Introduction. MWIR Materials Summary

Transcription

1 Comparative study of advanced IR transmissive materials Comparative study of advanced IR transmissive materials J.A. Cox, D. Greenlaw, G. Terry, K. McHenry, and L. Fielder J.A. Cox, P. Greenlaw, G. Terry, K. McHenry, and L. Fielder Honeywell, Inc. PO Box 1361 Minneapolis.MN 554 Honeywell, Inc. PO Box 1361 Minneapolis,MN 554 Abstract Six advanced MWIR transmissive materials have been selected for a comprehensive investigation to measure and compare their optical and mechanical properties. The six materials are: sapphire; A1ON (5A1N*9A13); cubic zirconia (Zr02 *9.4 %mo Y3); spinel (MgO *A13) fabricated by three processes; lanthana -doped yttria (Y3 *9' %La3) fabricated by three processes; and yttria (Y3). The following experiments have been performed on samples of each material: thermal variation of transmittance thermal variation of fracture strength color center absorption A summary of the measured data is presented for each material. Introduction There are increasing demands for optical quality, IR transmissive materials which provide high durability to severe thermal, mechanical, and erosive environments. Recent technology development in this area has progressed the most for those materials relevant to the 3-5 micrometer atmospheric window and has focused primarily on ceramics. The recent books by Savagel and Musikant2 give an excellent account of the current technology status for many advanced materials. A thorough study completed by Musikant et al 3.4 at General Electric four years ago identified a large number of ceramic materials as potentially feasible for stressing applications. We have selected six materials (Table 1) from the GE study to characterize and test experimentally, and in this paper we present data from some of the measurements and experiments completed to date. Table 1. Abstract Six advanced MWIR transmissive materials have been selected for a comprehensive investigation to measure and compare their optical and mechanical properties. The six materials are: sapphire; AN (5AlN*9Al3); cubic zirconia (Zr02*9.4%mo 3); spinel (MgO*Al2C>3) fabricated by three processes; lanthana-doped yttria (Y2 3*^La2 3^ fabricated by three processes; and yttria ( 3). The following experiments have been performed on samples of each material: thermal variation of transmittance thermal variation of fracture strength color center absorption A summary of the measured data is presented for each material. Introduction There are increasing demands for optical quality, IR transmissive materials which provide high durability to severe thermal, mechanical, and erosive environments. Recent technology development in this area has progressed the most for those materials relevant to the 3-5 micrometer atmospheric window and has focused primarily on ceramics. The recent books by Savage* and Musikant^ give an excellent account of the current technology status for many advanced materials. A thorough study completed by Musikant et al ^»4 at General Electric four years ago identified a large number of ceramic materials as potentially feasible for stressing applications. We have selected six materials (Table 1) from the GE study to characterize and test experimentally, and in this paper we present data from some of the measurements and experiments completed to date. Table 1. MWIR Materials Summary MWIR Materials Summary Material Material Structure/ Process Structure/ Process Number of Samples Number of Samples V endor Vendor 1. A1 3 () 1. A13 () 0 Crystal 65 Crystal Systems ( "Hemlite ") 2. MgO-Al 3 (Spinel) 2. Mg0A13 Polycrystalline Raytheon Research (Spinel) (pressed) 3. Cubic zirconia (Zr iaotfY 2 03 ) Polycrystalline Coors Porcelain (not pressed) Polycrystalline Coors Porcelain (hot isostatic pressed) 3.Cubic zirconía Crystalline Ceres Corporation (2r02 9.4mo %Y3) (stabilized cubic) 4. Y *La3 ( Lanth ana-do pe d yttria) 4. Y3.9 %La3 Polycrystalline 23 GTE (Lanthana -doped ( "Standard yttria) pressed ") 4. a Y2 03 (yttria) Polycrystalline 7 GTE (Reduced OH process) Polycrystalline GTE (Toughened process) 4.a Y3 (yttria) Polycrystalline 5 Raytheon Research 5. 5A1N-9 A1 3 (AN) Crystal ("Hemlite") Poly crystalline (pressed) Poly crystal line (not pressed) Poly crystal line (hot isostatic pressed) Poly crystal line ("Standard pressed") Poly crystal line (Reduced OH process) Poly crystalline (Tougnened process) Poly crystal line Poly crystalline (pressed) 5. 5A1N9 A13 Polycrystalline Raytheon Research (AlON) (pressed) 65 Crystalline (stabilized cubic) Crystal Systems Raytheon Research Coors Porcelain Coors Porcelain Ceres Corporation GTE GTE GTE Raytheon Research Raytheon Research SPIE Vol. 683 Infrared and Optical Transmitting Materials (1986) / 49 SPIE Vol. 683 Infrared and Optical Transmitting Materials (1986) / 49 Downloaded From: on 02/18/16 Terms of Use:

2 When completed, our program will examine the thermal variation of optical (transmittance, absorption, reflectance, scatter) and mechanical (fracture strength, hardness) properties, thermal shock resistance, erosion resistance, and environmental susceptibility (humidity, radiation, etc.) of all six materials. In addition, the sensitivity of these properties to the fabrication process and related parameters (such as impurity concentration and type) will be explored. Currently, we have completed the first sequence of experiments to measure thermal variation of transmittance (and absorption) and fracture strength and to examine the absorption strength of color centers associated with specific impurities and lattice defects. These results are given here. When completed, our program will examine the thermal variation of optical (transmittance, absorption, reflectance, scatter) and mechanical (fracture strength, hardness) properties, thermal shock resistance, erosion resistance, and environmental susceptibility (humidity, radiation, etc.) of all six materials. In addition, the