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Helium implantation in surfaces is of interest for plasma-facing materials and other nuclear applications. Vanadium as both a representative bcc material and a material relevant for fusion applications is implanted using a Helium ion beam microscope, and the resulting swelling and nanomechanical properties are quantified. These values are put in correlation to data obtained from micro-residual stress measurements using a focused ion beam-based ring-core technique. We found that the swelling measured is similar to literature values. Further, we are able to measure the surface stress caused by the implantation and find that it approaches the yield strength of the material at blistering doses. The simple calculations performed in the present work, along with several geometrical considerations deduced from experimental results confirm the driving force for blister formation comes from bulging resulting mainly from gas pressure buildup, rather than solely stress-induced buckling. Graphic abstract: [Figure not available: see fulltext.]

The effect of thermal oxide layer on He implanted 316L stainless steel was studied to evaluate experimentally how thermal oxidation affects the diffusion and distribution of He in the material. In the case of thermal oxidation of a He implanted sample, with an increase in oxidation time, the max swelling height increases logarithmically as a function of time and finally saturates for all samples except for the lowest dose of implanted He. Concerning TEM results, two void regions are identified. Similar to the calculation, the total irradiated depth was around 250 nm and the large void region was formed around 100–150 nm depth. On the other hand, the small void region was observed immediately under oxide layer from the thermal oxidation. In contrast, there were no voids in the altered zone near the metal/oxide interface in the non-thermal oxidized/He implanted sample. This description of the phenomena was justified using the Kirkendall effect and the Point Defect Model.

Composites formed by SiC fiber reinforcement of a SiC matrix are materials of interest for use in high temperature, high strength, and high irradiation condition applications. These materials have been considered for use in both fusion and fission reactors due to their excellent physical and neutronic properties. Recent attention has focused on such materials’ ability to act as accident-tolerant fuel cladding in light water reactors. The work presented herein studies the swelling behavior of these materials using a novel rapid helium-ion implantation approach. Localized helium implantation was conducted to a dose of 5 × 1017 ions/cm2 in fibers and matrix independently. The results showed that the height increase measured using atom force microscopy (AFM) was significantly less for the fibers than the matrix, potentially due to their finer microstructure.

Three distinct composite architectures of silicon carbide (SiC) and pyrocarbon (PyC) were incorporated into the SiC coating layer of tristructural-isotropic (TRISO) nuclear fuel particles. The composite architectures are meant to increase the resistance of SiC coating layer to cracking and fission product attack during operation and accident scenarios. All composite layers were produced using the existing fluidized bed chemical vapor deposition apparatus that is used for production of TRISO fuel particles without modifications. Detailed characterization of the composite microstructure was carried out via optical and electron microscopy. Nano-indentation examination confirms that mechanical properties of the SiC phase was not affected in the composite architectures, however, the resistance to crack propagation in this coating layer was greatly increased in all cases when compared to the reference monolithic coating layer. The stress required to debond the SiC-inner PyC interface in the reference TRISO particles was determined to be ∼1 GPa using micropillar compression technique. The high strength may explain the ease of crack propagation from the inner PyC to SiC in the reference design. In the composite architectures, the means of crack deflection were effectively incorporated at this interface. Finite element analysis of stress evolution in the fuel particles during normal operation with the reference and composite SiC coating layer architectures did not show any significant differences between the variants.

Microstructural changes induced by helium implantation in materials lead to volumetric swelling and mechanical property changes. How these properties are linked and establishing direct relationships can be difficult due to the underlying material’s microstructure evolution. Some materials also experience a phase change due to irradiation damage making them even more complex to analyze. Here, single crystalline Si (100) was used to establish a relationship among these parameters. The swelling height as a function of implantation fluence can equally fit a linear relationship. Solely irradiation induced defects are observed at low fluence below 5.0 × 1016 ions/cm2. An abrupt amorphous and crystalline mixed layer of ∼200 nm thick within a highly damaged polycrystalline matrix is observed when implantation fluence exceeds 5.0 × 1016 ions/cm2, leading to the appearance of irradiation induced swelling and hardening behavior. As the fluence increases beyond 1.0 × 1017 ions/cm2, the amorphous layer expands in size and the bubble size distribution takes the form of a Gaussian distribution with a maximum size of up to 6.4 nm, which causes a further increase in the height of swelling. Furthermore, irradiation induced softening appeared due to the enlarged bubble size and amorphization.

Helium-ion-induced swelling and blistering of single-crystal tungsten is investigated using a Helium Ion Microscope for site-specific dose-controlled irradiation (at 25 keV) with analysis by Helium Ion Microscopy, Atomic Force Microscopy and Transmission Electron Microscopy (cross-sectioning by Focused Ion Beam milling). Our measurements show that the blister cavity forms at the depth of the helium peak and that nanobubbles coalesce to form nanocracks within the envelope of the ion stopping range, causing swelling of the blister shell. These results provide the first direct experimental evidence for the interbubble fracture mechanism proposed in the framework of the gas pressure model for blister formation.

This work features the synthesis of nanoscale cavities using a He ion beam implantation method. A multitechnique (AFM, TEM, nanoindentation) post synthesis approach allows for the characterization of a 200 nm thin implantation region containing nanocavities. It was found that at lower doses, the cavities arrange themselves in a superlattice and are close to equilibrium pressure while at higher doses the self-organization of the nanoscale cavities is lost, forming cavities that change in nature resulting in a degradation of the mechanical properties. Close correlation between theory and experimental observation is achieved considering for the observed phenomena.

The mechanical properties of a well-characterized irradiated light water reactor fuel pin with an average burnup of 72 MWd/kgU were investigated by utilizing nano-indentation technique. Young's modulus and hardness are reported as functions of radial positions covering from mid radial position in the urania pellet to high burnup structure, fuel-cladding chemical interaction zone and zirconium-based cladding. By probing microstructure of the high burnup zone, the mechanical properties of sub-grains and the interaction between them were examined at the nanometer scale. Force-displacement curves from each of these individual sub-grains suggest they resemble single crystal UO2. However, these restructured grains are weakly bonded to their neighbors.

