The Effect of Hot Pressing Parameters on the

Tehran International Congress on Manufacturing Engineering (TICME2005) December 12-15, 2005, Tehran, Iran

The Effect of Hot Pressing Parameters on the Microstructure and Infrared Transparency of MgF2 Ceramics M. Nofar1, H. R. Madaah Hosseini2, H. A. Shivayee3 Corresponding Author Address: Department of Materials Science and Engineering, Sharif University of Technology, P.O. Box 11365-9466, Tehran, Iran Corresponding Author E-mail: [email protected]

Abstract In this work, the effect of most important parameters of hot pressing, including temperature, time and pressure on densification, microstructure and infrared transparency of MgF2 ceramics have been discussed. The results show that by increasing the temperature or the pressure, transparency in infrared wavelengths increases due to the rise in density. By increasing the hot pressing time, a continuous increase in density and infrared transparency occurs and the amount of porosity decreases. Cold pressing and preheating before the hot pressing have minor effects on the density and infrared transparency of MgF2. Keywords: Magnesium Fluoride, Infra-red windows, Hot pressing, Optical properties

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Introduction

Hot pressed magnesium fluoride has been used as dome materials for many devices having infrared sensors due to excellent transmittance in the infrared region of spectrum, high mechanical strength and thermal stability. However, transparency, thermal stability and high mechanical strength of these materials are influenced by imperfections such as pores and grain boundaries [1- 3]. Hot pressing which involves simultaneously pressing and heating a powder mass, is a widespread technique employed for the production of optical polycrystalline ceramics. Tetragonal MgF2 could be prepared by hot pressing in air, but the material would be transparent only if the pressure applies after the green powder has attained the hot pressing temperature [4- 6]. The losses of transmission are mainly due to the scattering by pore boundaries, grain boundaries, and surface roughness; absorption by small electronic energy gap and vibration between impurity- matrix bound state. Scattering by residual pores is the main reason for the attenuation of the intensity of the light on its passage through a slice of 1

- B. Sc. - Associate Prof. 3 - M. Sc. 2

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specimen. The grain scattering becomes significant when the porosity is extremely low. The surface contribution could be neglected after a careful polishing treatment [1, 2]. The major problem associated with the production of polycrystalline materials is the requirement for the virtual complete elimination of porosities or impurities which may segregate at grain boundaries and cause scattering zones [4]. Since the refraction index of MgF2 is near air, it could be a valuable material for fabrication of optical ceramics [1]. This study is focused on the dependence of microstructure and transparency in the range of infrared wavelengths with densification behavior of the powders.

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Experimental procedure

MgF2 powders with an average particle size of 10 micrometer were produced by Fast Mill method. The powders were hot pressed at 650, 750 and 800 °C under 200, 250 and 270 MPa pressure for 20, 30 and 40 minutes, respectively. The die material for hot pressing was GTD 111, super alloy. In some cases, 40 MPa pressure was applied for 10 minutes at room temperature before hot pressing. Before applying the hot pressure, preheating was applied at hot pressing temperature for 20- 30 minutes. The specimens for SEM studies were prepared after a fine polishing and etching by a H2SO4 and B2O3 solution with 1/10 ratio [7]. The densities of the samples were measured by Archimedes method with Sartorius CP 324 S instrument accordance with ISO 2738 standard. Transmittance in IR wavelengths region was measured by FTIR, Maston 100 model.

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Results and Discussion

High transparency in infrared wavelengths could be obtained by increasing the density and eliminating the porosities which are the main causes for scattering. Figures. 1 and 2 show the effect of temperature on densification of hot pressed MgF2 under different pressures and pressing times, respectively. By increasing the temperature or pressing time, relative density increases continuously. The maximum density has been obtained at 800 °C and 270 MPa. Figures 3 and 4 show the scanning electron micrographs of the specimens after hot pressing at 250 MPa for 30 min, but one in 650 °C and the other in 750 °C. It could be seen that the amount of pores and the average pore size are reduced by increasing the temperature. This result is in good agreement with the density trends in Figures. 1 and 2. Reduction in pore size and the amount of pores is due to a proper sintering during hot pressing. Figure 5 shows the effect of hot pressing time on the relative density. By increasing the pressing time, a significant increase in density is occurred. The linear trend of densification in Figure 5 could be explained by a general kinetic relationship for densification of the form:

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96

95

(P=250MPa)

94

(P=200MPa)

95 Relative Density(%)

Relative Density (%)

96

93 92 91 90

(t=40min) (t=30min)

94 93 92 91 90

89

89 600

88 600

650

700

750

800

850

650

700

750

800

850

Temperature,(°C)

Temperature, (° C)

Figure 1: Temperature dependency of relative density of the samples after hot pressing under 200 and 250 pressure and for 30 min hot pressing time.

Figure 2: Temperature dependency of relative density of the samples after hot pressing under 250 MPa pressure for 30 and 40 min pressing times. (1)

ρ = ρ * + KLn(t )

The density (ρ) is directly proportional to the logarithm of time (t), where K and ρ * are the constants. The value of ρ when t = 1, is equal to the initial density represented by ρ ° [8]. The SEM micrograph in Figures 6 and 7 show that by increasing the hot pressing time from 20 to 30 min, a considerable reduction in pore size is occurred. A similar trend for pressure could be seen in Figure8. By increasing the pressure, a linear increase in relative density has been occurred. It is well known that increasing the pressure reduces the porosities due to the localized plastic flow of the powders, giving a further modification to the densification rate equation [9]. The SEM macrographs in Figures. 9 and 10 show the important role of pressure in elimination of porosities and shifting toward full densities.

Figure 4: The scanning electron micrograph of a sample after hot pressing at 750° C under 250 MPa for 30 min.

