture strength of 351 � 32 MPa could be prepared via pressureless sintering by increasing the free oxygen in the glass-ceramics. This result and the discovery of free oxygen promoting lithium disilicate crystallization are of great significance for preparing high-strength lithium disilicate glassceramics.
[4]
[5]
[6]
[7]
[8]
Acknowledgements [9]
This work is supported by the Opening Project of State Key Labora tory of High Performance Ceramics and Superfine Microstructure (SKL201514SIC), the National Natural Science Foundation of China (No. 51778098), the 2016 Dalian City Construction Science & Technology Project ([2016] 415), and Dalian Science & Technology Innovation Fund (2018J12SN066).
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Effectively promoting the crystallization of lithium disilicate glass-ceramics by free oxygen in the glass Zhengjie Shan a, Yanan Deng b, Jingxiao Liu a, *, Fei Shi a, **, Ming Liu a, Jing Zhou a, Haojie Zhang a, Chengtie Wu c, Tianshuang Liu d a
School of Textile and Materials Engineering, Dalian Polytechnic University, Dalian, 116034, China School of Information Science and Engineering, Dalian Polytechnic University, Dalian, 116034, China Shanghai Institute of Ceramics, Chinese Academy Sciences, Shanghai, 20050, China d Dalian Stomatological Hospital, Dalian, 116021, China b c
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Lithium disilicate glass-ceramics with high fracture strength were fabricated. � Introducing bigger Rb/Cs into the glassceramics increases the “free oxygen” content. � The “free oxygen” in the glass can pro mote the crystallization of lithium disilicate.
A R T I C L E I N F O
A B S T R A C T
Keywords: Fracture strength Lithium disilicate glass-ceramics Pressureless sintering Free oxygen
Lithium disilicate glass-ceramics were prepared with SiO2–Al2O3–Li2O–CeO2–P2O5-R2O as the base system through the traditional melting method and modified with K2O, Rb2O, Cs2O, and Tb4O7. The microstructure of the lithium disilicate glass-ceramics was characterized through differential thermal analysis, X-ray diffraction and scanning electron microscopy. The whiteness and fracture strength of the lithium disilicate glass-ceramics were investigated to validate their potential applications as dental materials. Results indicated that the mixed alkali effect exerted by the introduction of K2O, Rb2O, and Cs2O greatly improved the fracture strength of the modified lithium disilicate glass-ceramics. Furthermore, X-ray photoelectron spectroscopy and whiteness anal ysis proved that free oxygen increased through the introduction of larger alkali metal ions and terbium element in the glass-ceramics. The increase in free oxygen content through the introduction of Rb2O or Cs2O into the lithium disilicate glass-ceramics could effectively promote the formation of lithium disilicate crystals and the crystallization of lithium disilicate glass-ceramics. Research results indicated that lithium disilicate glassceramics with high fracture strength under pressureless sintering had been successfully fabricated.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Liu), [email protected] (F. Shi). https://doi.org/10.1016/j.matchemphys.2019.122131 Received 14 June 2019; Received in revised form 31 August 2019; Accepted 3 September 2019 Available online 4 September 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
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Materials Chemistry and Physics 240 (2020) 122131
1. Introduction
oxygen content could improve the crystallization and fracture strength of lithium disilicate glass-ceramics. This work and the research results are important for the development of high-strength lithium disilicate glass-ceramics.
