Structures and microwave dielectric characteristics of compounds in vicinity of CaNdAlO 4 in CaO–Nd 2 O 3

Abstract

Single phase ceramics in the vicinity of CaNdAlO₄ in CaO–Nd₂O₃–Al₂O₃ ternary system, such as ternary oxide compound CaNdAl₃O₇ and binary oxide compounds NdAlO₃, CaAl₂O₄, Ca₃Al₂O₆ and Ca₁₂Al₁₄O₃₃, were prepared by the standard solid state reaction route. The phases and structure of the ceramics were analysed through X-ray diffraction and scanning electron microscopy. The microwave dielectric characteristics of the ceramics were studied using a network analyser. High Qf values were obtained in CaNdAlO₄, CaNdAl₃O₇, NdAlO₃ and CaAl₂O₄, while the Qf values in Ca₃Al₂O₆ and Ca₁₂Al₁₄O₃₃ were much lower. The best microwave dielectric characteristics (ϵ_r = 8·9, Qf = 90 800 GHz at ∼9·40 GHz and τ_f≈−55·2 MK⁻¹) were achieved for CaAl₂O₄ ceramics sintered at 1425°C in air for 3 h. It could be expected as a promising new candidate for microwave dielectric ceramics. Moreover, the present results showed the direction for modification of CaNdAlO₄ microwave dielectric characteristics through composition tailoring.

Keywords

Microwave dielectric properties Microstructure

Introduction

Microwave dielectric ceramics have wide applications in the microwave communication systems as the key materials for microwave resonators, filters and antennas.1 So far, a large number of microwave dielectric ceramics have been investigated and developed, such as Ba_6−3xR_8+2xTi₁₈O₅₄, (Zr,Sn)TiO₄, Ba₂Ti₉O₂₀, Ba(Mg_1/3Ta_2/3)O₃, Ba(Zn_1/3Ta_2/3)O₃, etc.1^–4 With the development trend of microwave communication towards millimetre wave range, the microwave dielectric ceramics with low dielectric permittivity and low dielectric loss have attracted increasing scientific attention.4, 5

Recently, MRAlO₄ (M = Sr and Ca; R = La, Nd, Sm and Y) ceramics with the tetragonal K₂NiF₄ structure have been proposed and investigated as promising candidates for high band microwave applications.6^–12 The MRAlO₄ compounds with K₂NiF₄ structure consist of the stacking of rock salt layer and perovskite layer.10, 13 According to previous work,10 the measured Qf values for MRAlO₄ ceramics are much lower than the calculated ones, and this suggests that the microwave dielectric characteristics, especially the Qf value for the present ceramics, should be significantly improved by microstructure modification. Actually, the microwave dielectric characteristics for CaSmAlO₄ ceramics largely depend on the microstructures, especially the phase constitution.11 However, the microwave dielectric characteristics of the phases in the vicinity of MRAlO₄ in the MO–R₂O₃–Al₂O₃ ternary systems have been rarely reported before. Therefore, it should be an important issue to investigate the phase equilibrium and the microwave dielectric characteristics for the compositions in the vicinity of MRAlO₄ in the MO–R₂O₃–Al₂O₃ ternary systems.

Researches on the phase equilibrium of CaO–Al₂O₃, Nd₂O₃–Al₂O₃ and the isothermal section of CaO–Nd₂O₃–Al₂O₃ ternary system at 1300°C were reported before.14^–16 It was indicated that the compounds of NdAlO₃, CaAl₂O₄, CaNdAl₃O₇, Ca₃Al₂O₆ and Ca₁₂Al₁₄O₃₃ in the vicinity of CaNdAlO₄ were all stable phase. In the present work, these compounds and CaNdAlO₄ in the CaO–Nd₂O₃–Al₂O₃ ternary system were prepared, and then, their structures were determined together with the microwave dielectric characteristics.

