O 3-based Dielectric Ceramics for Silver Co-sintering Applications

The Ca(Zn1/3Nb2/3)O3 (CZN) complex perovskite oxide has been studied for its attractive dielectric properties (εr=34, Qxf=15 890GHz, τf=-48 ppm.°C) for applications such as multilayer ceramics capacitors or hyperfrequency resonators. Nevertheless, high temperatures (>1250°C) are required to obtain well dense CZN ceramic, prohibiting any silver co-sintering (Tf (Ag) = 961°C). For that reason, the sintering temperature lowering of CZN by glass phase’s additions has been investigated. This material is finally sinterable at low temperature with combined glass phase –lithium salt additions, and exhibits, at 1MHz, very low dielectric losses, a relatively high dielectric constant with a good stability versus temperature. The 2%weight of ZnO-SiO2-B2O3 glass phase and 1%wt of LiF added CZN sample sintered at 920°C exhibits a relative density higher than 95% and attractive dielectric properties: a dielectric constant εr of 22, low dielectrics losses (tan (δ)< 10), a temperature coefficient of the permittivity τε<100 ppm.°C, and an insulating resistivity higher than 10Ω.cm. Its interesting properties and its co-sinterability with silver electrodes make this ceramic suitable for L.T.C.C applications.


Introduction
The low temperature co-fired ceramic (L.T.C.C) multilayer devices, made of alternating dielectric and internal metallic electrode layers, are intensively studied by the ceramics community [1,2].In these devices, the silver is commonly used as inner electrodes because of its high stability at high temperature (around 900°C) in air.Moreover, silver constitutes very good electrodes owing to both its high conductivity and relatively low cost.The drawback is of course its low melting point, i.e. 961°C, which imposes to develop high performance ceramics sinterable at low temperature, around 900°C.The Ca(Zn 1/3 Nb 2/3 )O 3 (CZN) complex perovskite, exhibits interesting microwave dielectric properties: ε r =34, Qxf=15 890GHz, τ f =-48 ppm.°C -1 [3], fitting to the requirements of LTCC and silver based multilayer capacitors manufacturing.However, the CZN compound requires a sintering temperature higher than 1350°C to achieve a satisfying density.For that reason, the lowering of the sintering temperature of this ceramic has been investigated.Glass additions are known for their effect on the sinterability of several dielectric system, e.g.(Zr,Sn)TiO 4 [4], BaTi 4 O 9 [5], (Ca,Mg)TiO 3 [6], MgO-SiO 2 -TiO 2 [7].Moreover, combined additions such as CuO-B 2 O 3 [8], CuO-Bi 2 O 3 [9], CuO-V 2 O 5 [10], ZnO-B 2 O 3 -Li 2 O-CuO [11] and B 2 O 3 -LiF [12,13] can be used as sintering aids, due to the presence of a liquid phase during stage.
In this work, different kinds of sintering aids (glass phases, combined sintering aids additions) have been investigated in order to decrease the CZN sintering temperature, in order to enable the CZN-Ag co-sintering.The effects of these additions on sintering temperature, microstructure, composition, and dielectric properties have been hence investigated.

Experimental procedure
The Ca(Zn 1/3 Nb 2/3 )O 3 compound was prepared by solid state reaction using reagent grades powders of CaCO 3 , ZnO and Nb 2 O 5 (purity >99%).The precursors were appropriately weighted according to the Ca(Zn 1/3 Nb 2/3 )O 3 stoechiometry.The mixing was performed in an ammoniac solution at pH=11 using zircon balls in a Teflon jar for 2 hours.These conditions were found to be optimal to obtain very well-dispersed slurry [14].The slurry was subsequently dried under infra-red lamps and the powder was manually reground and heat treated at 1000°C and 1150°C for 2 hours in air.The powder was finally reground using the same process as the previous one, for 1 hour.
These glasses were grinded in a planetary grinder for 45 minutes to obtain a fine powder.Ceramic powders and glass powders were mixed in a planetary grinder for 45 minutes in absolute ethanol.To manufacture pellets, an organic binder (Polyvinyl alcohol at 5 volume %) was manually added to the powder and disks (8 or 6 mm in diameter, 2 mm thick) were shaped by uni-axial pressing with a load of about 2100 kg.The green samples were finally sintered in air in a tubular furnace for two hours at a dwell temperature determined by TMA (Thermo-Mechanical analysis, Setaram TMA 92), with heating and cooling rates of 150°C/h.The density of the sintered samples was characterised using a He pycnometer (Accupyc 1330).The dielectric properties were determined using a RLC bridge (PM6306) versus temperature (from -60°C to 160°C).The crystallised phase composition has been identified by X-ray diffraction (XRD) technique using the Cu X-ray radiation (Philips X' Pert) and the microstructures were observed using a Scanning Electron Microscop (SEM Philips XL'30).