Glass ceramics have a wide range of applications in the field of biomeidical materials, such as tissue regeneration [1,2] and dental restoration materials, etc. In restoration materials field, several com panies, including Ivoclar Vivadent AGm, have developed lithium dis ilicate glass-ceramics for use as dental restoration materials [3]. Commercial dental products based on lithium disilicate glass-ceramics, such as IPS Empress 2, have typical fracture strengths of 400 � 40 MPa. The fracture strength of these ceramics remains at approximately 400 MPa even after 17 years of use [4,5]. Previous works have shown that the fracture strength of lithium disilicate glass-ceramic prepared in the laboratory through pressureless sintering can reach 676–828 MPa, but they have not provided the fracture strengths of lithium disilicate glass-ceramics measured through the conventional three-point bending method [6,7]. Glass-ceramics fabricated through pressureless sintering have three-point fracture strengths of only 224–307 MPa [8–10]. Other techniques have been applied to increase the fracture strength of lithium disilicate glass-ceramics. For example, hot pressing sintering could improve the compactness and increase the fracture strength of glass-ceramics to 308–400 MPa [4,11,12]. Lithium disilicate glass-ceramics with fracture strength of 350 MPa can be pre pared through the reaction sintering method [13]. Additionally, ce ramics with fracture strength of 340 MPa could be fabricated through zirconia toughening technology [14]. However, these techniques only enhance the fracture strength of lithium disilicate glass-ceramics externally. Therefore, how to improve the fracture strength of glass-ceramics through pressureless sintering needs to be focused on. At present, the fracture strength of lithium disilicate glass-ceramics can only be increased by adjusting their composition. For example, the addition of P2O5 as a nucleator can promote the phase separation and reduce the crystallization temperature of lithium disilicate. The maximum fracture strength of lithium disilicate glass-ceramics with 1 mol% P2O5 content could reach 310 MPa [15,16]. The introduction of Al2O3 could cause lithium silicate to crystallize during the initial heat treatment stage and then change into lithium disilicate at increased temperatures [17,18]. K2O is an alkali metal oxide that is commonly used to fabricate lithium disilicate glass-ceramics. The preparation of nearly all lithium disilicate glass-ceramics requires K2O, which can reduce the viscosity and softening point of glass and can effectively promote the formation of lithium silicate [19,20]. Alkali metal ions mainly break Si–O bonds in glass-ceramics. The reduction in field strength and oxygen-binding ability with the increment in alkali metal ion content increases free oxygen generation. The equation for lithium disilicate production and previous works show that lithium disilicate formation will consume free SiO2 in the glass [10,21]. In this work, the effects of alkali metal oxides, such as K2O, Rb2O, and Cs2O on the crystallization behavior, microstructure, and fracture strength of lithium disilicate glass-ceramics were investigated by using the SiO2–Li2O–Al2O3–P2O5-R2O glass system as the base glass. The presence of free oxygen in the glass samples was confirmed, and the increment in the free oxygen content of glasses upon the introduction of alkali metal ions that are larger than Kþ ions verified that increasing free
2. Experimental The lithium disilicate glass-ceramics based on SiO2–Li2O–Al2O3R2O–P2O5–CeO2 system were prepared according to the composition listed in Table 1, in which R2O represents alkali metal oxides such as K2O, Rb2O or Cs2O. The analytic grade SiO2, Li2CO3, R2CO3 (R ¼ K, Rb, Cs), Al(OH)3, NH4H2PO4, TiO2, ZrO2, Tb4O7 and CeO2 were chosen as the raw materials. After the batches were melted in a platinum crucible in an electric furnace at 1600 � C for 1 h in the air, the melt was poured into a preheated 30 mm � 100 mm graphite mould to get glass speci mens. The temperature measurement error of the melting furnace is �10 � C. Then the casting glass specimens were annealed immediately at 500 � C for 1.5 h with natural cooling to room temperature so as to relieve internal stress. Finally, the two-stage crystallization heattreatment was performed according to the DTA analysis results. In the experiment, the glass samples were first nucleated at 530 � C for 1 h, and then were heat-treated with two-stage crystallization at 700 � C and 700–850 � C for 1 h, respectively. The temperature error of the muffle furnace is assumed as � 5 � C. The crystallization temperatures of the glass-ceramics were deter mined by differential thermal analyzer (DTA, WCR-2D, China). The parent glass was milled and sieved to obtain the particles less than 300 μm. Then the parent glass powders were heated from room tem perature to 1100 � C with a heating rate of 10 � C min 1, 15 � C min 1 and 20 � C min 1 in the air, respectively. The temperature measurement error of the DTA is assumed as � 5 � C. The phase composition of the glassceramics was determined by X-ray diffractometer (XRD, D/max-38, Japan) using the Cu Kα (λ ¼ 0.154059 nm) at 40 kV and 30 mA with the scanning from 10� to 70� at a scanning speed of 5� min 1. The morphology of the glass-ceramics was observed by a scanning electron microscopy (SEM, JEOL JSM-7800F, Japan), using the broken surface of the specimens which were etched with 2 vol% HF solution for 30 s. The surface composition of the sample and the binding energies of Tb in the sample were determined by X-ray photoelectron spectroscopy (XPS, ESCALAB 250). The fracture strength tests were conducted for 5 times on a universal strength tester (Three-point bending tests, DZS-1, China) with a crosshead speed of 1 mm min 1. Five glass-ceramic samples with 2 � 10 � 25 mm were used for the three-point bending tests at room temperature, and then the corresponding results were used for calcu lating the average fracture strength and the standard deviation. The measurement error is assumed as � 5 N. The whiteness of the glassceramics was determined by whiteness tests (whiteness tests, HY-BDY, China).
