| Potassium-sodium niobate-based lead-free piezoelectric ceramics are one of lead-free piezoelectric ceramics that have been widely studied.However,there are still some shortcomings in KNN-based ceramics.The sintering temperature window is relatively narrow,usually only in the range of 20-30?.The alkali metal elements are easily volatilized during the sintering process.The conventional sintering methods are difficult to prepare dense ceramics with good performance.Here,K0.5Na0.5NbO3(KNN)powders of submicron/nanoscale were prepared by high-energy ball milling.Effect of different preparation parameters on particle size of the powders was studied.The K0.5Na0.5NbO3 powder with particle size about 120 nm obtained by high-energy ball milling was cold sintered.The effects of annealing temperature,deionized water content,and liquid phase type on the structure and performance of ceramics were investigated.The crystal structure,microstructure,dielectric,ferroelectric and other electrical properties of the ceramic samples were systematically studied.In addition,K0.48Na0.52NbO3-xmol%KF ceramics were prepared by a traditional sintering method,and influence of KF addition on structure and electrical properties of the ceramics was studied.The content is as follows:(1)Ko0.5Na0.5NbO3(KNN)with a simple composition was selected as the research object.In order to obtain sub-micron/nano-scale powders,high-energy ball milling was used.The raw materials were weighed and then high-energy ball milled(called as the first ball milling).The powders after the first ball milling were calcined at different temperatures.The effects of the ball milling time and calcination temperature on phase of the powders were investigated.The calcined powders were again high-energy ball milled(called as the second ball milling).The effect of second ball milling time on particle sizes of the powders was explored.It was found that as the ball milling time increased,the calcination temperature for obtaining the pure KNN phase gradually decreased.XRD results showed that all powders could not be pure phase as calcined at 550?.Except the powders milled for 1 h and 2 h,the other powder samples showed pure phase.When the temperature was 650,700,and 800?,the powders except ball milled for 1h,show pure phase.The powders milled for various ball milling time and calcined at various temperatures were chosen for the second ball milling.For the 4h-650 ? sample,the particle size of the powder was significantly reduced after the second ball milling for 1 h and 2 h.The further increase in the second ball milling could not further decrease particle sizes obviously.The average particle size of the powders second ball-milled for 16 h was about 200 nm.The same results were also observed for the powders of 2h-700? and 8h-600?.(2)K0.5Na0.5NbO3 powders were used for cold sintering.Effects of variables such as annealing temperature,deionized water content,and liquid phase type on the structure and performance of the ceramics were studied.The cold sintering process includes two steps,i.e.,the cold sintering stage and the annealing stage.The effects of the addition of deionized water content,the type of liquid phase,and the annealing temperature on the structure and electrical properties of the obtained ceramics were studied in detail.In the case of given deionized water content and cold sintering temperature,different annealing temperatures have a great impact on the electrical properties of the ceramic samples.The sample 1100-180-20(i.e.,the cold sintering temperature 180?,water content 20 wt%annealing temperature 1100?)shows dielectric loss tan?=0.03,dielectric constant ?r=345,the maximum dielectric constant 6076,the breakdown field strength is high than92.5 kV/cm.Under 92.5 kV/cm,the 1100-180-20 sample shows Pr,Pm,and Ec of 26.4 ?C/cm2,32.5 ?C/cm2,and 12.3 kV/cm,respectively.The KNN ceramics with different contents of deionized water were successfully prepared via cold sintering.The relative densities of all samples are above 94%.The grains of all samples were between 3 and4 ?m.The 1100-180-0 sample has a large coercive field Ec=17.1 kV/cm,a small residual polarization intensity Pr=20.8?C/cm2,and a maximum polarization intensity Pm 26.4 ?C/cm2.With the addition of different deionized water content,the coercive field of KNN decreases,and the residual polarization intensity and maximum polarization intensity increase.The ferroelectric performance of the samples added with the deionized water during cold sintering process was better than that of the samples without deionized water(1100-180-0).The results show d33*=140 pm/V for the 1100-10-180 sample,d33*=149 pm/V for the 1100-15-180 sample,d33*=146 pm/V for the 1100-20-180 sample.At E=50 kV/cm,the d33*of the samples with the various deionized water content during cold sintering were between 150 and 180 pm/V,which are higher than the value of 1100-180-0 sample(d33*=138 pm/V).The KNN ceramics with different suspensions were successfully prepared via cold sintering.The samples have dense microstructures.At room temperature,the values of dielectric loss tan? of the samples 1100-180-10,1100-180-10-K and 1100-180-10-Na are 0.09,0.05,and 0.08,respectively,and the room temperature dielectric constants ?r are 326,391,and 422,respectively.The electrical performances are better than those of the 1100-1 80-0 sample.The 1100-1 80-10 and 1100-180-10-Na samples show the coercive field Ec 10.8 kV/cm and 10.7 kV/cm,respectively;the residual polarizations Pr 23.8?C/cm2 and 27.5?C/cm2,respectively;the maximum polarization intensities Pm 29.0 ?C/cm2 and 32.1?C/cm2,respectively.(3)The K0.48Na0.52NbO3-xmol%KF ceramics were prepared by means of a conventional solid-state reaction method.The ceramics were sintered at 965?1065? for 2 hours.The ceramic KNN-KF exhibits a denser microstructure than that of pure KNN.Accordingly,KNN-KF shows more excellent electrical performance,which has d33=105 pC/N,?r=496,tan?=0.03,Pm=27.3 ?C/cm2,and Pr=21.6?C/cm2.The PFM results show that the electrical domain in the KNN-KF ceramic can be easily switched as the electric fields change,corresponding to its excellent ferroelectric properties. |
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