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Investigation on low-temperature sinterable behavior and tunable dielectric properties of BLMT glass-Li2ZnTi3O8 composite ceramics
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
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Original Article
Investigation on low-temperature sinterable behavior and tunable dielectric properties of BLMT glass-Li2ZnTi3O8 composite ceramics ⁎
Haishen Rena,b, , Liang Haoa,c, Haiyi Penga,b, Mingzhao Danga,b, Tianyi Xiea,c, Yi Zhanga, Shaohu Jianga, Xiaogang Yaoa, Huixing Lina, Lan Luoa a
Key Laboratory of Inorganic Functional Material and Device, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 200050, China University of Chinese Academy of Sciences, Beijing, 100049, China c Department of materials, Chongqing University of Technology, Chongqing, 400050, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: B2O3-La2O3-MgO-TiO2 Glass Li2ZnTi3O8 ceramic Sintering behavior Dielectric properties LTCC
The novel low-temperature sinterable ceramic composites were fabricated by mixing B2O3-La2O3-MgO-TiO2 (BLMT) glass with Li2ZnTi3O8 ceramic. All composites could be well sintered at 900 °C for 2 h through liquidphase sintering and viscous sintering process. With BLMT glass increasing, the main phase of composites changed from Li2ZnTi3O8 to LaBO3 phase crystallized from glass. Nevertheless, the rutile phase was observed in composites with ≥10 wt% glass, which could adjust the temperature coefficient of resonant frequency (τf) to near-zero owing to the opposite τf value to other phases. Simultaneously relative permittivity (εr) and quality factor (Q× f) could be controlled by varying the content of Li2ZnTi3O8 ceramic and BLMT glass. The composite with 20 wt% glass exhibited excellent dielectric properties: εr = 22.7, Q × f = 19,900 GHz, and τf = 0.28 ppm/ °C. In addition, the good chemical compatibility between the composite with 5 wt% glass and Ag electrode made it as a potential candidate for LTCC technology.
1. Introduction With the recent rapid development of the Tactile Internet (5th generation wireless systems), the Industrial Internet, Internet of Things, electronic warfare, satellite broadcasting and intelligent transport systems, the new advanced integration, packaging and interconnection technology is being strongly required to realize the microwave components (filter, resonator, antenna, capacitor, etc.) with high-miniaturization, high-reliability, multifunctional performance and usable at higher frequency range [1,2]. Low-temperature co-fired ceramic (LTCC) technology is turn out to be the upmost approach which widely used for multilayer circuit from fabrication to integrate, as well as build the different types of passive components and conductor into the ceramic [2,3]. To be applicable to the LTCC technology, the LTCC materials not only should possess an appropriate relative permittivity (εr, high for miniaturization and low for fast signal transmission), a high quality factor (Q × f), a near-zero temperature coefficient of resonant frequency (τf) but also can co-fired with little conductor loss, low electrical resistance at high frequencies and cost-effective Ag inner electrode (the melting point 961 °C) below 900 °C [1–3]. Nowadays, there are three dominant methods widely applied for simultaneously achieving the densification temperature of LTCC
⁎
materials below 900 °C and possessing the excellent dielectric properties. The first approach is based on the glass-ceramic or filled glassceramic composites such as CaO–B2O3–SiO2 glass-ceramic system from Ferro and lead borosilicate glass/Al2O3 system of Dupont [4,5], in which crystallized glass matrix acts as a main constituent (greater than 50 vol%) and/or fillers (Al2O3, SiO2, mullite, cordierite) can modulate the dielectric properties, sintering process, mechanical strength and coefficient of thermal expansion (CTE) etc. [6–9]. The second approach is to lower the sintering temperature of present microwave dielectric ceramic through adding an optimal amount of the sintering additive by virtue of liquid-phase sintering mechanism, for instance B2O3, BaCu (B2O5), BaO-B2O3-ZnO glass [10–12].The last approach is developing and searching for new microwave dielectric ceramic systems with intrinsic low sintering temperature (so-called free-glass LTCC material) based on the low-melting oxide like TeO2-based compounds, MoO3based compounds, V2O5-based compounds and Bi2O3-based compounds, etc [2,13–18]. However, many problems still exist in free-glass LTCC material, such as expensive and toxic raw materials, chemical incompatibility with Ag electrodes and large τf values, which is a huge barrier for them to be further applied in LTCC [2,16,19,20]. Hence, seeking for new suitable glass-ceramic system and lowering the sintering temperature of present ceramic are best choice for achieving the
Corresponding author at: Key Laboratory of Inorganic Functional Material and Device, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 200050, China. E-mail address: [email protected] (H. Ren).
https://doi.org/10.1016/j.jeurceramsoc.2018.03.053 Received 9 January 2018; Received in revised form 29 March 2018; Accepted 30 March 2018 0955-2219/ © 2018 Elsevier Ltd. All rights reserved.
Please cite this article as: Ren, H., Journal of the European Ceramic Society (2018), https://doi.org/10.1016/j.jeurceramsoc.2018.03.053
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steel mold, and then were pressed at 2 MPa by hydraulic pressing, followed by sintering between 500 °C and 920 °C for 2 h in air at a heating rate of 5 °C/min. The diff ;erential thermal analyzer (DTA) curve of the glass and composite powders was collected in a computerized system (DSC 404 C, Netzsch Instruments, Germany) at a heating rate of 10 °C/min from room temperature to 1100 °C. The crystalline phase present in sintered samples was identified by X-ray diff ;raction analysis (XRD, D8 ADVANCE, Bruker, Germany) using a Cu/Kα radiation, and was further analyzed by energy dispersive spectroscopy (EDS, Magellan 400, FEI Company, USA) and element-distribution mapping (EDM). The microstructure characteristics of the sintered samples was observed by field emission scanning electron microscope (FESEM, Magellan 400, FEI Company, USA). The sintering process was measured with 18 × 5.0 × 5.0 cm3 “green” samples by using a horizontal-loading dilatometer with alumina rams and boats (DIL 402 C, Netzsch Instruments, Germany) with a heating rate of 10 °C/min. The bulk density of sintered samples was measured applying the Archimedes method. The relative permittivity and tan δ (dielectric loss) of the samples with the diameter of 12 mm and the height of 6 mm were collected by the Hakki-Coleman dielectric resonator method in the TE011 mode using an Agilent E8363A PNA series network analyzer. The Q value were calculated from the value in the light of the Q = 1/ tan δ. The τf value was measured over the range from 25 to 85 °C heating through the temperature test cabinet (VTL7003, Vötsch, Germany), and was calculated by following equation:
cost-effective commercially available LTCC materials. Recently, the dielectric ceramics in the Li2O–ZnO–TiO2 ternary system have drawn more attention due to its good microwave dielectric properties and the excellent chemical compatibility with Ag inner electrodes. And among them, Li2ZnTi3O8 ceramic has excellent microwave dielectric properties with middle-relative permittivity (25.6), excellent quality factor (72,000 GHz) and negative temperature coefficient of resonant frequency (−11.2 ppm/°C) [21,22]. Although Li2ZnTi3O8 ceramic could be completely densified at 1075 °C without sintering additives, it is a requisite to lower the sintering temperature to below 900 °C for being co-fired with Ag electrode. In order to reducing the sintering temperature, many work has been done by adding lowmelting glasses or compounds, such as B2O3, Bi2O3, ZnO–B2O3–SiO2 glass and ZnO–La2O3–B2O3 glass etc [10,23–25]. However, these sintering aids often bring about the deterioration τf value, for instance −19.5 ppm/°C for Bi2O3 and −13.4 ppm/°C for ZnO–La2O3–B2O3 glass. On other the hand, the above-mentioned work have focused on lowering temperature sintering below 900 °C in the presence of an optimal amount of liquid phase without any designed phase formation to modulate microwave dielectric properties. Many work shows that glass plus ceramic is a favorable method to improve dielectric properties for the fabrication of low-temperature co-fired ceramics [26,27]. Wang et al reported that the LTCC materials base on BaO-B2O3-SiO2/BaTiO3 system could be densified at 900 °C and relative permittivity was effectively adjustable from 5 to 30 by changing the mass percent of BaTiO3 from 60 to 90. We also reported similar work that BBZ/BaTi4O9 composites sintered at 925 °C and as BBZ glass increased from 5 to 30 wt%, the εr and τf values decreased from 33 to 25 and from +25.44 to −3.19 ppm/°C, respectively. Therefore, if there is a glass that possess low Tg and Tm for lowering sintering temperature of Li2ZnTi3O8 ceramic and crystallization phase for adjusting relative permittivity and τf values, the microwave dielectric properties of the final LTCC materials could be improved by varying the content of glass. It can be found from our previous work that B2O3-La2O3-MgO-TiO2 (BLMT) glass is proved to be a good candidate duo to its low transformation temperature (644 °C) and great potential for LTCC applications [28,29].In addition, the crystal phases formed of the BLMT glass including LaBO3 (εr = 12.5, Q × f = 76,000 GHz) and TiO2 (εr = 108, Q × f = 44,000 GHz, τf =+456 ppm/°C) shows optimum dielectric properties. The major concern of this paper is to thoroughly investigate sintering behavior, crystallization, microstructures and dielectric properties of BLMT glass-Li2ZnTi3O8 composite ceramics, and to develop some adjustable middle relative permittivity material systems sintered at 900 °C for LTCC through mixing designed Li2ZnTi3O8 and B2O3-La2O3MgO-TiO2 glass.