Table 1 The glass compositions of the lithium disilicate glass-ceramics (mol%). Samples name
SiO2
Li2O
K2O
Rb2O
Cs2O
Al2O3
P2O5
CeO2
TiO2
La2O3
ZrO2
Tb4O7
SiO2/Li2O
K-2.4 R-2.4 C-2.4 RC-2.4 KRC-2.4 K-2.36 R-2.36 Tb–K Tb-RC
67.49 67.49 67.49 67.49 67.49 66.37 66.37 66.37 66.37
28.09 28.09 28.09 28.09 28.09 28.10 28.10 28.65 28.65
1.90 – – – 0.63 1.88 – 2.00
– 1.90 – 0.95 0.63 – 1.88
– – 1.90 0.95 0.63 – –
1.00
1.00
1.01 1.01 1.01 1.01 1.01 1.28 1.28 1.50 1.50
1.10 1.10 1.10 1.10 1.10 1.10 1.10 1.10 1.10
0.41 0.41 0.41 0.41 0.41 0.33 0.33 0.33 0.33
– – – – – 0.36 0.36 – –
– – – – – 0.07 0.07 – –
– – – – – 0.46 0.46 – –
– – – – – 0.05 0.05 0.05 0.05
2.40 2.40 2.40 2.40 2.40 2.36 2.36 2.31 2.31
2
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3. Results and discussion
introduced in the glass-ceramics, which may be owing to the in teractions of various alkali ions in the glasses. It is reported that the mixed-alkali effect has important influences on the conductivity and diffusivity of glasses [23,24]. As for the C-2.4 sample, the crystallization activation energy of lithium disilicate is 220.0 � 55.6 kJ mol 1, which is slightly lower than the research result of 230 � 8 kJ mol 1 reported by Thieme et al. [25,26].
3.1. DTA analysis The DTA results for the different base glass powders heated at 10 � C min 1 in air are shown in Fig. 1. In the DTA curves, Tg is the glass transition temperature; Tp1, Tp2, and Tp3 represent the crystallization temperatures of lithium silicate, lithium disilicate, and quartz, respec tively; and Tm is the melting temperature. The measurement error is assumed to be �5 � C. As shown in Fig. 1, the crystallization tempera tures of lithium silicate and lithium disilicate gradually increased as the radii of the introduced alkali metal ions increased (Csþ > Rbþ > Kþ). However, the crystallization temperature of the RC-2.4 sample was be tween that of the R-2.4 sample and C-2.4 sample, and the crystallization temperature of the KRC-2.4 sample was close to that of the R-2.4 sample. These results illustrate that the radii of alkali metal ions will directly affect the crystallization temperature of the lithium disilicate glassceramics. The absence of Tp3 from the DTA test curve of KRC-2.4 and K-2.4 but not from those of other samples indicates that the presence of Kþ ions will weaken the quartz crystal peak. The Tm of the glassceramics indicates that the introduction of different alkali metal ions will negligibly change the melting temperature of the glass, and the silicon oxide network structure of the glass is similar. The activation energies that corresponded to each DTA crystalliza tion event (Ek) were calculated in accordance with the Kissinger model given by Eq. (1) [22]: ln
β ¼ T 2p
Ek þ ln A RTp
3.2. XRD analysis The XRD patterns of different samples after heat treatment at different temperatures are shown in Fig. 2. The XRD results for K-2.4 samples are provided in Fig. 2a. Lithium disilicate began to crystallize at 700 � C. The number of lithium disilicate crystals gradually increased and that of lithium silicate crystals gradually decreased with the incre ment in temperature. Lithium disilicate crystals had almost completely crystallized when the temperature was increased to 800 � C and did not show any further changes even when the temperature reached 850 � C. The K-2.4 sample had lithium disilicate as the main crystalline phase and contained a small amount of quartz crystals. By contrast, lithium dis ilicate in the R-2.4 samples (Fig. 2b) began to crystallize at 750 � C. A small amount of lithium silicate remained when the temperature was increased to 800 � C. Lithium silicate completely transformed into lithium disilicate when the temperature was increased to 850 � C. The final crystal phase of the R-2.4 sample was the same as that of the K-2.4 