Samples of fine-grain, transparent polycrystalline alumina (CeraNova Corp) and multispectral zinc sulfide (Cleartran) were tested to determine mechanical strength and slow crack growth parameters. Mechanical strength measurements of coupons were fit to a Weibull equation that describes the material strength and its distribution. Slow crack growth parameters were calculated using the procedure set forth by Weiderhorn.(1) This paper describes the derivation of Weibull and slow crack growth parameters from strength measurements over a range of stress rates and how these parameters are used to predict window lifetime under stress. Proof testing is employed to ensure that a window begins its life with a known, minimum strength.

Transparent ceramics are finding increasing use in optical applications with demanding operating conditions. Polycrystalline ceramics provide a unique combination of mechanical, dielectric and optical properties for sensor window applications that were previously not possible. The mechanical strength of CeraNova's transparent alumina and spinel was measured by an equibiaxial strength test method. The results of the tests and their analysis, included those at elevated temperatures for transparent alumina, will be presented.

Mechanical strength measurements of transparent ceramic window material coupons are customarily fit to a Weibull equation that describes the strength and its distribution. Predictions of window lifetime under stress are commonly based on slow crack growth parameters obtained by measuring the mechanical strength of coupons over a range of constant stress rates. This tutorial paper describes how to derive Weibull and slow crack growth parameters from strength measurements and how to use those parameters to predict window lifetime under stress. Proof testing is employed to ensure that a window begins its life with a known, minimum strength.

Irradiation of sapphire with fast neutrons (0.8–10 MeV) at a fluence of 1022/m2 increased the c-axis compressive strength and the c-plane biaxial flexure strength at 600 °C by a factor of ∼2.5. Both effects are attributed to inhibition of r-plane twin propagation by damage clusters resulting from neutron impact. The a-plane biaxial flexure strength and four-point flexure strength in the c- and m-directions decreased by 10–23% at 600 °C after neutron irradiation. Neutron irradiation had little or no effect on thermal conductivity, infrared absorption, elastic constants, hardness, and fracture toughness. A featureless electron paramagnetic resonance signal at g=2.02 was correlated with the strength increase: This signal grew in amplitude with increasing neutron irradiation, which also increased the compressive strength. Annealing conditions that reversed the strengthening also annihilated the g=2.02 signal. A signal associated with a paramagnetic center containing two Al nuclei was not correlated with strength. Ultraviolet and visible color centers also were not correlated with strength in that they could be removed by annealing at temperatures that were too low to reverse the compressive strengthening effect of neutron irradiation.

Neutron irradiation of sapphire with I X 1022 neutrons( greater than or equal to1 MeV)/m(2) increases the c-axis compressive strength by a factor of 3 at 600degreesC. The mechanism of strength enhancement is the retardation of r-plane twin propagation by radiation-induced defects. I OB and Cd shielding was employed during irradiation to filter out thermal neutrons (:! I eV), thereby reducing residual radioactivity in the sapphire to background levels in a month. Yellow-brown irradiated sapphire is nearly decolorized to pale yellow by annealing at 600degreesC with no loss of mechanical strength. Annealing at sufficiently high temperature (such as 1200degreesC for 24 h) reduces the compressive strength back to its baseline value. Neutron irradiation decreases the flexure strength of sapphire at 600degreesC by 0-20% in some experiments. However, the c-plane ring-on-ring flexure strength at 600degreesC is doubled by irradiation. Elastic constants of irradiated sapphire are only slightly changed by irradiation. Infrared absorption and emission and thermal conductivity of sapphire are not affected by irradiation at the neutron fluence used in this study. Defects that might be correlated with strengthening were characterized by electron paramagnetic resonance spectroscopy. Color centers observed in the ultraviolet absorption spectrum were not clearly correlated with mechanical response. No radiation-induced changes could be detected by x-ray topography or x-ray diffraction.

C-axis compressive strengths of pure and titanium-doped sapphire were measured at 600°C in air. Single crystal sapphire was doped with both titanium (3+) and titanium (4+) ions from mixtures initially containing 0.05% wt. to 0.25% wt. Ti2O3 in Al2O3. It was shown that both valence state and concentration of titanium were important in slowing down twin boundary movement under c-axis compression. Titanium (4+) doping did not adversely affect the thermal conductivity and infrared optical properties of sapphire.

The flexure and compressive strengths of sapphire are dependent on crystal orientation and temperature. Most notably, the c ‐axis compressive strength decreases below the tensile strength at temperatures >400°C and falls to 2% of the room‐temperature compressive strength at 800°C. Loss of compressive strength complicates the interpretation of flexure tests. Four‐point flexure specimens with no component of c ‐axis compression increase in strength at temperatures >500°C; however, specimens that have c ‐axis compression decrease in strength. It has been observed that c ‐axis compression causes twinning on rhombohedral crystal planes. Intersection of twins on different rhombohedral planes causes fracture that leads to mechanical failure.

Properties of 3–5 μm infrared-transmitting window materials are reviewed, with an emphasis on durable materials for applications in environments involving moisture, impact by solid and liquid particles, high temperatures and rapid heating rates. Infrared, visible and ultraviolet transmission windows are compared for MgF2, aluminum oxynitride, sapphire, spinel, MgO, Y2O3, calcium aluminate, SiO2, CaF2, LiF, ZnS, ZnSe, GaAs, GaP, Si and Ge. Emission at elevated temperature, reflection and optical scatter are also discussed. A comparison of mechanical and thermal properties is given, as is a brief discussion of rain and particle erosion resistance.

