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Spark plasma sintering (SPS) was utilized to achieve the densification of ZnSe ceramics with a simple 3D ball-milling powder processing. The morphology evolution of powders during the ball milling process and the densification and microstructure of fabricated ceramics were investigated. The effective ball-milling of 8h obviously reduced the particle size and the full densification of ZnSe ceramics was achieved at the sintering temperature of 900 °C. The lower heating rate could obtain the denser ceramics with fewer pores and fine microstructure but serious carburization, and higher heating rate accelerated the grain growth. The optimized heating rate of 5 °C/min obtained the fine grain size of ~5.0 μm. Some challenging issues of fundamentals and technologies were proposed to improve the optical quality of ZnSe ceramics by SPS.
© 2017 Optical Society of America
1. Introduction
ZnSe has excellent optical transmission in the middle and long-wave infrared (IR) regions (2-15 μm) [1,2], and it has been widely used in the detection of organic molecules, thermal imaging as windows and lenses, LED and semiconductor lasers due to its “molecular fingerprint” transmission and low absorption in this region, and direct wide-band gap (2.7 eV) semiconductor features [3,4]. Meanwhile, transition metals (TM) ions doped ZnSe has the great potential to fill the capability gap that the lack of suitable laser sources in 2-5 μm, which plays an important role in optical communication, infrared countermeasures and laser-illuminated imaging fields [5–8].
Generally, the high-quality ZnSe bulk materials are prepared by the seeded physical vapor transport [9–11] and solid-phase recrystallization techniques [12]. However, the performance and durability of ZnSe-based devices depended on the surface conditions [13,14], and further treatment is necessary for the low threshold, high quantum efficiency and long lifetime devices. Furthermore, another widely used chemical vapor transport (CVT) has the shortcoming of long growth cycle, high cost and unsatisfactory mechanical properties in spite of its easy operation and scale-up capability [14]. Therefore, the preparation method for ZnSe is urgently needed to achieve densification with a short period and lower processing temperature.
Spark plasma sintering (SPS) as a new sintering technology is developed in recent years [15]. It combines the advantages of plasma activation, hot pressing (HP) and resistance heating due to ON-OFF direct current pulse, and the fast heating rate (>200 °C/min), short holding time (2~10 min) and mechanical pressure significantly inhibit the grain growth (grain size equivalent from HIP) and achieve ceramics densification at a lower temperature (200°C lower than normal sintering) [16]. SPS has been successfully applied to the preparation of transparent ceramics such as Al2O3 [17], MgAl2O4 [18], Lu2O3 [19] and ZnS [20]. Although the densification of ZnSe ceramics fabricated by SPS has already been reported [21,22], these fewer papers have just focused on the sintering temperature, time and pressure effects.
As known, the ball-milling (BM) is a simple and versatile advanced materials processing technique as well as economically viable in the laboratory and industry. However, BM often increased the contamination level and some undesirable phases could form if milling time is too long in spite of the substantial reduction in particle size [23,24]. The widely used BM is planetary or horizontal. Their working principles are impacting and friction force, and the grinding tank can just move in 2D directions by rotation on its own axis and revolution on the machine axis (in fact, the horizontal BM has only rotation), and some mixed powder is often adhered to the wall of grinding tank. In addition, the heating rate as a major factor of thermal nature significantly affects the material transport during SPS: surface diffusion, grain boundary diffusion, and power-law creep [25]. A higher heating rate could reduce the duration of densification-noncontributing surface diffusion, it favors powder sinterability, and the densification is intensified by grain-boundary diffusion with diminished grain growth [26] but also easily leads to the intragranular pores. More importantly, these impacts are dependent on particle sizes, and the interplay of heating rates and initial particle sizes on the densification behaviors during the electric current-assisted SPS has not been deeply understood [25,27].
In this study, a simple 3D BM processing was used to refine particle size of raw ZnSe powders. The new developed 3D BM, with another name of “shaker-mixer”, is especially suitable for the homogeneous mixing of powders with different specific weights and particle sizes. It sets grinding tank into 3D movement that exposes the mixture to always changing, rhythmically pulsing motion by the rotation, translation, and inversion, exhibiting the exceptional efficiency to fulfill the highest requirements in a minimum time. In addition, the heating rate of SPS on the densification and microstructure of ceramics was systematically investigated and some challenging issues for high optical quality were proposed.
