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Sintering and piezoelectric properties of K0.5Na0.5NbO3 glass microspheres
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Sintering and piezoelectric properties of K0.5 Na0.5 NbO3 glass microspheres Firas J. Hmood a,c,∗ , Jens Guenster a,b , Juergen G. Heinrich a a b c
Clausthal University of Technology, Department for Engineering Ceramics, Germany Federal Institute for Materials Research and Testing (BAM), Germany University of Babylon, Department for Construction Materials and Ceramics Engineering, Iraq
a r t i c l e
i n f o
Article history: Received 2 May 2015 Received in revised form 29 July 2015 Accepted 30 July 2015 Available online xxx Keywords: Transparent piezoceramics Glass microspheres Viscous flow sintering K0.5 Na0.5 NbO3
a b s t r a c t Laser-fused K0.5 Na0.5 NbO3 (KNN) powder of 75% transparent fraction has been sintered by pressureless sintering, spark plasma sintering (SPS), and hot isostatic pressing (HIP). The laser-fused KNN has a fictive temperature of 503 ◦ C and an onset crystallization temperature of around 529 ◦ C. The results have shown that sintering of the laser-fused KNN powder utilizing the viscous flow of the transparent microspheres (amorphous content)–at the kinetic window (26 ◦ C)–is possible. The highest yield relative density is around 83% at a sintering temperature of 525 ◦ C and at a sintering pressure of 280 MPa. Limited density has been reached because of formation of crystalline surface layers around the amorphous areas. The samples hipped at 525 ◦ C have low piezoelectric coefficient d33 of 5 pC/N because of the residual porosity that led to early dielectric breakdown during the polarization. The sintering behavior, the resulting microstructure as well as the measured properties will be discussed. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction This paper deals with the sintering behavior of the laser-fused K0.5 Na0.5 NbO3 (KNN) [1] utilizing the viscous flow of the amorphous content at the glass transition range. The viscous flow represents the kinetic path to sinter glass particles. During the viscous flow, the neck formation and the shrinkage are taking place concurrently. The shrinkage will take place at the end of the sintering process in sintering of crystalline particles by the diffusion process at relatively high sintering temperatures [2]. The big issue associated with this kind of sintering is concurrent sintering-crystallization of the glass particles especially at the outer surfaces that prohibits the full densification at low sintering temperatures. However, this route seems to be useful by decreasing the time of glass-ceramics preparation utilizing the surface crystallization of the glass particle [3]. The surface finishing of the glass particles plays an important role in the viscous sintering. The experimental work of Zanotto et al. [4] indicated that the fired surfaces show little tendency to surface nucleation. It also revealed that an existing of few crystalline grains glued to the glass surfaces could accelerate the surface nucleation. Normally, sintering of glass particles is performed at tempera-
∗ Corresponding author at. Postal address: P.O. Box 213, Hilla—Babylon, Iraq. E-mail address: fi[email protected] (F.J. Hmood).
tures between the glass transition temperature and that of the crystallization start. The difference between both temperatures is called kinetic window. In most silicate glass ceramic systems the kinetic window is wide enough to achieve full densification with well controlling of crystallization. While systems that are free from glass-forming oxides have small kinetic windows that increase the probability of crystallization [5]. Therefore, consolidation of glass particles from the later systems will be a competition between the sintering and the crystallization. If crystallization wins, microstructures with various densities will be obtained [6]. In this study, more than one sintering method was involved in order to prohibit or delay the surface nucleation during sintering. Keeping the sintering temperature at the glass transition range as well as maximizing of the sintering pressure are helpful conditions to maintain the sintering process until reaching a full densification. Theoretically, spark plasma sintering (SPS) seems to be a suitable method to consolidate glass microbeads. By applying this technology, high heating rates combined with short sintering time can be realized. Ramond et al. [7] succeeded by sintering soda-lime glass (SLG) microspheres without any crystallization at 522 ◦ C using SPS in 10 min with a heating rate of 100 K/min and at sintering pressure of 34 MPa. However, the high temperature concentration at the neck regions between the particles as well as the temperature gradient through the glass particles can cause other reasons for hindering sintering by the viscous flow [8].
http://dx.doi.org/10.1016/j.jeurceramsoc.2015.07.035 0955-2219/© 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: F.J. Hmood, et al., Sintering and piezoelectric properties of K0.5 Na0.5 NbO3 glass microspheres, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.07.035
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K0.5 Na0.5 NbO3 (KNN) glass microspheres were prepared by laser fusing where a 5 kW—CO2 laser beam was used. The transparent fraction of the laser-fused KNN granules has been calculated to be 68.5% and the mean particle size (d50 ) was 62.5 m. According to the Differential Scanning Colorimeter (DSC), the glass transition temperature (Tf ) has been determined to be 503 ◦ C and the onset crystallization temperature (Tx ) was 529 ◦ C with a kinetic window of 26 ◦ C [1]. A lot of efforts were devoted to separate the transparent laser-fused granules from the opaque ones depending on their density. However, it was very difficult because they have approximately the same density. Even by hand, it was difficult because they are just some microns in size. Therefore, the laser-fused KNN as a mixture of transparent and opaque microspheres was sintered in order to show whether or not such technical route can lead to transparent ceramics with piezoelectric properties. 2. Experimental 2.1. Transparent fraction Preparation and characterization of the transparent microspheres can be found in details in reference [1]. As it has been cleared in reference [1], the highest transparent fraction reached was around 68.5% and it is raised to 75% during working on this part of the study. Rising of the transparent microspheres is important for the sintering and for the transparency as well. A lot of experiments were conducted to separate the transparent microbeads a part from the other components according to the density or even by hand. However, no one was successful. Therefore, using an extra ultrasonic cleaning combined with moderate vacuum in distilled water was an alternative method. The water was used because it is cheap as a cleaning media. The increase of the transparent fraction can be attributed to mechanically destroying the sintered particles/porous granules to smaller particles. After that, they were separated easily according to their density. However, this is not the end and working on this part of the study is continuous. 2.2. Samples preparation Because of the high flowability of as-received KNN microspheres, compacts from the as-received microspheres were impossible. With respect to the Pressureless sintering and the Hot Isostatic Pressing (HIP), the laser-fused KNN granules were ball milled to d50 of 2 m. The milled microspheres were first mixed with an organic binder, pressed in an axial press and finally cold isostatically post densified at a pressure of 200 MPa. The prepared samples were heated up to 300 ◦ C for 10 h in order to remove the organics from the pressed samples. At this stage, the maximum temperature was kept at 300 ◦ C to ensure that no crystallization would occur. For the SPS, there was no need for milling because the microspheres can be supplied to the die without preforming or pre-pressing. 2.3. Sintering methods
100 K/min; starting powder: as-received (d50 : 62 m) laser-fused KNN and milled (d50 : 2 m) laser-fused KNN. • Hot Isostatic Pressing (HIP): sintering temperatures: 525 ◦ C, 1000 ◦ C; sintering pressure: 280 MPa; heating rate: 10 K/min; starting powder; milled laser-fused KNN (d50 : 2 m). 2.4. Equipment • Dilatometer (Baehr 802L thermo-analysis—Germany) was used to investigate the sintering behavior of the laser-fused KNN microspheres. The heating rate in this experiment was 10 K/min. • Spark plasma sintering (SPS) machine (HHPD25, FCT Systeme GmbH, Germany) was also involved to assemble the laser-fused KNN microspheres into tablets with diameter of 20 mm. The die was lined up with graphite foils in order to prevent the reaction of the powder with the die. • Hot isostatic pressing (HIP) machine (EPSI, Belgium) with a maximum sintering pressure of 280 MPa were conducted. In the HIP experiments, the samples were encapsulated in stainless steel capsules (Type: 1.4301). Nd-YAG laser was used to weld the capsules in order not to develop a lot of heat during the encapsulation that can lead to start the crystallization before the sintering process. • The density of the sintered samples was calculated as mass to volume ratio. Unless the samples hipped at 1000 ◦ C, their density was measured by Archimedes principle because they got irregular shape after hipping. • Microstructures of the sintered samples were examined by a field emission scanning electron microscopy (FESEM) (Helios NanoLab 600, FEI, Netherlands) and by a transmission electron microscopy (TEM) (JEM 2100 von JEOL, USA) with operating voltage of 200 kV. The specimens of TEM were prepared by using focused ion beam, which is a compatible tool with the FESEM. The thickness of the specimens was around 100 nm. • Phase composition was investigated by using X-ray diffraction (XRD, Diffractometer D5000 Kristalloflex, Siemens, Germany). Cu-K␣ radiation with wavelength of 0.154 nm was used and a scanning angle of 0.02◦ . • Measuring of the piezoelectric properties was just done on the hipped samples that were ground to 0.4 mm in thickness. The samples were contacted with silver paste which was fired at 700 ◦ C. The polarization was done in silicon oil at room temperature at direct current. An electrical field as high as 2.5 kV/mm was applied for the samples sintered at 1000 ◦ C and 0.4 kV/mm was applied for the samples sintered at 525 ◦ C. The period of polarization was around 5 min for all samples. The poled samples were aged for 24 hour after the polarization process. The planar coupling factor p was determined with an impedance analyzer (4294A Agilent—Malaysia). The piezoelectric coefficient d33 was measured directly with a d33 device (Channel Technologies Group, USA). While the dielectric constant was calculated mathematically according to the following equation at 1 kHz: 33/ о = (t × c × 144)/(da2 - di2 )Where t: thickness of the ceramic sample (mm), c: capacity of the ceramic sample (pF), da: outer diameter (mm), di: inner diameter (mm).