sample. The XRD test results for the C-2.4 samples (Fig. 2c) show that the main crystal phase was lithium metasilicate when the heat treatment temperature was 700 � C. Lithium disilicate crystals began to crystallize at 800 � C. Comparing the XRD patterns of K-2.4, R-2.4, and C-2.4 sam ples revealed that the crystallization temperature of lithium disilicate increased with the increase in the radii of the alkali metal ions. This tendency is consistent with the DTA results. The DTA test indicates that the temperature of lithium disilicate crystallization for RC-2.4 was between that for R-2.4 and C-2.4 and that for KRC-2.4 was close to that for R-2.4. According to the above rules, the temperature of lithium disilicate crystal formation for RC-2.4 should be between 750 � C and 800 � C, whereas that for KRC-2.4 was almost close to 750 � C. However, the XRD results contradict these inferences. Spe cifically, lithium disilicate crystals began to crystallize at 700 � C and remained in RC-2.4 after the final 850 � C heat treatment (Fig. 2d). KRC2.4 underwent the same phenomenon as RC-2.4 (Fig. 2e). The almost complete formation of lithium disilicate crystals in KRC-2.4 at 700 � C indicates that crystallization temperature was considerably lower than 700 � C. Lithium silicate crystals were present as secondary crystal pha ses in RC-2.4 and KRC-2.4 along with quartz crystals after heat treat ment at 850 � C.
(1)
where β is the heating rate, Tp is the peak temperature, Ek is the acti vation energy, R is the molar gas constant (R ¼ 8.314 J mol 1 K 1), and A is the pre-exponential factor. The values of crystallization activation energy (Ek1, Ek2, and Ek3) and the first, second, and third exothermic peak (TP1, TP2, and TP3 respectively) are listed in Table 2. Table 2 shows that the crystallization activation energy of lithium disilicate gradually decreased as the radii of the introduced alkali metal ions increased when only one type of alkali metal ion was present in the glass-ceramics. These effects facilitated the generation of lithium dis ilicate. The introduction of Csþ ions will promote the formation of lithium silicate. By contrast, the introduction of alkali metal ions with a larger radius than Kþ ions can inhibit the formation of quartz. Inhibiting quartz formation is advantageous for improving the fracture strength of lithium disilicate glass-ceramics because the secondary quartz crystals cannot form pin-like interlocking structures with lithium disilicate crystals. However, the crystallization activation energy of lithium dis ilicate and quartz crystals increased when various alkali metal ions were
3.3. SEM analysis Fig. 3a–c shows the SEM images of K-2.4, R-2.4, and C-2.4 after heat treatment at 850 � C. The images show that K-2.4 had crystal diameters of between 0.5 and 0.9 μm and a crystal length of approximately 3 μm. The K-2.4 sample had small grain sizes and a broad size distribution range. The lithium disilicate crystal diameter of R-2.4 was larger than that of K-2.4. Sample R-2.4 had crystals with diameter of approximately 0.7–1.0 μm and length of approximately 3 μm. The crystals in sample C2.4 had a width of approximately 1 μm and a length of approximately 3 μm. The partially enlarged image of the sample (Fig. 3d) reveals the presence of a large number of coral-like crystals on the surfaces of lithium disilicate crystals. The above mentioned XRD analysis indicates that this coral-like crystal is quartz or lithium silicate crystals. The SEM images of RC-2.4 and KRC-2.4 are provided in Fig. 4 and those of RC-2.4 are given in Fig. 4a and b. Rod-like lithium disilicate crystals were almost absent from the SEM images. Instead, irregular spherical crystals were present in the images. The magnified SEM images show that the crystal diameter fell in the range of 100–300 nm.