2. Materials and experimental details
The commercial high purity ZnSe powders (99.999%, Russia) was served as the raw material and a 3D BM machine (Turbula-T2F, Willy A. Bachofen, Switzerland) was used to process powders under the speed of 72 r/min. Weighed powders (36.0 g) was ball-milled with high-purity ZrO2 balls in ethyl alcohol. Two different sizes of ZrO2 ball were used: 3 mm (49.52 g) and 5 mm (99.78 g), respectively. Four different BM times were 2, 4, 8 and 10 h, respectively. Then the slurry was dried to remove alcohol at 50 °C for two days in an oven. After being sieving through 200 mesh screen, the powders were loaded into a graphite die (Φ = 20 mm), and the powder compact and the internal surface of the graphite die was separated by a piece of graphite paper to SPS (SP25-10, GT, USA). The up and down punches of the graphite die were suffered uniaxial pressure (5 MPa) and low-voltage electric current pulse. The five different heating rates of 2, 5, 10, 20 and 50 °C/min and pressure of 80 MPa were used to reach the sintering temperature of 900 °C for 5 min. The temperature was measured by a thermocouple inserted into a cavity of graphite mold. SPSed ZnSe ceramics (3 mm) were mirror-polished on both sides using the cerium oxide slurry.
The morphology of ZnSe powders and microstructure of ceramics were represented by scanning electron microscope (SEM, JMS-6510, Japan). The density of ceramics was measured by Archimedes method. The phase composition was determined by X-ray diffraction (XRD, Bruker D2; German) under Kα = 0.1541 nm, the voltage of 30 kV, and step size of 0.01°. All measurements were performed at the room temperature.
3. Results and discussion
Figure 1 shows SEM image of commercial ZnSe powders. Some big aggregation and agglomeration of particles were observed and they contained small particles (less than 0.5 μm) and big particles (larger than 20.0 μm) which all exhibited typical cubic structure. The particles seemed to have the sphere shape even though they were aggregated. This inhomogeneous size distribution was not beneficial to high forming density and uniform microstructure of green body, making lots of big pores left in ceramics after sintering. Besides of high purity, high dispersion and high sintering activity are the important two features for high density and uniform microstructure of ceramics [28]. Therefore, a simple 3D BM technology was employed to homogenize powders.
Figure 2 shows SEM images of ZnSe powders with different 3D BM times. Obviously, the mean particle size decreased gradually from ~3 μm to ~0.4 μm with the increase of BM time from 2h to 10h, and all particles had the regular shape. Although some bigger particles (aggregates) up to 1.0 μm still existed at each BM time, its number was decreased with the prolonged BM time. During 3D BM process, the powder materials were suffered severely from high-energy impacts in the process of ball-to-ball and ball-to-vial wall collisions of the grinding media. The collision resulted in the deformation, fracture and welding behavior among particles, and then the particle size decreased. However, the powder particles were also easily agglomerated in BM processing due to the reduced particle size, high surface energy and an increasing number of defects of surface. When the BM time of 10h (Fig. 2(d)), there was nearly no difference in particle size and distribution compared with that of 8h (Fig. 2(c)). Therefore, BM time of 8h was selected as the optimized parameter because the prolonged BM time would lead to more wear and tear of ZrO2 balls into ZnSe slurry, greatly decreasing powders purity and then deteriorating optical quality.
The phase comparison between ZnSe powders under BM time of 8 hours and raw powders is shown in Fig. 3. All the diffraction peaks were well indexed by cubic ZnSe phase with space group F43m (PDF #65-7409) without hexagonal ZnSe phase and any impurities. However, there was a remarkable difference in diffraction intensity and width. Raw powders had sharp and intense peaks showing that they were well crystallized and had bigger grain size. But the obvious weakening and broadening at 2θ = 27.17° indicated that 3D BM process effectively decreased the grain size of powders, and the crystallization was also degraded due to the increased surface defects.
Figure 4 shows the displacement shrinkage of ZnSe ceramics during SPS in the temperature range of 820-920 °C. ZnSe ceramics reached the maximum contraction point at the sintering temperature of 880 °C. This was much lower than those of other sintering methods of ZnSe ceramics [29]. Microv SB et.al [30] have reported the sintering temperature of ZnSe ceramics was ~1200 °C in the HP sintering, and such high temperature would increase the carburizing rate and then decrease the optical quality of ceramics. Here, SPS could achieve the densification of ZnSe ceramics at a temperature which was lower ~300 °C compared with HP method. However, from the viewpoint of sintering kinetics, ZnSe ceramics could not achieve complete densification at 880 °C because the higher heating rate and the thermodynamically nonequilibrium state would result in the inadequate grain growth and residual pores in ceramics under this temperature. Therefore, the sintering temperature of 900 °C was selected to optimize the heating rate of SPS for complete densification and high quality of ZnSe ceramics.