SiC-SiC composites exhibit exceptional high temperature strength and oxidation properties making them an advantageous choice for accident tolerant nuclear fuel cladding. In the present work, small scale mechanical testing along with AFM and TEM analysis were employed to evaluate PyC interphase properties that play a key role in the overall mechanical behavior of the composite. The Mohr-Coulomb formulation allowed for the extraction of the internal friction coefficient and debonding shear strength as a function of the PyC layer thickness, an additional parameter. These results have led to re-evaluation of the Mohr-Coulomb failure criterion and adjustment via a new phenomenological equation.

The mechanical properties of zirconium hydrides were studied using nano-indentation technique between 25 and 400 °C. Temperature dependency of reduced elastic modulus and hardness of δ- and e-zirconium hydrides were obtained by conducting nanoindentation experiments on bulk hydride samples with independently heating capability of indenter and heating stage. The reduced elastic modulus of δ-zirconium hydride (H/Zr ratio = 1.61) decreased from ∼113 GPa at room temperature to ∼109 GPa at 400 °C, while its hardness decreased significantly from 4.1 GPa to 2.41 GPa in the same temperature range. For e-zirconium hydrides (H/Zr ratio = 1.79), the reduced elastic modulus decreased from 61 GPa at room temperature to 54 GPa at 300 °C, while its hardness from 3.06 GPa to 2.19 GPa.

A novel light water reactor fuel has been designed and fabricated at the University of California, Berkeley; irradiated at the Massachusetts Institute of Technology Reactor; and examined within the Radiochemical Processing Laboratory at the Pacific Northwest National Laboratory. This fuel consists of U0.17ZrH1.6 fuel pellets core-drilled from TRIGA reactor fuel elements that are clad in Zircaloy-2 and bonded with lead-bismuth eutectic. The performance evaluation and post irradiation examination of this fuel are presented here.

Three hydride mini-fuel rods were fabricated and irradiated at the MIT nuclear reactor with a maximum burnup of 0.31% FIMA or ∼5 MWd/kgU equivalent oxide fuel burnup. Fuel rods consisted of uranium-zirconium hydride (U (30 wt%)ZrH1.6) pellets clad inside a LWR Zircaloy-2 tubing. The gap between the fuel and the cladding was filled with lead-bismuth eutectic alloy to eliminate the gas gap and the large temperature drop across it. Each mini-fuel rod was instrumented with two thermocouples with tips that are axially located halfway through the fuel centerline and cladding surface. In-pile temperature measurements enabled calculation of thermal conductivity in this fuel as a function of temperature and burnup. In-pile thermal conductivity at the beginning of test agreed well with out-of-pile measurements on unirradiated fuel and decreased rapidly with burnup.

Nanostructured metals are a promising candidate for future applications in irradiative environments, such as nuclear energy facilities, due to a conceivable tolerance against radiation damage. As the presence of helium irradiation is frequently unavoidable, e.g. in nuclear fusion facilities, the effect of helium on the properties of nanostructured materials is of immanent interest. In this work, ultra-fine grained (UFG; 100 nm grain size) and nanocrystalline (NC; 20 nm grain size) Cu-Fe-Ag samples have been implanted with various fluences of helium and were investigated regarding helium-induced modifications using atomic force microscopy, nanoindentation and transmission electron microscopy. While for these nanostructured materials a tolerance against radiation damage has been reported earlier, we find that the influence of helium on swelling and mechanical properties is not negligible. The increased amount of closely spaced interfaces in the NC material provides swift diffusion paths of helium, thereby facilitating bubble nucleation in the early stages of irradiation. For high fluences of helium, however, the smaller grain size and larger amount of nucleation sites in the NC composite restrict the growth of individual bubbles, which has a positive effect on swelling and counteracts mechanical property degradation compared to UFG and conventional coarse-grained materials. As such, our investigations on immiscible Cu-Fe-Ag nanocomposites pave a promising strategy for designing novel highly radiation enduring materials for irradiative environments.

Helium-ion-induced swelling and blistering of single-crystal tungsten is investigated using a Helium Ion Microscope for site-specific dose-controlled irradiation (at 25keV) with analysis by Helium Ion Microscopy, Atomic Force Microscopy and Transmission Electron Microscopy (cross-sectioning by Focused Ion Beam milling). Our measurements show that the blister cavity forms at the depth of the helium peak and that nanobubbles coalesce to form nanocracks within the envelope of the ion stopping range, causing swelling of the blister shell. These results provide the first direct experimental evidence for the interbubble fracture mechanism proposed in the framework of the gas pressure model for blister formation.

Monophase thorium–zirconium hydrides (ThZr2Hx) have been fabricated starting from a metallic alloy and the hydrogen stoichiometry determined by X-ray diffraction. Incoherent Quasielastic Neutron Scattering (IQNS) on the hydrides was conducted over the temperature range 650–750 K at the Backscattering Silicon Spectrometer (BASIS) at the Spallation Neutron Source (SNS) at ORNL. The isotropic Chudley–Elliott model was utilized to analyze the quasielastic linewidth broadening data as function of momentum transfer. The diffusion coefficient and average jump distance of hydrogen atoms in ThZr2H5.6 and ThZr2H6.2 were extracted from the measurements.

Three sets of independent experiments were conducted to determine the kinetics of hydrogen desorption from and adsorption on δ-zirconium hydrides. One method involved measurement of hydrogen gas pressure-buildup as a result of dehydriding in a closed vessel. The other two involved thermogravimetric experiments, measuring the rate of mass loss during dehydriding under vacuum. Zeroth-order desorption and first order (with respect to gas-phase hydrogen) adsorption kinetics was determined. The Arrhenius dependence of rate constants showed excellent agreement among different experimental data. The activation energies for desorption and adsorption processes were determined as 205 ± 8 and 86 ± 15 kJ mol −1 respectively.

