A low-firing melilite ceramic Ba2CuGe2O7 and compositional modulation on microwave dielectric properties through Mg substitution

Abstract: A melilite Ba2CuGe2O7 ceramic was characterized by low sintering temperature and moderate microwave dielectric properties. Sintered at 960 ℃, the Ba2CuGe2O7 ceramic had a high relative density 97%, a low relative permittivity (εr) 9.43, a quality factor (Q×f) of 20,000 GHz, and a temperature coefficient of resonance frequency (τf) –76 ppm/℃. To get a deep understanding of the relationship between composition, structure, and dielectric performances, magnesium substitution for copper in Ba2CuGe2O7 was conducted. Influences of magnesium doping on the sintering behavior, crystal structure, and microwave dielectric properties were studied. Mg doping in Ba2CuGe2O7 caused negligible changes in the macroscopic crystal structure, grain morphology, and size distribution, while induced visible variation in the local structure as revealed by Raman analysis. Microwave dielectric properties exhibit a remarkable dependence on composition. On increasing the magnesium content, the relative permittivity featured a continuous decrease, while both the quality factor and the temperature coefficient of resonance frequency increased monotonously. Such variations in dielectric performances were clarified in terms of the polarizability, packing fraction, and band valence theory. 

Keywords: ceramics; dielectric properties; melilite structure; compositional modulation

1 Introduction

The development of wireless communication and broadband network technology has dramatically increased the demand for microwave dielectric materials, especially those being of small volume, low weight, high stability, and fast propagation speed [1,2]. The primary performance parameters that should be concerned are: (1) suitable relative permittivity εr depending on specific application scenarios, (2) high quality factors Q×f (or low dielectric loss), and (3) low-temperature drift of the resonance frequency (τf
[1–3]. Particularly, the official commercialization of the fifth-generation (5G) networks is expanding the operational frequency to microwave and millimeter waves, and even terahertz bands [4,5]. To realize rapid signal propagation under so high frequencies, low relative permittivity is a prerequisite given that the decay for electromagnetic signal transmission in dielectric carriers is in proportion to the relative permittivity [6]

Up to now, there have been many studies on the exploration of low-εr dielectric materials, most of which highlight silicates such as MgSiO3r = 6.7), Zn2SiO4r = 6.6), and Ba2ZnSi2O7r = 8.09) because of the low ionic polarizability of silicon (~0.87 ų) [7–9]. Among them, Si-based melilites with a chemical formula [A2BSi2O7] have attracted intensive attention as microwave dielectric materials due to the ease with which the structures can accommodate different combinations of cations in the A and B sites [9–11]. The structure of A2BSi2O7 melilite is represented as a two-dimensional framework of tetrahedral sheets of corner-sharing [SiO4] and [BO4] tetrahedra, in such a way that the [SiO4] tetrahedra link at one corner to form [Si2O7] dimers and the four remaining corners in the dimer are linked to the [BO4] tetrahedra. Consequently, a set of distorted pentagonal rings is formed and the big A ions enter these sites, providing the connection between adjacent sheets [12,13]. The compositional and structural flexibility of melilites provides wide space and potential for tunability in their physical properties. As regards, in our previous work, Ge substitution for Si was proposed to modify the sintering behavior and dielectric performance of A2BSi2O7. Consequently, some Ge-based melilites A2BGe2O7 (with A = Sr, Ba, and B = Zn, Mg) have been reported to possess encouraging microwave dielectric properties with εr = 7.76–9.0, Q×f = 13950–35700 GHz, and τf = (–84.4)–(–55) ppm/℃ [14,15]

Recent reports have featured that CuO, either as a low-melting-point (1086 ℃ ) ingredient or as a sintering aid, usually facilitates sintering and uprates dielectric behaviors [16,17]. For example, adding an appropriate amount of CuO in Mg2SiO4 stabilized the crystal structure and lowered the dielectric loss to some extent [18]. Wang et al. reported similar enhancement in sintering behavior and dielectric performances of Li3Mg2NbO6 by CuO doping [19]. This inspires us to investigate melilites which would further yield a reduction in sintering temperature and improvement in dielectric properties.

Herein, Ba2CuGe2O7, a member of melilites, was studied on the dielectric properties at microwave frequencies with an attempt to explore novel materials as potential candidates for microwave communication. Besides, given the identical valence and effective ionic radius for Mg2+ and Cu2+ in a tetra-coordinated group, the substituted magnesium in Ba2CuGe2O7 is expected to induce certain structural variation on an either macroscopic or microscopic scale, which in turn engenders property evolutions. Doping effects on the sintering behavior, crystal structure, and microwave dielectric properties were studied. 

