Study On Low-temperature Sintering Mechanism Of Ti-based Dielectric Ceramics With Medium-k And High-k

The low temperature co-fired ceramics?LTCC?technology is a vital method promoting the development of components towards miniaturization,integration,high reliability and low cost.It has become one of the core technologies of the emerging manufacturing in electronic information industry.To meet the demand of the booming evolution of communication technology,investigations on serialized dielectric constant LTCC ceramic materials with middle/high dielectric constant are urgent.Although numerous LTCC materials have been explored over the decades,the mechanism of low-temperature sintering of those ceramics is still ambiguous,which only explained by simple analysis in the available reports.There is no validly theoretical guidance about solubility of ceramics in the liquid phase at high temperatures which is reckon as crucial factor influencing the low-temperature sintering due to the lack of appropriate characterization methods for a long time.The designed dissolution experiment in this paper characterizes the solubility of different ceramics in the dopant liquid phase at high temperature.The effects of the solubility and dissolution rate in liquid phase sintering on the low temperature sintering of ceramics are systematically investigated.For enhancing the universality of the experiment and popularizing the self-designed experiment method,the middle/high dielectric Ti-based ceramic systems with different structures are specifically selected in this paper.After adding low melting point dopants in ceramics,the sintering temperature of those ceramics was reduced to less than 950°C.The effects of dopant content and sintering temperature on the microstructure,densification,phase composition and dielectric properties of Ti-based ceramics are analyzed in depth.Furthermore,the mechanism of low-temperature sintering is deeply studied based on wettability,activation energy and self-designed solubility experiments.The composite materials with different dielectric constant??_r?,high quality factor?Q×f?,and adjustable near zero temperature coefficient of frequency??_f?have been obtained,which further enriched the systems of LTCC ceramic materials with middle/high dielectric constant.There are the dominated achievements.?1?The sintering temperature of Ca_0.61La_0.26Ti O₃?CLT?ceramic is reduced to950°C with 3 wt.%La₂O₃-B₂O₃-Zn O?LBZ?glass.V₂O₅-Cu O?VC?glass can promote the melting of LBZ glass and inhibit the formation of the second phase.The glass of3LBZ+1VC?3L+1V?can lower the sintering temperature of CLT ceramic to 900°C by liquid phase sintering.And CLT+3L+1V ceramic shows good microwave dielectric properties:?_r?98.9,Q×f?4900 GHz,?_f?300.2 ppm/^oC.Ca_0.6Nd_0.267Ti O₃-Li_0.5Nd_0.5Ti O₃?CNT-LNT?and CLT ceramics have similar perovskite structure.So the sintering temperature of CNT-LNT ceramic is lower to 1000°C after adding 5 wt.%LBZ,while the glass of 3L+1V can further reduce the sintering temperature to 950°C.CNT-LNT+3L+1V ceramic exhibits the properties of?_r?97.4,Q×f?2300 GHz,?_f?-2.5 ppm/^oC.Especially,a near-zero frequency temperature coefficient is obtained.The solubility and dissolution rate of CLT and CNT-LNT ceramics in LBZ+VC glass are better than those of in single glass.In LBZ+VC glass,the dissolution rate of CLT ceramic is higher than CNT-LNT,contributing to a lower sintering temperature.?2?To improve the microwave dielectric properties,Ti⁴⁺was substituted for Sn⁴⁺and(Nd_1/2Nb_1/2)⁴⁺in Zn_0.15Nb_0.3Ti_0.55O₂ system,respectively.After substitution,Sn⁴⁺improves the Q×f value of Zn_0.15Nb_0.3Ti_0.55-xSn_xO₂?ZNTSx?ceramics to 15456 GHz,but the?_f value is still too high.The second phases appear,with increasing content of doping(Nd_1/2Nb_1/2)⁴⁺into Zn_0.15Nb_0.3Ti_0.55O₂.Because of the existence of the second phases,ZNTNNx ceramics show the near-zero?_f value,while the?_rvalue is badly deteriorated.Zn_0.15Nb_0.3-xTa_xTi_0.55O₂?ZNTTx?solid solution is prepared by the substitution of Ta⁵⁺for Nb⁵⁺ions.According to the P-V-L theory,?