Figure 3: The scanning electron micrograph of a sample after hot pressing in 650° C under 250 MPa for 30 min. 3


There are other lenient effective parameters in hot pressing process which cause fluctuation in density and so in IR region transparency. Two of these less effective but important factors are cold pressing pressure and preheating time before hot pressing. Effect of cold pressing pressure is shown in figure 11 which show that by increasing the cold pressing pressure, the relative density increases. The cold pressing provides more junctions between particles and better heat transfer in comparison to a spongy sample [10].

Relative Density, (%)

98 97

(P=250MPa)

96

(P=200MPa)

95 94 93 92 91 90 89 10

20

30

40

50

Time, (min)


Figure 5: Effect of pressing time on the relative density of the samples after hot pressing at 750 ° C under 200 and 250 MPa.

Relative Density(%)

98 97 96 95 94 93 92 175

195

215

235

255

275

295

Pressure, (MPa)


Figure 8: Effect of pressure on the relative density of the samples after hot pressing at 800° C for 30min.

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Figure 12 shows the relation between preheating time and relative density. It could be seen that increasing in time of preheating, reduces the density. Preheating of raw MgF2 is advantageous for homogenizing the temperature between green compact and mold, but increasing preheating time above a convenient limit causes some pore and grain growth which is malign for IR transmittance.

Figure 9: The scanning electron micrograp of a sample after hot pressing at 800° under 250 MPa for 30 min.

Figure 10: The scanning electron micrograp of a sample after hot pressing at 800° under 270 MPa for 30 min.

96

91.5

95

Relative density(%)

Relative Density(% )

92

91 90.5 90

94 93 92 91 90

89.5

89

0

10

20

30

40

50

0

30

60

90

120

Time, (min)

Pressure, (MPa)

Figure 22: Effect of preheating time on relative density of the samples after hot pressing at 750° C under 250 MPa for 30 min.

Figure 31: Effect of cold pressure on relative density of the samples after hot pressing at 650 ° C under 250 MPa for 30 min.

By considering the optimal range of the discussed factors, the size of porosities approach to a nanometer size, as shown in figure 13. Figures 14- 17 show the transparency in IR wavelengths region for MgF2 after hot pressing at different conditions. These results emphasize the SEM observations in this study. Figures.14 and 15 show that increasing the pressure of hot pressing makes a considerable increase in 5


transmittance. It could also be seen in Figures.15 and 16 that increasing the temperature makes an increase in transmittance. Figures 16 and 17 show the influence of hot pressing time on IR transparency, which is the same as temperature and pressure.

Figure 53: The scanning electro micrograph of sample after hot pressin at 800° C under 270 MPa for 30 min. Figure 44: Transparency in IR wavelengths for a sample after hot pressing at 750° C under 250 MPa for 30 min hot pressing time after 30 min preheating.

Figure 65: Transparency in IR wavelengths for a sample after hot pressing at 750° C under 270 MPa for 30 min hot pressing time after 30 min preheating. 6


Figure 87: Transparency in IR wavelengths for a sample after hot pressing at 800° C under 270 MPa for 40 min hot pressing time, after 30 min preheating.

Figure 76: Transparency in IR wavelengths for a sample after hot pressing at 800° C under 270 MPa for 30 min hot pressing time after 30 min preheating.

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Conclusion

By increasing the hot pressing pressure to about 270 MPa, temperature to 800 °C, and hot pressing time to 40 min, a relative density of 98.7 % and high transparency in IR wavelengths region were achieved. The cold pressing before hot pressing was advantageous on modification of density and transmittance, but increasing the preheating time before hot pressing has a negative role on densification and transparency of hot pressed MgF2.

References [1]

C. Chen-Shen, H. Min-Hsiung, Y. Sheng-Jenn; “Effect of grain growth of hotpressed magnesium fluoride on optical transmittance”, Japanese Journal of Applied Physics, V 30, N 3, Mar 1991, pp. 506-510.

[2]

C. Chen-Shen, H. Min-Hsiung, Y. Sheng-Jenn; “Effect of Calcinations and hotpressing temperature of magnesium fluoride powder on its optical property”, Japanese Journal of Applied Physics, V 29, N 12, December, 1991, pp. 2751-2754.

[3]

S. E. Hatch; “Emittance measurements on infrared windows exhibiting wavelength dependent diffuse transmittance”, Appl. Opt., No.5, Vol.1, 1962 Sep, pp. 595-601.

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[4]

J.A. Savage; “Infrared Optical Materials and Their Antireflection Coatings”, Adam Hillier, Bristol, Volume 32, Number 12, 1985, pp. 950-954.

[5]

W. Donald, P.W. McMillan; “Review Infra-red transmitting materials”, Department of physics, University of Warwick, Coventry, UK Journal of Materials Science 13 (1987) pp. 1151-1176.

[6]

H. M. Hsiung; “Texture effect of hot-pressed magnesium fluoride on optical transmittance”, Materials Chemistry and Physics, v 81, n 1, Jul20, 2003, pp. 27-32.

[7]

T. F. Yen, Y. H. Chang; “Diffusion bonding of MgF2 optical ceramics”, National Cheng Kung University, Materials Science and Engineering A Volume 147, Issue 1 , 30 October 1991, pp. 121-128.

[8]

R. M. German; “Sintering Theory and Practice”, John Wiley & Sons 1996, pp. 314368.

[9]

J. M. Vieira, R. J. Brook; “Kinetics of Hot Pressing: The Semi logarithmic Law”, Journal of American Ceramic Society, Vol. 67, No. 4, 1984, pp. 245-249.

[10] A. Ezis, J. A. Rubin; “Hot pressing”, ASM handbook vol. 7; powder metallurgy, 1978, pp. 501-521.

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