Results and discussion
Fig. 1 shows XRD pattern of the powders obtained after the calcination step at 1000, 1150 and 1200ºC.Heat treated at 1000ºC, the powder is a mixture of Ca(Zn 1/3 Nb 2/3 )O 3 and secondary phases.With increasing the calcination temperature, the secondary phases disappear gradually, and the single phase Ca(Zn 1/3 Nb 2/3 )O 3 is obtained after calcination at 1150°C for 3 hours.Fig. 2 depicts the thermomechanical analyses performed on the pure CZN compound and on the glass added samples.It is shown that the glass phases additions are really efficient as sintering agents, since the sintering temperatures of different compositions decrease systematically from 1270ºC (pure CZN sintering temperature) to 1050°C.The most interesting result is obtained for ZSB-LiF combined addition for which, the densification starts at 650°C and ends around 920°C.Taking into account the thermomechanical analyses for the dwell temperature, pellets of each composition have been sintered in air for 2hours.Thanks to this heat treatment, the disks are well dense, the densities reaching at least 94% of the theoretical one.
XRD analyses have been performed on crushed sintered samples and are shown Fig. 3.The sintered samples are exclusively composed by the CZN perovskite phase without any crystallised secondary phase.
Fig. 4(a) exhibits SEM micrograph of the CZN-ZSB-LiF ceramic sintered at 920°C.The liquid phase sintering effect can clearly be observed in the grain morphology.The ceramic reveals a relatively dense microstructure with a heterogeneous grain size distribution, exhibiting both very large and very small grain sizes.This phenomenon is due to the accelerated diffusion mechanism, due to the liquid phase sintering.Dielectric constant versus temperature of the sintered samples is given Fig. 5.The relative permittivities of CZN and CZN-ZSB-LiF are respectively 25 and 22 at room temperature, at 1MHz.In both cases, these permittivities have a linear dependence with temperature.The permittivity temperature coefficient depending on the samples compositions.The pure CZN compound exhibits a negative temperature coefficient τ ε of -70ppm/°C, whereas the CZN-ZSB-LiF temperature coefficient is 98ppm/°C (Table I).In both cases, the dielectric losses are lower than 3.10 -2 at 1MHz at the studied temperature (Fig. 6) and the insulating resistivities are higher than 10 13 Ω.cm.A prototype of silver / CZN-ZSB-1LiF based capacitor has been obtained, with a sintering temperature of 920°C for a dwell time of 2 hours.The cross section of this sample has been observed by SEM (Fig. 4(b)).No silver diffusion into the ceramic has occurred, showing a satisfying compatibility between the silver electrode and the ceramic.This component exhibits very attractive dielectric properties, i.e. a room temperature relative permittivity around 23, a permittivity temperature coefficient τ ε < 100ppm/°C and dielectric losses lower than 10 -3 , at 1MHz.These properties make this formulation suitable to fabricate silver based multilayer ceramic capacitors.

Conclusion
In this work, the lowering of the sintering temperature has been investigated by glass phases and lithium salt additions, in order to co-sinter CZN samples with silver electrodes.The sintered samples show the interesting dielectrics properties, and do not exhibit crystallized secondary phases.The best results are obtained with the 2w t % ZnO-SiO 2 -B 2 O 3 + 1wt%LiF addition.The sintered ceramic reaches density higher than 94 % of the theoretical one and exhibit interesting dielectric properties: a relative permittivity of 22, a temperature coefficient τ ε < 100ppm/°C, dielectric losses lower than 10 -3 , and an insulating resistivity higher than 10 13 Ω.cm.These good dielectric properties combined with the low sintering temperature make CZN based ceramic suitable for silver based multilayer structure for L.T.C.C. applications.