Different sintering methods were employed to get as high as relative density at the glass transition range of the transparent KNN which is around 503–529 ◦ C [1]. The sintering methods are:
3. Results and discussion
• Pressureless sintering: sintering temperatures: 475 ◦ C, 500 ◦ C, 525 ◦ C, 1150 ◦ C; sintering pressure: 0.1 MPa; heating rate: 10 K/min; starting powder: milled laser-fused KNN (d50 : 2 m). • Spark Plasma Sintering (SPS): sintering temperatures: 525 ◦ C, 700 ◦ C, 1000 ◦ C; sintering pressure: 50 MPa; heating rate:
Sintering experiments were done with different conditions in order to get on one hand a high relative density and on the other hand a high residual glass content after sintering. The small kinetic window of the transparent laser-fused KNN microspheres (26 ◦ C) has limited the viscous flow during sintering. In the follow-
3.1. Sintering methods
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Fig. 1. TEM images of transparent laser-fused KNN microspheres.
Fig. 2. Linear shrinkage of the laser-fused KNN. Fig. 3. Linear shrinkage of the laser-fused KNN within the kinetic window.
ing paragraphs, the transparent microspheres will be called glass microspheres because our evidence yet indicate that this melt is a glass where the TEM images (see Fig. 1) reveal an amorphous structure. However, this work is not to verify whether the transparent beads are real glass or metastable quenched liquid. More investigations on this issue are required. The question shall be discussed, is whether it is possible that necks can be formed between the particles during sintering of the laser-fused KNN at the kinetic window. The answer may be driven from Fig. 1. It shows two TEM images (surface and center) of transparent KNN microspheres. On the left hand image, the surface does not exhibit any crystallites within the amorphous structure. On the right hand image, the structure is similar. This implies that forming of necks during the viscous flow of the laser-fused KNN granules at the kinetic window is possible. Oelgardt et al. [9] found an existence of a crystalline surface layers around the laser-fused Al2 O3 -Y2 O5 -ZrO2 (AYZ) microspheres and as a result neck forming was impossible during sintering, because the crystalline structure has higher viscosity than the amorphous structure for the same material at the same sintering temperature. Hereafter, the sintering results of the laser-fused KNN according to the sintering methods will be discussed. • Pressureless sintering Fig. 2 exhibits the dilatometry curves of the starting KNN powder (curve 1) and that of the laser-fused KNN (curve 2). The starting KNN powder curve exhibits normal behavior of a crystalline material which can be sintered at around 1050 ◦ C where the sintering
temperature of the KNN powder is between 950 ◦ C and 1050 ◦ C depending on the kind of sintering [10,11]. The onset of sintering is at around 1050 ◦ C and proceeds up to 1150 ◦ C with a total linear shrinkage of 10%. With respect to curve 2, it exhibits two steps of shrinkages. First at the glass transition range of the laser-fused KNN, which starts at around 503 ◦ C with linear shrinkage of around 0.7–1%. The viscous flow of the glass particles resulted in the first densification (first shrinkage). The second shrinkage takes place at around 1000 ◦ C and takes the same path of curve 1. In order to maintain the viscous flow, samples from laser-fused KNN microspheres were sintered at 475 ◦ C, 500 ◦ C, and 525 ◦ C for a dwell time of one hour. The heating rate was kept constant at 10 K/min. Fig. 3 shows that the resulting curves exhibit limited linear shrinkage with a maximum value of 1.2% for the samples sintered at 500 ◦ C. This result indicates that the pressureless sintering cannot maintain the viscous flow for a long time. Therefore, modifying the sintering conditions was important to reach higher densification. • Spark plasma sintering (SPS) SPS was involved in this study to combine higher heating rate with higher sintering pressure in a short time. Thanks to the SPS equipment, sintering curves can result concurrently with the sintering method. Fig. 4 shows sintering curves of three experiments as a relationship between the punch displacement through the graphite die and the sintering temperatures. The experiments can be achieved with any particle size; either as-received microspheres (d50 : 62 m) or as milled microspheres (d50 : 2 m) because the
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ing temperature of densification to around 400 ◦ C with the milled microspheres. Despite of this result, the densification is still poor. In fact, declination of the microspheres leads to faster crystallization because the jagged particles offer more surface nucleation sites in comparison to the smooth surfaces of the glass microspheres and this hinders more densification [4]. A heating rate of 100 K/min at SPS is not promoting densification of the laser-fused KNN. Boccaccini et al. [12] indicated that increasing the heating rate during glass particle sintering promotes densification. One or two of the following reasons might explain this difference: • Boccaccini et al. [12] used barium magnesium aluminosilicate glass powder which has a good ability to form a glass, while the system used here has a 25% crystalline fraction (crystalline beads) which could hardly be decreased to almost zero during the preparation stage. • Crystallization especially at the surface of the glass particles (see Fig. 7) is a hindering factor for further sintering.