Fig. 1. DTA curves of basic glasses modified with different alkali metal ions. 3
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Table 2 Crystallization temperature and crystallization activation energies of different samples of glass-ceramics. Samples name
Tg (� C)
Tp1 (� C)
Ep1 (kJ mol
K-2.4 R-2.4 C-2.4 RC-2.4 KRC-2.4
475 481 493 491 481
657 676 700 683 667
203.4 � 23.9 214.5 � 12.1 150.8 � 8.0 234.6 � 16 175.2 � 4.3
1
)
Tp2 (� C)
Ep2 (kJ mol
804 839 895 849 835
381.2 � 23.5 314.9 � 48.3 220.0 � 55.6 434.7 � 9.9 451.9 � 0.6
1
)
Tp3 (� C)
Ep3 (kJ mol 1)
Tm (� C)
– 895 914 882 –
– 392.9 � 27.3 288.8 � 49.9 669.9 � 41.6 –
984 988 987 981 989
Note: The data were obtained with heating rate β ¼ 10 � C min 1; Tg–glass transition temperature; Tp1 and Ek1–crystallization temperature and activation energy of Li2SiO3; Tp2 and Ek2–crystallization temperature and activation energy of Li2Si2O5; Tp3 and Ek3–crystallization temperature and activation energy of SiO2. The temperature measurement error of the DTA is assumed as � 5 � C.
Fig. 2. XRD patterns of the glass-ceramics modified with different alkali metal ions after heat treatment at different temperatures (Li2Si2O5, Lithium disilicate, JCPDS 40–0376; Li2SiO3, Lithium silicate, JCPDS30-0766; SiO2, Cristobalite, JCPDS 27–0605 and Quartz, JCPDS 14–0654).
Combined XRD analysis reveals that RC-2.4 samples contained high numbers of lithium silicate and quartz crystals after heat treatment. Therefore, the irregular spherical crystals were considered to be lithium silicate and quartz crystals. Only a large number of spherical crystals
were observed through SEM because the lithium disilicate crystals were occluded by numerous lithium silicate and quartz crystals. Fig. 4c and d presents the SEM images of KRC-2.4. The sizes of the lithium disilicate crystals exceeded 1.0 μm. Some of the lithium 4
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Fig. 3. SEM images of different lithium disilicate glass-ceramics after heat treatment at 850 � C (a: K-2.4, b: R-2.4, c and d: C-2.4).
Fig. 4. SEM images of RC-2.4 and KRC-2.4 samples after heat treatment at 850 � C (a and b are the RC-2.4 sample, and c and d are the KRC-2.4 sample).