Photographs of ZnSe ceramics (3 mm thick) fabricated from different heating rates of SPS are displayed in Fig. 5. They exhibited very different colors depending on the heating rate. The highest heating rate one of 50 °C/min was brown color and the 20 °C/min one was dark yellow, and the ceramic color became blacker with decreasing the heating rate. Since the die was made of graphite, the carbon diffusion into ceramic could not be avoided under the high temperature and long dwelling time. The sample color became deep as the heating rate decreased and ZnSe ceramic with the heating rate of 2 °C/min was the darkest one because of the longest thermal history time, while carbon diffusion slightly affected the ceramics with higher heating rate.
Figure 6 displays XRD patterns of ZnSe ceramics with different heating rates of SPS (900 °C for 5 min). As expected, all ceramics exhibited the cubic structure of space group F43m (PDF #65-7409) without any hexagonal phase. In fact, the transforming temperature of ZnSe from cubic to hexagonal phase was about 1425 °C [31]. Here, the lower sintering temperature of 900 °C by SPS to achieve the densification kept cubic ZnSe phase very stable. However, it is noted that the diffraction peaks occurred an obvious shift to smaller angle gradually with the increase of heating rate. Though the conclusion is easily obtained that the higher heating rate results in the bigger lattice parameters according to diffraction Eq. (2) dsinθ = nλ, d is interplanar distance, θ is diffraction angle, λ is X-ray wavelength), the reason why it can happen is hard to understand and need the further study. Another possible calculation from XRD is grain size according to the Scherrer equation (D = Kλ/ (Bcosθ), D is grain size, K is a constant of 0.89 if B is the full width at half maximum (FWHM) of diffraction peak). The grain size was seemed to decrease due to the decreased θ value, but the narrowed FWHM was also noticeable with the increase of heating rate in Fig. 6, and it would result in the large of grain size. Therefore, it is also hard to evaluate the micron-level grain size because of the reasonable application range of Scherrer equation just in nano-level (< 100 nm) [32]. However, SEM of etching surface of ceramics (not shown here) could present clear grain size of ceramic (in Fig. 7).
Fig. 7 Relative densities and mean grain sizes (from SEM of etched ceramics surface) of ZnSe ceramics with different heating rates.
All grain sizes of ZnSe ceramics with different heating rates of SPS were in the range of 5~10 μm. However, it was not expected to monotonously decrease with the increase of heating rate and short thermal history time under the same sintering temperature. Instead, a gradual increase was observed when the heating rate was higher than 10 °C/min. This means that the higher heating rate is not always favorable to limit the grain growth. According to Brook’s model of grain growth under an equilibrium state, the relationship between the grain size (D) and the sintering time (t) has a typical Arrhenius behavior given by Dn-D0n = [kexp(-Q/RT)]t [33], where Q is the activation energy for grain growth, k is the Boltzmann constant, and T is the absolute temperature. Generally, Q is regarded as constant. In this case, the grain growth is only dependent on the sintering time of t. However, in this study, SPS is used to sinter the ceramic powders, and the fast increased “ON-OFF” electronic current will greatly activate the powder when the heating rate is higher. Thus, Q value will never be constant and it will dramatically decrease. The grain growth will be easier and controlled by the smaller Q. This means, the grain size will grow bigger even at the same final temperature for same sintering temperature (in this study, all samples 900 °C for 5 min.) when a higher heating rate is used. Therefore, when the heating rate is lower, the decrease of Q can be neglectful. The grain size of ceramic with a lower heating rate is bigger due to the nearly two times of thermal history time. (2→ 5 °C/min in Fig. 7). However, when the heating rate is higher, the more activated powders greatly decrease Q value of grain growth, the grain size is increased with the increase of heating rate (5→ 10→ 20 °C/min in Fig. 7). It should be noted that the Brook’ model is just be used during the holding stage (not the heating stage) to evaluate the grain growth. The different heating rate just affects the activation energy (Q) under the equilibrium state. The optimized heating rate for fine grain size and microstructure was 5 °C/min in this study.