Fig. 4. Sintering behavior of the laser-fused KNN by SPS machine.
powder can be poured inside the die without any pre-pressing. The general sintering behavior of the laser-fused material can be seen in Fig. 4. It shows an increase in the densification with increasing the sintering temperature until reaching the saturation density, at which there is no more increment in the density. Applying of the sintering pressure can lower the sintering temperature and can also promote the viscous flow between the glass microspheres. The onset of sintering of the as-received KNN microspheres is close to 500 ◦ C while sintering of milled microspheres exhibits earlier densification close to 400 ◦ C. The figure reveals also that the densification continues inside the kinetic window for a while and then stops. Combination of the results of the sintering behavior for both methods (Figs. 2–4) indicates that sintering of the laser-fused KNN within the kinetic window is possible. It is clear that at low sintering temperatures, viscous flow is retarded. Moreover, 50 MPa sintering pressure during SPS has no visible influence on the overall densification. However, this sintering pressure lowered the start-
• Hot Isostatic Pressing (HIP) In this method, the sintering pressure was increased to 280 MPa using the HIP parallel with using milled laser-fused microbeads. The heating rate was 10 K/min. The sintering temperatures were 525 ◦ C and 1000 ◦ C. Fig. 5 reveals the relative density of all the sintering methods employed in this study. It summarizes the sintering conditions for each sintering method. It reveals how the densification of the transparent laser-fused KNN at the kinetic window has been improved. The maximum relative density reached is 83% using the HIP while it fluctuates between 55% and 65% using pressureless sintering and between 55% and 75% using the SPS. Other experiments that were done using the SPS at 670 ◦ C exhibit densities between 75% and 85%. The same figure shows that combination of the SPS for 30 min with hipping for an hour leads to samples with densities around 98.3%. The samples hipped at 1000 ◦ C at sintering pressure of 280 MPa show a relative density of 98.5% which is the highest density for the selected sintering conditions. The sintering conditions are not completely unified, because each sintering method is different from one another. The goal behind the sintering process has been to
Fig. 5. Relative density using different sintering methods.
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Fig. 6. FESEM images of fracture surface of hipped laser-fused KNN (HIP, at 525 ◦ C and at 280 MPa).
Fig. 7. FESEM image of spsed laser-fused microspheres (SPS, at 525 ◦ C and at 50 MPa).
get on one hand high relative density and on the other hand high amorphous contain after sintering.
Fig. 8. TEM image of a spsed sample at a neck area (SPS, at 525 ◦ C, at 50 MPa).
when such crystalline areas grow and meet together, viscous flow is restrained as well [13]. 3.3. Phase composition
3.2. Microstructure Fig. 6 shows FESEM micrographs of the samples hipped at 525 ◦ C for an hour at sintering pressure of 280 MPa. The left image is a fracture surface of a hipped sample. It reveals that there are radial lines (Wallner-lines) that exhibit some similarity with a glass fracture surface. It does not indicate that the hipped sample is a glass but it can indicate that this sample has an amorphous content after the HIP. A higher magnification (right hand image) of this fracture surface shows well deformed and well connected grains with some regions of melted glass. Fig. 5 reveals that the relative density of this microstructure is 83%. Indeed, this is an encouraging result to improve this technical route in the near future. Fig. 7 exhibits a micrograph of a SPS sample sintered at 525 ◦ C for 5 min at a sintering pressure of 50 MPa. At this relative low sintering temperature and this short sintering time, crystalline surface layers have been developed between the glass—glass microspheres. As a result, the progress of viscous flow was stopped and the samples remained porous. This figure emphasizes that the crystallization starts at the particle surface and then heads towards the center of the glass microspheres. Another TEM image of a sample from milled microspheres sintered by SPS (see Fig. 8) exhibits new lattice structures that formed during sintering inside a neck area. This means
In this section the phase composition of the SPS samples will be discussed. In the previous publication [1], it has been cleared that the phase of the starting KNN powder is pure orthorhombic perovskite phase with a chemical formula of K0.5 Na0.5 NbO3 . After sintering and due to the high diffusivity of sodium ions Na+1 , the main phase that firstly formed is tetragonal with a chemical formula of Na0.9 K0.1 NbO3 , as it is shown in Fig. 9. Besides this compound, other phases appeared which belong to K4 Nb8 O21 and K8 Nb6 O17 as well as an unidentified phase. It is worth mentioning that there are differences between the secondary phases that are formed during the laser fusing process [1], during heat treatment of the transparent laser-fused microspheres [1] and during sintering. The main chemical formula for most of the second phases is Kx Nby Oz which is potassium niobate. However, after each treatment, it appears with different stoichiometric ratio. In fact, there is no clear reason for this variation but it is possible that this compound is very sensitive to slight changes in its composition, e.g. as a result of the loss of alkalis, within each heating step [14,15]. It was very difficult to separate transparent from opaque microspheres after sintering. Therefore, the content of the amorphous phase after each sintering step was calculated utilizing the area under the biggest amorphous peaks (humps) in the X-ray diffrac-
Please cite this article in press as: F.J. Hmood, et al., Sintering and piezoelectric properties of K0.5 Na0.5 NbO3 glass microspheres, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.07.035
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Fig. 9. X-ray diffraction patterns of spsed laser-fused KNN.
Table 1 Piezoelectric properties of normal KNN and the hipped samples. Property
Na0.5 K0.5 NbO3 [16]
Laser-fused KNN sintered at 525 ◦ C
Laser-fused KNN sintered at 1000 ◦ C
Planar coupling factor, Kp % Piezoelectric coefficient, d33 pC/N Dielectric constant, 33 /o Relative density, %
45 160 420 >99
– 5 1547 83
15 27 413 98.4
tion spectra. The amorphous content was calculated as a ratio of the area under each amorphous hump after sintering to that of the laser-fused KNN at 2 kW laser power (see Fig. 10). The area under the amorphous hump of the laser-fused KNN was considered in this calculation to be 100%. This quantization is not standard. However, it can give an idea about the influence of sintering on the residual amorphous amount. The relationship between the residual amorphous amount in percent and the sintering temperature was plotted in Fig. 11. As a matter of fact, the amorphous content decreases with increasing the sintering temperature. It shows that at the glass transition range, there is a steep decrement in the glass amount due to its consumption during generation of new crystallites for all the sintering methods. Fig. 11 indicates also that for a constant sintering temperature and sintering time, the residual amorphous
amount is increased with increasing the particle size. A difference of around 10% amorphous amount can be found between samples prepared from as-received and milled microspheres which were spark plasma sintered at 525 ◦ C for 5 min. A possible reason may belong to the difference in size of sintered particles and their difference in nucleation speed. 3.4. Piezoelectric properties Piezoelectric properties were investigated for samples with the highest relative density i.e. those were hipped at 525 ◦ C and at a sintering pressure of 280 MPa. They were firstly crystallized at 670 ◦ C to remove the amorphous phase, because it has low electrical resistivity and this made the polarization process impossible. Fig. 11 shows that these samples contain a residual amorphous amount
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of around 20% of that of the laser-fused KNN at 2 kW laser power. The measurement has shown that these samples have a piezoelectric constant (d33 ) of 5 pC/N, which is very small in comparison to 160 pC/N of the normal KNN [16]. The planar coupling factor p for these samples was close to zero. It is possible that the impedance analyzer could not sense this weak effect. The dielectric constant 33 /o has been 1547. Moreover, the samples hipped at 1000 ◦ C showed a planar coupling factor p of 15%, a piezoelectric constant (d33 ) of 27 pC/N and a dielectric constant of 413. Table 1 summarizes piezoelectric properties of regular KNN and laser fused KNN samples. The difference in dielectric constant 33 /o might originate from the different porosities of the hipped samples. Hikita et al. [17] reported that the poling becomes difficult with existing of porosity. Where the isolated air in the pores has led to early breakdown during polarization process. Jaffe et at. [18] reported that the best piezoelectric properties can be determined at a relative density more than 95%. The hysteresis loops of the samples were unsaturated loops. Therefore, they are not shown in this paper. In fact, Hikita [17] and Kakimoto [19] reported opposite results about the piezoelectric constant (d33 ) in relation to the porosity. Hikita [17] found that d33 slightly increases with the porosity content while Kakimoto [19] found that d33 decreases with the porosity content. As a result, the big difference of d33 and 33 /o for the two hipped samples may belong to one or two of the following reasons: the relative density difference and unsaturated polarization. The well-deformed grains of the microstructure of these samples were changed after the crystallization as it is shown in Fig. 12. It is clear that the shape of particles as well as the pore’s shape were changed after the crystallization. The main phase appeared after sintering is Na0.9 K0.1 NbO3 beside other phases that belong to the family of KNbO3 . These phases are considered to be another reason for the low piezoelectric properties. In fact, the best piezoelectric properties appearing in the Fig. 10. Estimation of amorphous phase amount utilizing the XRD patterns.