2 Experimental 

2. 1 Material synthesis 

A simple solid-state route was employed to fabricate Ba2Cu1-xMgxGe2O7 ceramics (x = 0, 0.2, 0.4, 0.6, and 0.8) using high-purity BaCO3, CuO, GeO2, and MgO (99.9%, Shanghai Aladdin, China) as raw materials, which were weighted according to the stoichiometry. Powder mixing via ball milling was conducted at a speed of 200 r/min for 4 h. After calcined at 900 ℃ for 4 h, the powders were secondly milled for particle uniformity, after which an appropriate amount of polyvinyl alcohol binder (5 wt%) was added. Then green pellets were pressed into cylinders with a diameter of 12 mm and 6 mm of thickness under a uniaxial pressure of 100 MPa. All green pellets firstly sintered at 550 ℃ with a heating rate 1.5 ℃/min for 6 h to remove the organic binder. And then, sintering behavior was performed in a temperature range from 900 to 1140 ℃ to achieve the best density, which was measured based on the Archimedes method.

2. 2 Characterizations 

The phase formation and purity were determined by X-ray diffraction using a Bruker Corporation diffractometer (D8 ADVANCE03030502, Karlsruhe, Germany). The Rietveld parameters of the scale factor, zero shift, unit cell parameters, background polynomial, profile parameters, atomic positional coordinates, and isothermal factors were refined step-by-step to improve the value of reliability factors. The microstructures were performed on the polished and thermal etched surfaces of the Ba2CuGe2O7 (BCG) ceramics and measured by a scanning electron microscope (JSM6701 F, Akishima City, Japan). Before SEM measurements, gold sputtering was done using a sputter coater (Ted Pella, USA). Micro Raman Spectroscopy System (Witec, Ulm, Germany) is used for obtaining the Raman spectra with an argon ion laser (λ = 514.5 nm) as the excitation light. Raman shifts were measured with a precision of ~0.3 cm–1. The spectral resolution is of the 1 cm–1 order and the range of it is 150–1200 cm–1. Microwave dielectric properties were measured by a Vector Network Analyzer (Rohde & Schwarz, Munich, Germany). According to the Hakki–Coleman method, the relative permittivity (εr) and quality factor (Q×f) were measured in the frequency range 10–16 GHz with a TE011 resonant mode. The temperature coefficient of resonance frequency (τf) was determined by recording the frequency drift over a temperature range from 25 to 85 ℃ in a temperature cavity and calculated based on the equation:

3 Results and discussion 

3. 1 Crystal structure and dielectric properties of Ba2CuGe2O7 

Figure 1 shows the room-temperature XRD pattern and the calculated profile based on the Rietveld refinement performed on the Ba2CuGe2O7 ceramic sintered at 960 ℃ for 6 h. A tetragonal melilite structure model with space group P-421m (113) was adopted for refinement. The calculated pattern fits well with the observed one, yielding low reliable factors with Rp = 6.02% and Rwp = 7.73%. This result confirms the phase purity and the crystal structure of Ba2CuGe2O7. The refined lattice parameters are a = b = 8.5133 Å, c = 5.4495 Å, and V = 390.2468 ų.

Fig. 1 Room-temperature XRD pattern and Rietveld refinement plots for Ba2CuGe2O7

To explore the sintering behavior of Ba2CuGe2O7 ceramics, the microstructure evolution and density variation as a function of sintering temperature was recorded and shown in Fig. 2. Evident grain growth was detected accompanied by a decrease in porosity as the sintering temperature increased, both signatures for densification. Consequently, a dense microstructure with grains about 5–7 μm was obtained after sintered at 960 ℃ (Fig. 2(d)). Correspondingly, the density (Fig. 2(f)) experienced an obvious increase from 4.65 g/cm³(~92% theoretical density) at 900 ℃ to 4.93 g/cm³(~97%) at 960 ℃. The slight decrease in density (4.89 g/cm³) when sintered at 980 ℃ might be related to the large grains that give rise to re-entrant pores, as shown in Fig. 2(e).

Fig. 2 SEM micrographs of the polished and thermal etched surfaces of the Ba2CuGe2O7 ceramics sintered at (a) 900 ℃, (b) 920 ℃, (c) 940 ℃, (d) 960 ℃, and (e) 980 ℃; (f) the bulk density and relative density as a function of sintering temperature. 