Experimental

Figure 1 gives the tentative phase diagram of the CaO–Nd₂O₃–Al₂O₃ ternary system,14^–16 where one can find the previously reported compounds in the vicinity of CaNdAlO₄. CaNdAlO₄, NdAlO₃, CaNdAl₃O₇, CaAl₂O₄, Ca₃Al₂O₆ and Ca₁₂Al₁₄O₃₃ ceramics were synthesised by a standard solid state reaction process using high purity CaCO₃ (99·99%), Al₂O₃ (99·99%) and Nd₂O₃ (99·9%) powders as the raw materials. Because rare earth oxides, such as Nd₂O₃, are easily hygroscopic,17 the preheat treatment for Nd₂O₃ raw powders was carried out at 900°C in air for 2 h before weighting to remove any hydroxides. The process conditions including calcination and sintering conditions are listed in Table 1. The raw materials with the corresponding stoichiometric compositions were ball milled in a polyethylene jar for 24 h with zirconia media and ethanol. After drying, the mixtures were put in corundum crucible and calcined in air at the relevant temperature for 3 h, and then, the calcined powders with zirconia media and ethanol were again ground by the ball mill for 24 h. Then, the powders, with 6–16 wt-%PVA as the binder, were pressed into discs that are 12 mm in diameter and 2–6 mm in thickness under a uniaxial pressure of 98 MPa. These discs were placed on sacrificial powder of the same composition and sintered at the relevant temperature in air for 3 h to yield the ceramics. After cooling from the sintering temperature to 800°C at a rate of 2°C min⁻¹, the sintered ceramics were further cooled with the furnace.

Figure 1

Tentative phase diagram of CaO–Nd₂O₃–Al₂O₃ ternary system

Table 1

Process conditions for present ceramics

Compound	Calcining temperature/°C	Sintering temperature/°C	Quantum of PVA/wt-%
CaNdAlO₄	1300	1400–1475	6
NdAlO₃	1250	1450–1600	8
CaNdAl₃O₇	1350	1500–1600	6
CaAl₂O₄	1250	1375–1475	6
Ca₃Al₂O₆	1300	1325–1375	16
Ca₁₂Al₁₄O₃₃	1300	1300–1350	12

The bulk density of the sintered ceramics was determined geometrically. The phase constitutions of crushed and ground powders of the sintered ceramics were identified by powder X-ray diffractometry using Cu K_α (λ = 0·15406 nm) radiation (Rigaku D/max 2550PC; Rigaku Co., Tokyo, Japan) operated at 40 kV and 250 mA with 2θ increments of 0·02°. The scanning electron microscopy (SEM) images of the surfaces of sintered discs were observed with a scanning electron microscope (FE-SEM; Sirion-100; FEI, USA). The relative dielectric constant ϵ_r and quality factor Q were evaluated by the Hakki–Coleman method18, 19 and the cavity method20 respectively, where a vector network analyser (E8363B; Agilent Technologies, Palo Alto, CA, USA) was used. Because the Q factor generally varied inversely with frequency f in the microwave region, the product Qf was used to evaluate the dielectric loss instead of Q. The temperature coefficient of the resonant frequency τ_f at microwave frequency was calculated with the following formula (1) where f₈₅ and f₂₀ were the TE₀₁₁ resonant frequencies at 85 and 20°C respectively.

Results and discussion

As shown in Fig. 2, the room temperature powder X-ray diffraction patterns for the present ceramics sintered at different temperatures all indicate the single phase structures, and they can be assigned into the compounds of CaNdAlO₄, NdAlO₃, CaNdAl₃O₇, CaAl₂O₄, Ca₃Al₂O₆ and Ca₁₂Al₁₄O₃₃ in space groups of I4/mmm, , , P2₁/n, and , according to the Joint Committee on Powder Diffraction Standards cards 81-0743, 71-1596, 50-1807, 70-0134, 38-1429 and 70-2144 respectively. The CaNdAlO₄ and CaNdAl₃O₇ ceramics crystallised in a tetragonal structure, the Ca₃Al₂O₆ and Ca₁₂Al₁₄O₃₃ ceramics crystallised in a cubic structure and the NdAlO₃ and CaAl₂O₄ ceramics crystallised in rhombohedral and monoclinic structures respectively. For CaNdAl₃O₇ and CaAl₂O₄, since there are no data for 2θ >60° in the Joint Committee on Powder Diffraction Standards cards, the rest relevant indices of crystallographic planes are not shown here.