_r gradually decreases with the ionicity of the Nb/Ta–O bond,and the Q×f value is doubled due to the larger lattice energy.The reduction of oxygen octahedron distortion improves the structure stability of ZNTT ceramic and reduces the?_f value.The optimal dielectric properties of ZNTTx is?_r?75.9,Q×f?22000 GHz,?_f?234.6 ppm/^oC.Besides,Li₂O-B₂O₃-Si O₂?LBS?glass shows excellent wettability to ZNTT ceramics.On the other hand,the large dissolution and fast dissolution rate of ZNTT in LBS glass accelerate the mass transfer and reduce the activation energy of ZNTT ceramics.So ZNT+1L ceramic can be sintered at 900°C,with a dielectric properties:?_r?72.1,?_f?188.3 ppm/^oC,Q×f?14800GHz.It is interesting that Ba O-Zn O-B₂O₃?BZB?glass can also wet ZNTT,but the ceramic cannot be dissolved in BZB.This phenomenon proves that wettability is not the only condition for liquid phase sintering.?3?The sintering temperature of Ba_3.75Nd_9.5Ti₁₈O₅₄?BNT?ceramic can be lowered to 900°C by adding 3 wt.%LBS+3 wt.%BZB?3L+3B?glass.The synergistic effect of LBS,BZB and Ba₂Li B₅O₁₀ contributed to a low melting temperature and good wettability of LBS+BZB glass.Meanwhile,the dissolution rates of BNT ceramics in LBS+BZB glass are greater than in LBS or BZB glass.Thus,the activation energy of BNT+3L+3B ceramics decrease and the densify of BNT ceramics are promoted.But excessive glasses separate out from grain boundaries,leading to the degradation of dielectric performance.Microwave dielectric properties of BNT+3L+3B ceramic are:?_r?69.8,Q×f?4400 GHz,?_f?46.1 ppm/^oC.?4?With the purpose of adjusting?_f in Ba₃Nb₄Ti₄O₂₁ ceramic,Ti⁴⁺is replaced by(Cu_1/3Nb_2/3)⁴⁺.This method will not change the phase composition of Ba₃Nb₄Ti_4-x(Cu_1/3Nb_2/3)_xO₂₁?BNTCNx?ceramics,but it will expand the cell volume.The?_f values of BNTCNx ceramics varies from+90 ppm/?to-22 ppm/?with the increase of unit cell volume.The decrease of?_rcan be explained by the unit cell volume and polarizability.Due to the low melting point of Cu O,the sintering temperature of BNTCNx ceramic is lowered to 1050?and the grain size is also reduced.BNTCNx ceramics obtain near-zero?_f value when x=2,with the properties of?_r?56.3,Q×f?5300GHz,?_f?1.0 ppm/?.It has been demonstrated that BZB glass has a good wettability to BNTCN ceramics.Moreover,BNTCN ceramics can be dissolved in BZB glass.The fast dissolution rate contributes to the mass transfer rate of BNTCN ceramics and reduces the activation energy.BNTCN+1B ceramic sintered at 925?displays a good temperature stability of?_r?55.6,Q×f?5700 GHz,?_f?3.0 ppm/?.Similarly,excessive BZB results in abnormal grain growth and degrades properties.?5?4 wt.%BZB glass can reduce the sintering temperature of Ba Ti₅O₁₁?BT?ceramics to 900°C.EDS,Mapping and XRD analysis prove that BT ceramic can be wetted well by BZB glass and any chemical reaction is not exited.Ba,Ti and O elements can be transferred quickly in the liquid phase,corresponding to the large solubility and dissolution rate and lower activation energy.The?_r and?_f values are reduced due to the low?_r and negative?_f value of BZB glass.The Q×f values of BT ceramics are improved attributing to the increase of grain size.At 900°C,BT+4B ceramic shows properties of?_r?35.4,Q×f?28000 GHz,?_f?36.8 ppm/^oC.The investigation about solubility of BZB glasses with different formulations prove again that wettability is not the only condition for liquid phase sintering.?6?According to the solubility experiments of the mentioned Ti-based ceramics,the sintering mechanism of microwave ceramics with dopants at low temperature is further clarified in this paper.Both a lower melting temperature and excellent wettability between glass and ceramics are necessary for choosing the appropriate additives.Especially,there is the phenomenon that the additives with satisfied wettability may not reduce the sintering temperature of the ceramics.The ceramics should have a large solubility in the liquid phase.In addition,the large solubility and dissolution rate can reduce the sintering activation energy of the ceramics system and contribute to the lower temperature,without the reaction.