Enhancing the Quality of the Characteristic Transmittance Curve in the Infrared Region of Range 2.5–7 μm of the Optical Magnesium Fluoride (MgF2) Ceramic Using the Hot-Pressing Technique in a Vacuum Environment

is paper carries out an experimental study to enhance the quality of the characteristic transmittance curve in the infrared range of 2.5–7 μm of the optical magnesium fluoride (MgF2) ceramic. is work evaluates the effectiveness of using vacuum environment in the MgF2 ceramic hot-pressing technique by comparing the density and the quality of the transmittance characteristic curve of two types of specimens manufactured by the hot-pressing technique corresponding to open air and vacuum environment under the same technological parameters including temperature 650°C, time 30min, and pressure 200MPa. e results present that, in comparison with using the hot-pressing technique in the open air, when using the hot-pressing technique in a vacuum environment, the transmittance increases about 4–10%, the characteristic transmittance curve is smoother and has fewer peak points, and the wide of peak points are smaller. When hot-pressing in a vacuum environment, the temperature can be up to 750°C in order to obtain the magnesium fluoride (MgF2) ceramic with the highest quality of characteristic line in the infrared range of 2.5–7 μm. Seventeen specimens are manufactured to conduct the experimental study in order to figure out the optimal set of technological parameters (temperature T� 640.9°C, time t� 33.0min, and pressure P � 231.4MPa) when hot-pressing in a vacuum environment by the experimental planning approach.


Introduction
e optical polycrystalline magnesium fluoride (MgF 2 ) ceramic is used widely in infrared optical filters due to its high transmittance in the infrared region and mechanicalphysical-thermal strengths. e most important characteristic of magnesium fluoride (MgF 2 ) is the ability to allow well-infrared radiation at the wavelength range of 2.5-7 µm. Recently, the hot-pressing technique has been a popular way to manufacture this optical ceramic. ere have been numerous works dealing with this technology, for example, Chang and Hon [1,2] and Buckner [3] studied the magnesium fluoride (MgF 2 ) hot-pressing technology in the open air. In [4], it is presented clearly that the optimal set of technological parameters when hot-pressing MgF 2 in the open air were temperature T � 650°C, time t � 30 min, and pressure P � 200 MPa.
In comparison with single-crystal magnesium fluoride (MgF 2 ) ceramic, which is created by a single-crystal growth method, the hot-pressed polycrystalline magnesium fluoride allows infrared light in the wavelength range of 0.1-2.5 µm to be transmitted less well [2]. Moreover, when using the hotpressing technique in the open air, the transmittance in the wavelength regions 2-4 µm and 6-7 µm is mostly not as good as that in the wavelength region 4-6 µm. e cause is that there is the formation of pores inside crystal particles during the hot-pressing process in the open air. ere is the fact that it is hard to remove these pores by using other approaches, for instance, hot isostatic pressing (HIP) was employed in [5,6] by Tsai and Yashina, respectively. When investigating the characteristic transmittance curve in the infrared range of magnesium fluoride (MgF 2 ) ceramic, we can capture the peak points corresponding to the absorption stretches. ere are some important point as follows. e peak point at 2.8 µm wavelength corresponds to the hydroxylate absorption stretching band (OH stretching), the peak point at 4.3 µm wavelength corresponds to the carbon dioxide absorption stretching band, the peak point at 5 µm wavelength corresponds to the bifluoride absorption stretching band, the peak point at 6.7 µm wavelength corresponds to the OH-banding, and other peak points at 3 µm and 6.1 µm wavelengths correspond to the bands caused by the humility [7]. In general, it is difficult to manufacture magnesium fluoride ceramic without infrared absorption bands by operating the hot-pressing technique in the open air. erefore, in order to cut down the formulation of pores during the manufacturing process, scientists consider using the hot-pressing technique in a vacuum environment. At that time, the optimal technological parameters such as temperature, time, and pressure need to be adjusted. It is the main motivation for our team to carry out an experimental study to find out the optimal function of the working system with the highest quality of the characteristic transmittance curve of hot-pressed magnesium fluoride (MgF 2 ) ceramic. Besides, we also evaluate the effect of the hot-pressing technique in a vacuum environment on the density and quality of the characteristic transmittance curve in the infrared range of 2.5-7 µm.
is paper is divided into 4 main sections. Section 1 presents briefly some works dealt with the hot-pressing technique in the open air as well as its disadvantages. e research object and governing equations are introduced in section 2. Section 3 presents the experimental results and discussions. Some important points are concluded in the last one, Section 4.