Figure 2

X-ray diffraction patterns of crushed powders of a CaNdAlO₄, b NdAlO₃, c CaNdAl₃O₇, d CaAl₂O₄, e Ca₃Al₂O₆ and f Ca₁₂Al₁₄O₃₃ ceramics sintered at 1425, 1500, 1600, 1425, 1375 and 1350°C for 3 h in air respectively

Figures 3 and 4 show the densification curves of CaNdAlO₄, NdAlO₃, CaNdAl₃O₇, CaAl₂O₄, Ca₃Al₂O₆ and Ca₁₂Al₁₄O₃₃ ceramics and their SEM images. With increasing sintering temperature, the relative densities of the first three compositions all increase gradually, and the values of their densification are all above 96% theoretical density (TD) at last. However, for CaAl₂O₄ ceramics, the relative densities turn to decrease slightly to ∼94·45% TD at sintering temperature of 1475°C. As shown in Fig. 4a–f, the grains of the ceramics grow gradually with increasing sintering temperature in the CaNdAlO₄, NdAlO₃ and CaNdAl₃O₇ compositions, which is especially observed in the latter two compositions. In NdAlO₃ ceramics, the obvious ledges are observed in the grain, which indicates the type of growth of crystal. As shown in Fig. 4g, there may be a layer of glass phase with many microcracks on the surface of the CaAl₂O₄ ceramic, and these microcracks may develop during the specimen cooling. It seemed that the densification of Ca₃Al₂O₆ is very difficult, and only 88·25% TD can be reached for the ceramics sintered at 1375°C, and further increasing the sintering temperature will lead the ceramics to partial melting. According to the SEM images of the Ca₃Al₂O₆ ceramics (see Fig. 4h–i), lots of pores are observed, which is in accordance with the densification. The relative densities and the growth of crystal of the Ca₁₂Al₁₄O₃₃ ceramics (see Fig. 4j–k) exhibit a similar evolution as in CaNdAlO₄, but between the grain boundaries, there are fewer gaps. In addition, the value of its densification can reach 99·85% TD at sintering temperature of 1350°C. It must be noted that all the theoretic densities are calculated from the X-ray data.

Figure 3

Relative density of ceramics as functions of sintering temperature

Figure 4

Secondary electron images of sintered surface of a CaNdAlO₄ sintered at 1400°C, b CaNdAlO₄ sintered at 1425°C, c NdAlO₃ sintered at 1500°C, d NdAlO₃ sintered at 1550°C, e CaNdAl₃O₇ sintered at 1500°C, f CaNdAl₃O₇ sintered at 1550°C, g CaAl₂O₄ sintered at 1475°C, h Ca₃Al₂O₆ sintered at 1325°C, i Ca₃Al₂O₆ sintered at 1350°C, j Ca₁₂Al₁₄O₃₃ sintered at 1300°C and k Ca₁₂Al₁₄O₃₃ sintered at 1325°C

Figure 5 shows the microwave dielectric properties of the CaNdAlO₄, NdAlO₃, CaNdAl₃O₇, CaAl₂O₄, Ca₃Al₂O₆ and Ca₁₂Al₁₄O₃₃ ceramics as functions of the sintering temperature. The microwave dielectric properties of the CaNdAlO₄ ceramics as observed in Fig. 5a are much better than those reported in the previous work.8 The improved properties are possibly due to the better densification and the usage of high purity raw materials and preheat treatment for Nd₂O₃ raw powder in the present work. It is observed that ϵ_r increases from 17·5 to 18·4 with sintering temperature increasing from 1400 to 1475°C, while the Qf value at ∼6·9 GHz increases first, reaches the maximum of 65 200 GHz at 1425°C and then turns to decrease slightly with further increasing sintering temperature. The negative τ_f decreases in quantity with increasing sintering temperature, reaches the minimum at 1425°C and turns to increase again with further increasing sintering temperature. The best combination of microwave dielectric properties of CaNdAlO₄ ceramics is obtained at the sintering temperature of 1425°C: ϵ_r = 17·9, Qf = 65 200 GHz and τ_f = −12·3 MK⁻¹.