τf = f85 −f25 60 × f × 106 (ppm / ℃) 25
(1)
where ƒ85 and ƒ25 represent the resonant frequencies at 85 °C and 25 °C, respectively. 3. Results and discussion 3.1. Sintering behavior On the basis of previous studies [6–9,12,26–29],there may be some physical and chemical changes during the sintering process of the glass/ ceramic LTCC system, such as the glass transition, devitrification of glass, the melting of phases or glass and reactions between glass and ceramic. In order to find the physical and chemical changes of BLMTLi2ZnTi3O8 composites during sintering process, the DTA curves of BLMT-Li2ZnTi3O8 composites with different BLMT glass and pure BLMT glass are measured at a heating rate of 10 °C/min, as shown in Fig. 1. It
2. Experimental Li2ZnTi3O8 phase was prepared by the solid-state-reaction method. Stoichiometry of Li2CO3, ZnO and TiO2 (99.9%) were weighted and mixed in a Nylon tank using ethyl alcohol and ZrO2 balls as media by planetary ball mill for 2 h. The mixture was then dried and calcined at 900 °C for 8 h to form Li2ZnTi3O8 phase. The BLMT glass with the molar composition of 42.9B2O3-17.1La2O3-25.7MgO-14.3TiO2 was prepared by a conventional glass fabrication process. The glass batch about 300 g was melted in a platinum crucible at 1350 °C for 2 h, and then the melts were quenched in water. The quenched glass was planetary-milled in aluminum jar with ethyl alcohol and ZrO2 balls for 2 h. After being dried and screened through a 200-mesh sieve, the BLMT glass powder was obtained. Last, the BLMT glass powder and calcined Li2ZnTi3O8 powder were weighed with the ratio of x BLMT-(100-x) Li2ZnTi3O8 (2.5≤ × ≤80wt%) and planetary-milled with ZrO2 balls and ethyl alcohol for 2 h. After drying, the mixture was granulated by adding 8 wt% poly(vinyl butyral) (PVB) solution for getting the uniformity particle size and good fluidity power. Preformed pellets of 15 mm in diameter and 8 mm in height were obtained from the powder using a cylindrical
Fig. 1. DTA curves of the BLMT-Li2ZnTi3O8 composites with different content of BLMT glass: a) 5, b) 10, c) 20, d) 40, e) 60, f) 70, g) 80 wt% and h) BLMT glass at a heating rate of 10 °C/min. 2
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Fig. 2. The linear shrinkage curve of the BLMT-Li2ZnTi3O8 composites with different content of BLMT glass at a heating rate of 10 °C/min.
Fig. 3. Relative density of BLMT-Li2ZnTi3O8 composites with different content of BLMT glass sintered at 625–920 °C for 2 h.
can be easily seen that the endothermic peak located at about 640 °C become more apparent with the increasing BLMT content and corresponds to glass transition temperature (Tg) of the glass compared with the DTA of BLMT glass in Fig. 1h. The exothermic peaks of crystallization (Tc) at about 700 °C shows an increase in the relative intensity with increasing glass content in the composites, which is different from the crystallization behavior of pure BLMT glass that has two crystallization peaks (Tc): 749 °C and 854 °C, as shown in Fig. 1h. In addition, two melting peaks at 810 °C (Tm1) and 845 °C (Tm2) appear when the content of BLMT glass is higher than 20 wt% in samples. Corresponding to the increase of BLMT glass content, the relative intensity of Tm1 presents a trend of rise first then fall and Tm2 demonstrates gradually decrease. These results point out that Li2ZnTi3O8 ceramic can somewhat influence the crystallization behavior and melting point of glass. On the other hand, above discussion also makes clear that the relative amount of glass and ceramic has a significant effect on sintering process and chemical reaction of BLMT-Li2ZnTi3O8 composites. In order to understand the sintering behavior of composites, the linear shrinkage curves for the composites with different contents of BLMT glass at a heating rate of 10 °C/min are measured, as shown in Fig. 2. It can be observed that the shrinkage of pure Li2ZnTi3O8 ceramic starts at about 950 °C, while the onset of shrinkage dramatically decreases to about 640 °C for composites added with BLMT glass, which is in line with our previous reports that the shrinkage of composites containing the BLMT glass starts at Tg [29,30]. However, the composites with lower glass (5 wt%) show only one-step shrinkage and display slow shrinkage process, while the composites with higher than 10 wt% BLMT glass show in contrast two-step shrinkage and the dominant step will change as the increasing of BLMT glass from 10 to 80 wt%. The composites with higher than 10 wt% BLMT glass in the first stages of sintering temperatures at the Tg should bring about shrinkage due to the shrinkage characteristic of BLMT glass. Then a platform occurs after 700 °C probably due to the beginning of the BLMT glass crystallization process that retards the further shrinkage of composite. The onset temperature of second step shrinkage process demonstrates gradually increase with the increasing of BLMT glass content. It may be related to the change in Tm1 and Tm2 as BLMT glass content rises in composites according to the DTA results shown in Fig. 1. It was reported that the densification mechanisms of the glass/ceramic systems can be divided into two major categories: liquid-phase sintering for the coexistence of ceramic and glass, and viscous sintering for the large amount of glass [6,30]. For our study, it is believed that the liquid-phase sintering dominates the sintering process of composites with 5 wt% BLMT glass. Small amount of melting glass could not wet the Li2ZnTi3O8 particles completely, and the solid-state sintering process therefore happens due
to directing contact between Li2ZnTi3O8 particles, resulting in that particles move and rearrange slowly [6,30]. For the composites with higher than 10 wt% BLMT glass, sintering process is controlled by liquid-phase sintering and viscous sintering, however the liquid-phase sintering process play a leading role in shrinkage process of the composites with 10–40 wt% BLMT glass, then the viscous sintering process dominates the shrinkage process of the composites with 60–80 wt% BLMT. These observations are further supported by the relative density of composites with different content of BLMT glass sintered at 625–920 °C for 2 h in Fig. 3. It can be seen that the relative density of composites depends strongly on the sintering temperature and BLMT glass content. Obviously, the change in density has a tendency similar to that liner shrinkage. The relative density of composite with low content of BLMT glass (5 wt%) keep a slow increasing tendency as sintering temperature increasing from 650 to 900 °C and reaches a maximum value sintered at 900 °C, then slightly decreases due to over-heating. When the contents of glass increases higher than 10 wt%, the bulk density of composites slightly increase between 600 and 650 °C, then remain almost unchanged as sintering temperature increases from 650 to 820 °C, after that increase again with sintering temperature increasing to 900 °C. All the results from Figs. 2 and 3 demonstrate that the densification of BLMT-Li2ZnTi3O8 composites can be obtained by sintering at 900 °C for 2 h.