Two uranium–thorium–zirconium hydrides, (UTh 4 Zr 10 )H 1.9 and (U 4 Th 2 Zr 9 )H 1.5 , have been fabricated and characterized. Fabrication involved arc melting of the constituent pure metals to form homogenous alloys, followed by hydriding at elevated temperatures in a hydrogen gas environment. The compounds were characterized by X-ray powder diffractometry as well as scanning and transmission electron microscopy. These methods revealed a multi-phase mixture of δ-zirconium hydride (ZrH 1.6+ x ), thorium–zirconium hydride (ThZr 2 H 7− x ), and uranium metal. The elastic modulus was mapped across the microstructure using nanoscale dynamic stiffness mapping. The elastic modulus of ThZr 2 H 7− x phase is found to be 172 GPa.

The aging-related deterioration of the fracture properties of bone, coupled with higher life expectancy, is responsible for increasing incidence of bone fracture in the elderly; consequently, an understanding of how these fracture properties degrade with age is essential. In this study, ex vivo fracture experiments have been performed to quantitatively assess the effect of age on human cortical bone in the proximal–distal orientation, i.e., longitudinally along the osteons. Because cortical bone exhibits rising crack-growth resistance with crack extension, the toughness is evaluated in terms of resistance-curve (R-curve) behavior, measured for bone taken from wide range of age groups (34–99 years). Using this approach, both the crack-initiation and crack-growth toughness are determined and are found to deteriorate with age; the initiation toughness decreases some 40% over six decades from 40 to 100 years, while the growth toughness is effectively eliminated over the same age range. The reduction in crack-growth toughness is considered to be associated primarily with a degradation in the degree of extrinsic toughening, in particular, involving crack bridging in the wake of the crack. An examination of the micro-/nano-structural changes accompanying the process of aging, using optical microscopy, X-ray tomography, nanoindentation and Raman spectroscopy, is shown to support such observations.

This study compares changes in bone microstructure in 6-month-old male GC-treated and female ovariectomized mice to their respective controls. In addition to a reduction in trabecular bone volume, GC treatment reduced bone mineral and elastic modulus of bone adjacent to osteocytes that was not observed in control mice nor estrogen-deficient mice. These microstructural changes in combination with the macrostructural changes could amplify the bone fragility in this metabolic bone disease.Patients with glucocorticoid (GC)-induced secondary osteoporosis tend to fracture at higher bone mineral densities than patients with postmenopausal osteoporosis. This suggests that GCs may alter bone material properties in addition to BMD and bone macrostructure.Changes in trabecular bone structure, elastic modulus, and mineral to matrix ratio of the fifth lumbar vertebrae was assessed in prednisolone-treated mice and placebo-treated controls for comparison with estrogen-deficient mice and sham-operated controls. Compression testing of the third lumbar vertebrae was performed to assess whole bone strength.Significant reductions in trabecular bone volume and whole bone strength occurred in both prednisolone-treated and estrogen-deficient mice compared with controls after 21 days (p0.05). The average elastic modulus over the entire surface of each trabecula was similar in all the experimental groups. However, localized changes within the trabeculae in areas surrounding the osteocyte lacunae were observed only in the prednisolone-treated mice. The size of the osteocyte lacunae was increased, reduced elastic modulus around the lacunae was observed, and a "halo" of hypomineralized bone surrounding the lacunae was observed. This was associated with reduced (nearly 40%) mineral to matrix ratio determined by Raman microspectroscopy. These localized changes in elastic modulus and bone mineral to matrix ratio were not observed in the other three experimental groups.Based on these results, it seems that GCs may have direct effects on osteocytes, resulting in a modification of their microenvironment. These changes, including an enlargement of their lacunar space and the generation of a surrounding sphere of hypomineralized bone, seem to produce highly localized changes in bone material properties that may influence fracture risk.

With the potential to map mechanical properties of heterogeneous materials on a micrometer scale, there is growing interest in nanoindentation as a materials characterization technique. However, nanoindentation has been developed primarily for characterization of hard, elasto-plastic materials, and the technique has not been validated for very soft materials with moduli less than 5 MPa. The current study attempted to use nanoindentation to characterize the elastic moduli of soft, elastomeric polydimethylsiloxane (PDMS) samples (with different degrees of crosslinking) and determine the effects of adhesion on these measurements using adhesion contact mechanics models. Results indicate that nanoindentation was able to differentiate between elastic moduli on the order of hundreds of kilo-Pascals. Moreover, calculations using the classical Hertz contact model for dry and aqueous environment gave higher elastic modulus values when compared to those obtained from unconfined compression testing. These data seem to suggest that consideration of the adhesion energy at the tip-sample interface is a significantly important parameter and needs to be taken into account for consistent elastic modulus determination of soft materials by nanoindentation.

The mechanical properties of SCS-6 SiC fibers were measured as a function of fiber radius using nanoindentation techniques. Hardness and Young's modulus were characterized for the material in all of the major regions of these fibers: the carbon core, the graphitic core coating, the inner SiC sheath, and the outer SiC sheath. The carbon core of the fibers was determined to be uniform in properties but extremely compliant. Young's modulus of 28 GPa and a hardness of 4.2 GPa were measured. The graphitic core coating was found to exhibit considerable anelasticity and to have both a low modulus (21 GPa) and a low hardness (1.7 GPa). The inner sheath of the fiber, which contained a varying chemistry, showed a sharp increase in stiffness and hardness from the inner core. Modulus and hardness increased by an order of magnitude over just 1 or 2 μm when transversing radially away from the core into the SiC. This change in properties was pronounced and clearly defined. The outer sheath, which contained a uniform chemistry and microstructure, was consistently stiff and hard when transversing radially. The average modulus and hardness for the full fiber was 333 GPa. The values reported for Young's modulus and hardness clearly showed that the mechanical properties of SCS SiC fibers exhibit dramatic changes across their diameters.