disilicate crystals even have diameters of 1.5 μm as inferred from an enlarged image of KRC-2.4. This characteristic indicates that lithium disilicate in the KRC sample grew rapidly, and thick lithium disilicate crystals were produced at the same heat treatment temperature and time. These results indicate that the rapid growth of lithium disilicate in KRC-2.4 accounts for the larger sizes of the lithium disilicate crystals in KRC-2.4 than those in other samples at the same heat treatment tem perature and time. It is possible that the cointroduction of various alkali metal ions (Kþ/Rbþ/Csþ ions or Rbþ/Csþ ions) into the glass-ceramics can effectively promote the crystallization of lithium disilicate
crystals. However, the cause of this phenomenon needs to be further investigated. 3.4. Fracture strength analysis Fig. 5 shows the fracture strength of different samples after heat treatment at 700 � C-900 � C. The lithium disilicate glass-ceramics that have been heat-treated at 850 � C presented the highest fracture strength among all samples. The maximum fracture strengths of RC-2.4 and KRC2.4 were 262 � 12 MPa and 271 � 14 MPa, respectively. However, the 5
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excessively high heat treatment temperatures. In fact, 900 � C is close to the glass melting point. 3.5. XPS analysis and whiteness analysis Fig. 6a shows the full XPS spectra of Tb–K and Tb–RC samples before and after heat treatment at 850 � C. The spectra were calibrated by using the C1s spectrum obtained at 284.6 eV. The signal peaks of C1s, Si2s, Si2p, O1s, Tb3d5, and Tb3d3 were the major peaks. The O1s signal peak was attributed to a large number of O elements, and the Tb3d3, Tb3d5, Si2s, and Si2p signal peaks were attributed to Tb or Si elements in the glass. The signal peaks of Cs3d3 and Cs3d5 in the XPS test spectrum of the Tb–RC sample were attributed to the introduction of elemental Cs. However, the peaks of other elements, such as P, Al, and Ce, from the full XPS spectrum were poorly resolved due to their lower content. Fig. 6b–e shows the narrow spectrum of elemental Tb in the Tb–K and Tb–RC samples before and after heat treatment. The narrow spectra of the Tb element in the Tb–K sample before and after heat treatment (Fig. 6bc) and in the Tb–RC (Fig. 6d) sample before heat treatment show almost the same Tb3d5 peak, which shows the superposition of two small peaks. Fitting the Tb3d5 peaks reveals that the binding energy of the two peaks were 1246.66 and 1240.37 eV. The Tb3d5 peak of the heat-treated Tb–RC sample (Fig. 6e) was different from that of the other three samples. The binding energies of
Fig. 5. Fracture strength of lithium disilicate glass-ceramic samples subjected to different heat treatment temperatures.
fracture strength of K-2.4 modified with only Kþ ions was only 175 � 9 MPa. Therefore, these results show that the introduction of Rbþ and Csþ can effectively improve the fracture strength of lithium dis ilicate glass-ceramics, particularly when Kþ, Rbþ, and Csþ were cointroduced. Fracture strength decreased when the heat treatment tem perature reached 900 � C because lithium disilicate crystals dissolve at
Fig. 6. (a–e) XPS test results for Tb–K and Tb–RC before and after heat treatment at 850 � C and (f) images of Tb–K and Tb-RC after heat treatment at 850 � C. 6
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the Tb–RC sample (Fig. 6e), were 1247.97 and 1241.97 eV. The shift in the Tb3d5 binding energy of the Tb–RC sample from the low state before heat treatment to the high state after crystallization indicates that the valence of the Tb element in the Tb–RC sample may have changed from Tb (III) to Tb (IV) [27]. Fig. 6f shows the photographs of Tb–K and Tb–RC after heat treatment. Tb–K and Tb–RC were white and yellowish in color, respectively. Fig. 7 shows the whiteness test results for different samples after heat treatment at 850 � C. The whiteness values of samples Tb–K and Tb–RC were lower than those of lithium disilicate glass-ceramic glasses without Tb. The yellowish colors of Tb–K and Tb–RC cannot be ascribed to their different Li2O content and instead can be attributed to the introduction of Tb and the increase in Al content. The color and whiteness of Tb–K were different from those of Tb–RC. Notably, these samples were modified with different alkali metal elements. Nevertheless, the white color of K-2.4 and RC-2.4 shows that the color of the glass-ceramics is unaffected by the type of alkali metal oxide used as a modifier. Thus, only the interaction between different alkali metal elements and Tb el ements results in color change. Therefore, as inferred from XPS results, color change may be attributed to the valence transition of Tb from Tb (III) to Tb (IV) due to oxidation during heat treatment.