Fig. 11. Amorphous phase amount after different sintering methods.
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Fig. 12. FESEM images of hipped sample at 525 ◦ C and at 280 MPa. Left image: before crystallization; Right images: after crystallization.
potassium sodium niobate oxide are associated with this chemical formula K0.5 Na0.5 NbO3 . As it is already described in references [15] and [20], the piezoelectric properties of the rest compositions are between the piezoelectric properties of KNbO3 and NaNbO3 which considered the end members of KNN phase diagram. Since Na0.9 K0.1 NbO3 contains more Na+1 ions than K+1 ions, its piezoelectric properties are weak because the composition is close to the NaNbO3 side in the phase diagram where NaNbO3 is considered to be an antiferroelectric compound. However, Shirane et al. [21] reported that the behavior of this compound is more similar to KNbO3 rather than to NaNbO3 . On the other hand, the microstructure comprises other phases that belong to K4 Nb6 O17 , K2 Nb8 O21 plus unidentified phase. No evidence has been found from our side whether or not the secondary phases have piezoelectric properties. In spite of the poor piezoelectric properties obtained, this technical way shows a possibility to produce samples at low sintering temperature 525 ◦ C with piezoelectric effect. This research is continuing to improve the phase composition and the microstructure as well as the piezoelectric properties.
4. Conclusions Fusing of ceramics using lasers as energy sources is one of the promising processes for the future of ceramic technologies. Due to the need of high-tech or smart materials the aim of this work is developing of a transparent KNN ceramics with piezoelectric properties by sintering—crystallization of glass particles, which can be used e.g. for sound damping. This work has shown: 1. The transparent laser-fused KNN microspheres can be sintered by viscous flow at 525 ◦ C. This temperature is inside the kinetic window of the KNN glass. 2. The sintering experiments have shown that pressureless sintering and spark plasma sintering lead to low relative density. The results indicated that the surface crystallization and the viscous flow happen concurrently. The maximum relative density reached with these sintering processes is between 55% and 75%. Even rising the heating rate to 100 K/min during spark plasma sintering has not promoted the densification. 3. Increase the sintering pressure during hot isostatic up to 280 MPa has intensified the viscous flow and has increased the relative density to 83% at a sintering temperature of 525 ◦ C and at a dwell time for an hour. 4. The samples hipped at 525 ◦ C exhibit a piezoelectric coefficient (d33 ) of 5 pC/N. These samples did not record a coupling factor p . In addition, the samples hipped at 1000 ◦ C show d33 of 27 pC/N and p of 15%.
Maximizing the transparent fraction of the laser-fused KNN to 100% and raising the sintering pressure are promising strategies for reaching maximum transparency from KNN. Acknowledgments The authors would like to thank Mrs. Peggy Knospe in the Institute of Particle Technology-Clausthal University of Technology for doing the TEM investigation, Mr. Jan Raethel at IKTS-Dresden, Germany, for making the Spark Plasma Sintering (SPS) experiments, Dr. Nadja Kratz FGK-Höher-Grenzhausen, Germany, for making the Hot Isostaic Pressing (HIP) experiments, Dr. Schreiner and his team in CeramTec-Lauf for the kind assistance to achieve the piezoelectric properties measurements. Special thank to the DAAD and the Iraqi ministry of higher education and scientific research for funding this work. References [1] F.J. Hmood, C. Oelgardt, R. Görke, J.G. Heinrich, Preparation of transparent microspheres in the K0.5 Na0.5 NbO3 system by laser fusing, J. Ceram. Sci. Technol. 4 (2013) 41–48. [2] H. Djohari, J.I. Martinez-Herrera, J.J. Derby, Transport mechanisms and densification during sintering: I. viscous flow versus vacancy diffusion, Chem. Eng. Sci. 64 (2009) 3799–3809. [3] M.O. Prado, E.D. Zanotto, Glass sintering with concurrent crystallization, C R Chimie 5 (2002) 773–786. [4] E.D. Zanotto, Experimental studies of surface nucleation and crystallization of glasses, Ceram. Trans. 30 (1993) 65–74. [5] A. Rosenflanz, M. Fery, B. Endres, T. Anderson, E. Richareds, C. Schardt, Bulk glasses and ultrahard nanoceramics based on alumina and rare-earth oxides, Nature 430 (2004) 761–764. [6] E.D. Zanotto, A bright future for glass-ceramics, Am. Ceram. Soc. Bull. 89 (2010) 19–27. [7] L. Ramond, G. Bernard-Granger, A. Addad, C. Guizard, Sintering of soda-lime glass microspheres using spark plasma sintering, J. Am. Ceram. Soc. 94 (2011) 2926–2932. [8] L. Perriere, M.T. Thai, S. Tusseau-Nenez, M. Bletry, Y. Champion, Spark plasma sintering of a Zr-based metallic glass, Adv. Eng. Mater 13 (2011) 581–586. [9] C. Oelgardt, Laserbasierte Herstellung tarnsparentner Mikrokugeln der ternären eutektischen Zusammensetzung Al2 O3 -Y2 O3 -ZrO2 und deren Sinterverhalten (PhD Thesis, in German), Clausthal University of Technology, Germany, 2013. [10] B. Zhang, J. Li, K. Wang, H. Zhang, Compositional dependence of piezoelectric properties in Nax K1-x NbO3 lead-free ceramics preparede by spark plasma sintering, J. Am. Ceram. Soc. 89 (2006) 1605–1609. [11] D. Jenko, A. Bencan, B. Malic, J. Holc, M. Kosec, Electron microscopy studies of potassium sodium niobate ceramics, Microsc. Microanal. 11 (2005) 572–580. [12] A.R. Boccaccini, W. Stumpfe, D.M.R. Taplin, C.B. Ponton, Densification and crystallization of glass powder compacts during constant heating rate sintering, Mate Sci. Eng. 219 (1996) 26–31. [13] R. Müller, S. Reinsch, Viscous-phase silicate processing, in: N.P. Bansal, A.R. Boccaccini (Eds.), Ceramics and composites processing methods, John Wiley & Sons Inc., 2012, pp. 75–144. [14] H. Du, Z. Li, F. Tang, S. Qu, Z. Pei, W. Zhou, Preparation and piezoelectric properties of (K0. 5 Na0. 5 )NbO3 lead-free piezoelectric ceramics with pressure-less sintering, Mater Sci. Eng. B 131 (2006) 83–87.
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[15] K. Kakimoto, I. Masuda, H. Ohsato, Lead-free KNbO3 piezoceramics synthesized by pressure-less sintering, J. Eur. Ceram. Soc. 25 (2005) 2719–2722. [16] R.E. Jaeger, L. Egerton, Hot pressing of potassium-sodium niobates, J. Am. Ceram. Soc. 45 (1962) 209–213. [17] K. Hikita, K. Yamada, M. Nishioka, M. Ono, Effect of porous structure to piezoelectric properties of PZT ceramics, Jpn. J. Appl. Phys. 22 (1983) 64–66. [18] B. Jaffe, W. Cook, H. Jaffe, Piezoelectric ceramics, in: J.P. Roberts, P. Popper (Eds.), Non-Metallic Solids, Academic Press Inc., 1971, pp. 253–269.