The mechanical properties of a tissue can be evaluated by determining the response of the structure to mechanical loading. This can be accomplished only when the tissue has been prepared with minimum to no artifacts, thus preserving its structure. In this study it was hypothesized that the structure of cementum is inhomogeneous, contributing to a significant variation in mechanical properties of cementum. Therefore, the goals of the study were to identify potential artifacts generated by conventional sample preparation techniques such as polishing and ultrasectioning and subsequently characterize the prepared specimens using an atomic force microscope (AFM) and an AFM-nanoindenter. Comparisons between cryofractured, ultrasectioned and polished specimens concluded that ultrasectioned surfaces have significantly lower average surface roughness 'R(a)' (p

The purpose of this study was to determine if dental calcified junctions (DEJs/CDJs) in human teeth contain different compositional phases compared to the adjacent dental calcified tissues. Peak positions and intensities were determined from micro-Raman spectra for PO and the CH modes and compared among the mineralized tissues and their junctions. Values of width were determined from the intersections of intensity regression lines through the junctions and in the adjacent tissues. The peaks were measured in 1-μm steps along a l00-μm line across the junction. High-resolution analysis revealed that PO band peaks for dentin, the DEJ, enamel, the CDJ, and cementum were at the same position (959 cm−1), while for the CH stretching mode a significant shift of 4.6 cm−1 was found between enamel, the DEJ, and dentin. The mean width of the DEJ was 7.6 (± 2.8) μm using the PO band and 8.6 (± 3.6) μm using the CH stretching mode. Across the DEJ, the mineral content monotonically decreased from enamel to dentin while the organic component monotonically increased. The DEJ width was in agreement with prior nanoindentation studies. No width estimate was possible for the CDJ because the compositional differences between cementum and dentin were small. © 2004 Wiley Periodicals, Inc. J Biomed Mater Res 69A: 286–293, 2004

Osteoporosis is a syndrome of excessive skeletal fragility that results from both the loss of trabecular bone mass and trabecular bone connectivity. Recently, bFGF has been found to increase trabecular bone mass in osteoporotic rats. The purpose of this study was to compare how trabecular bone architecture, bone cell activity, and strength are altered by two different bone anabolic agents, bFGF and hPTH(1–34), in an osteopenic rat model. Materials and Methods: Six-month-old female Sprague-Dawley rats (n = 74) were ovariectomized (OVX) or sham-operated (sham) and maintained untreated for 2 months. Then OVX rats were subcutaneously injected with basic fibroblast factor (bFGF; 1 mg/kg, 5 days/week), human parathyroid hormone [hPTH(1–34); 40 μg/kg, 5 days/week], or vehicle for 60 days (days 60–120 ). Sham-operated and one group of OVX animals were injected with vehicle. Biochemical markers of bone turnover (urinary deoxypyridinoline cross-links; Quidel Corp., San Diego, CA, USA) and serum osteocalcin (Biomedical Technologies, Stroughton, MA, USA) were obtained at study days 0, 60, 90, and 120 and analyzed by ELISA. At death, the right proximal tibial metaphysis was removed, and microcomputed tomography was performed for trabecular bone structure and processed for histomorphometry to assess bone cell activity. The left proximal tibia was used for nanoindentation/mechanical testing of individual trabeculae. The data were analyzed with Kruskal Wallis and post hoc testing as needed. Results: Ovariectomy at day 60 resulted in about a 50% loss of trabecular bone volume compared with sham-treated animals. By day 120 post-OVX, OVX + vehicle treated animals had decreased trabecular bone volume, connectivity, number, and high bone turnover compared with sham-operated animals [p < 0.05 from sham-, hPTH(1–34)-, and bFGF-treated groups]. Treatment of OVX animals with bFGF and hPTH(1–34) both increased trabecular bone mass, but hPTH(1–34) increased trabecular thickness and bFGF increased trabecular number and connectivity. Histomorphometry revealed increased mineralizing surface and bone formation rate in both bFGF and hPTH(1–34) animals. However, osteoid volume was greater in bFGF-treated animals compared with both the hPTH(1–34) and OVX + vehicle animals (p < 0.05). Nanoindentation by atomic force microscope was performed on approximately 20 individual trabeculae per animal (three animals per group) and demonstrated that elastic modulus and hardness of the trabeculae in bFGF-treated animals were similar to that of the hPTH-treated and sham + vehicle-treated animals. Conclusion: Both hPTH(1–34) and bFGF are anabolic agents in the osteopenic female rat. However, hPTH(1–34) increases trabecular bone volume primarily by thickening existing trabeculae, whereas bFGF adds trabecular bone mass through increasing trabecular number and trabecular connectivity. These results suggest the possibility of sequential treatment paradigms for severe osteoporosis.

Temperature programmed decomposition and complimentary microscopy/spectroscopy techniques were performed on lithium hydroxide with micron-sized grains. The lithium hydroxide grains thermally decomposed into Li2O, releasing H2O, following a three dimensional phase boundary moving from the surface inward. The energy barriers measured for the decomposition of surface and near-surface lithium hydroxide are noticeably smaller than those of bulk counterpart. The conversion of Li2O grains back to lithium hydroxide during moisture exposure was also found to proceed from the surface inward such that surface states are filled before bulk states. In a different set of experiments, nanometer-scale composite grains composed of LiD inner cores and LiOH outer layers were observed to form on top of pressed polycrystalline LiD upon moisture exposure. A diffusion coefficient on the order of 10−23 m2/s was measured for the diffusion controlled reaction of LiOH with LiD in the nanopowder at room temperature in a dry environment. The measured kinetics were used to model the evolution of the LiD/LiOH composite system in a dry environment.