into lithium disilicate glass-ceramics. For example, activation energy decreased from 381.2 kJ mol 1 to 314.6 kJ mol 1 owing to the intro duction of Kþ ions and to 220 kJ mol 1 owing to the introduction of Rbþ and Csþ ions. The amount of generated free oxygen increased with the increase in the radii of the alkali metal ions. This effect favored the formation of lithium disilicate crystals. The crystallization temperature of lithium disilicate also gradually increased as the radius of the intro duced alkali metal ions increased. Nevertheless, excessively high crys tallization temperatures will cause glass-ceramics to melt. Experimental results show that lithium disilicate glass-ceramics will melt at the heat treatment temperature of 900 � C. Therefore, although the introduction of Csþ ions could produce additional free oxygen, the heat treatment temperature of glass must be excessively high or even close to the melting point so as to enable lithium disilicate crystallization. This requirement is disadvantageous for the preparation of lithium disilicate glass-ceramics through the CAD method. However, the crystallization activation energy of lithium disilicate rapidly increased when two or more alkali ions were simultaneously introduced into the glass samples, such as RC-2.4 and KRC-2.4 samples. By contrast, the crystallinities of glasses modified with two or more alkali ions were higher than those of glasses modified with Rbþ or Csþ only. The reason for this phenomenon remains unclear. Nevertheless, in the glass-ceramics modified with two or more alkali ions, lithium disilicate has high crystallinity and the glassceramics exhibit high fracture strength. The ability of free oxygen to promote lithium disilicate crystalliza tion can also be proved through SEM and XRD analyses. The SEM images show that K-2.4 had the smallest lithium disilicate crystal sizes of only 0.5–0.9 8. The lithium disilicate crystal sizes of Rb-2.4 were between 0.7 and 1.0 μm and those of C-2.4 were approximately 1.0 μm. Crystal size increased with the increase in alkali metal ion radius. However, the presence of numerous small point-like crystals on the surfaces of the lithium disilicate crystals in Cs-2.4 may be attributed to the increment in the crystallization temperature of lithium disilicate crystal upon the introduction of Csþ and the incomplete generation of lithium disilicate at 850 � C [28]. The whiteness of lithium disilicate glass-ceramics modified with different alkali metal ions and terbium element can be explained through XPS, which indicated that the glass-ceramics contain free oxy gen and terbium element. The tendency of Tb (III) undergoing conver sion to Tb (IV) causes the color of the glass samples modified with Tb/ Rbþ or Tb/Csþ to change during heat treatment. Modifying lithium disilicate glass-ceramics with only Kþ ions did not change the valence of Tb and the color of the glass-ceramic under the same experimental conditions. Thus, free oxygen production can be increased by intro ducing Rbþ and Csþ into lithium disilicate glass-ceramics. Lithium disilicate glass-ceramics are usually prepared from a com plex glass composition system composed of multiple elements [3]. In order to further prove that the introduction of large radius alkali metal ions can effectively improve the fracture strength of lithium disilicate glass-ceramics, we designed four new lithium disilicate glass-ceramic systems (e.g. K-2.36, R-2.36, Tb–K and Tb-RC samples in Table 1) based on previous laboratory studies. In the new glass system more el ements are introduced, such as La2O3 as a toughening agent, ZrO2 and TiO2 as second nucleating agents, and Tb4O7 as a coloring agent. Table 3 shows the comparison of the fracture strengths of previously reported glass-ceramics with those obtained in the present study. The fracture strength of Kþ-modified K-2.36 in our work was 270 � 21 MPa, which was lower than the fracture strength of 310 MPa reported by Wang et al. However, the introduction of Rbþ ions increased fracture strength to 351 � 32 MPa, which exceeded the fracture strengths of lithium dis ilicate glass-ceramics prepared through pressureless sintering and even the fracture strengths of hot-pressed sintered lithium disilicate glass-ceramics. It can be expected that the fracture strength can be effectively increased through the production of free oxygen as long as Rbþ or Csþ ions are added to other lithium disilicate glass-ceramics with other compositions.
