Dependence of microwave dielectric properties on ceramic connectivity in ultralow-ε_r Al₂O₃ porous ceramics

Abstract

Introducing pores is an effective approach for fabricating ultralow-ε_r ceramics; however, it is still unclear how the microstructures affect the microwave dielectric properties. In the present work, Al₂O₃-A and Al₂O₃-B porous ceramics were prepared by incomplete sintering at 1,000-1,650 °C and sintering at 1,650 °C with a porogen, respectively, so that the effects of porosity and ceramic connectivity can be clarified. The introduction of pores led to a significant decrease in ε_r, Qf, and |τ_f|, and Al₂O₃-B ceramic exhibits a slightly higher ε_r, a larger |τ_f|, and a much higher Qf value compared to the Al₂O₃-A counterpart with a similar relative density. For Al₂O₃-A, increasing the sintering temperature and relative density notably enhanced ceramic connectivity, while Al₂O₃-B's ceramic connectivity remained relatively insensitive to density variations. The improved ceramic connectivity in Al₂O₃-B enhanced the contribution of the ceramic phase to ε_r and τ_f, while also increased the Qf value by reducing ceramic-pore interfaces. Notably, Al₂O₃-B, with a low relative density of 48.31%, demonstrated a good combination of microwave dielectric properties, with ε_r = 4.16, Qf = 38,400 GHz, and τ_f = -44.3 ppm/°C. These findings reveal the strong dependence of microwave dielectric properties on ceramic connectivity in porous ceramics, and also inspire the development of ultralow-ε_r materials by regulating both the porosity and ceramic connectivity.

Keywords

Porous ceramics / Al₂O₃ / ceramic connectivity / ultralow dielectric constant / microwave

References

[1] Rappaport TS,Mayzus R.Millimeter wave mobile communications for 5G cellular: It will work!.IEEE Access2013;1:335-49

[2] Sebastian MT,Jantunen H.Low-loss dielectric ceramic materials and their properties.Int Mater Rev2015;60:392-412

[3] Raveendran A,Raman S.Applications of microwave materials: a review.J Electron Mater2019;48:2601-34

[4] Li L,Chen XM.Where can the low dielectric constant go in dense inorganic materials?.J Materiomics2023;9:980-3

[5] Reaney IM.Microwave dielectric ceramics for resonators and filters in mobile phone networks.J Am Ceram Soc Soc2006;89:2063-72

[6] Došler U,Suvorov D.The synthesis and microwave dielectric properties of Mg₃B₂O₆ and Mg₂B₂O₅ ceramics.J Eur Ceram Soc2010;30:413-8

[7] Chang S,Tseng C.Microwave dielectric properties of ultra-low temperature fired Li₃BO₃ ceramics.J Alloy Compd2017;698:814-8

[8] Zhou D,Wang D,Reaney IM.High quality factor, ultralow sintering temperature Li₆B₄O₉ microwave dielectric ceramics with ultralow density for antenna substrates.ACS Sustain Chem Eng2018;6:11138-43

[9] Sun H,Yang H.(Ca_1-xMg_x)SiO₃: a low-permittivity microwave dielectric ceramic system.Mater Sci Eng B2007;138:46-50

[10] Bian J.Sintering behavior and dielectric properties of SiO₂-BPO₄ glass-fluxed ceramics.J Eur Ceram Soc2018;38:2747-52

[11] Kamutzki F,Barowski J,Hanaor DA.Silicate dielectric ceramics for millimetre wave applications.J Eur Ceram Soc2021;41:3879-94

[12] Hou H,Yang H.LiGaSiO₄: an ultra-low permittivity dielectric material enabling application in patch antenna.Ceram Int2024;50:7758-66

[13] Surendran K,Mohanan P.(1-x)MgAl₂O_4-xTiO₂ dielectrics for microwave and millimeter wave applications.Appl Phys A2005;81:823-6

[14] Wan Jalal WN,Zulfakar MS,Shaari S.ZnAl₂O₄-based microwave dielectric ceramics as GPS patch antenna: a review.Trans Indian Ceram Soc2013;72:215-24

[15] Xie M,Lai Y.Phase evolution and microware dielectric properties of high-entropy spinel-type (Mg_0.2Co_0.2Ni_0.2Li_0.4Zn_0.2)Al₂O₄ ceramics.J Eur Ceram Soc2024;44:284-92

[16] Bian JJ,Hong KS.Microwave dielectric properties of A₂P₂O₇ (A = Ca, Sr, Ba; Mg, Zn, Mn).Jpn J Appl Phys2004;43:3521

[17] Thomas D,Sebastian MT.Casting and characterization of LiMgPO₄ glass free LTCC tape for microwave applications.J Eur Ceram Soc2013;33:87-93

[18] Bian J,Xie Y.Structural evolution, sintering behavior and microwave dielectric properties of Al_(1-x)(S_i0.5Zn_0.5)_xPO₄ ceramics.J Eur Ceram Soc2019;39:4139-43

[19] Zhou D,Wang H,Yao X.Microwave dielectric ceramics in Li₂O-Bi₂O₃-MoO₃ system with ultra-low sintering temperatures.J Am Ceram Soc2010;93:1096-100