The attachment between cementum and dentin has been given several definitions and nomenclature, including: interzonal layer, intermediate cementum, collagen hiatus, Hopewell-Smith's hyaline layer, and more commonly, cementum-dentin junction (CDJ). Understanding the attachment of two structurally dissimilar hard tissues such as cementum and dentin defined by a junction may provide information necessary to engineer functionally graded materials that can be used for efficient tooth restorations in clinical dentistry and other bioengineering applications. Hence, in this study, as a first step toward understanding the CDJ using a biomechanical approach, it was hypothesized that the CDJ between cementum and dentin is a wide zone with mechanical properties significantly lower than the neighboring tissues. The structure of the CDJ was studied using an atomic force microscope (AFM), and site-specific mechanical response of the three regions; cementum, CDJ, and dentin were determined using an AFM-nanoindenter under dry and wet conditions. The AFM results of the CDJ demonstrated a valley under dry conditions and a peak under wet conditions. The magnitude of the depth of the valley was approximately the same as the height of the peak of the CDJ, ranging from 10 to 40 microm. The nanomechanical properties under dry conditions indicated no significant difference (p > 0.05) in elastic modulus and hardness of the CDJ (Er = 17.5 +/- 2.7 GPa, H = 0.6 +/- 0.1 GPa) and cementum (Er = 18.7 +/- 2.5 GPa, H = 0.6 +/- 0.1 GPa). The mechanical properties of the CDJ were significantly lower (p << 0.05) than dentin (Er = 19.9 +/- 2.9 GPa, H = 0.6 +/- 0.1 GPa) under dry conditions. However, under more relevant hydrated conditions, the mechanical properties of CDJ (Er 3.0 +/- 0.7 GPa, H = 0.1 +/- 0.0 GPa) were significantly lower (p << 0.05) than those of cementum (Er 6.8 +/- 1.9 GPa, H = 0.2 +/- 0.1 GPa) and dentin (Er 9.4 +/- 2.3 GPa, H = 0.3 +/- 0.1 GPa). Based on the results from this study, it can be concluded that the CDJ can be regarded as a wide zone containing large quantities of proteins including collagen that contribute to hydration and significantly reduce mechanical properties, compared with the adjacent hard tissues, cementum, and dentin. The lower mechanical properties of the CDJ may make it possible for it to redistribute occlusal loads to the alveolar bone.

Teeth contain two major calcified tissues, enamel and dentin, that are joined by an interface known as the dentin–enamel junction (DEJ). Enamel is the hard and brittle outer portion of the tooth that cuts and grinds food and dentin is composed of a tougher biological composite, that can absorb and distribute stresses. The DEJ is a complex and critical structure uniting these two dissimilar calcified tissues and acts to prevent the propagation of cracks from enamel into dentin. The DEJ has a three-level structure, 25–100 μm scallops with their convexities directed toward the dentin and concavities toward the enamel; 2–5 μm microscallops; and a smaller scale structure. Mechanical properties measurements, chemical differences and imaging have been used to determine the functional width of the DEJ. AFM based nanoindentation gave values of 11.8 μm, microRaman yielded a width of 7.0 μm, while the smaller probe used for AFM nanoscratching yielded 2.0 μm, and values from dynamic modulus mapping were less than 1 μm. The unique architecture of the DEJ may account for this variation based on enamel–dentin phase intermixing. The ultimate goal is to use the DEJ as a biomimetic model for other interfaces joining dissimilar materials.

Dentin collagen fibrils were studied in situ by atomic force microscopy (AFM). New data on size distribution and the axial repeat distance of hydrated and dehydrated collagen type I fibrils are presented. Polished dentin disks from third molars were partially demineralized with citric acid, leaving proteins and the collagen matrix. At this stage collagen fibrils were not resolved by AFM, but after exposure to NaOCl(aq) for 100-240 s, and presumably due to the removal of noncollagenous proteins, individual collagen fibrils and the fibril network of dentin connected to the mineralized substrate were revealed. High-aspect-ratio silicon tips in tapping mode were used to image the soft fibril network. Hydrated fibrils showed three distinct groups of diameters: 100, 91, and 83 nm and a narrow distribution of the axial repeat distance at 67 nm. Dehydration resulted in a broad distribution of the fibril diameters between 75 and 105 nm and a division of the axial repeat distance into three groups at 67, 62, and 57 nm. Subfibrillar features (4 nm) were observed on hydrated and dehydrated fibrils. The gap depth between the thick and thin repeating segments of the fibrils varied from 3 to 7 nm. Phase mode revealed mineral particles on the transition from the gap to the overlap zone of the fibrils. This method appears to be a powerful tool for the analysis of fibrillar collagen structures in calcified tissues and may aid in understanding the differences in collagen affected by chemical treatments or by diseases.

The properties of femtosecond pulsed laser deposited GaAs nanoclusters were investigated. Nanoclusters of GaAs were produced by laser ablating a single crystal GaAs target in vacuum or Ar gas. Atomic force and transmission electron microscopies showed that most of the clusters were spherical and ranged in diameter from 1 nm to 50 nm, with a peak size distribution between 5 nm and 9 nm, depending on the Ar gas pressure or laser fluence. X-ray diffraction, solid-state nuclear magnetic resonance, Auger electron spectroscopy, electron energy loss spectroscopy, and high-resolution transmission electron microscopy revealed that these nanoclusters were randomly oriented GaAs crystallites. An oxide outer shell of ∼2 nm developed subsequently on the surfaces of the nanocrystals as a result of transportation in air. Unpassivated GaAs nanoclusters exhibited no detectable photoluminescence. After surface passivation, these nanoclusters displayed photoluminescence energies less than that of bulk GaAs from which they were made. Our photoluminescence experiments suggest an abundance of sub-band gap surface states in these GaAs nanocrystals.