Figure 3 shows the variations in the dielectric properties (εr, Q×f, and τf) of the Ba2CuGe2O7 ceramics sintered at a temperature range from 900 to 980 ℃. As the sintering temperature increased both εr and Q×f revealed a similar change trend resembling the density (Fig. 2(f)). A saturated value of εr ≈ 9.43 and Q×f ≈ 20,000 GHz was obtained in the sample sintered at 960 ℃. It is well known that synergetic contributions from the extrinsic and intrinsic factors engender the overall variation in dielectric performances [20–22]. Herein, the strong correlation of the relative permittivity and quality factor on the density reveals a predominant role of density on the dielectric properties of Ba2CuGe2O7. By contrast, the τf was weakly dependent on the sintering temperature and fluctuated around –76 ppm/℃. In summary, a composition sintered at 960 ℃ possessed the optimal microwave dielectric properties with εr = 9.43, Q×f = 20,000 GHz, and τf = –76 ppm/℃. 

Fig. 3 Microwave dielectric properties of Ba2CuGe2O7 ceramics sintered at different temperatures (900–980 ℃). 

The lower densification temperature of Ba2CuGe2O7 ceramic than the melting point of the Ag electrode suggests that it has a promising application prospect in LTCC technology. Figure 4 shows XRD patterns, backscattering micrograph (BSEM), and EDS profile of the cofired Ba2CuGe2O7 ceramic with the silver electrode at 960 ℃. By elaborate comparison with the standard JCPDF cards, the diffraction peaks of silver are separated from those of the melilite. Two kinds of grains with different morphologies and element contrasts (bright grains and dark grains) can be detected. Combined with the EDS analysis, the bright grains are confirmed to be the Ag electrode.

Fig. 4 (a) XRD and (b) SEM micrograph of Ba2CuGe2O7 ceramic cofired with Ag electrode at 960 ℃ (EDS analysis of Ag is shown in the inset of Fig. 4(b)). 

By comparison with Ba2MgGe2O7r = 7.76, Q×f = 20,700 GHz, and τf =-55 ppm/℃), the present Cu-based compound has a higher relative permittivity, while its quality factor and the thermal stability of resonance frequency are inferior to those of Ba2MgGe2O7[14]. Thus, Mg substitution for Cu in Ba2CuGe2O7 was conducted with an attempt to tune the dielectric performances of Ba2CuGe2O7 and to correlate such dielectric evolutions to the composition and crystal structure. 

3. 2 Effects of magnesium substitution on crystal structure and microstructure 

The equivalent substitution of Mg for Cu caused no change in the macroscopic crystal structure of melilite Ba2CuGe2O7 but indeed induced continuous variation in lattice parameters. Figure 5(a) shows XRD patterns recorded on the calcined Ba2Cu1–xMgxGe2O7 (x = 0, 0.2, 0.4, 0.6, and 0.8) powders at 900 ℃. A similar pattern is observed for all compositions and by indexing with ICDD # 89-5260 for Ba2CuGe2O7, all peaks can be assigned and no additional peak is detected. The compositional-induced shift of (211) peak to the higher angle provides evidence of magnesium solution, despite the shift is not sizeable. The lattice parameters refined by the least-square method are obtained from Rietveld refinements. As presented in Fig. 5(b), an evident decrease in a is accompanied by a continuous increase in c, giving rise to a slight and monotonous decline in the unit cell by 0.18%. Otherwise, the linear variation in lattice parameters validates the Vegard’s law for a solid solution. These results indicate the scheduled magnesium could completely dissolve into the lattice of melilite to form an infinite solid solution. 

Fig. 5 (a) Room-temperature XRD patterns for Ba2Cu1–xMgxGe2O7 (x =0, 0.2, 0.4, 0.6, and 0.8) powders calcined at 900 ℃; (b, c) variations in the lattice parameters as a function of composition x. 

Rietveld refinements, shown in Fig. 6, further verifying the phase purity and structural stability, are characterized by the good match between the calculated and experimental profiles and the reliable residual factors. It should be noted that the structural model was established based on the parent Ba2CuGe2O7 and the sequences for refinements were set as scale factor, zero shift, background, lattice parameters, peak function parameters, and atomic positions, etc. Importantly, the atomic position for magnesium was fixed in the Cu positions and their distribution was set as random. 

Fig. 6 Rietveld refinement plots for Ba2Cu1–xMgxGe2O7: (a) x = 0.2; (b) x = 0.4; (c) x = 0.6; (d) x = 0.8. 