3.2. Phases evolution and microstructure Fig. 4 shows XRD patterns of BLMT-Li2ZnTi3O8 composites with different content of BLMT glass (a) 5, b) 10, c) 20, d) 40, e) 60, f) 70 and g) 80 wt%) sintered at 900 °C for 2 h. The phases of sample with 5 wt% BLMT glass consists of Li2ZnTi3O8 (JCPDS no. 44-1037) and LaBO3 (JCPDS file no. 12-0762). There is new phase, rutile (TiO2, JCPDS file no. 21-1276), formed beside above-mentioned phase in these samples with the addition of glass higher than 10 wt%. According to our previous report [29,30], the pure BLMT glass can crystallizes forming LaBO3, LaB3O6, rutile at 749 °C and MgLaB5O10 phase at 850 °C. It means that the LaB3O6 and MgLaB5O10 phase seems to be suppressed with the addition of Li2ZnTi3O8 phase, which indicates that Li2ZnTi3O8 ceramic has an influence on the crystallization behavior of glass as discussed in DTA study. Moreover, it can be easily seen that the peak intensity of LaBO3 and rutile enhances as the content of BLMT glass increased, whereas Li2ZnTi3O8 weakens gradually, which indicates that the content of LaBO3 and rutile phase in composite increases with the increasing of BLMT glass. With the glass content in excess of 60 wt%, 3
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Table 1 EDX data of composite marked in Fig. 5. Element
B O Mg Ti Zn La
Atomic% A
B
C
D
E
– 65.65 0.92 25.31 8.12 –
20.51 60 – – – 19.49
38.89 59.76 1.11 0.07 0.16 –
19.08 56.77 14.14 – 10.02 –
– 65.67 – 34.33 – –
as the BLMT glass is increased to 80 wt%, the rod-like TiO2 grains with a width size of about 5 um are observed. In addition, there are light black and black grains appearing in these samples with higher than 40 wt% BLMT glass. The EDS data of point C and D in Table 1 indicates that the black phase is residual boron amorphous phase and the light black phase is the residual magnesium zinc boron amorphous phase. On the other hands, as the contents of BLMT glass is increased from 40 to 80 wt%, the average size of the grain tend to decrease as shown in Fig. 5c,d. Particularly the composite with 80 wt% contents of BLMT glass, most of the grain sizes are less than 10 um.
Fig. 4. XRD patterns of the BLMT-Li2ZnTi3O8 composites with different content of BLMT glass: a) 5, b) 10, c) 20, d) 40, e) 60, f) 70 and g) 80 wt% sintered at 900 °C for 2 h.
the main phase of the composite has been transformed from Li2ZnTi3O8 into LaBO3. Fig. 5 shows the backscattered electron micrograph of the composites with different contents of BLMT glass sintered at 900 °C for 2 h. All samples have dense microstructures with low porosity, and the changes in the phases and grain size can be seen in the BSE micrographs with increasing BLMT glass. It can be found from Fig. 5a that these composites with 5 wt% BLMT glass exhibit compact microstructure with grain sizes in the range 10–20 um and are composed of two kinds of grains with different contents. The gray grains are consistent with the Li2ZnTi3O8 phase and the white grains belong to the LaBO3 phase according to the EDX data in Table 1. With further increasing the content of BLMT glass higher than 40 wt%, the amount and grain size of LaBO3 gradually increases, while Li2ZnTi3O8 decreases gradually. In particular, small tetragonal rutile TiO2 are clearly observed (red circle) and
3.3. Mircowave dielectric properties Fig. 6 shows the relative permittivity (εr) and Q × f value of BLMTLi2ZnTi3O8 composites as a function of sintering temperature and different contents of BLMT glass. It can be easily seen from Fig. 6a that the relative permittivity of the composites has the tendency of increasing with the increasing sintering temperature owing to the decrease in the pores and the densification of composites. The optimum sintering temperature (900 °C) of each composites could be adjusted to the range of 24.6–15.3 with the increasing of BLMT glass content. In generally, the εr values of composite ceramics is mainly affected by its phase composition and sintering densification according to the equation of mixture rules:
lnεr = x1 ln εr1 + x2 ln εr 2 + …+x i ln εri
(2)
Fig. 5. Backscattered electron (BSE) micrograph of the BLMT-Li2ZnTi3O8 composites with 5 (a), 40 (b), 60 (c) and 80 wt% (d) of BLMT glass sintered at 900 °C for 2 h. 4
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Fig. 6. Relative permittivity (a) and Q × f value (b) of BLMT-Li2ZnTi3O8 composites with different content of BLMT glass as a function of sintering temperature.
composites should generally decrease with increasing the content of the BLMT glass. In fact, the Q × f value of the composites is not only related to the phase composition but also is closely associated with imperfections such as porosity and grains size [35,36]. As a result, it is other reason of decreasing the Q × f value that the grains uniformity and grains size of composites descend with increasing the content of the BLMT glass. For the above reasons, it can be found that the Q × f value of composites is approximately in the range 5800–49,000 GHz. The variations of the τf valve of composites with different contents of BLMT glass are shown in Fig. 7. It can be easily seen that the τf valve of composites first decreases from −12.07 to −14.21 ppm/°C with the contents of BLMT glass increasing from 2.5 to 7.5 wt%, and then increases from −14.21 to +75.5 ppm/°C as the glass further increases to 80 wt%. The τf of composites is well known to be influenced by the composition and their relative contents [21,22]. It is easily found that the τf valve first decreases due to the appearance and increasing of LaBO3 phase (−52 ppm/°C), and then increases owing to the formation and increasing of rutile phase (+456 ppm/°C). The results for shown in Figs.7 and 8 illustrate that the microwave dielectric properties of BLMTLi2ZnTi3O8 composites are related to the microstructure and composition which directly correlate with the content of BLMT glass. On the whole, the promising microwave dielectric properties of εr = 22.7, Q × f = 19,900 GHz, and τf = 0.28 ppm/°C can be obtained when the composites doped with 20 wt% BLMT glass is sintered at 900 °C for 2 h. It indicates that the BLMT glass chosen in this paper can lower the sintering temperature of Li2ZnTi3O8 ceramic and modify to near-zero τf value simultaneously. Compared with other Li2ZnTi3O8 LTCC materials as listed in Table 3, 20BLMT-80Li2ZnTi3O8 (in wt%) composite can be sintered at a lower temperature and possesses a near-zero temperature coefficient of resonant frequency without extra positive τf- tailoring materials.