sensitivity of these properties to the fabrication process and related parameters (such as impurity concentration and type) will be explored. Currently, we have completed the first sequence of experiments to measure thermal variation of transmittance (and absorption) and fracture strength and to examine the absorption strength of color centers associated with specific impurities and lattice defects. These results are given here. Table 1 lists the materials by vendor and fabrication process which we are investigating. All samples are approximately 1 inch diameter and 0.1 inch thick. Both surfaces of every sample were polished to optical quality (scratch/dig not exceeding /). Table 1 lists the materials by vendor and fabrication process which we are investigating. All samples are approximately 1 inch diameter and 0.1 inch thick. Both surfaces of every sample were polished to optical quality (scratch /dig not exceeding /). Thermal variation of transmittance Thermal variation of transmittance Procedure Transmittance measurements as a function of temperature were done using Beckman 42 and 52 spectrophotometers equipped with a furnace cell. The two spectrophotometers cover the spectral range of 0.2 to micrometers. The furnace cell was specially built by Honeywell and was instrumented with thermocouples and a servo -system to maintain stable temperatures. Our standard procedure consisted of spectral scans from micrometers at temperatures of C, 2 C, and 0 C. Thermal variation of fracture strength The effect of temperature on the fracture strength was determined by means of four -point bend experiments. Samples measuring approximately cm x cm x (length determined from position in coupon) were diamond cut from the polished vendor -supplied coupons. The edges of the specimens were broken lightly using 0 -grit SiC paper to minimize the effect of edge- checking flaws produced during cutting of the specimens from the coupons. The measurements were made on samples in the "as- polished" condition; no effort was made to relieve surface stress that may have been induced during polishing. Samples were tested at room temperature (21 C), 2 C, and 0 C at a displacement rate of cm /min. The four -point bend fixture had an outer span of 1.9 cm and an inner span of 0.57 cm. The measurements were performed on an Instron TM testing machine. Elevated temperatures were obtained with a Kanthal wire -wound tube furnace adapted to the testing machine. A minimum of three samples per material were broken at each temperature. The samples were oriented during the fracture test such that one of the polished surfaces was the tensile surface. Color center absorption Procedure Transmittance measurements as a function of temperature were done using Beckman 42 and 52 spectrophotometers equipped with a furnace cell. The two spectrophotometers cover the spectral range of 0.2 to micrometers. The furnace cell was specially built by Honeywell and was instrumented with thermocouples and a servo-system to maintain stable temperatures. Our standard procedure consisted of spectral scans from micrometers at temperatures of C, 2 C, and 0 C. Thermal variation of fracture strength The effect of temperature on the fracture strength was determined by means of four-point bend experiments. Samples measuring approximately cm x cm x (length determined from position in coupon) were diamond cut from the polished vendor-supplied coupons. The edges of the specimens were broken lightly using 0-grit SiC paper to minimize the effect of edge-checking flaws produced during cutting of the specimens from the coupons. The measurements were made on samples in the "as-polished" condition; no effort was made to relieve surface stress that may have been induced during polishing. Samples were tested at room temperature (21 C), 2 C, and 0 C at a displacement rate of cm/min. The four-point bend fixture had an outer span of 1.9 cm and an inner span of 0.57 cm. The measurements were performed on an Instron TM testing machine. Elevated temperatures were obtained with a Kanthal wire-wound tube furnace adapted to the testing machine. A minimum of three samples per material were broken at each temperature. The samples were oriented during the fracture test such that one of the polished surfaces was the tensile surface. Color center absorption Color centers were generated by exposing the samples to gamma radiation. For our experiments a Co gammacell in the field flattened configuration was used. This arrangement uses lead attenuators to make a uniform deposition in the samples. At least one coupon for each material and material process was exposed to 0 krad(si). Coupons of sapphire, cubic zirconia, and AN were also exposed to 0 krad(si) and 00 krad(si). Color centers were generated by exposing the samples to gamma radiation. For our experiments a Co gammacell in the field flattened configuration was used. This arrangement uses lead attenuators to make a uniform deposition in the samples. At least one coupon for each material and material process was exposed to 0 krad(si). Coupons of sapphire, cubic zirconia, and AlON were also exposed to 0 krad(si) and 00 krad(si). Visual observations were made of the sample color immediately after exposure. Spectral transmittance scans from micrometers were made before and after irradiation. For one set of samples the spectral scans were started six days after irradiation. Otherwise, spectral scans were started 1 day after irradiation and repeated at 3, 5, 7, 11, and 15 days in order to determine long term bleaching coefficients. Visual observations were made of the sample color immediately after exposure. Spectral transmittance scans from micrometers were made before and after irradiation. For one set of samples the spectral scans were started six days after irradiation. Otherwise, spectral scans were started 1 day after irradiation and repeated at 3, 5, 7, 11, and 15 days in order to determine long term bleaching