Figures 7(a) and 7(b) illustrate the variation in bulk density as a function of sintering temperature and composition. All compositions exhibit an analogous dependence on sintering temperature of the density. An initial substantial increase in bulk density explains the crucial role of sintering temperature in promoting densification. A maximum bulk density (ρm) was achieved at a characteristic temperature (recognized as the optimized temperature, Tm) depending on composition. The compositional dependence of the ρm and Tm values is shown in Fig. 7(b), from which we can see that all compositions have a high relative density (> 96%), suggesting they are applicable for subsequent dielectric characterizations. The much higher melting point of MgO (2852 ℃) than that of CuO (1026 ℃) accounts for the increase in sintering behavior. 

Fig. 7 (a) Variations in bulk density as a function of sintering temperature for Ba2Cu1–xMgxGe2O7; (b) compositional dependence of bulk density and sintering temperature with the relative densities showing in the inset. 

Figures 8(a)–8(e) show SEM images of the polished and thermal etched surfaces of Mg-doped BCG along with their respective grain size distribution. Dense microstructures with few pores were developed for each compound, coincident with their high relative density. The similar grain morphology validated no structural transformation by Mg doping, which coincides with the XRD observation. Besides, the grain size distribution (in the inset of Fig. 8) for all compounds is alike, giving rise to their close average grain size, as shown in Fig. 8(f). 

Fig. 8 (a–e) SEM images and the grain size distributions for Ba2Cu1–xMgxGe2O7 sintered at their optimum temperature; (f) the change of average grain size as a function of x value. 

Table 1 Wyckoff sites for atoms and the corresponding Raman active modes in Ba2Cu1–xMgxGe2O7

Raman scattering is known to be highly sensitive to the local structure variations, especially those induced by composition [12,23]. For a tetragonal P-421m unit cell in Ba2CuGe2O7, theoretically, the Raman active vibration modes are predicted based on the factor group theory and listed in Table 1 along with the Wyckoff sites. A total number of 45 (Γ = 10A1+ 7B1+10B2+18E) Raman active modes are estimated. Figure 9(a) shows the room-temperature Raman spectra of the Ba2Cu1–xMgxGe2O7 samples. Due to the low density and overlapping of some modes, only a limited number of Raman modes (11–16) are observed via the Gauss–Lorenzian deconvolution (Fig. 9(b)), among which the three strongest peaks appear around 775 cm–1, 500 cm–1, and 258 cm–1, respectively. As well known, the Raman spectroscopy includes the internal vibrations of the framed polyhedron (either tetrahedron or octahedron for oxides) and the external ones from translational and liberational moves [23]. Generally, the binding energy in the polyhedral is larger than the intergroup or crystal energy, and thus the internal Raman modes usually appear at high-frequency bands. Thus, the strongest peak around 775 cm–1 is assigned as the symmetric stretching vibration of [GeO4], and the Raman mode around 500 cm–1 and 258 cm–1 corresponds to the asymmetric and symmetric bending of [GeO4], respectively. Nevertheless, the Raman modes related to the translation of Cu2+/Mg2+locate around 200–225 cm–1 and 280–300 cm–1, as previously reported [12]. It should be noted that in the structure of melilite Ba2CuGe2O7, the [CuO4] tetrahedra are corner-linked to the [Ge2O7] dimers. Thus, the variation in the [CuO4] tetrahedra by Mg doping, either distortion/titling or volume expansion/shrinkage would cause a corresponding change in the neighboring [GeO4] tetrahedra. Hence, by recording the alteration in the [GeO4] tetrahedra, the local structure evolution by Mg substitution can be manifested. 

Fig. 9 (a) Raman spectroscopies for Ba2Cu1–xMgxGe2O7 along with an enlarged profile around 775 cm–1; (b) a representative deconvolution fitting for the x = 0.8 composition; (c, d) variations in the Raman shift and FWHM for the three strongest Raman modes at 775 cm–1, 500 cm–1, and 258 cm–1

The change of Raman shift and the full width at half maximum (FWHM) of the strongest three modes are shown in Figs. 9(c) and 9(d). As shown, the 775 cm–1 mode is markedly red-shifted, while the 500 cm–1 and 258 cm–1 modes are quasi-invariant with increasing composition x. The red-shift in the 775 cm–1 mode indicates the decreased band energy of the [GeO4] tetrahedra. This can be explained by the increased bond length of Ge–O caused by the expansion in the [GeO4] because of the compress of the adjacent [Cu/MgO4] tetrahedra. On the other hand, the width of the 500 cm–1 and 258 cm–1 modes monotonously decreased with increasing Mg content, while the 775 cm–1 mode was firstly broadened and then narrowed, resulting in the x = 0.4 composition featuring the highest FWHM. These results revealed that magnesium substitution induced local disorder, which becomes more distinct around x = 0.4. 