NaOCl aq is used as a cleansing and non-specific deproteinizing agent in endodontic treatment, as a component of new chemomechanical caries treatment, and is under study for its alterations of dentin bonding characteristics. We sought to determine the microstructural and nanomechanical changes with such treatments and to test if NaOCl aq removed dentin collagen without microstructural or nanomechanical alteration of underlying mineralized dentin. Polished human dentin disks were prepared with a double reference technique that allowed changes to be determined following 10% citric acid etching for 15 s and subsequent treatment of the etched and unetched portions of the sample with 6.5% NaOCl aq , using atomic force microscopy (AFM) (Nanoscope III, Digital Instruments, Santa Barbara, CA). Images and measurements were made at intervals up to 1800 s. A Triboscope (Hysitron, Minneapolis, MN) on the AFM was used to measure nanohardness and the reduced elastic modulus. The double reference method allowed measurements immediately following etching and at intervals during deproteinization. Etching caused deep peritubular dentin removal and a small depth change of hydrated intertubular dentin as mineral was removed and left a remnant collagen matrix. NaOCl aq removed collagen over time, during which individual fibrils could be resolved; the underlying mineralized dentin was left with a unique porous surface containing numerous channels that are not normally observed in etched or fractured dentin. This could provide an attractive bonding substrate because of the increased surface area and high mineral content, if toughness is not reduced too much. Nanomechanical measurements showed that the reduced elastic modulus and hardness were 75% of original values after removal of the exposed collagen. Current dentin bonding systems rely on hybrid layer formation in which hydrophilic primers/polymers penetrate the opened collagen matrix exposed by etching. However some research suggests that preventing hybrid layer formation by deproteinization, in some cases, can give good bonding, and as shown here, alters the substrate microstructure. There are significant changes as indicated by ANOVA in the reduced elastic modulus that require further study to determine if these changes might affect clinical efficacy.

Atomic force microscopy (AFM) combined with a nano-indentation technique was used to reveal the structure and to perform site-specific mechanical testing of the enamel of third molars. Nano-indentations (size

The mechanical and optical properties of healthy and transparent root dentin are compared using atomic force microscopy (AFM), micro-Raman and emission spectroscopies and fluorescence microscopy. The elastic modulus and hardness of intertubular and peritubular transparent and healthy dentin did not differ appreciably. The tubule filling material in the transparent zone, however, exhibited values between peritubular and intertubular dentin. Raman spectroscopy revealed a shift in the 1066 cm−1 band to 1072 cm−1 from normal to transparent intertubular dentin. The material filling the tubule lumen in transparent dentin showed an increase in frequency of the band near 1070 cm−1 as well. The emission spectral characteristics under 351 nm photoexcitation indicate differences between normal and transparent intertubular dentin. A transition region of about 300 μm between normal and transparent dentin was identified. In this region the intertubular emission properties were the same as for normal dentin, but tubules were filled. The filling material had emission characteristics closer to the normal intertubular than to transparent intertubular dentin. © 2001 Kluwer Academic Publishers

This article summarizes the results of a three-dimensional study of changes in the morphology of the L6 rat vertebra at 120 days after ovariectomy (OVX), with estrogen replacement therapy used as a positive control. Synchrotron radiation microtomography was used to quantify the structural parameters defining trabecular bone architecture, while finite-element methods were used to explore the relationships between these parameters and the compressive elastic behavior of the vertebrae. There was a 22% decrease in trabecular bone volume (TBV) and a 19% decline in mean trabecular thickness (Tb.Th) with OVX. This was accompanied by a 150% increase in trabecular connectivity, a result of the perforation of trabecular plates. Finite-element analysis of the trabecular bone removed from the cortical shell showed a 37% decline in the Young's modulus in compression after OVX with no appreciable change in the estrogen-treated group. The intact vertebrae (containing its trabecular bone) exhibited a 15% decrease in modulus with OVX, but this decline lacked statistical significance. OVX-induced changes in the trabecular architecture were different from those that have been observed in the proximal tibia. This difference was a consequence of the much more platelike structure of the trabecular bone in the vertebra.

The local mechanical properties of silica-reinforced silicone composites were investigated using a modified atomic force microscopy technique. Elastic modulus measurements (1.5 ± 0.1 MPa) are consistent with bulk measurements (1.9 MPa), and changes in the modulus at the surface of the composite samples (E = 1.5 to 3.5 MPa) were observed as a result of α-irradiation (dose = 1.7 × 1010 to 2.0 × 1012 α/cm2). The sensitivity of the technique was demonstrated by a detectable change in modulus at even the small dose of 1.7 × 1010 α/cm2. The penetration depth of the α-particles into the material, estimated to be 22 ± 2 μm from the sample edge, was determined by cross-section depth profiling; and modeling of the ion penetration depth using transport of ions in matter codes (24.4 ± 0.4 μm) closely matched experimental observations.

The dentinoenamel junction (DEJ) is a complex and poorly defined structure that unites the brittle overlying enamel with the dentin that forms the bulk of the tooth. In addition, this structure appears to confer excellent toughness and crack deflecting properties to the tooth, and has drawn considerable interest as a biomimetic model of a structure uniting dissimilar materials. This work sought to characterize the nanomechanical properties in the region of the DEJ using modified AFM based nanoindentation to determine nanohardness and elastic modulus. Lines of indentations traversing the DEJ were made at 1-2 microm intervals from the dentin to enamel along three directions on polished sagittal sections from three third molars. Nanohardness and elastic modulus rose steadily across the DEJ from bulk dentin to enamel. DEJ width was estimated by local polynomial regression fits for each sample and location of the mechanical property curves for the data gradient from enamel to dentin, and gave a mean value of 11.8 microm, which did not vary significantly with intratooth location or among teeth. Nanoindentation was also used to initiate cracks in the DEJ region. In agreement with prior work, it was difficult to initiate cracks that traversed the DEJ, or to produce cracks in the dentin. The fracture toughness values for enamel of 0.6-0.9 MPa . m(1/2) were in good agreement with recent microindentation fracture results. Our results suggest that the DEJ displays a gradient in structure and that nanoindenation methods show promise for further understanding its structure and function.