Table 2 Microwave dielectric properties, density and sintering temperature of compounds in composites.
LaBO3 Li2ZnTi3O8 Rutile (TiO2)
εr
Q × f (GHz)
τf (ppm/°C)
Density (g/ cm3)
Ts(°C)
Ref.
12.5 25.6 104
56000 72,000 44,000
−52 −11.2 +456
4.25 3.974 5.304
1200 1075 1200
[31] [21] [23]
Ts : Sintering temperature. Ref.: Reference.
Fig. 7. The τf valve of the BLMT-Li2ZnTi3O8 composites with different content of BLMT glass sintered at 900 °C for 2 h.
where εri and xi are the εr and volume fraction of the i phase, respectively. The relative permittivity of the air equals 1, which indicates that more porosity would inevitably lead to the decline in relative permittivity of composites. On the basis of the previous phase and microstructure study as shown in Fig. 4 and Table 2, the decrease in εr at 900 °C with the contents of BLMT glass can be explained by the formation of more of the lower εr phases, such as LaBO3 (12.5) and amorphous phase, and the decrease of Li2ZnTi3O8 in composites. The variation of the Q × f values with respect to sintering temperature and BLMT glass contents has a similar tendency with relative permittivity shown in Fig. 6b. The increasing Q × f values of the composites with the increasing sintering temperature is still due to the decrease in the pores and the densification of composites. As the singlephase Li2ZnTi3O8 has a much bigger Q × f value (72,000 GHz) than that of others ceramic as shown in Table 1, the Q × f value of the
3.4. Compatibility with Ag electrodes In order to confirm the chemical compatibility of the composite with Ag electrode, the well-mixed powder of 95 wt% Li2ZnTi3O8 −5 wt % BLMT composite mixes with 95% Ethyl Alcohol and Xylene as solvent, Herring oil as dispersant, Benzyl butyl phthalate (S160) as plasticizer and Poly(vinyl butyral) B-98 (PVB-B98) as binder to obtain slurry. After vacuuming, the flat and astomatous green tape is prepared using tape-casting the slurry with a thick of 600 um by doctor blade. The silver conducting plate (Shanghai Miracle Materials Technology Co. LTD.) is printed onto the LTCC green sheet by 200 mesh sieve. The printed LTCC green sheets (3 layers) are laminated, hot isostatic laminated and cut. Last, the components are sintered at 900 °C for 2 h in air at a heating rate of 5 °C/min, after burning out organics completely at 450 °C for 12 h. The Fig. 8a shows the cross-section SEM micrograph 5
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Fig. 8. (a) The SEM micrograph of the cross-section of 95 wt% Li2ZnTi3O8 −5 wt% BLMT composite co-fired with Ag; (b) Surface morphology of a silver film on 95 wt% Li2ZnTi3O8 −5 wt% BLMT composite substrate. Table 3 Comparison of sintering temperature and dielectric properties of recently reported Li2ZnTi3O8 LTCC materials with this study. Composition
εr
Q × f (GHz)
τf (ppm/℃)
Ts (℃)
Ref.
1.5 wt% BaCu(B2O5) + Li2ZnTi3O8 0.5 wt% B2O3 + Li2ZnTi3O8 2 wt% Bi2O3 + Li2ZnTi3O8 0.75 wt% ZBS glass + Li2ZnTi3O8 0.25 wt% ZB glass + Li2ZnTi3O8 1 wt% ZLB glass + Li2ZnTi3O8 1 wt% CBS glass + 4 wt% TiO2 + Li2ZnTi3O8 1 wt% LZB glass + 3.5 wt% TiO2 + Li2ZnTi3O8 4 wt% Li2WO4 + 4 wt% TiO2 + Li2ZnTi3O8 1.5 wt% B2O3 + 3 wt% TiO2 + Li2ZnTi3O8 1 wt% LAB glass + 17 mol% TiO2 + Li2ZnTi3O8 1 wt% LBS glass + 3.5 wt% TiO2 + Li2ZnTi3O8 1 wt% ZBS glass + 0.6Li2ZnTi3O8-0.4Li2TiO3 20BLMT-80Li2ZnTi3O8 (in wt%)
23.1 25 27.8 25.61 25.34 24.34 26.9 26.1 27.1 25.9 26.8 26 25.4 22.7
22732 50917 36387 51615 61660 41360 23563 45168 51123 46487 28000 44023 86400 19,900
−17.6 −17.8 −19.5 −11.78 −12.98 −13.4 −1.5 −4.1 −3.8 −0.35 +2.5 −4.4 −1.0 0.28
925/4 h 875/4 h 950/4 h 925/4 h 950/4 h 925/4 h 900/4 h 900/4 h 860/4 h 900/4 h 900/4 h 950/4 h 900/4 h 900/2 h
[32] [33] [23] [24] [34] [25] [35] [36] [37] [10] [38] [39] [40] This paper
and EDS line scan of the composite co-fired with Ag electrode. It is can be seen from cross-section of composite and Ag electrode film that there is a clear interface between the composite and the Ag electrode film. And EDS line scan shows that Ag diffusion no occur during the co-fired processing. The surface SEM micrographs of silver film on 95 wt% Li2ZnTi3O8 −5 wt% BLMT composite substrate exhibits that the Ag film has a fully densified and an overall smoother microstructure, as shown in Fig. 8b. Combining the excellent microwave dielectric properties, it can be proposed that the composites is a very promising candidate material for the LTCC applications.
the εr and Q × f value decreases from 24.6 to 15.3 and from 49,000 to 5800 GHz, respectively. The τf valve first decreases from −12.07 to −14.21 ppm/°C, and then increases from −14.21 to +75.5 ppm/°C. Excellent microwave dielectric properties of composites with 20 wt% glass obtained by sintering at 900 °C/2 h with εr = 22.7, Q × f = 19,900 GHz, and τf = 0.28 ppm/°C. This results show that adjustable middle relative permittivity material system for LTCC application can be obtained by mixing Li2ZnTi3O8 and B2O3-La2O3-MgO-TiO2 glass. In addition, the good chemical compatibility between the composite with 5 wt% glass and Ag electrode makes it as a potential candidate for LTCC technology.