More importantly, an extra mode in the parent Ba2CuGe2O7 phase arises at 804 cm–1 and its intensity becomes weaker on the increasing magnesium content. This mode is assigned as the asymmetric stretching vibration of [GeO4]. By comparing the Raman modes for Ba2MgGe2O7 and Ba2ZnGe2O7, a pretty evident difference lies in the absence or presence of this 804 cm–1 mode. In consideration of their same structure and atomic distribution, this distinction in Raman spectroscopy is believed to be related to the local structure distortion by Zn replacement for Mg. Similar phenomenon observed in this work validates the influences of magnesium substitution on the local structure. 

In summary, magnesium substitution in Ba2CuGe2O7 does not cause macroscopic phase transformation, or induce the second phase, or change the grain morphology and grain size distribution, but the evolutions in XRD peaks and Raman modes reveal that such doping indeed arouses local structural distortion, which could exert significant influences on dielectric properties. 

3. 3 Effects of magnesium substitution on dielectric properties 

Figures 10(a) and 10(b) represent the change of relative permittivity (εr) and quality factor (Q×f) as a function of sintering temperature. For each composition, both εr and Q×f value increased as the sintering temperature increased and then decreased slightly. Importantly, the variation tendency of εr and Q×f versus sintering temperature is similar to that of density, being an indicator for the crucial role of density on dielectric performances. Generally, the highest εr and Q×f values for each position are achieved in the densest sample. Figures 10(c) and 10(d) summarize the optimum εr and Q×f values for various compositions. Strong dependence on composition is visible for both quantities. Relative permittivity features a noticeable decrement with increasing Mg content from 9.05 at x = 0 to 7.92 at x = 0.8. Conversely, the quality factor exhibits a continuous rise from 20,000 to 31,370 GHz as the amount of Mg increased from 0 to 0.8. 

Fig. 10 Variations in the relative permittivity (εr) and quality factor (Q×f) of Ba2Cu1–xMgxGe2O7 (a, b) as a function of sintering temperature and (c, d) composition x value along with the change of the polarizability per unit volume (αTD/Vm) and packing fraction. 

As well known, the relative permittivity and losses at microwave frequency bands not only depend on the intrinsic factors, but also the extrinsic ones from density (or pore), grain size, phase constitution, and phase transition, etc. [20,24–26]. Evidenced from the crystal structure and microstructure, magnesium substitution did not cause phase transformation, or induce the second phase, or change the grain morphology and grain size distribution. Thus, effects from these extrinsic factors (density, grain size, phase transition, second phase) on the dielectric properties can be ruled out. Hence, it is rational to conclude that the compositional variation in relative permittivity and quality factor is mainly controlled by the intrinsic influences. 

Theoretically, the relative permittivity is inherently affected by the molecular polarizability (αTD) and unit volume (Vm) based on the Clausius–Mossotti equation εr=(1+2bαTD/Vm)/(1-bαTD/Vm) with b = 4π/3 as a constant, from which a direct correlation is accessible between εr and the polarizability per unit volume ( αTD/Vm ) [27]. Figure 10(c) shows the variation of αTD/Vm with composition, being characterized by a linear decrease. This result accounts for the underlying decrease of relative permittivity by magnesium substitution. Nevertheless, the calculated permittivities according to the Clausius–Mossotti equation are much smaller than the measured ones and the separation between them is about 7.1%–18.0%, decreasing with an increase of composition x, as shown in Fig. 10(c). Shannon ascribed such discrepancy as ionic or electronic conductivity, and/or structural distortion like rattling or compressed cations [27]. This large deviation, especially in Ba2CuGe2O7, is an indicator of the local structural distortion and Mg doping released such distortion, which would inevitably influence the physical properties, e.g., dielectric properties. 

Such local distortion on relative permittivity can be reflected by the bond characteristics estimated by the electronegativity difference (Δe). According to Pauling’ rule [28], the electronegativity difference for Ba2Cu1–xMgxGe2O7 is calculated as

Evidently, the electronegativity difference increases linearly with increasing x value, and the Δe are calculated as 1.900, 1.924, 1.947, 1.971, and 1.994 for x = 0, x = 0.2, x = 0.4, x = 0.6, and x = 0.8, respectively. The smaller Δe than 2 indicates the covalent bond feature which becomes more prominent with Mg content. The enhanced covalent bond should in principle dilute the polarizability (or permittivity) because of the more rigid structure. However, an opposite variation trend in the relative permittivity validates the existence of some rattling or compressed cations rooting in the local distortion with an increase in Mg content.