coefficients. Thermal variation of transmittance Thermal variation of transmittance Results Results Figure la -If shows the spectral infrared transmittance from 2.5 to 9 micrometers at temperatures of C, 2 C, and 0 C for AN, sapphire, spinel, cubic zirconia, yttria, and lanthana-doped yttria (standard process). In the case of spinel, nearly identical results were obtained for all three fabrication processes. Figure 2 shows by comparison with Figure If that there is also little variation in transmittance among the three fabrication processes of lanthana-doped yttria. The major difference lies in the presence of OH absorption in some of the standard and toughened materials. Figure la -1f shows the spectral infrared transmittance from 2.5 to 9 micrometers at temperatures of 0C, 2 C, and 0 C for AlON, sapphire, spinel, cubic zirconia, yttria, and lanthana -doped yttria (standard process). In the case of spinel, nearly identical results were obtained for all three fabrication processes. Figure 2 shows by comparison with Figure if that there is also little variation in transmittance among the three fabrication processes of lanthana -doped yttria. The major difference lies in the presence of OH absorption in some of the standard and toughened materials. It can be seen that transmittance always decreases with increasing temperature and that the effect is most pronounced near the long wavelength cutoff. It is well known that this behavior is due to increased lattice absorption arising from both enhanced anharmonic interaction coefficients and anharmonically induced broadening in the phonon density of states**. In those regions near the cutoff where the multiphonon absorption is greatest, we can calculate the optical absorption coefficients from the measured transmittance curves using the following expression It can be seen that transmittance always decreases with increasing temperature and that the effect is most pronounced near the long wavelength cutoff. It is well known that this behavior is due to increased lattice absorption arising from both enhanced anharmonic interaction coefficients and anharmonically induced broadening in the phonon density of states8. In those regions near the cutoff where the multiphonon absorption is greatest, we can calculate the optical absorption coefficients from the measured transmittance curves using the following expression 1/2 at = -In {0.5 [ -((l-r)/r) 2 /T + [((l-r)/r) 4 / T2 + 4/r 2 ]} 1/2 at = -ln f0.5 E -((1-r)/r)2/T + [((1-r)/r)4 / T2 + 4/r2 ]} / SP /E Vol. 683 Infrared and Optical Transmitting Materials (1986) / SPIE Vol. 683 Infrared and Optical Transmitting Materials (1986) Downloaded From: on 02/18/16 Terms of Use:

3 where a = absorption coefficient r = [(n-l)/(n+l)]2 n = refractive index t = sample thickness T = transmittance. where a = absorption coefficient r = [(n- 1)/(n +1)]2 n = refractive index t = sample thickness T = transmittance. Table 2 lists the absorption coefficients calculated by this method at several wavelengths within the cutoff region. For all materials except yttria, the absorption increases by a factor between 2 and 3. For the yttria samples, the increase is between a factor of 3 and 6. The values in parentheses in the part of Table 2 covering sapphire were reported by Musikant et al^. At room temperature, our value for the absorption coefficient (0.8 cm-1) at 5 micrometers is greater than that found by both Musikant et al 3 (0.6 cm-1) and Gryvnak and Burch^ (0.7 cm-1) but less than that found by Oppenheim and Even' (0.9 cm-1). Table 2 lists the absorption coeffieients calculated by this method at several wavelengths within the cutoff region. For all materials except yttria, the absorption increases by a factor between 2 and 3. For the yttria samples, the increase is between a factor of 3 and 6. The values in parentheses in the part of Table 2 covering sapphire were reported by Musikant et al3. At room temperature, our value for the absorption coefficient (0.8 cm -1) at 5 micrometers is greater than that found by both Musikant et al 3 (0.6 cm -1) and Gryvnak and Burch6 (0.7 cm -1) but less than that found by Oppenheim and Even? (0.9 cm-1). Table 2. Absorption Coefficients (cm" 1 ) of ALON,, Spinel, Cubic,, and Lanthana-Doped at C, 2 C, and 0 C (The data for sapphire in parentheses are from Ref. 3) Table 2. Absorption Coefficients (cm-1) of ALON,, Spinel, Cubic,, and Lanthana -Doped at C, 2 C, and 0 C (The data for sapphire in parentheses are from Ref. 3) Material Material Temp (C) Wavelength (µm) AN AlON (0.6) (1.4) (2.1) 5.1 Spinel Spinel Cubic Cubic Lanthana doped (std) Lanthana doped (tghn'd) Lanthanadoped (std) Lanthanadoped (tghn'd) Lanthanadoped (OH red.) Temp (C) Wavelength ( um) Lanthana doped (OH red.) (0.6) 1.3(1.4) 2.1(2.1) Thermal variation of fracture strength Thermal variation of fracture strength The fracture strengths of the various materials as a function of temperature are given in Table 3. The fracture surface of all specimens was microscopically examined to determine fracture origins. The data were eliminated from tests where fracture was assessed to originate at corners from edge-checking flaws. In all tests, the fracture process was brittle in nature as determined from the load-time traces. The fracture strengths of the various materials as a function of temperature are given in Table 3. The fracture surface of all specimens was microscopically examined to determine fracture origins. The data were eliminated from tests where fracture was assessed to originate at corners from edge- checking flaws. In all tests, the fracture process was brittle in nature as determined from the load -time traces. The results demonstrate a marked decrease in fracture strength with increasing temperature for all materials except zirconia and lanthana-doped yttria. This phenomenon is a result of environmentally and thermally assisted slow crack growth due to the presence of moisture in the test