[20] Varghese J,Nelo M,Jantunen H.Microwave dielectric properties of low-temperature sinterable α-MoO₃.J Eur Ceram Soc2018;38:1541-7

[21] Song X,Li J.Low-fired fluoride microwave dielectric ceramics with low dielectric loss.Ceram Int2019;45:279-86

[22] Krupka J,Tobar M,Geyer RG.Complex permittivity of some ultralow loss dielectric crystals at cryogenic temperatures.Meas Sci Technol1999;10:387-92

[23] Li L,Xiao Q,Wang N.Microwave dielectric properties of fused silica prepared by different approaches.Int J Appl Ceram Technol2014;11:193-9

[24] Akkasoglu U,Arslan İ,Cicek B.Structural, thermal and dielectric properties of low-alkali borosilicate glasses for electronic applications.J Mater Sci Mater Electron2021;32:22629-36

[25] Wu S,Liu P.Effect of TiO₂ doping on the structure and microwave dielectric properties of cordierite ceramics.J Am Ceram Soc2015;98:1842-7

[26] Lou W,Song K.Low permittivity cordierite-based microwave dielectric ceramics for 5G/6G telecommunications.J Eur Ceram Soc2022;42:2820-6

[27] Penn SJ,Templeton A.Effect of porosity and grain size on the microwave dielectric properties of sintered alumina.J Am Ceram Soc1997;80:1885-8

[28] Jin F,Shen L,Chu PK.Micro-structural and dielectric properties of porous TiO₂ films synthesized on titanium alloys by micro-arc discharge oxidization.Mater Chem Phys2006;100:31-3

[29] Xia Y,Jiang D.Dielectric and mechanical properties of porous Si₃N₄ ceramics prepared via low temperature sintering.Ceram Int2009;35:1699-703

[30] Hou Z,Liu L.Effects of pore shape and porosity on the dielectric constant of porous β-SiAlON ceramics.J Eur Ceram Soc2015;35:4115-20

[31] Chen Y,Luo Y,Zhang L.Microwave and terahertz properties of porous Ba₄(Sm,Nd,Bi)_28/3Ti₁₈O₅₄ ceramics obtained by sacrificial template method.J Am Ceram Soc2021;104:5679-88

[32] Cao M,Wu SY.Dominant role of ceramic connectivity in microwave dielectric properties of porous ceramics.Acta Mater2023;258:119207

[33] Xu Y,Tu KN.Dielectric property and microstructure of a porous polymer material with ultralow dielectric constant.Appl Phys Lett1999;75:853-5

[34] Dong X,Zhao J.Method to predict effective dielectric constant of porous silica low dielectric constant materials.Phys B2010;405:3551-4

[35] Luo W,Randall C.Effect of porosity and microstructure on the microwave dielectric properties of rutile.Mater Lett2017;200:101-4

[36] Meng F,Zhang J.Study on the structure and properties of fine-grained alumina fast sintered with high heating rate.Mater Res Bull2008;43:3521-8

[37] Li L,Chen G.Status and prospect of garnet/polymer solid composite electrolytes for all-solid-state lithium batteries.J Energy Chem2020;50:154-77

[38] Levassort F,Desmare R.Effective electroelastic moduli of 3-3(0-3) piezocomposites.IEEE Trans Ultrason Ferroelectr Freq Control1999;46:1028-34

[39] Cao M,Li L,Chen XM.Obtaining greatly improved dielectric constant in BaTiO₃-epoxy composites with low ceramic volume fraction by enhancing the connectivity of ceramic phase.ACS Appl Mater Interfaces2022;14:7039-51

[40] Alford NM.Sintered alumina with low dielectric loss.J Appl Phys1996;80:5895-8

[41] Penn S,Breeze J.Layered Al₂O₃-TiO₂ composite dielectric resonators with tuneable temperature coefficient for microwave applications.IEE Proc Sci Meas Technol2000;147:269-73

[42] Kolodiazhnyi T,Spreitzer M.Development of Al₂O₃-TiO₂Al₂O₃-TiO₂ composite ceramics for high-power millimeter-wave applications.Acta Mater2009;57:3402-9

[43] Mollá J,Vila R.Effect of humidity on microwave dielectric losses of porous alumina.J Appl Phys1999;85:1727-30

[44] Kajfez D.Measurement of material properties with a tunable resonant cavity.Electron Lett1993;29:1936-7

[45] Zakri T,Vauclin M.Theoretical evidence for `Lichtenecker's mixture formulae' based on the effective medium theory.J Phys D Appl Phys1998;31:1589-94

[46] Stroud D.Generalized effective-medium approach to the conductivity of an inhomogeneous material.Phys Rev B1975;12:3368-73

[47] Heidinger R.Influence of porosity on the dielectric properties of AlN in the range of 30. 40 GHz.Powder Metall Int1988;20:30-2Available from: https://www.osti.gov/etdeweb/biblio/6526366 [Last accessed on 6 May 2025]

[48] Bosman AJ.Temperature dependence of dielectric constants of cubic ionic compounds.Phys Rev1963;129:1593-600