In fact, there are a few conflicting results about the effect of the heating rate on the grain size. Stanciu LA et al. [34] have reported that the grain size of alumina decreased with increasing heating rate, whereas Murayama N et al. [35] reported the opposite results that are consistent with the present study. Although the origin of the conflicting results has been unclear, their respective different sintering conditions of SPS and different powders may provide a clue for understanding the conflicting results. Generally, the rapid heating and rapid deformation during densification would produce a high concentration defect, because large the DC current for rapid heating would generate high-temperature plasma on particle surfaces to assist the defect formation [35,36]. This deformation induced defect is well known to induce the dynamic grain growth [37]. The defect induced by both heating and deformation may therefore accelerate the grain growth during SPS. As shown in Fig. 7, the accelerated grain growth for high heating rates was confirmed, although the quantitative investigation of defect concentration was not conducted in the present study, and the defect related mechanism seemed to be reasonable for heating rate dependence of grain size.
In addition, Olevsky EA [25] thought the different heating rate could decide the mass transport mechanism during SPS. The high heating rate reduced the duration time of densification-noncontributing surface diffusion, it favored the powders sinterability and the densification was intensified by grain boundary diffusion, which could also accelerate the grain growth and densification.
The relative densities of ZnSe ceramics with different heating rates of SPS are also shown in Fig. 7. The densities of most samples were up to 94% (theory density 5.27 g/cm3). With the increase of heating rate, it monotonously decreased and the sample with 2 °C/min had the highest density. It is easily understood the lower heating rate and the longer heating history time can favor the elimination of pores and obtain more complete densification of ceramics, which also agreed with nearly all researches of SPS fabrication of Al2O3 and MgAl2O4 [36,38]. Another probable reason was that the higher heating rate resulted in the untimely welding among particles at the beginning stage of sintering and did not benefit to the flowability and deformation of powders for higher density.
Figure 8 shows SEM images of fracture surfaces of ZnSe ceramics with different heating rates of SPS. The fracture mode of all samples was mainly transgranular fracture and no clear grain boundaries were observed. The porosity of ZnSe ceramics was expected to increase with the increase of heating rate, and there were numerous intercrystalline and intracrystalline pores with size of 0.1~0.3 μm at the heating rate up to 50 °C/min (Fig. 8(d)), leading to a low relative density in Fig. 7. The sample with the heating rate of 2 °C/min (Fig. 8(a)) had the highest densification and nearly no pores were observed. However, its color was darkest shown in Fig. 5. Therefore, it seems to be contradictory between the densification and carburization depended on the heating rate. Not only the low density with more pores but also the diffused carbon as scattering centers will deteriorate the optical quality of ceramics. The carburization problem under the present SPS conditions should be urgently solved to obtain the high-quality transparent ceramics in future, and the graphite die should be replaced by the metal material to weaken the carburization effect.
Fig. 8 SEM images of fracture surfaces of ZnSe ceramics with different heating rates of SPS (a) 2 °C/min, (b) 10 °C/min, (c) 20 °C/min, (d) 50 °C/min.
4. Conclusion
In this paper, a simple but efficient 3D ball-milling process was used to refine raw powders and the heating rate of SPS was optimized for the densification of ZnSe ceramics. A suitable 3D ball-milling time of 8h effectively decreased the particle size with less wear and tear of ZrO2 balls. The full densification of ZnSe ceramics was achieved at the sintering temperature of 900 °C, which was lower ~300 °C compared with HP method. The lower heating rate resulted in a serious carburization and the sintered ZnSe ceramic was black color. All ZnSe ceramics exhibited cubic structure and the grain size depended on the heating rate. Higher heating rate greatly activated the raw powder and reduced the activation energy for grain growth, and the defects caused by both heating and deformation accelerated the grain growth. The optimized heating rate of 5 °C/min obtained the fine grain size of ~5.0 μm. The lower heating rate could obtain the denser ceramics with fewer pores and fine microstructure in spite of carburization effect. In order to further improve the optical quality and application of ZnSe ceramics by SPS, the challenging issues should be solved that the graphite mold and paper should be replaced by other metal materials such as molybdenum, and the deep understanding of complicated sintering mechanism to explain the unexpected phenomenon during SPS such as the increased lattice parameters of ZnSe ceramics under the higher heating rate.
Funding
National Natural Science Foundation of China (NSFC) (51402133, 11274144, 51302115, 61177045, U1430111); Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD); Special Project for Technology Innovation of Xuzhou City (KC16GZ014, KC16HQ236, KC16HQ237).
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