[19] K. Kakimoto, T. Imura, Y. Fukui, M. Kuno, K. Yamagiwa, T. Mitsuoka, K. Ohbayashi, Processing of piezoelectric (Li,Na,K)NbO3 porous ceramics and (Li,Na,K)NbO3 /KNbO3 composites, Jpn. J. Appl. Phys. 46 (2007) 7089–7093. [20] L. Egerton, D.M. Dillon, Piezoelectric and dielectric properties of ceramics in the system potassium-sodium niobate, J. Am. Ceram. Soc. 42 (9) (1959) 438–442. [21] G. Shirane, R. Newnham, R. Pepinsky, Dielectric properties and phase transition of NaNbO3 and (Na, K)NbO3 , Phys. Rev. 96 (1954) 581–588.
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Sintering and piezoelectric properties of K0.5 Na0.5 NbO3 glass microspheres Firas J. Hmood a,c,∗ , Jens Guenster a,b , Juergen G. Heinrich a a b c
Clausthal University of Technology, Department for Engineering Ceramics, Germany Federal Institute for Materials Research and Testing (BAM), Germany University of Babylon, Department for Construction Materials and Ceramics Engineering, Iraq
a r t i c l e
i n f o
Article history: Received 2 May 2015 Received in revised form 29 July 2015 Accepted 30 July 2015 Available online xxx Keywords: Transparent piezoceramics Glass microspheres Viscous flow sintering K0.5 Na0.5 NbO3
a b s t r a c t Laser-fused K0.5 Na0.5 NbO3 (KNN) powder of 75% transparent fraction has been sintered by pressureless sintering, spark plasma sintering (SPS), and hot isostatic pressing (HIP). The laser-fused KNN has a fictive temperature of 503 ◦ C and an onset crystallization temperature of around 529 ◦ C. The results have shown that sintering of the laser-fused KNN powder utilizing the viscous flow of the transparent microspheres (amorphous content)–at the kinetic window (26 ◦ C)–is possible. The highest yield relative density is around 83% at a sintering temperature of 525 ◦ C and at a sintering pressure of 280 MPa. Limited density has been reached because of formation of crystalline surface layers around the amorphous areas. The samples hipped at 525 ◦ C have low piezoelectric coefficient d33 of 5 pC/N because of the residual porosity that led to early dielectric breakdown during the polarization. The sintering behavior, the resulting microstructure as well as the measured properties will be discussed. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction This paper deals with the sintering behavior of the laser-fused K0.5 Na0.5 NbO3 (KNN) [1] utilizing the viscous flow of the amorphous content at the glass transition range. The viscous flow represents the kinetic path to sinter glass particles. During the viscous flow, the neck formation and the shrinkage are taking place concurrently. The shrinkage will take place at the end of the sintering process in sintering of crystalline particles by the diffusion process at relatively high sintering temperatures [2]. The big issue associated with this kind of sintering is concurrent sintering-crystallization of the glass particles especially at the outer surfaces that prohibits the full densification at low sintering temperatures. However, this route seems to be useful by decreasing the time of glass-ceramics preparation utilizing the surface crystallization of the glass particle [3]. The surface finishing of the glass particles plays an important role in the viscous sintering. The experimental work of Zanotto et al. [4] indicated that the fired surfaces show little tendency to surface nucleation. It also revealed that an existing of few crystalline grains glued to the glass surfaces could accelerate the surface nucleation. Normally, sintering of glass particles is performed at tempera-
∗ Corresponding author at. Postal address: P.O. Box 213, Hilla—Babylon, Iraq. E-mail address: fi[email protected] (F.J. Hmood).
tures between the glass transition temperature and that of the crystallization start. The difference between both temperatures is called kinetic window. In most silicate glass ceramic systems the kinetic window is wide enough to achieve full densification with well controlling of crystallization. While systems that are free from glass-forming oxides have small kinetic windows that increase the probability of crystallization [5]. Therefore, consolidation of glass particles from the later systems will be a competition between the sintering and the crystallization. If crystallization wins, microstructures with various densities will be obtained [6]. In this study, more than one sintering method was involved in order to prohibit or delay the surface nucleation during sintering. Keeping the sintering temperature at the glass transition range as well as maximizing of the sintering pressure are helpful conditions to maintain the sintering process until reaching a full densification. Theoretically, spark plasma sintering (SPS) seems to be a suitable method to consolidate glass microbeads. By applying this technology, high heating rates combined with short sintering time can be realized. Ramond et al. [7] succeeded by sintering soda-lime glass (SLG) microspheres without any crystallization at 522 ◦ C using SPS in 10 min with a heating rate of 100 K/min and at sintering pressure of 34 MPa. However, the high temperature concentration at the neck regions between the particles as well as the temperature gradient through the glass particles can cause other reasons for hindering sintering by the viscous flow [8].
http://dx.doi.org/10.1016/j.jeurceramsoc.2015.07.035 0955-2219/© 2015 Elsevier Ltd. All rights reserved.
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K0.5 Na0.5 NbO3 (KNN) glass microspheres were prepared by laser fusing where a 5 kW—CO2 laser beam was used. The transparent fraction of the laser-fused KNN granules has been calculated to be 68.5% and the mean particle size (d50 ) was 62.5 m. According to the Differential Scanning Colorimeter (DSC), the glass transition temperature (Tf ) has been determined to be 503 ◦ C and the onset crystallization temperature (Tx ) was 529 ◦ C with a kinetic window of 26 ◦ C [1]. A lot of efforts were devoted to separate the transparent laser-fused granules from the opaque ones depending on their density. However, it was very difficult because they have approximately the same density. Even by hand, it was difficult because they are just some microns in size. Therefore, the laser-fused KNN as a mixture of transparent and opaque microspheres was sintered in order to show whether or not such technical route can lead to transparent ceramics with piezoelectric properties. 2. Experimental 2.1. Transparent fraction Preparation and characterization of the transparent microspheres can be found in details in reference [1]. As it has been cleared in reference [1], the highest transparent fraction reached was around 68.5% and it is raised to 75% during working on this part of the study. Rising of the transparent microspheres is important for the sintering and for the transparency as well. A lot of experiments were conducted to separate the transparent microbeads a part from the other components according to the density or even by hand. However, no one was successful. Therefore, using an extra ultrasonic cleaning combined with moderate vacuum in distilled water was an alternative method. The water was used because it is cheap as a cleaning media. The increase of the transparent fraction can be attributed to mechanically destroying the sintered particles/porous granules to smaller particles. After that, they were separated easily according to their density. However, this is not the end and working on this part of the study is continuous. 2.2. Samples preparation Because of the high flowability of as-received KNN microspheres, compacts from the as-received microspheres were impossible. With respect to the Pressureless sintering and the Hot Isostatic Pressing (HIP), the laser-fused KNN granules were ball milled to d50 of 2 m. The milled microspheres were first mixed with an organic binder, pressed in an axial press and finally cold isostatically post densified at a pressure of 200 MPa. The prepared samples were heated up to 300 ◦ C for 10 h in order to remove the organics from the pressed samples. At this stage, the maximum temperature was kept at 300 ◦ C to ensure that no crystallization would occur. For the SPS, there was no need for milling because the microspheres can be supplied to the die without preforming or pre-pressing. 2.3. Sintering methods
