Low Temperature Sintering of ZnAl2O4 Ceramics with CuO-TiO2-Nb2O5 Composite Oxide Sintering Aid

doi:10.15541/jim20240517

Abstract

Abstract:

ZnAl₂O₄ and ZnAl₂O₄-based ceramics have attracted much attention from researchers due to their good microwave dielectric, thermal and mechanical properties. In this work, the influence of 5% (in mass) CuO-TiO₂-Nb₂O₅ (CTN) ternary composite oxide additives with different composition ratios on sintering behavior and properties of ZnAl₂O₄ microwave dielectric ceramics was investigated. When the molar fraction ranges of Cu, Ti and Nb elements in 5% CTN additives are 0.625-0.875, 0-0.250 and 0.125-0.625, respectively, sintering temperature of ZnAl₂O₄ ceramics can be reduced from above 1400 ℃ to below 1000 ℃. The sintering additives CN (Cu : Nb=1 : 1, molar ratio) and CTN (Cu : Ti : Nb=4 : 1 : 3, molar ratio) can reduce sintering temperature of ZnAl₂O₄ ceramics to 975 and 1000 ℃, respectively, while maintaining good dielectric properties (dielectric constant ε_r=11.36, quality factor Q×ƒ=8245 GHz and ε_r=9.52, Q×ƒ=22249 GHz) and flexural strengths (200 and 161 MPa), which are expected to be applied in preparation of low temperature co-fired ceramic (LTCC) materials with copper electrodes. Low-temperature sintering of the ZnAl₂O₄+CTN system is characterized as activated sintering. Nanometer-level amorphous interfacial films containing Cu, Ti, and Nb elements are observed at the grain boundaries, which may provide fast diffusion pathways for mass transportation during the sintering process. Valence changes of Ti and Cu ions, along with changes of oxygen vacancies, are confirmed, which provides a potential mechanism for reduced sintering temperature of ZnAl₂O₄ ceramics. In addition, a series of reactions occurring at the grain boundaries can activate these boundaries and further promote the sintering densification process. These results suggest a promising way to design a novel LTCC material with excellent properties based on the low temperature sintering of ceramics with the sintering aid of CuO-TiO₂-Nb₂O₅ composite oxide.

Key words: ZnAl₂O₄, CuO-TiO₂-Nb₂O₅, low-temperature sintering, microwave dielectric ceramic, low temperature co-fired ceramic

CLC Number:

TM282

YANG Yan, ZHANG Faqiang, MA Mingsheng, WANG Yongzhe, OUYANG Qi, LIU Zhifu. Low Temperature Sintering of ZnAl₂O₄ Ceramics with CuO-TiO₂-Nb₂O₅ Composite Oxide Sintering Aid[J]. Journal of Inorganic Materials, 2025, 40(6): 711-718.

Figures/Tables 13

Fig. 1 (a) Compositions of CTN additives for ZnAl2O4; (b) Nephogram of sintering temperature of ZnAl2O4 ceramics with 5% CTN additives and different compositions Colorful figures are available on website

Table 1 Sintering temperatures of ZnAl2O4 ceramics mixed 5% CTN additives with different formulas

Sample ID	Sintering temperature/℃	Molar fraction
Sample ID	Sintering temperature/℃	Cu	Ti	Nb
S1	1150	0.500	0.500	0
S2	1150	0.500	0.375	0.125
S3	1100	0.500	0.250	0.250
S4	1000	0.500	0.125	0.375
S5	975	0.500	0	0.500
S6	1150	0.375	0.125	0.500
S7	1250	0.250	0.250	0.500
S8	1250	0.125	0.375	0.500
S9	1250	0	0.500	0.500

Fig. 2 XRD patterns of the sintered ZnAl2O4 ceramics mixed with 5% CTN additives according to different formulas

Fig. 3 Secondary electron SEM images of fracture surface of the sintered samples S4 (a) and S5 (b)

Fig. 4 XPS spectra of sample S4 sintered at 1000 ℃ for 2 h (a) and (b-e) detailed XPS spectra of Cu ion (b), Ti ion (c), Nb ion (d) and O1s (e) Colorful figures are available on website

Fig. 5 HRTEM images of grain boundary complexion of sample S4 sintered at 1000 ℃ for 2 h (a, b) and sample S5 sintered at 975 ℃ for 2 h (c, d)

Fig. 6 EDS element mappings of sample S4 sintered at 1000 ℃ for 2 h

Fig. S1 Camera images of the sintered samples with different compositions of CTN additives

Table S1 Properties of ZnAl2O4 ceramics with different formulas of CTN additives

Sample ID	Sintering temperature/℃	Density/(g·cm^-3)	ε_r(@10 GHz)	Q×ƒ/GHz	CTE/(×10^-6, ℃^-1) (25-350 ℃)	Flexural strength/MPa
S1	1150	4.46±0.17	10.18	17811	7.66	246±34
S2	1150	4.49±0.02	9.00	41673	7.34	230±10
S3	1100	4.49±0.01	10.10	17150	7.65	233±10
S4	1000	4.54±0.03	9.52	22249	7.49	161±25
S5	975	4.50±0.04	11.36	8245	7.56	200±14
S6	1150	4.54±0.01	9.67	30021	7.45	186±10
S7	1250	4.17±0.02	-	-	7.47	215±20
S8	1250	4.19±0.02	-	-	7.53	199±8
S9	1250	3.51±0.03	-	-	7.29	195±17

Fig. S2 Backscattered electron SEM images of the fracture surface of samples S4 and S5

Fig. S3 XPS spectra of sample S5 sintered at 1000 ℃ for 2 h (a) and detailed XPS spectra of Cu ion (b), Nb ion (c) and O1s (d)

Fig. S4 EDS element mappings of sample S5 sintered at 975 ℃ for 2 h (a) and EDS spectra of the grain phase (Area 1) and the adjacent second phase (Area 2) (b)

Table S2 EDS analysis results for the grain (Area 1) and the adjacent second phase (Area 2)

Element		Al (K)	Zn(K)	O (K)	Nb (K)	Cu (K)
Atomic/%	Area 1	28.63	13.50	57.47	0.00	0.38
Atomic/%	Area 2	1.44	4.43	88.90	4.04	1.17
Uncertainty/%	Area 1	0.31	0.42	0.49	100.00	0.05
Uncertainty/%	Area 2	0.11	0.43	1.14	0.78	0.19

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Low Temperature Sintering of ZnAl₂O₄ Ceramics with CuO-TiO₂-Nb₂O₅ Composite Oxide Sintering Aid

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