4. Conclusions
References
In this study, a novel low-temperature co-fired ceramic is prepared by x BLMT-(100-x) Li2ZnTi3O8 (2.5≤ × ≤80 wt%) composite system sintered at 900 °C. The sintering behavior, phase composition, microstructure and dielectric properties of the composites are investigated. The results show that the sintering process of composites with low BLMT glass (< 10 wt%) can be achieved by liquid-phase sintering and phase composition including LaBO3 phase crystallized from glass and Li2ZnTi3O8. In contract, viscous sintering and liquid-phase sintering process could control the sintering process of the composites with high glass (≥10 wt%). And the phase of composites consist of Li2ZnTi3O8, LaBO3 and rutile. Corresponding to the increase of BLMT glass content,
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Original Article
Investigation on low-temperature sinterable behavior and tunable dielectric properties of BLMT glass-Li2ZnTi3O8 composite ceramics ⁎
Haishen Rena,b, , Liang Haoa,c, Haiyi Penga,b, Mingzhao Danga,b, Tianyi Xiea,c, Yi Zhanga, Shaohu Jianga, Xiaogang Yaoa, Huixing Lina, Lan Luoa a
Key Laboratory of Inorganic Functional Material and Device, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 200050, China University of Chinese Academy of Sciences, Beijing, 100049, China c Department of materials, Chongqing University of Technology, Chongqing, 400050, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: B2O3-La2O3-MgO-TiO2 Glass Li2ZnTi3O8 ceramic Sintering behavior Dielectric properties LTCC
The novel low-temperature sinterable ceramic composites were fabricated by mixing B2O3-La2O3-MgO-TiO2 (BLMT) glass with Li2ZnTi3O8 ceramic. All composites could be well sintered at 900 °C for 2 h through liquidphase sintering and viscous sintering process. With BLMT glass increasing, the main phase of composites changed from Li2ZnTi3O8 to LaBO3 phase crystallized from glass. Nevertheless, the rutile phase was observed in composites with ≥10 wt% glass, which could adjust the temperature coefficient of resonant frequency (τf) to near-zero owing to the opposite τf value to other phases. Simultaneously relative permittivity (εr) and quality factor (Q× f) could be controlled by varying the content of Li2ZnTi3O8 ceramic and BLMT glass. The composite with 20 wt% glass exhibited excellent dielectric properties: εr = 22.7, Q × f = 19,900 GHz, and τf = 0.28 ppm/ °C. In addition, the good chemical compatibility between the composite with 5 wt% glass and Ag electrode made it as a potential candidate for LTCC technology.
1. Introduction With the recent rapid development of the Tactile Internet (5th generation wireless systems), the Industrial Internet, Internet of Things, electronic warfare, satellite broadcasting and intelligent transport systems, the new advanced integration, packaging and interconnection technology is being strongly required to realize the microwave components (filter, resonator, antenna, capacitor, etc.) with high-miniaturization, high-reliability, multifunctional performance and usable at higher frequency range [1,2]. Low-temperature co-fired ceramic (LTCC) technology is turn out to be the upmost approach which widely used for multilayer circuit from fabrication to integrate, as well as build the different types of passive components and conductor into the ceramic [2,3]. To be applicable to the LTCC technology, the LTCC materials not only should possess an appropriate relative permittivity (εr, high for miniaturization and low for fast signal transmission), a high quality factor (Q × f), a near-zero temperature coefficient of resonant frequency (τf) but also can co-fired with little conductor loss, low electrical resistance at high frequencies and cost-effective Ag inner electrode (the melting point 961 °C) below 900 °C [1–3]. Nowadays, there are three dominant methods widely applied for simultaneously achieving the densification temperature of LTCC
⁎
materials below 900 °C and possessing the excellent dielectric properties. The first approach is based on the glass-ceramic or filled glassceramic composites such as CaO–B2O3–SiO2 glass-ceramic system from Ferro and lead borosilicate glass/Al2O3 system of Dupont [4,5], in which crystallized glass matrix acts as a main constituent (greater than 50 vol%) and/or fillers (Al2O3, SiO2, mullite, cordierite) can modulate the dielectric properties, sintering process, mechanical strength and coefficient of thermal expansion (CTE) etc. [6–9]. The second approach is to lower the sintering temperature of present microwave dielectric ceramic through adding an optimal amount of the sintering additive by virtue of liquid-phase sintering mechanism, for instance B2O3, BaCu (B2O5), BaO-B2O3-ZnO glass [10–12].The last approach is developing and searching for new microwave dielectric ceramic systems with intrinsic low sintering temperature (so-called free-glass LTCC material) based on the low-melting oxide like TeO2-based compounds, MoO3based compounds, V2O5-based compounds and Bi2O3-based compounds, etc [2,13–18]. However, many problems still exist in free-glass LTCC material, such as expensive and toxic raw materials, chemical incompatibility with Ag electrodes and large τf values, which is a huge barrier for them to be further applied in LTCC [2,16,19,20]. Hence, seeking for new suitable glass-ceramic system and lowering the sintering temperature of present ceramic are best choice for achieving the
Corresponding author at: Key Laboratory of Inorganic Functional Material and Device, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 200050, China. E-mail address: [email protected] (H. Ren).
https://doi.org/10.1016/j.jeurceramsoc.2018.03.053 Received 9 January 2018; Received in revised form 29 March 2018; Accepted 30 March 2018 0955-2219/ © 2018 Elsevier Ltd. All rights reserved.
Please cite this article as: Ren, H., Journal of the European Ceramic Society (2018), https://doi.org/10.1016/j.jeurceramsoc.2018.03.053
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steel mold, and then were pressed at 2 MPa by hydraulic pressing, followed by sintering between 500 °C and 920 °C for 2 h in air at a heating rate of 5 °C/min. The diff ;erential thermal analyzer (DTA) curve of the glass and composite powders was collected in a computerized system (DSC 404 C, Netzsch Instruments, Germany) at a heating rate of 10 °C/min from room temperature to 1100 °C. The crystalline phase present in sintered samples was identified by X-ray diff ;raction analysis (XRD, D8 ADVANCE, Bruker, Germany) using a Cu/Kα radiation, and was further analyzed by energy dispersive spectroscopy (EDS, Magellan 400, FEI Company, USA) and element-distribution mapping (EDM). The microstructure characteristics of the sintered samples was observed by field emission scanning electron microscope (FESEM, Magellan 400, FEI Company, USA). The sintering process was measured with 18 × 5.0 × 5.0 cm3 “green” samples by using a horizontal-loading dilatometer with alumina rams and boats (DIL 402 C, Netzsch Instruments, Germany) with a heating rate of 10 °C/min. The bulk density of sintered samples was measured applying the Archimedes method. The relative permittivity and tan δ (dielectric loss) of the samples with the diameter of 12 mm and the height of 6 mm were collected by the Hakki-Coleman dielectric resonator method in the TE011 mode using an Agilent E8363A PNA series network analyzer. The Q value were calculated from the value in the light of the Q = 1/ tan δ. The τf value was measured over the range from 25 to 85 °C heating through the temperature test cabinet (VTL7003, Vötsch, Germany), and was calculated by following equation:
cost-effective commercially available LTCC materials. Recently, the dielectric ceramics in the Li2O–ZnO–TiO2 ternary system have drawn more attention due to its good microwave dielectric properties and the excellent chemical compatibility with Ag inner electrodes. And among them, Li2ZnTi3O8 ceramic has excellent microwave dielectric properties with middle-relative permittivity (25.6), excellent quality factor (72,000 GHz) and negative temperature coefficient of resonant frequency (−11.2 ppm/°C) [21,22]. Although Li2ZnTi3O8 ceramic could be completely densified at 1075 °C without sintering additives, it is a requisite to lower the sintering temperature to below 900 °C for being co-fired with Ag electrode. In order to reducing the sintering temperature, many work has been done by adding lowmelting glasses or compounds, such as B2O3, Bi2O3, ZnO–B2O3–SiO2 glass and ZnO–La2O3–B2O3 glass etc [10,23–25]. However, these sintering aids often bring about the deterioration τf value, for instance −19.5 ppm/°C for Bi2O3 and −13.4 ppm/°C for ZnO–La2O3–B2O3 glass. On other the hand, the above-mentioned work have focused on lowering temperature sintering below 900 °C in the presence of an optimal amount of liquid phase without any designed phase formation to modulate microwave dielectric properties. Many work shows that glass plus ceramic is a favorable method to improve dielectric properties for the fabrication of low-temperature co-fired ceramics [26,27]. Wang et al reported that the LTCC materials base on BaO-B2O3-SiO2/BaTiO3 system could be densified at 900 °C and relative permittivity was effectively adjustable from 5 to 30 by changing the mass percent of BaTiO3 from 60 to 90. We also reported similar work that BBZ/BaTi4O9 composites sintered at 925 °C and as BBZ glass increased from 5 to 30 wt%, the εr and τf values decreased from 33 to 25 and from +25.44 to −3.19 ppm/°C, respectively. Therefore, if there is a glass that possess low Tg and Tm for lowering sintering temperature of Li2ZnTi3O8 ceramic and crystallization phase for adjusting relative permittivity and τf values, the microwave dielectric properties of the final LTCC materials could be improved by varying the content of glass. It can be found from our previous work that B2O3-La2O3-MgO-TiO2 (BLMT) glass is proved to be a good candidate duo to its low transformation temperature (644 °C) and great potential for LTCC applications [28,29].In addition, the crystal phases formed of the BLMT glass including LaBO3 (εr = 12.5, Q × f = 76,000 GHz) and TiO2 (εr = 108, Q × f = 44,000 GHz, τf =+456 ppm/°C) shows optimum dielectric properties. The major concern of this paper is to thoroughly investigate sintering behavior, crystallization, microstructures and dielectric properties of BLMT glass-Li2ZnTi3O8 composite ceramics, and to develop some adjustable middle relative permittivity material systems sintered at 900 °C for LTCC through mixing designed Li2ZnTi3O8 and B2O3-La2O3MgO-TiO2 glass.