100 K/min; starting powder: as-received (d50 : 62 m) laser-fused KNN and milled (d50 : 2 m) laser-fused KNN. • Hot Isostatic Pressing (HIP): sintering temperatures: 525 ◦ C, 1000 ◦ C; sintering pressure: 280 MPa; heating rate: 10 K/min; starting powder; milled laser-fused KNN (d50 : 2 m). 2.4. Equipment • Dilatometer (Baehr 802L thermo-analysis—Germany) was used to investigate the sintering behavior of the laser-fused KNN microspheres. The heating rate in this experiment was 10 K/min. • Spark plasma sintering (SPS) machine (HHPD25, FCT Systeme GmbH, Germany) was also involved to assemble the laser-fused KNN microspheres into tablets with diameter of 20 mm. The die was lined up with graphite foils in order to prevent the reaction of the powder with the die. • Hot isostatic pressing (HIP) machine (EPSI, Belgium) with a maximum sintering pressure of 280 MPa were conducted. In the HIP experiments, the samples were encapsulated in stainless steel capsules (Type: 1.4301). Nd-YAG laser was used to weld the capsules in order not to develop a lot of heat during the encapsulation that can lead to start the crystallization before the sintering process. • The density of the sintered samples was calculated as mass to volume ratio. Unless the samples hipped at 1000 ◦ C, their density was measured by Archimedes principle because they got irregular shape after hipping. • Microstructures of the sintered samples were examined by a field emission scanning electron microscopy (FESEM) (Helios NanoLab 600, FEI, Netherlands) and by a transmission electron microscopy (TEM) (JEM 2100 von JEOL, USA) with operating voltage of 200 kV. The specimens of TEM were prepared by using focused ion beam, which is a compatible tool with the FESEM. The thickness of the specimens was around 100 nm. • Phase composition was investigated by using X-ray diffraction (XRD, Diffractometer D5000 Kristalloflex, Siemens, Germany). Cu-K␣ radiation with wavelength of 0.154 nm was used and a scanning angle of 0.02◦ . • Measuring of the piezoelectric properties was just done on the hipped samples that were ground to 0.4 mm in thickness. The samples were contacted with silver paste which was fired at 700 ◦ C. The polarization was done in silicon oil at room temperature at direct current. An electrical field as high as 2.5 kV/mm was applied for the samples sintered at 1000 ◦ C and 0.4 kV/mm was applied for the samples sintered at 525 ◦ C. The period of polarization was around 5 min for all samples. The poled samples were aged for 24 hour after the polarization process. The planar coupling factor p was determined with an impedance analyzer (4294A Agilent—Malaysia). The piezoelectric coefficient d33 was measured directly with a d33 device (Channel Technologies Group, USA). While the dielectric constant was calculated mathematically according to the following equation at 1 kHz: 33/ о = (t × c × 144)/(da2 - di2 )Where t: thickness of the ceramic sample (mm), c: capacity of the ceramic sample (pF), da: outer diameter (mm), di: inner diameter (mm).
Different sintering methods were employed to get as high as relative density at the glass transition range of the transparent KNN which is around 503–529 ◦ C [1]. The sintering methods are:
3. Results and discussion
• Pressureless sintering: sintering temperatures: 475 ◦ C, 500 ◦ C, 525 ◦ C, 1150 ◦ C; sintering pressure: 0.1 MPa; heating rate: 10 K/min; starting powder: milled laser-fused KNN (d50 : 2 m). • Spark Plasma Sintering (SPS): sintering temperatures: 525 ◦ C, 700 ◦ C, 1000 ◦ C; sintering pressure: 50 MPa; heating rate:
Sintering experiments were done with different conditions in order to get on one hand a high relative density and on the other hand a high residual glass content after sintering. The small kinetic window of the transparent laser-fused KNN microspheres (26 ◦ C) has limited the viscous flow during sintering. In the follow-
3.1. Sintering methods
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Fig. 1. TEM images of transparent laser-fused KNN microspheres.
Fig. 2. Linear shrinkage of the laser-fused KNN. Fig. 3. Linear shrinkage of the laser-fused KNN within the kinetic window.
ing paragraphs, the transparent microspheres will be called glass microspheres because our evidence yet indicate that this melt is a glass where the TEM images (see Fig. 1) reveal an amorphous structure. However, this work is not to verify whether the transparent beads are real glass or metastable quenched liquid. More investigations on this issue are required. The question shall be discussed, is whether it is possible that necks can be formed between the particles during sintering of the laser-fused KNN at the kinetic window. The answer may be driven from Fig. 1. It shows two TEM images (surface and center) of transparent KNN microspheres. On the left hand image, the surface does not exhibit any crystallites within the amorphous structure. On the right hand image, the structure is similar. This implies that forming of necks during the viscous flow of the laser-fused KNN granules at the kinetic window is possible. Oelgardt et al. [9] found an existence of a crystalline surface layers around the laser-fused Al2 O3 -Y2 O5 -ZrO2 (AYZ) microspheres and as a result neck forming was impossible during sintering, because the crystalline structure has higher viscosity than the amorphous structure for the same material at the same sintering temperature. Hereafter, the sintering results of the laser-fused KNN according to the sintering methods will be discussed. • Pressureless sintering Fig. 2 exhibits the dilatometry curves of the starting KNN powder (curve 1) and that of the laser-fused KNN (curve 2). The starting KNN powder curve exhibits normal behavior of a crystalline material which can be sintered at around 1050 ◦ C where the sintering
temperature of the KNN powder is between 950 ◦ C and 1050 ◦ C depending on the kind of sintering [10,11]. The onset of sintering is at around 1050 ◦ C and proceeds up to 1150 ◦ C with a total linear shrinkage of 10%. With respect to curve 2, it exhibits two steps of shrinkages. First at the glass transition range of the laser-fused KNN, which starts at around 503 ◦ C with linear shrinkage of around 0.7–1%. The viscous flow of the glass particles resulted in the first densification (first shrinkage). The second shrinkage takes place at around 1000 ◦ C and takes the same path of curve 1. In order to maintain the viscous flow, samples from laser-fused KNN microspheres were sintered at 475 ◦ C, 500 ◦ C, and 525 ◦ C for a dwell time of one hour. The heating rate was kept constant at 10 K/min. Fig. 3 shows that the resulting curves exhibit limited linear shrinkage with a maximum value of 1.2% for the samples sintered at 500 ◦ C. This result indicates that the pressureless sintering cannot maintain the viscous flow for a long time. Therefore, modifying the sintering conditions was important to reach higher densification. • Spark plasma sintering (SPS) SPS was involved in this study to combine higher heating rate with higher sintering pressure in a short time. Thanks to the SPS equipment, sintering curves can result concurrently with the sintering method. Fig. 4 shows sintering curves of three experiments as a relationship between the punch displacement through the graphite die and the sintering temperatures. The experiments can be achieved with any particle size; either as-received microspheres (d50 : 62 m) or as milled microspheres (d50 : 2 m) because the
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ing temperature of densification to around 400 ◦ C with the milled microspheres. Despite of this result, the densification is still poor. In fact, declination of the microspheres leads to faster crystallization because the jagged particles offer more surface nucleation sites in comparison to the smooth surfaces of the glass microspheres and this hinders more densification [4]. A heating rate of 100 K/min at SPS is not promoting densification of the laser-fused KNN. Boccaccini et al. [12] indicated that increasing the heating rate during glass particle sintering promotes densification. One or two of the following reasons might explain this difference: • Boccaccini et al. [12] used barium magnesium aluminosilicate glass powder which has a good ability to form a glass, while the system used here has a 25% crystalline fraction (crystalline beads) which could hardly be decreased to almost zero during the preparation stage. • Crystallization especially at the surface of the glass particles (see Fig. 7) is a hindering factor for further sintering.