3.6. Free oxygen theory Fig. 8 shows the underlying mechanism of the generated free oxygen to promote the formation of lithium disilicate crystals upon the intro duction of alkali metal ions with large radii into the lithium disilicate glass-ceramics. Usually, lithium disilicate glass-ceramics are prepared as follows: the base glass is first fabricated and then heat-treated for transformation into lithium disilicate glass-ceramics. As shown in Fig. 8a and b, alkali metal elements mainly break Si–O bonds in the glass. When the alkali metal breaks the Si–O bonds in the glass, some small groups containing Si and O elements, which can be named as “dissociated SiO2 groups”, will be produced, and the oxygen in the “dissociated SiO2 groups” can be called as “free oxygen”. As can be seen from Equations (2) and (3), the dissociated SiO2 groups are the “raw material” for the formation of lithium silicate and lithium disilicate crystals. However, the field strength of alkali metal ions decreases with the increase of their radius, which leads to the weakening of their binding ability with free oxygen. Therefore, alkali metal ions with bigger radius will enhance the ability of glasses to generate free oxygen (see Fig. 8b and c). Thus, the amount of free oxygen has a significant effect on the crystallization of lithium disilicate glass-ceramics (see Fig. 8c and d). SiO2(liquid)þLi2O(liquid)—Li2SiO3(Crystal)
(2)
Li2SiO3(Crystal)þ SiO2(liquid)—Li2Si2O5(Crystal)
(3)
The results of DTA and the crystallization activation energy of lithium disilicate show that the activation energy of lithium disilicate has decreased after the introduction of alkali metal ions with large radii
Fig. 7. Whiteness test of different samples after crystallization at 850 � C. 7
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Materials Chemistry and Physics 240 (2020) 122131
Fig. 8. The underlying mechanism of the free oxygen formation and promoting the growth of lithium disilicate crystals. Table 3 Comparison of three-point fracture strengths obtained in previous works and the current study. Authors
Heat treatment
Samples composition
Alkali metal ions
Fracture strength (MPa)
Our K-2.36 Our R-2.36 Ting Zhao et al. [9] Fernandes et al. [19] Fernandes et al. [29] Wen et al. [12] Yuan et al. [9] Denry et al. [30] Drummond et al. [31] H€ oland et al. [4] Wang et al. [15]
Pressureless at 850 � C Pressureless at 850 � C Pressureless at 910 � C Pressureless at 900 � C Pressureless at 900 � C Hot pressing at 820 � C Hot pressing at 900 � C Hot pressing Hot pressing at 930 � C Hot pressing Pressureless at 830 � C
SiO2–Li2O–ZrO2–P2O5–Al2O3–K2O–CeO–Tb4O7–TiO2–La2O3 SiO2–Li2O–ZrO2–P2O5–Al2O3–Rb2O–CeO–Tb4O7–TiO2–La2O3 SiO2–Li2O–ZrO2–P2O5–Al2O3–K2O–CeO SiO2–Li2O–Al2O3–K2O SiO2–Li2O–Al2O3–K2O SiO2–Li2O–ZrO2–P2O5–Al2O3–K2O SiO2–Li2O–ZrO2–P2O5–Al2O3–K2O-others IPS Empress 2 LD GCs IPS Empress 2 SiO2–Li2O–ZrO2–P2O5–Al2O3–K2O–CeO–MgO–CaO
Kþ Rbþ Kþ Kþ Kþ Kþ Kþ Unknown Unknown Unknown Kþ
270 � 21 351 � 32 282 � 14 224 � 4 201 � 14 290 � 10 308 � 25 280 � 53 205 400 � 40 310
4. Conclusion [3]
In this work, lithium disilicate glass-ceramic containing various al kali metal ions were prepared. It has been clarified that free oxygen exists in the lithium disilicate glass-ceramics and the amount of free oxygen will increase with the increase of alkali metal ions radii. More over, free oxygen can increase the crystallinity and crystal size of lithium disilicate and thus is favorable for improving the fracture strength of lithium disilicate glass-ceramics. Particularly, the as-prepared lithium disilicate glass-ceramics with introducing Rb element shows higher fracture strength than other current studies. The experimental results indicate that lithium disilicate glass-ceramics with a fracture strength of 351 � 32 MPa could be prepared via pressureless sintering by increasing the free oxygen in the glass-ceramics. This result and the discovery of free oxygen promoting lithium disilicate crystallization are of great significance for preparing high-strength lithium disilicate glassceramics.
[4]
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[8]
Acknowledgements [9]
This work is supported by the Opening Project of State Key Labora tory of High Performance Ceramics and Superfine Microstructure (SKL201514SIC), the National Natural Science Foundation of China (No. 51778098), the 2016 Dalian City Construction Science & Technology Project ([2016] 415), and Dalian Science & Technology Innovation Fund (2018J12SN066).
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