τf = f85 −f25 60 × f × 106 (ppm / ℃) 25
(1)
where ƒ85 and ƒ25 represent the resonant frequencies at 85 °C and 25 °C, respectively. 3. Results and discussion 3.1. Sintering behavior On the basis of previous studies [6–9,12,26–29],there may be some physical and chemical changes during the sintering process of the glass/ ceramic LTCC system, such as the glass transition, devitrification of glass, the melting of phases or glass and reactions between glass and ceramic. In order to find the physical and chemical changes of BLMTLi2ZnTi3O8 composites during sintering process, the DTA curves of BLMT-Li2ZnTi3O8 composites with different BLMT glass and pure BLMT glass are measured at a heating rate of 10 °C/min, as shown in Fig. 1. It
2. Experimental Li2ZnTi3O8 phase was prepared by the solid-state-reaction method. Stoichiometry of Li2CO3, ZnO and TiO2 (99.9%) were weighted and mixed in a Nylon tank using ethyl alcohol and ZrO2 balls as media by planetary ball mill for 2 h. The mixture was then dried and calcined at 900 °C for 8 h to form Li2ZnTi3O8 phase. The BLMT glass with the molar composition of 42.9B2O3-17.1La2O3-25.7MgO-14.3TiO2 was prepared by a conventional glass fabrication process. The glass batch about 300 g was melted in a platinum crucible at 1350 °C for 2 h, and then the melts were quenched in water. The quenched glass was planetary-milled in aluminum jar with ethyl alcohol and ZrO2 balls for 2 h. After being dried and screened through a 200-mesh sieve, the BLMT glass powder was obtained. Last, the BLMT glass powder and calcined Li2ZnTi3O8 powder were weighed with the ratio of x BLMT-(100-x) Li2ZnTi3O8 (2.5≤ × ≤80wt%) and planetary-milled with ZrO2 balls and ethyl alcohol for 2 h. After drying, the mixture was granulated by adding 8 wt% poly(vinyl butyral) (PVB) solution for getting the uniformity particle size and good fluidity power. Preformed pellets of 15 mm in diameter and 8 mm in height were obtained from the powder using a cylindrical
Fig. 1. DTA curves of the BLMT-Li2ZnTi3O8 composites with different content of BLMT glass: a) 5, b) 10, c) 20, d) 40, e) 60, f) 70, g) 80 wt% and h) BLMT glass at a heating rate of 10 °C/min. 2
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Fig. 2. The linear shrinkage curve of the BLMT-Li2ZnTi3O8 composites with different content of BLMT glass at a heating rate of 10 °C/min.
Fig. 3. Relative density of BLMT-Li2ZnTi3O8 composites with different content of BLMT glass sintered at 625–920 °C for 2 h.
can be easily seen that the endothermic peak located at about 640 °C become more apparent with the increasing BLMT content and corresponds to glass transition temperature (Tg) of the glass compared with the DTA of BLMT glass in Fig. 1h. The exothermic peaks of crystallization (Tc) at about 700 °C shows an increase in the relative intensity with increasing glass content in the composites, which is different from the crystallization behavior of pure BLMT glass that has two crystallization peaks (Tc): 749 °C and 854 °C, as shown in Fig. 1h. In addition, two melting peaks at 810 °C (Tm1) and 845 °C (Tm2) appear when the content of BLMT glass is higher than 20 wt% in samples. Corresponding to the increase of BLMT glass content, the relative intensity of Tm1 presents a trend of rise first then fall and Tm2 demonstrates gradually decrease. These results point out that Li2ZnTi3O8 ceramic can somewhat influence the crystallization behavior and melting point of glass. On the other hand, above discussion also makes clear that the relative amount of glass and ceramic has a significant effect on sintering process and chemical reaction of BLMT-Li2ZnTi3O8 composites. In order to understand the sintering behavior of composites, the linear shrinkage curves for the composites with different contents of BLMT glass at a heating rate of 10 °C/min are measured, as shown in Fig. 2. It can be observed that the shrinkage of pure Li2ZnTi3O8 ceramic starts at about 950 °C, while the onset of shrinkage dramatically decreases to about 640 °C for composites added with BLMT glass, which is in line with our previous reports that the shrinkage of composites containing the BLMT glass starts at Tg [29,30]. However, the composites with lower glass (5 wt%) show only one-step shrinkage and display slow shrinkage process, while the composites with higher than 10 wt% BLMT glass show in contrast two-step shrinkage and the dominant step will change as the increasing of BLMT glass from 10 to 80 wt%. The composites with higher than 10 wt% BLMT glass in the first stages of sintering temperatures at the Tg should bring about shrinkage due to the shrinkage characteristic of BLMT glass. Then a platform occurs after 700 °C probably due to the beginning of the BLMT glass crystallization process that retards the further shrinkage of composite. The onset temperature of second step shrinkage process demonstrates gradually increase with the increasing of BLMT glass content. It may be related to the change in Tm1 and Tm2 as BLMT glass content rises in composites according to the DTA results shown in Fig. 1. It was reported that the densification mechanisms of the glass/ceramic systems can be divided into two major categories: liquid-phase sintering for the coexistence of ceramic and glass, and viscous sintering for the large amount of glass [6,30]. For our study, it is believed that the liquid-phase sintering dominates the sintering process of composites with 5 wt% BLMT glass. Small amount of melting glass could not wet the Li2ZnTi3O8 particles completely, and the solid-state sintering process therefore happens due
to directing contact between Li2ZnTi3O8 particles, resulting in that particles move and rearrange slowly [6,30]. For the composites with higher than 10 wt% BLMT glass, sintering process is controlled by liquid-phase sintering and viscous sintering, however the liquid-phase sintering process play a leading role in shrinkage process of the composites with 10–40 wt% BLMT glass, then the viscous sintering process dominates the shrinkage process of the composites with 60–80 wt% BLMT. These observations are further supported by the relative density of composites with different content of BLMT glass sintered at 625–920 °C for 2 h in Fig. 3. It can be seen that the relative density of composites depends strongly on the sintering temperature and BLMT glass content. Obviously, the change in density has a tendency similar to that liner shrinkage. The relative density of composite with low content of BLMT glass (5 wt%) keep a slow increasing tendency as sintering temperature increasing from 650 to 900 °C and reaches a maximum value sintered at 900 °C, then slightly decreases due to over-heating. When the contents of glass increases higher than 10 wt%, the bulk density of composites slightly increase between 600 and 650 °C, then remain almost unchanged as sintering temperature increases from 650 to 820 °C, after that increase again with sintering temperature increasing to 900 °C. All the results from Figs. 2 and 3 demonstrate that the densification of BLMT-Li2ZnTi3O8 composites can be obtained by sintering at 900 °C for 2 h.