environment. Because of the relatively low strain rate used, the fracture strengths indicated are lower and the effect of temperature on the fracture strengths is more pronounced than would be the case had a higher strain rate been used. The results demonstrate a marked decrease in fracture strength with increasing temperature for all materials except zirconia and lanthana -doped yttria. This phenomenon is a result of environmentally and thermally assisted slow crack growth due to the presence of moisture in the test environment. Because of the relatively low strain rate used, the fracture strengths indicated are lower and the effect of temperature on the fracture strengths is more pronounced than would be the case had a higher strain rate been used. SPIE Vol. 683 Infrared and Optical Transmitting Materials (1986) / 51 SPIE Vol. 683 Infrared and Optical Transmitting Materials (1986) / 51 Downloaded From: on 02/18/16 Terms of Use:

4 Table 3. Fracture Strength (MPa) of AN,, Spinel, Cubic, and Lanthana-Doped at 21 C, 2 C, and 0 C Table 3. Fracture Strength (MPa) of AlON,, Spinel, Cubic, and Lanthana -Doped at 21 C, 2 C, and 0 C Material Material Average (Std. Dev.) Fracture Strength (MPa) Average (Std. Dev.) Fracture Strength (MPa) 21 C 2 C 0 C 21 C 2 C 0 C AN AlON 6 (52) 167 (19) 143 (13) 2 () 211 () 163 (32) Spinel (Coors) Spinel 71 (22) 76 () 65 (16) (Coors) Spinel (Raytheon) Spinel 166 (8) 153 (9) 139 (29) (Raytheon) Cubic 6 (52) 2 () 71 (22) 166 (8) 111 (29) 167 (19) 211 () 76 () 153 (9) 96 () 143 (13) 163 (32) 65 (16) 139 (29) 115 (28) Cubic 111 (29) 96 () 115 (28) Lanthanadoped Lanthanadoped 68 (12) 62 (2) 68 (11) 68 (12) 62 (2) 68 (11) exhibited the highest fracture strength at each temperature but also showed the greatest sensitivity to temperature ( i.e., its loss in strength per degree increase in temperature was largest). The results for the spinel samples are listed separately for the two vendors because of the marked difference in fracture strengths and behavioral characteristics. The Coors samples exhibited a much coarser substructure as shown in Figure 3 and typically did not demonstrate catastrophic fracture. Load-time traces for the Coors samples revealed a gradual decay in the load as a function of time. The Raytheon samples exhibited a more characteristic rapid load decrease upon fracture and a finer substructure. In general, the fracture strengths of all materials seemed to correlate well with the degree of substructure development. exhibited the highest fracture strength at each temperature but also showed the greatest sensitivity to temperature ( i.e., its loss in strength per degree increase in temperature was largest). The results for the spinel samples are listed separately for the two vendors because of the marked difference in fracture strengths and behavioral characteristics. The Coors samples exhibited a much coarser substructure as shown in Figure 3 and typically did not demonstrate catastrophic fracture. Load -time traces for the Coors samples revealed a gradual decay in the load as a function of time. The Raytheon samples exhibited a more characteristic rapid load decrease upon fracture and a finer substructure. In general, the fracture strengths of all materials seemed to correlate well with the degree of substructure development. KoenigS has performed similar measurements on both sapphire and Coors spinel. His sapphire samples were from the same vendor but did not match ours in both crystal orientation and surface polish. It is possible to extrapolate his results, however, and on doing so we estimate from his data that the fracture strength of sapphire ( degree orientation, / finish) is 286 MPa at 21 C and 137 MPa at 93 C. These are in good agreement with our measurements. For Coors spinel, Koenig measured the fracture strength to be 93 MPa at 21 C and MPa at 538 C, which agree well with our measurements at the high temperature but not at room temperature. Koenig5 has performed similar measurements on both sapphire and Coors spinel. His sapphire samples were from the same vendor but did not match ours in both crystal orientation and surface polish. It is possible to extrapolate his results, however, and on doing so we estimate from his data that the fracture strength of sapphire ( degree orientation, / finish) is 286 MPa at 21 C and 137 MPa at 93 C. These are in good agreement with our measurements. For Coors spinel, Koenig measured the fracture strength to be 93 MPa at 21 C and MPa at 538 C, which agree well with our measurements at the high temperature but not at room temperature. Color center absorption Color center absorption Table 4 lists the exposure levels used to induce defect absorption in the materials and the color of the sample immediately after exposure. Note that the color represents the transmitted part of the visible spectrum, and thus the absorbed part of the spectrum is the complement of the observed color. As is well known, sapphire exhibits very little susceptibility to induced absorption, a fact attributed to the low impurity content in this crystalline material. Table 4 lists the exposure levels used to induce defect absorption in the materials and the color of the sample immediately after exposure. Note that the color represents the transmitted part of the visible spectrum, and thus the absorbed part of the spectrum is the complement of the observed color. As is well known, sapphire exhibits very little susceptibility to induced absorption, a fact attributed to the low impurity content in this crystalline material. The remaining materials showed varying degrees of induced absorption. In AN and cubic zirconia, the absorption increases with exposure level, but they show little variation among samples at the same exposure level. In spinel and yttria, however, we found wide variations in absorption among the samples exposed to the same total dose. The remaining materials showed varying degrees of induced absorption. In