S. Kubsky

Ion beam analysis of the hydration of tricalcium silicate

Tricalcium silicate is the major constituent of Portland cement, and the kinetics of its hydration is a major topic in concrete technology. Nuclear resonance reaction analysis using 15N has been applied to measure the evolution of the hydrogen profile. During the first few hours, the induction period, the hydrogen diffusion is controlled by a 10–20 nm thick surface layer. To observe this layer, the beam energy resolution must be on the order of 10 keV or less. This has been achieved at the dynamitron tandem accelerator at the Ruhr Universitat Bochum.

Gamma-ray detection with a 4π NaI spectrometer for material analysis

Abstract A γ-ray detection system with nearly 4π geometry for applications in materials science is described. The energy resolution of the NaI(Tl)-bore hole detector was found to be about 2% for 10 MeV γ-rays. The total detection efficiency has been determined to be above 80% for γ-energies between 0.5 and 20 MeV. A precision of the efficiency calibration of about 2% has been obtained for a wide range of γ-energies. The experimental results were found to be in excellent agreement with computer simulations using the GEANT computer code. The high efficiency of the system combined with its relatively high energy resolution opened a wide range of applications. Examples are the simultaneous detection of light isotopes in thin films by (d,pγ) nuclear reactions, high depth resolution profiling of isotopes by narrow, low-energy resonances in (p,γ) reactions, and hydrogen profiling using the 6.4 MeV resonance in the 1H(15N,αγ)12C reaction.

Ion beam synthesis of buried CoSi2-structures

Ion beam synthesis of buried CoSi2 microstructures was performed by high energy ion projection (HEIP) implantation of cobalt into silicon. Ion beam synthesis (mesotaxy) means stoichiometrically high-fluence ion implantation into a heated sample and subsequent annealing of the specimen. The use of the ion projection technique permits an in situ contact mask-free structuring with a sub-micron lateral resolution. The HEIP set-up consists of a superconducting solenoid lens, which projects the structures of a stencil mask onto the target and is driven by a 4 MV tandem accelerator. Because of the high demagnification of the structures the ion current density at the target is increased by two orders of magnitude for small structures. n nThe analysis of the produced structures by optical and electron microscopy, Rutherford backscattering spectroscopy and electrical characterisation shows the good applicability of the Bochum HEIP for IBS.

Heavy ion beam measurement of the hydration of cementitious materials.

The setting and development of strength of Portland cement concrete depends upon the reaction of water with various phases in the Portland cement. Nuclear resonance reaction analysis (NRRA) involving the (1)H((15)N,alpha,gamma)(12)C reaction has been applied to measure the hydrogen depth profile in the few 100 nm thick surface layer that controls the early stage of the reaction. Specific topics that have been investigated include the reactivity of individual cementitious phases and the effects of accelerators and retarders.

Concept and status of the new sample preparation and analyzing facility at Bochum

Abstract The technical conditions of the activities at the Dynamitron Tandem Accelerator Laboratory at Bochum in the field of ion beam modification and analysis of thin films will be improved. A new 500 kV accelerator with high energy resolution of the ion beams as well as a UHV system consisting of several chambers are presently being built up. The beam lines of the new accelerator and of the 4 MV Tandem are interconnected, providing a wide range of ion species and energies at the target sites. The UHV system not only allows the use of ion beam techniques but also provides standard electron spectroscopic techniques for surface analyses. For sample preparation techniques such as standard furnace evaporation, electron gun evaporation and rapid thermal processing are available.

Study of cement chemistry with nuclear resonant reaction analysis

Nuclear Resonance Reaction Analysis (NRRA) has been applied for the first time to measure the development of the hydrogen depth profile in the early stages of hydration of tricalcium silicate, the major constituent of Portland cement. To obtain the best spatial resolution, it is necessary to have good beam energy resolution. Three regions were observed in the profile, whose H concentrations were obtained by using the 1H(15N,αγ)12C reaction. By analogy with the hydration of alkali silica glasses, these are identified as the reaction-controlling surface layer, the gel layer, and the calcium-leached layer. The surface layer has an H concentration and thickness consistent with a few unit cells (1.1 nm) of tobermorite-like material. The inner regions exhibit diffusion-controlled growth with time until the hydrogen concentration approaches that of the surface layer at 4.25±0.07u200ahrs. This event marks the end of the induction period.

Production of Thin Epitaxial Films Using Ion Beam Deposition

An ion beam deposition system to produce isotopically pure epitaxial thin films of different materials has been designed and built. Using negative ions, problems due to mass interference with molecular ions could be significantly reduced, thus allowing the production, for instance, of 29 Si films of high purity. By including thermal processing and different analytical facihties to this system, in-situ studies of atomic transport as well as atomic exchange processes can be studied directly.

Progress in Nanoscale Studies of Hydrogen Reactions in Construction Materials

Nuclear resonance reaction analysis (NRRA) has been applied to measure the nanoscale distribution of hydrogen with depth in the hydration of cementitious phases. This has provided a better understanding of the mechanisms and kinetics of cement hydration during the induction period that is critical to improved concrete technology. NRRA was also applied to measure the hydrogen depth profiles in other materials used in concrete construction such as fly ash and steel. By varying the incident beam energy one measures a profile with a depth resolution of a few nanometers. Time-resolved measurements are achieved by stopping the chemical reactions at specific times. Effects of temperature, sulfate concentration, accelerators and retarders, and superplasticizers have been investigated. Hydration of fly ashes has been studied with synthetic glass specimens whose chemical compositions are modeled on those of actual fly ashes. A combinatorial chemistry approach was used where glasses of different compositions are hydrated in various solutions for a fixed time. The resulting hydrogen depth profiles show significant differences in hydrated phases, rates of depth penetration and amount of surface etching. Hydrogen embrittlement of steel was studied on slow strain rate specimens under different corrosion potentials.

Interconnected UHV facilities for materials preparation and analysis

Abstract A UHV system for in-situ preparation and analysis of ultra thin films has been built. The system includes a rapid thermal processing furnace which allows production of samples over a wide range of temperatures and pressures using isotopically enriched gases. XPS, AES, and LEED analyses provide information on the surface structure and composition. With a transportable UHV chamber, the samples can be transferred to a 4π γ-ray spectrometer facility (in UHV), where analytical ion beam methods can be used to determine isotopic depth profiles and total amounts of isotopes in the films. Furthermore, an ion beam deposition facility (in UHV) can produce isotopically enriched silicon films on Si substrates for in situ isotopic tracing.