3.2. Phases evolution and microstructure Fig. 4 shows XRD patterns of BLMT-Li2ZnTi3O8 composites with different content of BLMT glass (a) 5, b) 10, c) 20, d) 40, e) 60, f) 70 and g) 80 wt%) sintered at 900 °C for 2 h. The phases of sample with 5 wt% BLMT glass consists of Li2ZnTi3O8 (JCPDS no. 44-1037) and LaBO3 (JCPDS file no. 12-0762). There is new phase, rutile (TiO2, JCPDS file no. 21-1276), formed beside above-mentioned phase in these samples with the addition of glass higher than 10 wt%. According to our previous report [29,30], the pure BLMT glass can crystallizes forming LaBO3, LaB3O6, rutile at 749 °C and MgLaB5O10 phase at 850 °C. It means that the LaB3O6 and MgLaB5O10 phase seems to be suppressed with the addition of Li2ZnTi3O8 phase, which indicates that Li2ZnTi3O8 ceramic has an influence on the crystallization behavior of glass as discussed in DTA study. Moreover, it can be easily seen that the peak intensity of LaBO3 and rutile enhances as the content of BLMT glass increased, whereas Li2ZnTi3O8 weakens gradually, which indicates that the content of LaBO3 and rutile phase in composite increases with the increasing of BLMT glass. With the glass content in excess of 60 wt%, 3
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Table 1 EDX data of composite marked in Fig. 5. Element
B O Mg Ti Zn La
Atomic% A
B
C
D
E
– 65.65 0.92 25.31 8.12 –
20.51 60 – – – 19.49
38.89 59.76 1.11 0.07 0.16 –
19.08 56.77 14.14 – 10.02 –
– 65.67 – 34.33 – –
as the BLMT glass is increased to 80 wt%, the rod-like TiO2 grains with a width size of about 5 um are observed. In addition, there are light black and black grains appearing in these samples with higher than 40 wt% BLMT glass. The EDS data of point C and D in Table 1 indicates that the black phase is residual boron amorphous phase and the light black phase is the residual magnesium zinc boron amorphous phase. On the other hands, as the contents of BLMT glass is increased from 40 to 80 wt%, the average size of the grain tend to decrease as shown in Fig. 5c,d. Particularly the composite with 80 wt% contents of BLMT glass, most of the grain sizes are less than 10 um.
Fig. 4. XRD patterns of the BLMT-Li2ZnTi3O8 composites with different content of BLMT glass: a) 5, b) 10, c) 20, d) 40, e) 60, f) 70 and g) 80 wt% sintered at 900 °C for 2 h.
the main phase of the composite has been transformed from Li2ZnTi3O8 into LaBO3. Fig. 5 shows the backscattered electron micrograph of the composites with different contents of BLMT glass sintered at 900 °C for 2 h. All samples have dense microstructures with low porosity, and the changes in the phases and grain size can be seen in the BSE micrographs with increasing BLMT glass. It can be found from Fig. 5a that these composites with 5 wt% BLMT glass exhibit compact microstructure with grain sizes in the range 10–20 um and are composed of two kinds of grains with different contents. The gray grains are consistent with the Li2ZnTi3O8 phase and the white grains belong to the LaBO3 phase according to the EDX data in Table 1. With further increasing the content of BLMT glass higher than 40 wt%, the amount and grain size of LaBO3 gradually increases, while Li2ZnTi3O8 decreases gradually. In particular, small tetragonal rutile TiO2 are clearly observed (red circle) and
3.3. Mircowave dielectric properties Fig. 6 shows the relative permittivity (εr) and Q × f value of BLMTLi2ZnTi3O8 composites as a function of sintering temperature and different contents of BLMT glass. It can be easily seen from Fig. 6a that the relative permittivity of the composites has the tendency of increasing with the increasing sintering temperature owing to the decrease in the pores and the densification of composites. The optimum sintering temperature (900 °C) of each composites could be adjusted to the range of 24.6–15.3 with the increasing of BLMT glass content. In generally, the εr values of composite ceramics is mainly affected by its phase composition and sintering densification according to the equation of mixture rules:
lnεr = x1 ln εr1 + x2 ln εr 2 + …+x i ln εri
(2)
Fig. 5. Backscattered electron (BSE) micrograph of the BLMT-Li2ZnTi3O8 composites with 5 (a), 40 (b), 60 (c) and 80 wt% (d) of BLMT glass sintered at 900 °C for 2 h. 4
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Fig. 6. Relative permittivity (a) and Q × f value (b) of BLMT-Li2ZnTi3O8 composites with different content of BLMT glass as a function of sintering temperature.
composites should generally decrease with increasing the content of the BLMT glass. In fact, the Q × f value of the composites is not only related to the phase composition but also is closely associated with imperfections such as porosity and grains size [35,36]. As a result, it is other reason of decreasing the Q × f value that the grains uniformity and grains size of composites descend with increasing the content of the BLMT glass. For the above reasons, it can be found that the Q × f value of composites is approximately in the range 5800–49,000 GHz. The variations of the τf valve of composites with different contents of BLMT glass are shown in Fig. 7. It can be easily seen that the τf valve of composites first decreases from −12.07 to −14.21 ppm/°C with the contents of BLMT glass increasing from 2.5 to 7.5 wt%, and then increases from −14.21 to +75.5 ppm/°C as the glass further increases to 80 wt%. The τf of composites is well known to be influenced by the composition and their relative contents [21,22]. It is easily found that the τf valve first decreases due to the appearance and increasing of LaBO3 phase (−52 ppm/°C), and then increases owing to the formation and increasing of rutile phase (+456 ppm/°C). The results for shown in Figs.7 and 8 illustrate that the microwave dielectric properties of BLMTLi2ZnTi3O8 composites are related to the microstructure and composition which directly correlate with the content of BLMT glass. On the whole, the promising microwave dielectric properties of εr = 22.7, Q × f = 19,900 GHz, and τf = 0.28 ppm/°C can be obtained when the composites doped with 20 wt% BLMT glass is sintered at 900 °C for 2 h. It indicates that the BLMT glass chosen in this paper can lower the sintering temperature of Li2ZnTi3O8 ceramic and modify to near-zero τf value simultaneously. Compared with other Li2ZnTi3O8 LTCC materials as listed in Table 3, 20BLMT-80Li2ZnTi3O8 (in wt%) composite can be sintered at a lower temperature and possesses a near-zero temperature coefficient of resonant frequency without extra positive τf- tailoring materials.