Patterned and unpatterned thin films of Si/Cs/O nanoclusters have been synthesized by the technique of supersaturated thermal vaporization of Si and Cs in an oxygen background gas. These films, which were deposited onto conducting or semiconducting substrates, exhibit negative electron affinity (NEA) as evidenced by ultraviolet photoemission spectroscopy. Photo, secondary, and field electron emission properties of these nanocluster films were investigated with photoelectron emission microscopy, field electron emission microscopy, secondary electron microscopy, and current-voltage measurements. Flat cathodes covered with thin films of Si/Cs/O nanoclusters exhibited high current outputs and lower turn-on fields ({lt}8.7 V/{mu}m) than most NEA diamond surfaces and gated Si or Mo tip arrays. The films{close_quote} NEA is unaffected by air exposure and is stable to high-temperature annealing (550h{degree}C). The electron emission property of the NEA nanocluster films can be best explained by a subband gap abundant surface-state nanocluster model. A field-emission display unit with a simple diode structure containing a flat cathode coated with a thin film of Si/Cs/O nanoclusters has also been built to demonstrate the potential application of this material in cold cathode electron emitting devices, particularly field-emission flat panel displays. {copyright} {ital 1999} {ital The American Physical Society}

We report on an experimental and theoretical study of the growth of SiC films and of the profile development of the silicon underlayer. SiC features were grown via the direct reaction of the silicon substrate with C60 precursors. Two masking configurations were used to investigate the effects of bulk and surface diffusion on SiC film growth. Without a diffusion barrier (i.e., a patterned SiO2 mask with regions of silicon initially exposed directly to C60), voids formed in the substrate beneath the growing SiC layer, which ultimately controlled the final thickness of the SiC film. Pronounced faceting was observed at the early stages of growth on crystalline silicon. When a tungsten diffusion barrier was used to prevent bulk diffusion (i.e., tungsten covering silicon in non-SiO2 masked regions), significant undercutting resulted beneath the oxide and SiC layers without void formation. A profile simulation was developed to model the time evolution of the silicon underlayer when this diffusion barrier is used...

Using an atomic force microscope (AFM) with an attachment specifically designed for indentation, we measured the mechanical properties of demineralized human dentin under three conditions: in water, in air after desiccation, and in water after rehydration. The static elastic modulus (E(h)r = 134 kPa) and viscoelastic responses (tau(epsilon) = 5.1 s and tau(sigma) = 6.6 s) of the hydrated, demineralized collagen scaffolding were determined from the standard linear solid model of viscoelasticity. No significant variation of these properties was observed with location. On desiccation, the samples showed considerably larger elastic moduli (2 GPa), and a hardness value of 0.2 GPa was measured. Upon rehydration the elastic modulus decreased but did not fully recover to the value prior to dehydration (381 kPa).

Dentin, the inner hard tissue of teeth, is a complex hydrated composite and forms the substrate for much of restorative dentistry. Current efforts to improve restorative and preventive dentistry involve the development of effective bonds to this tissue. Modern bonding techniques involve etching the dentin to remove the mineral portion, leaving a collagen network that can be infiltrated with a monomer and polymerized in place to form a mechanical bond. The objective of this study was to investigate the nature of the interfacial zone, called the resin-dentin interdiffusion zone, using scanning electron microscopy for characterization and thickness measurements, X-ray tomographic microscopy for fracture characteristics and atomic force microscopy for the local modulus and hardness variations.

The authors would like to take this opportunity to rectify a derivational error in our recently published research article. Equation (12) (p. 2824) in the original manuscript pertains to the calculation of the Young's modulus from nanoindentation data considering a Johnson, Kendall, Roberts (JKR) adhesion model

Objectives: Dentin is a vital, hydrated composite material with structural components and properties that vary with location. These variations are reviewed along with alterations by physiological and pathological changes that allow classification into various forms of dentin. Structural characteristics and mechanical properties are reviewed and the limitations of our understanding of structure-property relationships for normal and modified forms of dentin are discussed with respect to their impact on dentin bonding. Recent progress in methods available to study dentin and its demineralization are emphasized with their promise to increase our understanding of dentin properties and structure. Data sources: Recent microstructural studies, focusing on scanning electron microscopy, atomic force microscopy and X-ray tomographic microscopy are included. A review of fundamental studies with emphasis on microstructurally sensitive methods, and prior reviews of basic mechanical properties are included with discussion of their correlation to composition and structure. Study selection and conclusions: Emphasis in this work was placed on the major structural components of the tissue, including the collagen based organic matrix and its mineral reinforcement, the distribution of these components and their microstructural organization as related to mechanical properties and response to demineralization. Little information is included on biochemical and developmental studies or on non-collagenous proteins and other organic components for which limited understanding is available with respect to their role in structure-property relations and influence on bonding. In spite of the fact that the complexity of dentin precluded a comprehensive review, it is clear that local structural variations influence properties and impact nearly all preventive and restorative dental treatments. Much more work is needed in order to understand differences between vital and non-vital dentin, and dentin from extracted teeth. Although our knowledge is rudimentary in certain areas, increasingly sophisticated methods of studying dentin should provide the necessary information to model structure-property relations, optimize dentin bonding, and improve many aspects of preventive and restorative dentistry.