Experiment
In this work, the experimental specimens are manufactured using the hot-pressing technique from the same kind of powder magnesium fluoride with size is about 80-100 nm and the purity is over 99%. e SEM of the MaF 2 powders is shown in Figure 1.
To evaluate the efficiency of the vacuum hot-pressing technique, we consider two types of the experimental specimen: MgF 2 ceramic type I (hot-pressed specimen in the open air with optimal parameters: temperature T � 650°C, time t � 30 min, and pressure P � 200 MPa) and MgF 2 ceramic type II (hot-pressed specimen in the 0.04 bar vacuum environment). By comparing the density and quality of the characteristic transmittance curve of two mentioned specimens, we can figure out the efficiency of the use of the vacuum hot-pressing technique.
Besides, in order to determine the optimal technological parameters (temperature, time, and pressure) of the vacuum hot-pressing technique by using the experimental planning approach, herein, we use the orthogonal second-order planning method; the nonlinear compatibility using the quadratic polynomial with three deformation components are expressed as follows: where x 1 , x 2 , and x 3 are the technological parameters corresponding to temperature T (°C), time t (min), and pressure P (MPa), respectively. By selecting the experimental planning option with the center given by Box and Wilson [8], the number of experiments is defined as follows [9]: where the structure has three impact factors k � 3, and the number of the center of the option is n 0 � 3. us, according to equation (2), we can find the number of the experiment is N � 17. In order to be convenient to calculate the experimental coefficients of the mathematical regression model and process the data, we convert to the nondimensional value encoding with the upper bound (+1), the lower bound (−1), and the average value (0) in the extended space α � 1, 215: Based on the optimal technological parameters of the hot-pressing technique in the open air, we choose the values of the technological parameters of the hot-pressing technique in the vacuum as listed in Table 1. en, we measure the transmittance of the specimens at the 4.5 µm wavelength to obtain the experimental planning values.
In order to conduct the experimental planning investigation, we manufacture seventeen specimens in the 0.04 bar vacuum environment following the data set as presented in Table 2.
e experimental cylinder specimens with Φ30 × 10 mm are created from the same hot processing mold, which is made from the heat-resistant Nickel alloy-Inconel 718, and the mold is covered with a boron nitride nonstick layer. e hot-pressing process does not need to use additional adhesive additives. e diagram of the hot-pressing technique in a vacuum environment is shown in Figure 2.
e specimens need to be ground and polished to ensure the following requirements: the degree of parallelism of two working surfaces is not over 30,' the surface roughness R does not exceed 0.050 µm, and the surface cleanness must be from level IV or higher based on GOST 11141 standard. e density of the specimens is measured by the hydrostatic weighing method on the device CY 323 GT. In order to obtain the characteristic transmittance curve, we measure the transmittance using a single-beam spectrometer with a Michelson interferometer. In detail, herein, we use the infrared spectrometer Nicolet Summit FTIR Spectrometers with the measuring spectral range of 1.28-28 µm. e transmittance is measured in the direction of hot-pressing.
From the transmittance of seventeen specimens at the 4.5 µm wavelength, we obtain the experiment planning data set by using the experimental planning method to determine the coefficients of the regression function as shown in equation (1). en, we use Maple 2016 application (Waterloo Maple Inc, Waterloo, Ontario, Canada) to find out the optimal value of the regression function and the optimal data set of the working system.