AlON and cubic zirconia, the absorption increases with exposure level, but they show little variation among samples at the same exposure level. In spinel and yttria, however, we found wide variations in absorption among the samples exposed to the same total dose. Figure 4a-4f illustrates typical spectral transmittance data in the visible region for AN, Raytheon spinel, Coors HP spinel, Coors HIP spinel, cubic zirconia, and yttria. In each plot, the top-most curve is the pre-irradiated sample, and then are shown the curves (from top to bottom) at 15, 7, 5, and 1 days after irradiation. It is seen that AN and the two Coors spinel samples show both the greatest amount of relative absorption initially and the largest amount of bleaching subsequently. In every case, the rate of bleaching is decreasing such that the transmittance approaches a limit below the initial value. Figure 4a -4f illustrates typical spectral transmittance data in the visible region for AlON, Raytheon spinel, Coors HP spinel, Coors HIP spinel, cubic zirconia, and yttria. In each plot, the top -most curve is the pre -irradiated sample, and then are shown the curves (from top to bottom) at 15, 7, 5, and 1 days after irradiation. It is seen that AlON and the two Coors spinel samples show both the greatest amount of relative absorption initially and the largest amount of bleaching subsequently. In every case, the rate of bleaching is decreasing such that the transmittance approaches a limit below the initial value. Color centers are formed by two basic mechanisms: the creation of lattice defects due to radiation induced damage and the trapping of ionized electrons at pre-existing defects. The induced absorption coefficient is proportional to the color center density. Annealing of the lattice damage will reduce the number of color centers formed by the first process; only bleaching will reduce the number of color centers formed by the second process. Assuming that the intrinsic and induced defects are the same for samples of the same fabrication process, the variation of color center density is attributed to variation in impurity concentration. This hypothesis is currently being tested by experiments on samples of varying impurity concentrations. The increase of annealing and bleaching rates at elevated temperatures will also be measured. Color centers are formed by two basic mechanisms: the creation of lattice defects due to radiation induced damage and the trapping of ionized electrons at pre -existing defects. The induced absorption coefficient is proportional to the color center density. Annealing of the lattice damage will reduce the number of color centers formed by the first process; only bleaching will reduce the number of color centers formed by the second process. Assuming that the intrinsic and induced defects are the same for samples of the same fabrication process, the variation of color center density is attributed to variation in impurity concentration. This hypothesis is currently being tested by experiments on samples of varying impurity concentrations. The increase of annealing and bleaching rates at elevated temperatures will also be measured. 52 / SPIE Vol. 683 Infrared and Optical Transmitting Materials (1986) 52 / SPIE Vol. 683 Infrared and Optical Transmitting Materials (1986) Downloaded From: on 02/18/16 Terms of Use:

5 Table 4. Gammacell Test Matrix with Record of Discoloration after Exposure Table 4. Gammacell Test Matrix with Record of Discoloration after Exposure Material Material Sample # Total Dose Visible Change S1 0 krd(si) no change S2 0 krd(si) no change S3 0 krd(si) no change S4 0 krd(si) no change S5 00 krd(si) no change S6 00 krd(si) no change Spinel STD SL 2 0 krd(si) 1. brown Spinel STD SL 3 0 krd(si) 1. brown Spinel HP SL 21 0 krd(si) brown Spinel HP SL 22 0 krd(si) brown Spinel HIP SL 41 0 krd(si) dark brown Spinel HIP SL 42 0 krd(si) dark brown AN AN AN AN AN ÁN AL 05 0 krd(si) purple AlON AL 19 0 krd(si) purple AlON AL 27 0 krd(si) deep purple AlON AL 38 0 krd(si) deep purple AlON AL krd(si) deep purple CZ 8 0 krd(si) light yellow CZ 35 0 krd(si) light yellow CZ 36 0 krd(si) yellow CZ 45 0 krd(si) yellow CZ krd(si) deep yellow PLN Sample # Total Dose SI S2 S3 S4 S5 S6 Spinel STD Spinel STD SL SL 2 3 Spinel HP Spinel HP Spinel HIP Spinel HIP SL 21 SL 22 SL 41 SL 42 AL 05 AL 19 AL 27 AL 38 AL 56 CZ 8 CZ 35 CZ 36 CZ 45 CZ 46 Y 21-9 PLN Y krd(si) orange STD LY STD LY 31-8 OH LY OH LY TGH LY 39-2 TGH LY krd(si) 0 krd(si) 0 krd(si) 0 krd(si) 00 krd(si) 00 krd(si) 0 krd(si) 0 krd(si) 0 krd(si) 0 krd(si) 0 krd(si) 0 krd(si) 0 krd(si) 0 krd(si) 0 krd(si) 0 krd(si) 00 krd(si) 0 krd(si) 0 krd(si) 0 krd(si) 0 krd(si) 00 krd(si) 0 krd(si) 0 krd(si) 0 krd(si) 0 krd(si) 0 krd(si) 0 krd(si) 0 krd(si) Visible Change no change no change no change no change no change no change 1. brown 1. brown brown brown dark brown dark brown purple purple deep purple deep purple deep purple light yellow light yellow yellow yellow deep yellow orange dark brown clear light brown gray clear clear STD LY krd(si) dark brown STD LY krd(si) clear OH LY krd(si) light brown OH LY krd(si) gray TGH LY krd(si) clear TGH LY krd(si) clear Figure 5a-5d shows the visible transmittance data for 4 lanthana-doped yttria samples before and after irradiation. Note the dramatic difference in absorption between the one standard process sample (LY12-11) and the other three samples made from all three processes. Figure 6a-6d presents the infrared transmittance data for the same 4 samples. Most evident in Figure 6 is the much stronger OH absorption band in sample LY The vendor of this material (GTE) has informed us that the OH radical tends to be pinned to impurities^, and thus the strength of the OH absorption tends to correlate with the impurity concentration. On this basis, sample LY12-11 should be richer in impurities, and the amount of induced absorption and subsequent bleaching should be correspondingly