Figure 5

Microwave dielectric properties of a CaNdAlO₄, b NdAlO₃, c CaNdAl₃O₇, d CaAl₂O₄, e Ca₃Al₂O₆ and f Ca₁₂Al₁₄O₃₃ ceramics as functions of sintering temperature

Figure 5b shows the variation of microwave dielectric properties with the sintering temperature for NdAlO₃ ceramics. As the sintering temperature increases, ϵ_r increases from 19·9 to 21·8, and Qf indicates the highest value of 25 700 GHz at ∼6·3 GHz at the sintering temperature of 1500°C, while τ_f varies in the range of −46·3 to −43·4 MK⁻¹. The best combination of microwave dielectric properties of NdAlO₃ ceramics is obtained at the sintering temperature of 1500°C: ϵ_r = 21·0, Qf = 25 700 GHz and τ_f = −45·8 MK⁻¹. It should be noticed that the decreased Qf value for NdAlO₃ with increasing sintering temperature above 1500°C should be caused by the inhomogeneous microstructures such as the ledges (see Fig. 4c and d), while the influence of grain boundaries on the microwave dielectric loss is complex, and it needs further investigation.

As shown in Fig. 5c, the good microwave dielectric properties are also revealed in the CaNdAl₃O₇ ceramics, which have been rarely reported before. When the sintering temperature increases from 1500 to 1600°C, ϵ_r increases slightly from 10·7 to 11·0, Qf soars linearly from 13 600 to 30 400 GHz at ∼8·60 GHz and τ_f changes from −32·2 to −31·7 MK⁻¹. Deduced from the chemical formula, CaNdAl₃O₇ is the combination of NdAlO₃ and CaAl₂O₄ with 1∶1 ratio, which both exhibit good microwave dielectric properties (see Fig. 5b and d). Therefore, the better microwave dielectric properties of CaNdAl₃O₇ are expected. The best combination of microwave dielectric properties of CaNdAl₃O₇ ceramics is obtained at the sintering temperature of 1600°C: ϵ_r = 11·0, Qf = 30 400 GHz and τ_f = −31·7 MK⁻¹.

CaAl₂O₄ is one of the most interesting compounds in the CaO–Nd₂O₃–Al₂O₃ ternary system because of its promising microwave dielectric properties (see Fig. 5d). With increasing sintering temperature from 1375 to 1475°C, ϵ_r decreases slightly from 9·0 to 8·8, and Qf increases from 77 500 GHz at 1375°C to the maximum value of 90 800 GHz (sintered at 1425°C) and then decreases to 86 300 GHz with the measurement frequency between 9·30 and 9·50 GHz. For the τ_f value, it varies between −57·5 and −55·2 MK⁻¹. The best combination of microwave dielectric properties of CaAl₂O₄ ceramics is obtained at the sintering temperature of 1425°C: ϵ_r = 8·8, Qf = 90 800 GHz and τ_f = −55·2 MK⁻¹. It should be noted that the microcracks are observed on the surface of the CaAl₂O₄ ceramics (see Fig. 4g), and further improved microwave dielectric properties are expected for crack free CaAl₂O₄ ceramics. Furthermore, it is an important issue to investigate the intrinsic loss of CaAl₂O₄ ceramics and improve the temperature stability in particular. For CaAl₂O₄ ceramics and the others, it is observed that the dielectric constant has a similar evolution as the relative density with increasing sintering temperature. This can be explained by the lower dielectric constant of pores existing in the ceramics and the similar analysis also reported by Penn et al.21 Considering the excellent microwave dielectric properties and the low cost of raw materials, CaAl₂O₄ will be a new promising candidate for low loss microwave dielectric ceramics, especially for millimetre wave applications.