Results and Discussions
Firstly, we obtain the results as follows. e density of the hot-pressed specimen in a vacuum environment is 3.178 g/ cm 3 , while the density of the hot-pressed specimen in the open air is 3.173 g/cm 3 . We can see clearly that by using the hot-pressing technique in a vacuum environment, the density of the specimen increases. e reason can be easily explained that when operating in the open air, the air will diffuse into the specimen structure and then the porosity of the pressed product increases. However, when employing the hot-pressing technique in a vacuum environment, we can overcome this disadvantage; as a result, the porosity of the pressed product decreases (Figure 3). Figure 4 presents the characteristic transmittance curve in two cases (specimen type I and specimen type II) at the same technological conditions: temperature 650°C, time 30 min, and pressure 200 MPa.
Secondly, we can see that the quality of the characteristic transmittance curve of specimen type II (the blue line, in a vacuum environment) is higher than that of the characteristic transmittance curve of specimen type I (the red line, in the open air). e hot-pressed specimen in a vacuum environment has a 4-10% higher transmittance in the range of 2-7 µm than that of the hot-pressed specimen in the open air with the same technological condition. Others depend on the wavelength.
Next, the characteristic transmittance curve of the hotpressed specimen in a vacuum environment is smoother than that of the hot-pressed specimen in the open air, it has fewer peak points, and the width of the peak points is smaller.
e first peak point of the characteristic transmittance curve of magnesium fluoride (MgF 2 ) ceramic can be observed at the 2.8 µm wavelength. is peak point is the most easily identifiable characteristic of the nature of MgF 2 ceramic compared with that of other optical ceramics. is can be explained that, when operating the system in the high-temperature environment, the powder hydrolysis phenomenon appears. In other words, there is an interaction between MgF 2 powder and the steam in the high-temperature environment. e finer the powder MgF 2 is, the stronger the thermal hydrolysis reaction is. e cause is the powder MgF 2 makes it easy to absorb the high-temperature steam. e interaction between MgF 2 powder and the steam and the formulation of the substance are expressed as follows [10,11]: (i) e reaction of steam absorption is based on the following chemical equation: (ii) e process of forming hydroxy fluoride complex compounds is based on the following chemical equation: e formation of hydroxyl compounds is the cause of the peak point corresponding to the 2.8 µm wavelength. From Figure 4, we can observe that, for the specimen type I (the red line, in the open air), this peak point is relatively wide, spreading to the 3 µm wavelength, while for the hotpressed specimen in a vacuum environment (the blue line), this peak point is very narrow and it is only located at the 2.8 µm wavelength. e decrease in transmittance at the 3 µm wavelength is due to the steam getting into the particles of the product in the open air. So, using the hotpressing technique in the vacuum can thoroughly resolve this drop. e other effect of the hydrolysis of ceramic powder MgF 2 is that, in addition to hydroxyl compounds, it also produces bifluoride (HF). is is the cause of the peak point corresponding to 5 µm wavelength. is effect has a relatively strong effect on the application scope of the optical MgF 2 ceramic, because the 5 µm wavelength is in the best transmittable infrared region (3-6 µm) of the MgF 2 optical ceramic. On the other hand, regarding the decrease in transmittance at the 5 µm wavelength, we need to pay attention to the decomposition temperature of hydroxy fluoride as follows [10,11]: e effects of thermal hydrolysis are greatly reduced when operating in a vacuum environment due to the steam restriction. e peak points corresponding to the 2.8 µm and 5 µm wavelengths can be clearly seen on the characteristic transmittance curve of the hot-pressed specimen in a vacuum environment and is narrower and much shorter than those of the hot-pressed specimen in the open air.   Advances in Materials Science and Engineering Besides, we can see the peak point at the 6.7 µm wavelength. is decrease is due to the OH bending. is peak point of the red line (the hot-pressed specimen in the open air) is not only deeper than that of the blue line (the hot-pressed specimen in a vacuum environment), but also there is a larger wide spreading to 7 µm wavelength. e reason is that, when hotpressing the fluoride ceramic, there is a formation of carbonate impurities due to the interaction of the hydroxide group (OH) with CO 2 in the air by the following reactions [12][13][14]: A decrease at the 7 µm wavelength of a vacuum-pressed specimen is virtually absent due to the removal of CO 2 in the pressing environment. Table 3 shows the experimental results at the 4.5 µm wavelength of seventeen hot vacuum-pressed specimens to obtain the optimal technological parameters in a vacuum environment by the experimental planning approach. e results of the regression coefficients in equation (1) are listed in Table 4. e regression function coefficient is confirmed based on the Student standard; firstly, we need to calculate the reproductive variance S th . According to the results of the experimental data table, we can find the reproductive variance S th as shown in Table 5.
After some efforts including finding the variances of coefficients in the regression function S bi , identifying statistical tests, and comparing with the Student standard t (0.05; 2) � 4.3, we get the regression function coefficient values as listed in Table 6. en, we obtain the regression function as follows: where and.
e results of the regression function test using the Fisher standard are as follows: (i) Residual variance: (ii) e Fisher standard:   Advances in Materials Science and Engineering 5 We can see that F < F α , so the function is compatible. erefore, the regression function written in equation (11) is verified. Now, we use Maple 2016 application (Waterloo Maple Inc, Waterloo, Ontario, Canada) to find out 86.25% highest transmittance at the optimal technological parameters as follows: Table 4: e results of the regression coefficients.     We can see that the optimal technological parameters in a vacuum environment are not much different than those in the open air. In the case of operating in a vacuum environment, the pressure is higher. erefore, the residual pressure decreases and the air in the particles is removed. Consequently, the density and quality of the product increase ( Figure 5). It can be seen the dependency of the transmittance on the technological parameters when hot-pressing in a vacuum environment is similar to that in the open air. e effect of temperature and pressure on the quality of the hot-pressed product is greater than the effect of time. We can see that the curvature of the surface in Figure 6 is greater than that in Figures 7 and 8.

Conclusions
By operating the hot-pressing technique in a vacuum environment, the impact of moisture and other disturbances in the open air on the quality of hot-pressed MgF 2 ceramic is overcome. e transmittance at the infrared band 2-7 µm of the hot-pressed specimen in a vacuum environment is about 4-10% higher than that of the hot-pressed specimen in the open air. Moreover, the characteristic transmittance curve of the hot-pressed specimen in a vacuum environment is smoother, has fewer peak points, and decreases. We also find out the optimal technological parameters of the system when running in a vacuum environment are as follows: temperature T � 640.9°C, time t � 33.0 min, and pressure P � 231.4 MPa.

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Data Availability e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that they have no conflicts of interest.