Fig. 4. Sintering behavior of the laser-fused KNN by SPS machine.
powder can be poured inside the die without any pre-pressing. The general sintering behavior of the laser-fused material can be seen in Fig. 4. It shows an increase in the densification with increasing the sintering temperature until reaching the saturation density, at which there is no more increment in the density. Applying of the sintering pressure can lower the sintering temperature and can also promote the viscous flow between the glass microspheres. The onset of sintering of the as-received KNN microspheres is close to 500 ◦ C while sintering of milled microspheres exhibits earlier densification close to 400 ◦ C. The figure reveals also that the densification continues inside the kinetic window for a while and then stops. Combination of the results of the sintering behavior for both methods (Figs. 2–4) indicates that sintering of the laser-fused KNN within the kinetic window is possible. It is clear that at low sintering temperatures, viscous flow is retarded. Moreover, 50 MPa sintering pressure during SPS has no visible influence on the overall densification. However, this sintering pressure lowered the start-
• Hot Isostatic Pressing (HIP) In this method, the sintering pressure was increased to 280 MPa using the HIP parallel with using milled laser-fused microbeads. The heating rate was 10 K/min. The sintering temperatures were 525 ◦ C and 1000 ◦ C. Fig. 5 reveals the relative density of all the sintering methods employed in this study. It summarizes the sintering conditions for each sintering method. It reveals how the densification of the transparent laser-fused KNN at the kinetic window has been improved. The maximum relative density reached is 83% using the HIP while it fluctuates between 55% and 65% using pressureless sintering and between 55% and 75% using the SPS. Other experiments that were done using the SPS at 670 ◦ C exhibit densities between 75% and 85%. The same figure shows that combination of the SPS for 30 min with hipping for an hour leads to samples with densities around 98.3%. The samples hipped at 1000 ◦ C at sintering pressure of 280 MPa show a relative density of 98.5% which is the highest density for the selected sintering conditions. The sintering conditions are not completely unified, because each sintering method is different from one another. The goal behind the sintering process has been to
Fig. 5. Relative density using different sintering methods.
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Fig. 6. FESEM images of fracture surface of hipped laser-fused KNN (HIP, at 525 ◦ C and at 280 MPa).
Fig. 7. FESEM image of spsed laser-fused microspheres (SPS, at 525 ◦ C and at 50 MPa).
get on one hand high relative density and on the other hand high amorphous contain after sintering.
Fig. 8. TEM image of a spsed sample at a neck area (SPS, at 525 ◦ C, at 50 MPa).
when such crystalline areas grow and meet together, viscous flow is restrained as well [13]. 3.3. Phase composition
3.2. Microstructure Fig. 6 shows FESEM micrographs of the samples hipped at 525 ◦ C for an hour at sintering pressure of 280 MPa. The left image is a fracture surface of a hipped sample. It reveals that there are radial lines (Wallner-lines) that exhibit some similarity with a glass fracture surface. It does not indicate that the hipped sample is a glass but it can indicate that this sample has an amorphous content after the HIP. A higher magnification (right hand image) of this fracture surface shows well deformed and well connected grains with some regions of melted glass. Fig. 5 reveals that the relative density of this microstructure is 83%. Indeed, this is an encouraging result to improve this technical route in the near future. Fig. 7 exhibits a micrograph of a SPS sample sintered at 525 ◦ C for 5 min at a sintering pressure of 50 MPa. At this relative low sintering temperature and this short sintering time, crystalline surface layers have been developed between the glass—glass microspheres. As a result, the progress of viscous flow was stopped and the samples remained porous. This figure emphasizes that the crystallization starts at the particle surface and then heads towards the center of the glass microspheres. Another TEM image of a sample from milled microspheres sintered by SPS (see Fig. 8) exhibits new lattice structures that formed during sintering inside a neck area. This means
In this section the phase composition of the SPS samples will be discussed. In the previous publication [1], it has been cleared that the phase of the starting KNN powder is pure orthorhombic perovskite phase with a chemical formula of K0.5 Na0.5 NbO3 . After sintering and due to the high diffusivity of sodium ions Na+1 , the main phase that firstly formed is tetragonal with a chemical formula of Na0.9 K0.1 NbO3 , as it is shown in Fig. 9. Besides this compound, other phases appeared which belong to K4 Nb8 O21 and K8 Nb6 O17 as well as an unidentified phase. It is worth mentioning that there are differences between the secondary phases that are formed during the laser fusing process [1], during heat treatment of the transparent laser-fused microspheres [1] and during sintering. The main chemical formula for most of the second phases is Kx Nby Oz which is potassium niobate. However, after each treatment, it appears with different stoichiometric ratio. In fact, there is no clear reason for this variation but it is possible that this compound is very sensitive to slight changes in its composition, e.g. as a result of the loss of alkalis, within each heating step [14,15]. It was very difficult to separate transparent from opaque microspheres after sintering. Therefore, the content of the amorphous phase after each sintering step was calculated utilizing the area under the biggest amorphous peaks (humps) in the X-ray diffrac-
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Fig. 9. X-ray diffraction patterns of spsed laser-fused KNN.
Table 1 Piezoelectric properties of normal KNN and the hipped samples. Property
Na0.5 K0.5 NbO3 [16]
Laser-fused KNN sintered at 525 ◦ C
Laser-fused KNN sintered at 1000 ◦ C
Planar coupling factor, Kp % Piezoelectric coefficient, d33 pC/N Dielectric constant, 33 /o Relative density, %
45 160 420 >99
– 5 1547 83
15 27 413 98.4
tion spectra. The amorphous content was calculated as a ratio of the area under each amorphous hump after sintering to that of the laser-fused KNN at 2 kW laser power (see Fig. 10). The area under the amorphous hump of the laser-fused KNN was considered in this calculation to be 100%. This quantization is not standard. However, it can give an idea about the influence of sintering on the residual amorphous amount. The relationship between the residual amorphous amount in percent and the sintering temperature was plotted in Fig. 11. As a matter of fact, the amorphous content decreases with increasing the sintering temperature. It shows that at the glass transition range, there is a steep decrement in the glass amount due to its consumption during generation of new crystallites for all the sintering methods. Fig. 11 indicates also that for a constant sintering temperature and sintering time, the residual amorphous
amount is increased with increasing the particle size. A difference of around 10% amorphous amount can be found between samples prepared from as-received and milled microspheres which were spark plasma sintered at 525 ◦ C for 5 min. A possible reason may belong to the difference in size of sintered particles and their difference in nucleation speed. 3.4. Piezoelectric properties Piezoelectric properties were investigated for samples with the highest relative density i.e. those were hipped at 525 ◦ C and at a sintering pressure of 280 MPa. They were firstly crystallized at 670 ◦ C to remove the amorphous phase, because it has low electrical resistivity and this made the polarization process impossible. Fig. 11 shows that these samples contain a residual amorphous amount
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of around 20% of that of the laser-fused KNN at 2 kW laser power. The measurement has shown that these samples have a piezoelectric constant (d33 ) of 5 pC/N, which is very small in comparison to 160 pC/N of the normal KNN [16]. The planar coupling factor p for these samples was close to zero. It is possible that the impedance analyzer could not sense this weak effect. The dielectric constant 33 /o has been 1547. Moreover, the samples hipped at 1000 ◦ C showed a planar coupling factor p of 15%, a piezoelectric constant (d33 ) of 27 pC/N and a dielectric constant of 413. Table 1 summarizes piezoelectric properties of regular KNN and laser fused KNN samples. The difference in dielectric constant 33 /o might originate from the different porosities of the hipped samples. Hikita et al. [17] reported that the poling becomes difficult with existing of porosity. Where the isolated air in the pores has led to early breakdown during polarization process. Jaffe et at. [18] reported that the best piezoelectric properties can be determined at a relative density more than 95%. The hysteresis loops of the samples were unsaturated loops. Therefore, they are not shown in this paper. In fact, Hikita [17] and Kakimoto [19] reported opposite results about the piezoelectric constant (d33 ) in relation to the porosity. Hikita [17] found that d33 slightly increases with the porosity content while Kakimoto [19] found that d33 decreases with the porosity content. As a result, the big difference of d33 and 33 /o for the two hipped samples may belong to one or two of the following reasons: the relative density difference and unsaturated polarization. The well-deformed grains of the microstructure of these samples were changed after the crystallization as it is shown in Fig. 12. It is clear that the shape of particles as well as the pore’s shape were changed after the crystallization. The main phase appeared after sintering is Na0.9 K0.1 NbO3 beside other phases that belong to the family of KNbO3 . These phases are considered to be another reason for the low piezoelectric properties. In fact, the best piezoelectric properties appearing in the Fig. 10. Estimation of amorphous phase amount utilizing the XRD patterns.