The intrinsic influence on quality factor can be reflected by the packing fraction, as empirically summarized by Kim et al. [29,30]. For a certain material system, the higher of the packing fraction, the smaller space for the thermal motion of cations or anions, thus leading to lower dielectric losses [31]. The packing fraction is defined as the total volume of packed ions over the volume of the unit cell and the calculated values are shown in Fig. 10(d). A consistent increase in packing fraction with the quality factor is seen because magnesium substitution in BCG heightened the packing fraction, leading to more close packing of ions, which in turn results in low dielectric losses. On the other hand, the release of local structural distortion with Mg doping partly accounts for the diminished dielectric loss. 

A linear increase in the temperature coefficient of resonance frequency (τf) is illustrated in Fig. 11 as the amount of Mg substitution increases from 0 to 0.8. It is well known that the τf value is correlated with the temperature coefficient of the relative permittivity (τε) by the relation τf = –(τε/2+αL) (αL represents the linear thermal expansion coefficient, which is usually a constant about 10 ppm/℃). Thereby, the composition dependence of τf can be analyzed by studying the respective variation in τε value. Derived from the definition for τε, Bosman and Havinga divided the formula as follows [32]:

where ε and αm represent the relative permittivity and polarizability, respectively. A is the direct dependence of the polarizability on temperature, being generally negative; B (generally positive) denotes the variation in polarizability concerning to the thermal change of volume; and C, in general negative, directly represents the volume change on temperature. The magnitudes of the variables B and C are similar and in opposing signs. Hence, the relative magnitude between A and (B+C) determines the signs and magnitude of τε values. Particularly, both B and C terms involve in the volume variation, which would be reflected through the bond valence of ions. According to the previous reports on the calculation of bond valence [33–36], the Mg substitution effect on the bond valence of Cu–O in BCG is evaluated and shown in Fig. 11. In contrast to the variation tendency in the τf value, a steady decrease occurs in the bond valence. The decreasing bond valence reveals the reduced structural distortion caused by magnesium doping. The thermal energy is preferential to recover the structural distortion, giving rise to a larger (B+C) than A term, and thus a negative τf value. Therefore, the present rising τf value can be explained by the decrease in the structural distortion, as indicated by the decreased bond valence. 

Fig. 11 Variations in the temperature coefficient of resonance frequency (τf) and the bond valence of Mg/Cu–O. 

Table 2 compares the sintering temperature and microwave dielectric properties of some melilite structured ceramics. As expected, either Si-based or Ge-based melilite compounds possess low permittivities (εr < 10), moderate Q×f values, and negative τf values.  Generally, Ba-containing compounds have relatively low sintering temperatures (~1200 ℃), while those Sr-containing counterparts have higher sintering temperatures (> 1300 ℃). What makes the present Ba2CuGe2O7 unique is its lowest sintering temperature (960 ℃) amongst the listed melilites, which makes it a potential candidate for LTCC technology when using Ag as the inner electrode [39–41]

Table 2 Sintering temperature and microwave dielectric properties of some melilite structure ceramics 

4 Conclusions 

In summary, we fabricated a series of magnesium substituted Ba2CuGe2O7 ceramics and detailed the effects of magnesium on the sintering behavior, structure (both macroscopic and microscopic), and microwave dielectric properties. XRD and SEM results revealed that magnesium doping induced limited variations in the macroscopic structure and microstructure features. On the contrary, the densification temperature was remarkably raised from 960 to 1120 ℃; meanwhile, Raman spectra provided an indicator of considerable change in the local structure, as characterized by the alteration in the Raman shift and width. Moreover, on increasing the magnesium content, the dielectric properties decreased from 9.43 to 7.92 which are attributed to the lower ionic polarizability of magnesium and the released local structural distortion, while the quality factory (Q×f) increased from 19,560 to 31,370 GHz because of the increasing packing fraction. A monotonous increase in τf was induced by magnesium substitution, which was explained by the decreased bond valence of Cu/Mg–O. This work provides a potential method to tune the thermal stability of Ba2CuGe2O7 realized by increasing the bond strength through substituting larger cations, e.g., Ni2+ for Cu2+ or Si4+ for Ge4+

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