Sapphire's loss of strength between 20 degrees and 1000 degrees Celsius depends on orientation and state of stress. The critical weakness of sapphire occurs in compression along the c-axis of the crystal. In flexure tests of sapphire that is not subject to c-axis compression, the strength actually increases between 20 degrees and 1000 degrees Celsius. Compression on the c-axis causes twinning on rhombohedral crystal planes. When twins on different planes intersect, a crack forms and the specimen is then subject to tensile failure. Doping with Mg2+, Ti4+, or introduction of a TiO2 second phase each doubled the c-axis compressive strength of sapphire at 600 degrees Celsius, probably by inhibiting twin propagation. X-ray topography was employed to investigate the relationship between surface and bulk defects and mechanical strength in sapphire. Low angle grain boundaries were not associated with mechanical weakness. Wide, transverse scratches that are evident to x-rays, but not obvious in optical microscopy, can weaken sapphire. Topography demonstrated that annealing reduces long range strain in polished sapphire.

: This report describes the results of testing six potential missile dome materials in simulated rain fields in whirling-arm facilities. Visual damage and changes of optical scatter are described.

Diamond films were grown on Si wafers at about 3 μm/hr by microwave plasma assisted chemical vapor deposition. The infrared transmission of a 3-μm thick film varied from near 61% at 5000 cm−1 to near 75% at 1000 cm−1.

Zinc aluminum sulfide (formula ZnAl2S4, if stoichiometric) was prepared from the elements or from ZnS, Al and sulfur in sealed ampoules. Samples prepared at 800–900°C with the atomic composition AlZn = 2 or 3 contained α, β and W phases, as identified by X-ray diffraction. Pure yellow α phase material was sometimes formed or could be hand-picked from some samples. Samples prepared at 1070°C with AlZn = 1.1 contained only the β phase. Crystals of a phase, designated γ′, were grown by iodine transport of α zinc aluminum sulfide powder (AlZn = 3) from 800 to 740°C. The infrared transmittance of a stack of γ′ crystals (<1 mm thick) is 66–68° from 2.5 to 9μm, at which wavelength the transmittance begins to decrease. A characteristic EPR spectrum of an Mn2+ impurity at tetrahedral sites of α zinc aluminum sulfide [g = 2.002, A = -64.6 × 10-4 cm-1, |D| = 50 (±20) × 10-4 cm-1] identified. A second Mn2+ spectrum was characteristic of samples that were mixtures of α, W and (sometimes) β phases. A symmetric 21 line EPR signal at g = 1.991 with A = 2.86 × 10-4 cm-1 is attributed to an extra electron trapped at a site containing a sulfide ion surrounded by four alumium ions in W-phase zinc aluminum sulfide.

The total copper content and Cu3+Cu2+ ratio were determined for three samples of the high-temperature (Tc = 90 to 100 K) superconductor, YBa2Cu3O8−x, prepared in different laboratories under various final heating conditions. The X-ray diffraction patterns for the samples gave similar orthorhombic unit cell parameters. Iodometric titration and thermogravimetric analysis under hydrogen were used to determine the x-value in the formula YBa2Cu3+3−2xCu2+2xO8−x. The final heating temperatures, cooling methods, and x-values from the titration measurements are: 900°C, O2-annealed/slow-cooled, x = 1.06(5); 950°C, air-quenched, x = 1.35(5); 930°C, air/slow-cooled, x = 1.13(4). The approximate Tc values, taken as onset temperatures from magnetic susceptibility curves, are 93, 98, and 100 K, respectively.

Student preparation of YBa2Cu3O8-x, a demonstration of its superconductivity, and an analytical experiment dealing with the oxidation state of copper in the material.

The total copper content and Cu3+Cu2+ ratio were determined for three samples of the high-temperature (Tc = 90 to 100 K) superconductor, YBa2Cu3O8−x, prepared in different laboratories under various final heating conditions. The X-ray diffraction patterns for the samples gave similar orthorhombic unit cell parameters. Iodometric titration and thermogravimetric analysis under hydrogen were used to determine the x-value in the formula YBa2Cu3+3−2xCu2+2xO8−x. The final heating temperatures, cooling methods, and x-values from the titration measurements are: 900°C, O2-annealed/slow-cooled, x = 1.06(5); 950°C, air-quenched, x = 1.35(5); 930°C, air/slow-cooled, x = 1.13(4). The approximate Tc values, taken as onset temperatures from magnetic susceptibility curves, are 93, 98, and 100 K, respectively.

Mercury telluride films were produced by photolysis of an organometallic compound already containing a mercury-tellurium bond. Films were deposited on a quartz surface by gas-phase photolysis of (t-butyl)HgTe(t-butyl) at 30 or 150°C in vacuum or in H2 using a low-pressure mercury lamp (254 nm).

Calcium sulfide powder containing iron as an impurity was irradiated with 580, 366 or 254 nm light at 77 K. Irradiation enhanced a broad (16 G peak-to-trough) electron paramagnetic resonance (EPR) signal at g = 2.017 and caused six sharp (∼1 G) lines to appear in the X-band EPR spectrum at 347, 529, 956, 1963, 3547 and 5376 G. Enrichment of CaS with Fe2+ produced samples with similar photochemistry. It is proposed that irradiation causes the reaction Fe2+ + trap → Fe3+ + trap−, whose products give rise to six sharp EPR lines assigned to Fe3+ and a broad line associated with trap−. Both hyperfine splitting by 57Fe (13 G) and superhyperfine splitting by 33S (11.4 G) are observed in the six line spectrum. The environment of the photo-generated Fe3+ has less than octahedral symmetry. V2+ was observed at octahedral sites in unirradiated CaS for the first time, and is characterized by the EPR parameters g = 1.961 and A (hyperfine coupling) = 74.6 × 10−4 cm−1. EPR signals due to Mn2+ and Cr3+ at octahedral sites and Fe3+ at a low symmetry site were also observed in unirradiated CaS.