Fig. 11. Amorphous phase amount after different sintering methods.
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Fig. 12. FESEM images of hipped sample at 525 ◦ C and at 280 MPa. Left image: before crystallization; Right images: after crystallization.
potassium sodium niobate oxide are associated with this chemical formula K0.5 Na0.5 NbO3 . As it is already described in references [15] and [20], the piezoelectric properties of the rest compositions are between the piezoelectric properties of KNbO3 and NaNbO3 which considered the end members of KNN phase diagram. Since Na0.9 K0.1 NbO3 contains more Na+1 ions than K+1 ions, its piezoelectric properties are weak because the composition is close to the NaNbO3 side in the phase diagram where NaNbO3 is considered to be an antiferroelectric compound. However, Shirane et al. [21] reported that the behavior of this compound is more similar to KNbO3 rather than to NaNbO3 . On the other hand, the microstructure comprises other phases that belong to K4 Nb6 O17 , K2 Nb8 O21 plus unidentified phase. No evidence has been found from our side whether or not the secondary phases have piezoelectric properties. In spite of the poor piezoelectric properties obtained, this technical way shows a possibility to produce samples at low sintering temperature 525 ◦ C with piezoelectric effect. This research is continuing to improve the phase composition and the microstructure as well as the piezoelectric properties.
4. Conclusions Fusing of ceramics using lasers as energy sources is one of the promising processes for the future of ceramic technologies. Due to the need of high-tech or smart materials the aim of this work is developing of a transparent KNN ceramics with piezoelectric properties by sintering—crystallization of glass particles, which can be used e.g. for sound damping. This work has shown: 1. The transparent laser-fused KNN microspheres can be sintered by viscous flow at 525 ◦ C. This temperature is inside the kinetic window of the KNN glass. 2. The sintering experiments have shown that pressureless sintering and spark plasma sintering lead to low relative density. The results indicated that the surface crystallization and the viscous flow happen concurrently. The maximum relative density reached with these sintering processes is between 55% and 75%. Even rising the heating rate to 100 K/min during spark plasma sintering has not promoted the densification. 3. Increase the sintering pressure during hot isostatic up to 280 MPa has intensified the viscous flow and has increased the relative density to 83% at a sintering temperature of 525 ◦ C and at a dwell time for an hour. 4. The samples hipped at 525 ◦ C exhibit a piezoelectric coefficient (d33 ) of 5 pC/N. These samples did not record a coupling factor p . In addition, the samples hipped at 1000 ◦ C show d33 of 27 pC/N and p of 15%.
Maximizing the transparent fraction of the laser-fused KNN to 100% and raising the sintering pressure are promising strategies for reaching maximum transparency from KNN. Acknowledgments The authors would like to thank Mrs. Peggy Knospe in the Institute of Particle Technology-Clausthal University of Technology for doing the TEM investigation, Mr. Jan Raethel at IKTS-Dresden, Germany, for making the Spark Plasma Sintering (SPS) experiments, Dr. Nadja Kratz FGK-Höher-Grenzhausen, Germany, for making the Hot Isostaic Pressing (HIP) experiments, Dr. Schreiner and his team in CeramTec-Lauf for the kind assistance to achieve the piezoelectric properties measurements. Special thank to the DAAD and the Iraqi ministry of higher education and scientific research for funding this work. References [1] F.J. Hmood, C. Oelgardt, R. Görke, J.G. Heinrich, Preparation of transparent microspheres in the K0.5 Na0.5 NbO3 system by laser fusing, J. Ceram. Sci. Technol. 4 (2013) 41–48. [2] H. Djohari, J.I. Martinez-Herrera, J.J. Derby, Transport mechanisms and densification during sintering: I. viscous flow versus vacancy diffusion, Chem. Eng. Sci. 64 (2009) 3799–3809. [3] M.O. Prado, E.D. Zanotto, Glass sintering with concurrent crystallization, C R Chimie 5 (2002) 773–786. [4] E.D. Zanotto, Experimental studies of surface nucleation and crystallization of glasses, Ceram. Trans. 30 (1993) 65–74. [5] A. Rosenflanz, M. Fery, B. Endres, T. Anderson, E. Richareds, C. Schardt, Bulk glasses and ultrahard nanoceramics based on alumina and rare-earth oxides, Nature 430 (2004) 761–764. [6] E.D. Zanotto, A bright future for glass-ceramics, Am. Ceram. Soc. Bull. 89 (2010) 19–27. [7] L. Ramond, G. Bernard-Granger, A. Addad, C. Guizard, Sintering of soda-lime glass microspheres using spark plasma sintering, J. Am. Ceram. Soc. 94 (2011) 2926–2932. [8] L. Perriere, M.T. Thai, S. Tusseau-Nenez, M. Bletry, Y. Champion, Spark plasma sintering of a Zr-based metallic glass, Adv. Eng. Mater 13 (2011) 581–586. [9] C. Oelgardt, Laserbasierte Herstellung tarnsparentner Mikrokugeln der ternären eutektischen Zusammensetzung Al2 O3 -Y2 O3 -ZrO2 und deren Sinterverhalten (PhD Thesis, in German), Clausthal University of Technology, Germany, 2013. [10] B. Zhang, J. Li, K. Wang, H. Zhang, Compositional dependence of piezoelectric properties in Nax K1-x NbO3 lead-free ceramics preparede by spark plasma sintering, J. Am. Ceram. Soc. 89 (2006) 1605–1609. [11] D. Jenko, A. Bencan, B. Malic, J. Holc, M. Kosec, Electron microscopy studies of potassium sodium niobate ceramics, Microsc. Microanal. 11 (2005) 572–580. [12] A.R. Boccaccini, W. Stumpfe, D.M.R. Taplin, C.B. Ponton, Densification and crystallization of glass powder compacts during constant heating rate sintering, Mate Sci. Eng. 219 (1996) 26–31. [13] R. Müller, S. Reinsch, Viscous-phase silicate processing, in: N.P. Bansal, A.R. Boccaccini (Eds.), Ceramics and composites processing methods, John Wiley & Sons Inc., 2012, pp. 75–144. [14] H. Du, Z. Li, F. Tang, S. Qu, Z. Pei, W. Zhou, Preparation and piezoelectric properties of (K0. 5 Na0. 5 )NbO3 lead-free piezoelectric ceramics with pressure-less sintering, Mater Sci. Eng. B 131 (2006) 83–87.
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