Compared with the above compositions, the poor microwave dielectric properties are indicated in the Ca₃Al₂O₆ and Ca₁₂Al₁₄O₃₃ ceramics (see Fig. 5e and f ). With increasing sintering temperature for the Ca₃Al₂O₆ ceramics, ϵ_r increases from 10·4 to 12·5, Qf increases from 9800 to 11 370 GHz with the measurement frequency ∼8·5 GHz and τ_f varies in the range of −347·7 to −358·4 MK⁻¹. According to Ref. 21, the effect of porosity on dielectric loss can be expressed by (2) where tan δ₀ is the loss when materials fully dense, A is a constant and p is fractional porosity. If the first term on right side in equation (2) is ignored, the Qf value should increase ∼190% with increasing relative density from 77·97 to 88·25%. However, the measured Qf value only increases with the amplitude of ∼16%. That is, tan δ₀ but not the porosity plays a role in the actual Qf value, and tan δ₀ may be mainly determined by the intrinsic loss.

For Ca₁₂Al₁₄O₃₃ ceramics, the permittivity between 7·6 and 7·8 with a low Qf of 8780–8130 GHz at ∼11 GHz is obtained at room temperature, and the τ_f is difficult to determine since the resonant peak at 85°C is too weak to be identified. It is indicated that the low Qf value of the Ca₁₂Al₁₄O₃₃ ceramics is mainly attributed to its high intrinsic loss since the density is near to the TD.

From the above discussion, in the vicinity of CaNdAlO₄ in the CaO–Nd₂O₃–Al₂O₃ ternary system, high Qf values were obtained in CaNdAlO₄, CaNdAl₃O₇, NdAlO₃ and CaAl₂O₄, while the Qf values were much lower for Ca₃Al₂O₆ and Ca₁₂Al₁₄O₃₃; the best combinations of microwave dielectric characteristics were shown in Table 2. These results could provide the guideline for property optimising in CaNdAlO₄ based ceramics by composition tailoring. Moreover, further investigation of the CaAl₂O₄ and CaNdAl₃O₇ ceramics and their structure and property modification should be carried on.

Table 2

Best combining microwave dielectric characteristics for present ceramics in CaO–Nd₂O₃–Al₂O₃ ternary system

Compounds	Sintering temperature/°C	ϵ _r	Qf/GHz	τ_f/MK⁻¹
CaNdAlO₄	1425	17·9	65 200	−12·3
NdAlO₃	1500	21·0	25 700	−48·5
CaNdAl₃O₇	1600	11·0	30 400	−31·7
CaAl₂O₄	1425	8·8	90 800	−55·2
Ca₃Al₂O₆	1375	12·5	11 370	−353·4
Ca₁₂Al₁₄O₃₃	1300	7·6	8780	…

Conclusion

The following ternary or binary oxide compounds were confirmed in the vicinity of CaNdAlO₄ in the CaO–Nd₂O₃–Al₂O₃ ternary system: CaNdAl₃O₇, NdAlO₃, CaAl₂O₄, Ca₃Al₂O₆ and Ca₁₂Al₁₄O₃₃. High Qf values were obtained in CaNdAlO₄, CaNdAl₃O₇, NdAlO₃ and CaAl₂O₄, while the Qf values were much lower for Ca₃Al₂O₆ and Ca₁₂Al₁₄O₃₃. The excellent microwave dielectric characteristics (ϵ_r = 8·9, Qf = 90 800 GHz at ∼9·40 GHz and τ_f≈−55·2 ppm °C⁻¹) suggested CaAl₂O₄ as a promising new candidate for low loss microwave dielectric ceramics for the millimetre wave applications. It should be an important issue to investigate the intrinsic loss of CaAl₂O₄ ceramics to improve its microwave ceramics properties and improve the temperature stability in particular. Moreover, the present results showed the direction for modification of CaNdAlO₄ microwave dielectric characteristics through composition tailoring.

Footnotes

Acknowledgements

The present work was supported by the Chinese National Basic Research Program under grant no. 2009CB623302. The authors gratefully acknowledge Ms L. Q. Peng and X. L. Zhu for their assistance in the Qf measurement and discussions.

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Structures and microwave dielectric characteristics of compounds in vicinity of CaNdAlO 4 in CaO–Nd 2 O 3 –Al 2 O 3 ternary system

Abstract

Keywords

Introduction

Experimental

Results and discussion

Conclusion

Footnotes

Acknowledgements

References