Objectives This investigation sought to determine: 1) if dentin demineralization rates are proportional to acid concentration for demineralization in phosphoric acid (10% or 1.76M, 0.025M, 0.0001 M, with pH=0.95, 2.0, 4.0 respectively); 2) if the etching characteristics are independent of dentin depth; and 3) if the etching characteristics for phosphoric acid were comparable to those for citric acid over a similar pH range. Methods Highly polished dentin disks from freshly extracted, non-carious, third molars were prepared with a reference layer. Samples were prepared from either superficial or deep coronal dentin. The samples were etched for periods of up to 30 min using phosphoric acid solutions (pH = 0.95, 2.0, 4.0) in a wet cell of an atomic force microscope (AFM). Depth changes with respect to the reference layer were determined for the intertubular and peritubular dentin to quantify structural changes. The results were compared with similar studies using citric acid (pH = 1.0, 2.15 or 3.4). Etching characteristics were statistically compared using 2-way repeated measures ANOVA at p Results The relation between time and recession for peritubular dentin was initially linear. The intertubular dentin recession started rapidly but then reached a plateau within a very short interval for etching solutions at pH = 2.0 and 4.0. At the highest concentration, the recession decreased with time, but a clear plateau was not established. There was no statistical difference between peritubular etching rates of superficial and deep dentin surfaces with phosphoric acid at any concentration. There was also no difference in the intertubular dentin recession at the location of the plateau that depended on dentin depth. Etching rates increased dramatically with decreased pH for both phosphoric and citric acids, but were higher for citric acid than for phosphoric acid. Significance The AFM allowed quantification of changes during etching of wet dentin. Peritubular dentin etching rates increased with decreasing pH, as expected, but changes were not linear and were different for the two acids studied over a similar pH range. Intertubular dentin surface recession was small and plateaued for low concentrations. The peritubular etching rate and intertubular dentin recession did not depend on dentin depth.

An ion beam etching study designed to characterize the kinetic and transport processes important in the ion-assisted etching of silicon in molecular chlorine was performed. Monoenergetic argon ion beams were directed normal to a silicon wafer that was simultaneously exposed to a background of molecular chlorine, thereby simulating a low temperature rf plasma etching process. The ion-induced etching yield scaled with the square root of the ion energy for a surface saturated with adsorbed chlorine. Moreover, the yield was found to depend strongly on the ion to neutral flux ratio which was varied over two orders of magnitude. A kinetic model in which the dissociative sticking probability and the yield scale linearly with surface coverage was developed and was found to be consistent with the yield data in the ion energy range of 90–300 eV. Its applicability to etching topography was tested with additional experiments where patterned silicon wafers (with oxide masks and with a range of features and linewidths)...

The mechanical properties of SiC films grown via C60 precursors were determined using atomic force microscopy (AFM). Conventional silicon nitride and diamond-tipped steel AFM cantilevers were employed to determine the film hardness, friction coefficient, and elastic modulus. The hardness is found to be 26 GPa by nanoindentation of the film with a Berkovich diamond tip. The friction coefficient for the silicon nitride tip on the SiC film is about one half to one third that for silicon nitride sliding on a silicon substrate. By combining nanoindentation and AFM measurements an elastic modulus of ~300 GPa is estimated for these SiC films.

The fast pace of MOSFET scaling is accelerating the introduction of smaller technology nodes to extend CMOS beyond 20nm as required by Moore’s law. To meet these stringent requirements, the industry is seeing an increase in the number of critical layers per reticle set as it move to lower technology nodes especially in a high volume manufacturing operation. These requirements are resulting in reticles with higher feature densities, smaller feature sizes and highly complex Optical Proximity Correction (OPC), built with using new absorber and pellicle materials. These rapid changes are leaving a gap in maintaining these reticles in a fab environment, for not only haze control but also the functionality of the reticle. The industry standard of using wet techniques (which uses aggressive chemicals, like SPM, and SC1) to repel reticles can result in damage to the sub‐resolution assist features (SRAF’s), create changes to CD uniformity and have potential for creating defects that require other means of removal or repair. Also, these wet cleaning methods in the fab environment can create source for haze growth. Haze can be controlled by: 1) Chemical free (dry) reticle cleaning, 2) In‐line reticle inspection in fab, and 3) Manage the environment where reticles are stored. In this paper we will discuss a dry technique (chemical free) to remove pellicle adhesive residue from advanced optical reticles. Samsung Austin Semiconductors (SAS), jointly worked with Eco‐Snow System (a division of RAVE N.P., Inc.) to evaluate the use of Dry Reactive Gas (DRG) technique to remove pellicle adhesive residue on reticles. This technique can significantly reduce the impact to the critical geometry in active array of the reticle, resulting in preserving the reticle performance level seen at wafer level. The paper will discuss results on the viability of this technique used on advanced reticles.

Thin films of Si nanoclusters passivated with oxygen or hydrogen, with an average size of a few nanometers, have been synthesized by thermal vaporization of Si in an Ar buffer gas, followed by subsequent exposure to oxygen or atomic hydrogen. High-resolution transmission electron microscopy and x-ray diffraction revealed that these nanoclusters were crystalline. However, during synthesis, if oxygen was the buffer gas, a network of amorphous Si oxide nanostructures (an-${\mathrm{SiO}}_{\mathit{x}}$) with occasional embedded Si dots was formed. All samples showed strong infrared and/or visible photoluminescence (PL) with varying decay times from nanoseconds to microseconds depending on synthesis conditions. Absorption in the Si cores for surface passivated Si nano- crystals (nc-Si), but mainly in oxygen related defect centers for an-${\mathrm{SiO}}_{\mathit{x}}$, was observed by photoluminescence excitation spectroscopy. The visible components of PL spectra were noted to blueshift and broaden as the size of the nc-Si was reduced. There were differences in PL spectra for hydrogen and oxygen passivated nc-Si. Many common PL properties between oxygen passivated nc-Si and an-${\mathrm{SiO}}_{\mathit{x}}$ were observed. Our data can be explained by a model involving absorption between quantum confined states in the Si cores and emission for which the decay times are very sensitive to surface and/or interface states. The emission could involve a simple band-to-band recombination mechanism within the Si cores. The combined evidence of all of our experimental results suggests, however, that emission between surface or interface states is a more likely mechanism. \textcopyright{} 1996 The American Physical Society.