Thomas M. Regan

Neutron irradiation of sapphire for compressive strengthening.: I. Processing conditions and compressive strength

Abstract Sapphire suffers a dramatic loss of c-axis compression strength at elevated temperatures. Irradiation of sapphire with fission-spectrum neutrons to an exposure of ∼1022 neutrons/m2 in the core of a 1 MW fission reactor increased the c-axis compression strength by a factor of ∼3 at 600 °C. Strength was similarly improved when 99% of slow neutrons (⩽0.1 eV) were removed by 10 B and Cd shields during irradiation. Annealing at 600 °C for 10 min changed the yellow–brown color of irradiated sapphire to pale yellow, but had no effect on compressive strength. Annealing irradiated sapphire at 1200 °C for 24 h reduced the compressive strength to its baseline value. Transmission electron microscopy suggests that fast-neutron-induced displacement damage inhibits the propagation of r-plane twins which are responsible for the low compressive strength. When irradiated with 10 B and Cd shielding, sapphire that was not grown in iridium crucibles is safe for unrestricted handling after 1 month.

Neutron irradiation of sapphire for compressive strengthening. II. Physical properties changes

Abstract 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.

TPV conversion of nuclear energy for space applications

This paper explores the possibility of achieving high conversion efficiencies in a closed loop TPV system by replacing the typical combustion thermal source with a nuclear thermal source such as a high temperature gas cooled reactor. Possible applications are nuclear electric propulsion, long‐term power for mission support systems, and terrestrial base nuclear power systems. The emitter portion of the system utilizes a novel low temperature processing technique, while the collector portion of the system utilizes a holographic concentrator/spectral splitter to reduce cell area and remove off‐band emissions.

Electron Irradiation of Transparent and Ceramics Window Materials

The use of energetic electrons to modify the optical and mechanical properties of several window materials was examined. The materials were exposed to fields of high-energy electrons (5 MeV at a dose of 1,000 MRad). In this paper, we will report on the electron irradiation effects on the following materials: alumina, ALON, ZnSe and ZnS. Alumina irradiated under these conditions revealed little if any changes in flexure strength at room temperature. Irradiation changes in ALON hardness were measured. The hardness fracture toughness of electron beam irradiated ZnS and ZnSe was examined by both indentation and known flaw methods. Toughness measured by both methods were then compared and contrasted to ascertain the effects induced by the irradiation. The electron irradiation produced changes in the fracture toughness of both the ZnS and the ZnSe. The optical properties of the ZnSe and ZnS were measured by FTIR indicated minor changes in the absorption spectra.

Compressive strengthening of sapphire by neutron irradiation

Neutron irradiation of sapphire with 1 x 1022 neutrons(<EQ MeV)/m2 increases the c-axis compressive strength by a factor of 3 at 600 degree(s)C. The mechanism of strength enhancement is the retardation of r-plane twin propagation by radiation-induced defects. 1-B and Cd shielding was employed during irradiation to filter our thermal neutrons (<EQ1 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 600 degree(s)C with no loss of mechanical strength. Annealing at sufficiently high temperature (such as 1200 degree(s)C for 24 h) reduces the compressive strength back to its baseline value. Neutron irradiation decreases the flexure strength of sapphire at 600 degree(s)C by 0-20% in some experiments. However, the c- plane ring-on-ring flexure strength at 600 degree(s)C 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.

Laser thermal shock testing of neutron-irradiated sapphire

Half of a set of sapphire disks was exposed to fast neutrons (0.8-10 MeV) at a fluence of 1022 neutrons/m2. Each 25-mm-diameter x 1-mm-thick disk was then exposed to a 10.6 μm CO2 laser (121 W/cm2) while the central 12.7-mm-diameter region of the disk was shielded from the laser. c-Plane disks that had not been exposed to neutrons survived 76% longer than a-plane disks that had not been exposed to neutrons. Neutron irradiation had no significant effect on time to failure of c-plane sapphire. However, neutron irradiation increased the survival time of a-plane sapphire by 30%-a result that was significant at the 99.9% confidence level. c-Plane disks were expected to fail in tension at the center of the disk. The calculated tensile stress at the mean failure time was ~700 MPa and the center temperature was ~400°C. By contrast, a-plane disks failed near the boundary between the shielded central region and the exposed outer annulus. The radial stress at this location is tensile and the hoop stress is compressive. Failure origins were at surface scratches. Rhombohedral twinning was observed in many a-plane disks, but there was no fractographic evidence that r-plane twinning caused failure. The mechanism by which neutron irradiation increases the time to failure of a-plane disks is unknown.