Table 2 Microwave dielectric properties, density and sintering temperature of compounds in composites.
LaBO3 Li2ZnTi3O8 Rutile (TiO2)
εr
Q × f (GHz)
τf (ppm/°C)
Density (g/ cm3)
Ts(°C)
Ref.
12.5 25.6 104
56000 72,000 44,000
−52 −11.2 +456
4.25 3.974 5.304
1200 1075 1200
[31] [21] [23]
Ts : Sintering temperature. Ref.: Reference.
Fig. 7. The τf valve of the BLMT-Li2ZnTi3O8 composites with different content of BLMT glass sintered at 900 °C for 2 h.
where εri and xi are the εr and volume fraction of the i phase, respectively. The relative permittivity of the air equals 1, which indicates that more porosity would inevitably lead to the decline in relative permittivity of composites. On the basis of the previous phase and microstructure study as shown in Fig. 4 and Table 2, the decrease in εr at 900 °C with the contents of BLMT glass can be explained by the formation of more of the lower εr phases, such as LaBO3 (12.5) and amorphous phase, and the decrease of Li2ZnTi3O8 in composites. The variation of the Q × f values with respect to sintering temperature and BLMT glass contents has a similar tendency with relative permittivity shown in Fig. 6b. The increasing Q × f values of the composites with the increasing sintering temperature is still due to the decrease in the pores and the densification of composites. As the singlephase Li2ZnTi3O8 has a much bigger Q × f value (72,000 GHz) than that of others ceramic as shown in Table 1, the Q × f value of the
3.4. Compatibility with Ag electrodes In order to confirm the chemical compatibility of the composite with Ag electrode, the well-mixed powder of 95 wt% Li2ZnTi3O8 −5 wt % BLMT composite mixes with 95% Ethyl Alcohol and Xylene as solvent, Herring oil as dispersant, Benzyl butyl phthalate (S160) as plasticizer and Poly(vinyl butyral) B-98 (PVB-B98) as binder to obtain slurry. After vacuuming, the flat and astomatous green tape is prepared using tape-casting the slurry with a thick of 600 um by doctor blade. The silver conducting plate (Shanghai Miracle Materials Technology Co. LTD.) is printed onto the LTCC green sheet by 200 mesh sieve. The printed LTCC green sheets (3 layers) are laminated, hot isostatic laminated and cut. Last, the components are sintered at 900 °C for 2 h in air at a heating rate of 5 °C/min, after burning out organics completely at 450 °C for 12 h. The Fig. 8a shows the cross-section SEM micrograph 5
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Fig. 8. (a) The SEM micrograph of the cross-section of 95 wt% Li2ZnTi3O8 −5 wt% BLMT composite co-fired with Ag; (b) Surface morphology of a silver film on 95 wt% Li2ZnTi3O8 −5 wt% BLMT composite substrate. Table 3 Comparison of sintering temperature and dielectric properties of recently reported Li2ZnTi3O8 LTCC materials with this study. Composition
εr
Q × f (GHz)
τf (ppm/℃)
Ts (℃)
Ref.
1.5 wt% BaCu(B2O5) + Li2ZnTi3O8 0.5 wt% B2O3 + Li2ZnTi3O8 2 wt% Bi2O3 + Li2ZnTi3O8 0.75 wt% ZBS glass + Li2ZnTi3O8 0.25 wt% ZB glass + Li2ZnTi3O8 1 wt% ZLB glass + Li2ZnTi3O8 1 wt% CBS glass + 4 wt% TiO2 + Li2ZnTi3O8 1 wt% LZB glass + 3.5 wt% TiO2 + Li2ZnTi3O8 4 wt% Li2WO4 + 4 wt% TiO2 + Li2ZnTi3O8 1.5 wt% B2O3 + 3 wt% TiO2 + Li2ZnTi3O8 1 wt% LAB glass + 17 mol% TiO2 + Li2ZnTi3O8 1 wt% LBS glass + 3.5 wt% TiO2 + Li2ZnTi3O8 1 wt% ZBS glass + 0.6Li2ZnTi3O8-0.4Li2TiO3 20BLMT-80Li2ZnTi3O8 (in wt%)
23.1 25 27.8 25.61 25.34 24.34 26.9 26.1 27.1 25.9 26.8 26 25.4 22.7
22732 50917 36387 51615 61660 41360 23563 45168 51123 46487 28000 44023 86400 19,900
−17.6 −17.8 −19.5 −11.78 −12.98 −13.4 −1.5 −4.1 −3.8 −0.35 +2.5 −4.4 −1.0 0.28
925/4 h 875/4 h 950/4 h 925/4 h 950/4 h 925/4 h 900/4 h 900/4 h 860/4 h 900/4 h 900/4 h 950/4 h 900/4 h 900/2 h
[32] [33] [23] [24] [34] [25] [35] [36] [37] [10] [38] [39] [40] This paper
and EDS line scan of the composite co-fired with Ag electrode. It is can be seen from cross-section of composite and Ag electrode film that there is a clear interface between the composite and the Ag electrode film. And EDS line scan shows that Ag diffusion no occur during the co-fired processing. The surface SEM micrographs of silver film on 95 wt% Li2ZnTi3O8 −5 wt% BLMT composite substrate exhibits that the Ag film has a fully densified and an overall smoother microstructure, as shown in Fig. 8b. Combining the excellent microwave dielectric properties, it can be proposed that the composites is a very promising candidate material for the LTCC applications.
the εr and Q × f value decreases from 24.6 to 15.3 and from 49,000 to 5800 GHz, respectively. The τf valve first decreases from −12.07 to −14.21 ppm/°C, and then increases from −14.21 to +75.5 ppm/°C. Excellent microwave dielectric properties of composites with 20 wt% glass obtained by sintering at 900 °C/2 h with εr = 22.7, Q × f = 19,900 GHz, and τf = 0.28 ppm/°C. This results show that adjustable middle relative permittivity material system for LTCC application can be obtained by mixing Li2ZnTi3O8 and B2O3-La2O3-MgO-TiO2 glass. In addition, the good chemical compatibility between the composite with 5 wt% glass and Ag electrode makes it as a potential candidate for LTCC technology.
4. Conclusions
References
In this study, a novel low-temperature co-fired ceramic is prepared by x BLMT-(100-x) Li2ZnTi3O8 (2.5≤ × ≤80 wt%) composite system sintered at 900 °C. The sintering behavior, phase composition, microstructure and dielectric properties of the composites are investigated. The results show that the sintering process of composites with low BLMT glass (< 10 wt%) can be achieved by liquid-phase sintering and phase composition including LaBO3 phase crystallized from glass and Li2ZnTi3O8. In contract, viscous sintering and liquid-phase sintering process could control the sintering process of the composites with high glass (≥10 wt%). And the phase of composites consist of Li2ZnTi3O8, LaBO3 and rutile. Corresponding to the increase of BLMT glass content,
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