greater. A spark source mass spectrographic analysis of impurity concentration was performed by GTE^ on the powder batches prior to making the samples. We found on examining GTE's data that sample LY12-11 was distinguished by the presence of 1 PPM calcium impurity compared to 7.5- PPM for the other irradiated LY samples. Figure 5a -5d shows the visible transmittance data for 4 lanthana -doped yttria samples before and after irradiation. Note the dramatic difference in absorption between the one standard process sample (LY12-11) and the other three samples made from all three processes. Figure 6a -6d presents the infrared transmittance data for the same 4 samples. Most evident in Figure 6 is the much stronger OH absorption band in sample LY The vendor of this material (GTE) has informed us that the OH radical tends to be pinned to impurities9, and thus the strength of the OH absorption tends to correlate with the impurity concentration. On this basis, sample LY12-11 should be richer in impurities, and the amount of induced absorption and subsequent bleaching should be correspondingly greater. A spark source mass spectrographic analysis of impurity concentration was performed by GTE9 on the powder batches prior to making the samples. We found on examining GTE's data that sample LY12-11 was distinguished by the presence of 1 PPM calcium impurity compared to PPM for the other irradiated LY samples. Summary Summary There are clear differences in the state of development of the six materials we have examined. For sapphire, the fabrication process and material quality control are relatively mature and understood. Similarly, for cubic zirconia the skull melting process is well established, but previously the demand has been for gemstones rather than optical elements, leading to different specifications for impurity control. The remaining materials, AN, spinel, yttria, and lanthana-doped yttria, are in the early developmental stage, and further improvements are expected in both the optical and mechanical properties. For these reasons, it is more appropiate at this point to compare the six materials in a relative sense. There are clear differences in the state of development of the six materials we have examined. For sapphire, the fabrication process and material quality control are relatively mature and understood. Similarly, for cubic zirconia the skull melting process is well established, but previously the demand has been for gemstones rather than optical elements, leading to different specifications for impurity control. The remaining materials, AlON, spinel, yttria, and lanthana -doped yttria, are in the early developmental stage, and further improvements are expected in both the optical and mechanical properties. For these reasons, it is more appropiate at this point to compare the six materials in a relative sense. AN, sapphire, and spinel provide good transmittance out to approximately 5 micrometers. All three materials have nearly the same refractive index and thus similar peak transmittances. is the benchmark material in terms of strength and low susceptibility to impurity absorption. However, being noncubic it exhibits scatter associated with birefringence. Furthermore, although it has the greatest strength of all materials tested, it also shows the greatest relative strength loss (35%) with temperature. AN appears to be a very promising material in terms of strength, both in its magnitude and in its ability to maintain strength with temperature. Impurity absorption does not appear to be a significant problem and can probably be controlled. AN does have the shortest cut-off wavelength, however. Spinel offers a longer cut-off wavelength at the price of somewhat reduced strength. Spinel loses only 16% of its strength from 21 C to 0 C, and thus it maintains strength better than either sapphire or AN. On the basis of our color center absorption measurements, it appears that significant variations in impurity concentrations can occur, and thus more control is needed. AlON, sapphire, and spinel provide good transmittance out to approximately 5 micrometers. All three materials have nearly the same refractive index and thus similar peak transmittances. is the benchmark material in terms of strength and low susceptibility to impurity absorption. However, being noncubic it exhibits scatter associated with birefringence. Furthermore, although it has the greatest strength of all materials tested, it also shows the greatest relative strength loss (35 %) with temperature. AlON appears to be a very promising material in terms of strength, both in its magnitude and in its ability to maintain strength with temperature. Impurity absorption does not appear to be a significant problem and can probably be controlled. AlON does have the shortest cut -off wavelength, however. Spinel offers a longer cut -off wavelength at the price of somewhat reduced strength. Spinel loses only 16% of its strength from 21 C to 0 C, and thus it maintains strength better than either sapphire or AlON. On the basis of our color center absorption measurements, it appears that significant variations in impurity concentrations can occur, and thus more control is needed. SPIE Vol 683 Infrared and Optical Transmitting Materials (1986) / 53 SPIE Vol. 683 Infrared and Optical Transmitting Materials (1986) / 53 Downloaded From: on 02/18/16 Terms of Use:

6 Cubic zirconia and the yttria materials transmit well to 6 micrometers. Both have higher refractive indices (around 2) and thus have lower peak transmittances. is about % stronger than yttria, and both materials hold their strength over temperature. As evidenced by color center absorption, impurity control is not a problem with zirconia, but it is a significant issue with lanthana-doped yttria. We have seen large variations in color center absorption among samples made with the same fabrication process but with different powder lots. The problem is particularly significant since this is the one case where impurities appear to affect transmittance in the infrared; namely, we see a direct correspondence between color center absorption in the visible and OH absorption in the infrared. Some impurities thus seem to play a role in pinning the OH radical in the lattice. More work is needed to identify and control the specific impurities responsible. Cubic zirconia and the yttria materials transmit well to 6 micrometers. Both have higher refractive indices (around 2) and thus have lower peak transmittances. is about % stronger than yttria, and both materials hold their strength over temperature. As evidenced by color center absorption, impurity control is not a problem with zirconia, but it is a significant issue with lanthana -doped yttria. We have seen large variations in color center absorption among samples made with the same fabrication process but with different powder lots. The problem is particularly significant since this is the one case where impurities appear to affect transmittance in the infrared; namely, we see a direct correspondence between color center absorption in the visible and OH absorption in the infrared. Some impurities thus seem to play a role in pinning the OH radical in the lattice. More work is needed to identify and control the specific impurities responsible. For those materials where different fabrication processes have been examined, we find little variation in the transmittance with temperature. However, as the data for spinel demonstrates, the fabrication process can dramatically affect the strength of the material, presumably through the control of grain size and structure. For those materials where different fabrication processes have been examined, we find little variation in the transmittance with temperature. However, as the data for spinel demonstrates, the fabrication process can dramatically affect the strength of the material, presumably through the control of grain size and structure. Acknowledgements Acknowledgements This work was funded by the U.S. Army Strategic Defense command under Contract No. DASG-85-C This work was funded by the U.S. Army Strategic Defense command under Contract No. DASG -85 -C Special thanks are due to Mr. Merle Gray of the Honeywell Systems & Research Center for his invaluable assistance in the laboratory. Special thanks are due to Mr. Merle Gray of the Honeywell Systems & Research Center for his invaluable assistance in the laboratory. We have benefitted from many technical discussions with Mr. C. E. Martin, Dr. C. E. Patty, Jr., and Mr. J. A. Wells. We have benefitted from many technical discussions with Mr. C. E. Martin, Dr. C. E. Patty, Jr., and Mr. J. A. Wells. References References 1. Savage, J. A., Infrared Optical Materials and their Antireflection Coatings, Adam Hilger Ltd Musikant, S., Optical Materials - An Introduction to Selection and Application, Marcel Dekker, Inc Musikant, S., et al., "Advanced Optical Ceramics, Phase II," AD-B0545, Annual Report for period 1 June May 19, ONR Contract No. N C-0466, 31 August Musikant, S., et al., "Advanced Optical Ceramics, Phase III," AD-B0645, Final Report for period 1 August January 1982, ONR Contract No. N C February Koenig, J. R., "Thermostructural Evaluation of Four Infrared Seeker Dome Materials," AD-B0936, Final Report for period , NWC Contract No. N5-83-C-0031, April Gryvnak, D. A. and Burch, D. E., JOSA, Vol 55, pp , Oppenheim, U. P. and Even, U., JOSA, Vol 52, pp , Bendow, B., Lipson, H. G., and Yukon, S. P., Phys. Rev. B, Vol 16, pp , Dr. W. H. Rhodes, GTE, private communication. 1. Savage, J. A., Infrared Optical Materials and their Antireflection Coatings, Adam Hilger Ltd Musikant, S., Optical Materials - An Introduction to Selection and Application, Marcel Dekker, Inc Musikant, S., et al., "Advanced Optical Ceramics, Phase II," AD- B0545, Annual Report for period 1 June May 19, ONR Contract No. N C -0466, 31 August Musikant, S., et al., "Advanced Optical Ceramics, Phase III," AD- B0645, Final Report for period 1 August January 1982, ONR Contract No. N C -0964, 28 February Koenig, J. R., "Thermostructural Evaluation of Four Infrared Seeker Dome Materials," AD- B0936, Final Report for period , NWC Contract No. N5-83 -C -0031, April Gryvnak, D. A. and Burch, D. E., JOSA, Vol 55, pp , Oppenheim, U. P. and Even, U., JOSA, Vol 52, pp , Bendow, B., Lipson, H. G., and Yukon, S. P., Phys. Rev. B, Vol 16, pp , Dr. W. H. Rhodes, GTE, private communication. Z o gl F z ce ~ J o= o_ N C ALON (2.7MM) 456 WRVELENGTH (microns) Figure la. AN. Figure la. AlON. Figure 1. Thermal variation in transmittance for the candidate materials at C, 2 C, and 0 C. Figure / SPIE Vol. 683 Infrared and Optical Transmitting Materials (1986) 54 / SPIE Vol. 683 Infrared and Optical Transmitting Materials (1986) Thermal variation in transmittance for the candidate materials at C, 2 C, and 0 C. Downloaded From: on 02/18/16 Terms of Use:

7 I 1 I I I I C 0 C C SAPPHIRE (2.8nn) WAVELENGTH WRVELENGTH (microns) 8 Figure Ib.. Figure lb.. 3 \ \ \ C 0 C---- \` \\ \ WRVELENGTH (microns) Figure Ic. Spinel. Figure lc. Spinel. 1 SPINEL (3.2nr4) - - E- r "1 -' BO, C Z o: ^~ CK UJ o. en CUBIC ZIRCONIA (2.0MM) CUBIC ZIRCONIA (2.0m) 456? WRVELENGTH (microns) Figure Id. Cubic zirconia. Figure ld. Cubic zirconia. Figure 1. Thermal variation in transmittance for the candidate materials at C, 2 C, and 0 C (continued). Figure 1. Thermal variation in transmittance for the candidate materials at C, 2 C, and 0 C (continued). SPIE Vol. 683 Infrared and Optical Transmitting Materials (1986) / 55 SPIE Vol. 683 Infrared and Optical Transmitting Materials (1986) / 55 Downloaded From: on 02/18/16 Terms of Use:

8 I, s T Figure le.. ~h \ti \, ti 0 C --"! 2 C 1 C to Y3 9% LA 2 03 STANDARD (3.2) L Figure lf. L_ 5 6 WHVELENGTH (microns) Figure If. Lanthana-doped, standard. Lanthana -doped, standard. Figure 1. Thermal variation in transmittance for the candidate materials at C, 2 C, and 0 C (concluded). Figure 1. Thermal variation in transmittance for the candidate materials at C, 2 C, and 0 C (concluded). se \ 5 6 HRVELENGTH (microns) Figure 2. Figure 2a. 56 / SPIE Vol. 683 Infrared and Optical Transmitting Materials (1986) Figure 2a. Lanthana-doped, toughened. Lanthana -doped, toughened. Figure 2. Thermal variation in transmittance for Lanthana-doped fabricated by two separate processes. 56 / SPIE Vol. 683 Infrared and Optical Transmitting Materials (1986) Thermal variation in transmittance for Lanthana -doped fabricated by two separate processes. Downloaded From: on 02/18/16 Terms of Use:

9 1 `1 2 C C 0 C-; Y2 03 Y3 9% LA3 U3 (OH(OH" REMOVED) REMOVED) 9% \\' <3,2wi) (3.2mm). I I I WflVELENGTH (microns) WRVELENGTH (microns) Figure 2b. Figure 2b. Figure 2. Figure 2. Figure Figure 3a. 3a. Lanthana-doped removed. Lanthana -doped,, OH~ OH- removed. Thermal variation Thermal -doped variation in in transmittance transmittance for for Lanthana Lanthana-doped fabricated by by two two separate separate processes processes (concluded). (concluded). fabricated Sample -39 (Coors (Coors HIP; HIP; 0X). Sample SL SL-39 0X). Figure Figure 33. Figure 3b. Figure 3b. Sample SL-17 0X), Sample SL -17 (Raytheon; (Raytheon; 0X). Photographs of grain structure structure differences Photographs of grain differences between between Coors Coors and and Raytheon spinal Raytheon spinel samples. samples. Photographs Photographs taken taken after after fracture fracture of of the the specimens. specimens (A) (A) AION AN Sample AL-05 Sample AL krad krad (Si) (Si) 0, I, 5, 7, 7, 15 0, 1, 5, 15 days days , HRVELENGTH (nanometers) WAVELENGTH (nanometers) Figure 4a. Figure 4a. Figure 4. Figure 4. MON. AN. Transmittance variations variations caused caused by by activated activated defects. defects. Transmittance SP /EVoL Vol Infrared Infraredand andoptical OpticalTransmitting Transmitting Materials11986) (1986)/ / 57 SPIE Materials 57 Downloaded From: on 02/18/16 Terms of Use:

10 9e se «5B 5e (B) Spinel (Raytheon) Spinel (Raytheon) Sample SL SL-02 0 krad (Si) 0. 1, 5, 7, 15 days WAVELENGTH HflVELENGTH (nanometers) ( Figure 4b. Spinel (Raytheon). SB Be ás. N Ñ SB 48 g 38 N (C) Spinel (Coors Hot Pressed) Sample SL SL-21 0 Krad krad (Si) WAVELENGTH HflVELENGTH (nanometers) (nanometers) Figure 4c. Ac. Spinel (Coors HP). 98 e 88 BB %,-_- y I- 38 N (D) Spinel (Coors HIP) Sample SL SL-41 0 krad (Si) I, I I I 1 I I I I HflVELENGTH WAVELENGTH (nanometers) I 0 Figure 4d. Spinel (Coors HIP). Figure 4. Transmittance variations caused by activated defects (continued). (continued), 58 / SP / SPIE /E Vol. 683 Infrared and Optical Transmitting Materials (1986) Downloaded From: on 02/18/16 Terms of Use:

11 1 0.1., Figure 4e. (E) Cubic Sample CZ-08 0 krad (Si) (E) Cubic Sample CZ krad (Si). I.. I.. I I HflVELENGTH (nanometers) WAVELENGTH (nanometers) Figure 4e. Cubic zirconia. Cubic zirconia. (F) (Raytheon) Sample Y krad (Si) 0/1, 5, 7, 15 days (F) (Raytheon) Sample Y krad (Si) 0,'1, 5, 7, 15 days HflVELENGTH (nanometer*) WAVELENGTH (nanometers) Figure 4f.. Figure 4f Figure 4. Transmittance variations caused by activated defects (concluded), Figure 4. Transmittance variations caused by activated defects (concluded). Be (A) T- I (A) Lanthana-doped (std) Sample LY krad (Si) Lanthana -doped (std) Sample LY krad (Si) 48 y 1, , WflVELENGTH (nanometers) WAVELENGTH ( nanometers) Figure 5a. Sample LY Figure 5a. Sample LY Figure 5. Transmittance variations in Lanthana-doped caused by activated defects, Figure 5. Transmittance variations in Lanthana -doped caused by activated defects. SPIE Vol. 683 Infrared and Optical Transmitting Materials (1986) / 59 SPIE Vol 683 Infrared and Optical Transmitting Materials 11986)/ 59 Downloaded From: on 02/18/16 Terms of Use:

12 1 I 1 I I 1 7 I , I I F C F- ó Ñ - É r t- 2 o! ~ - 2CC aw. N E- E _ C (B) Lanthana-doped (std) Sample LY krad (Si) (B) Lanthana -doped (std) Sample LY krad (Si) WRVELENGTH (nanometers) WAVELENGTH (nanometers) 1 L_1 Lj 0 Figure 5b. Sample LY Figure 5b. Sample LY J J (C) Lanthana-doped (OH red.) Sample LY krad (Si) (C) Lanthana -doped (OH red.) Sample LY krad (Si) WAVELENGTH (nanometers) WRVELENGTH (nanometers) I 0 Figure 5c. Sample LY Figure 5c. Sample LY T - (D) Lanthana-doped (tghn'd) Sample LY krad (Si) (D) Lanthana -doped (tghn'd) Sample LY krad (Si) 0, 1, 5, days WAVELENGTH (nanometers) WRVELENGTH (nanometers) 0 Figure 5d. Sample LY39-2. Figure 5d. Sample LY39-2. Figure 5. Transmittance variations in Lanthana-doped caused by activated defects (concluded). Figure 5. / SPIE Vol. 683 Infrared and Optical Transmitting Materials (1986) / SPIE Vol. 683 Infrared and Optical Transmitting Materials (1986) Transmittance variations in Lanthana -doped caused by activated defects (concluded). Downloaded From: on 02/18/16 Terms of Use:

13 B0 z o I 1 ^ cr &. H LJ Q. <" (A) Lanthana-doped (std) (A) Lanthana-doped (std) Sample LY12-11 Sample LY krad(si) 0 krad(si) 0, 1, 11, 15 days 0, 1, 11, 15 days WHVELENGTH (microns) Figure 6a. Sample LY Figure 6a. Sample LY K (B) Lanthana-doped (std) (B) Lanthana -doped (std) Sample LY31-8 Sample LY krad(si) 0 krad(si) 0, 11, 15 days 0, 11, 15 days 456; 5 6 WflVELENGTH (microns) Figure 6b. Sample LY31-8. Figure 6b. Sample LY31-8. T l S0 - Z?0 O I I - ^ <r (C) Lanthana -doped (OH red.) Sample LY Krad (Si) 0, 1, 11, 15 days (C) Lanthana-doped (OH red.) Sample LY krad (Si) 0, 1, 11, 15 days.ll -1 l i! I 1 1-1_L L 1.LLL i l, I I I I t WflVELENGTH (microns) Figure 6c. Sample LY Figure 6c. Sample LY I I I I 1_L. Figure 6. Infrared transmittance variations in Lanthana-doped caused by activated defects (continued). Figure 6. Infrared transmittance variations in Lanthana -doped caused by activated defects (continued). SPIE Vol. 683 Infrared and Optical Transmitting Materials (1986) / 61 SPIE Vol. 683 Infrared and Optical Transmitting Materials (1986) / 61 Downloaded From: on 02/18/16 Terms of Use: