Max Willi Hermann Braun

Epitaxy at {111}fcc/{110}bcc metal interfaces

Abstract This paper reviews a theory that justifies “lattice match” assumptions in epitaxy and the application of the theory to epitaxy at {111}fcc/{110}bcc metal interfaces. The basic assumptions of the model are (i) that the adatom-substrate interaction has the periodicity and symmetry of the substrate surface, (ii) that the overlayer is crystalline, (iii) that the adatom-adatom interaction is harmonic and (iv) that the ideal epitaxial configurations are the same as for rigid half crystals. The theory correlates epitaxy with lattice match and predicts the occurrence of the Nishiyama-Wassermann and Kurdjumov-Sachs orientations in thick overlayers with astonishing accuracy. In thin overlayers there is good qualitative agreement with empirical data. The theory stresses the need for quantifying the Fourier coefficients.

Experimental verification and validation of a computer model for light-tissue interaction.

Laser light is frequently used in both diagnostics and treatment of patients. For any laser treatment to be effective it is important to deliver the correct dose at the treatment site. Human skin scatters and absorbs laser light in the visible wavelength region, which results in a decrease in fluence some distance into the skin. Computer simulations can be used to predict the fluence at the treatment site. Liquid and solid phantoms were prepared and the optical properties were measured. These values were then used as input values to a commercial software package simulating the different layers of skin representing phantoms. The transmission and reflected fractions of the different phantoms were measured with an integrating sphere and compared with the computer simulations. The results showed very good agreement with the measured values and the model can therefore be used with confidence to predict fluence at any treatment site inside the skin.

Adatom-rystal intermtion potentials. Part 1: A tungsten adatom on a tungsten {110} substrate

Abstract The semi-empirical many-body potential of Finnis and Sinclair with the hardcore modification proposed by Ackland and Thetford, is extended for application to self-adsorption phenomena on a {110} W surface, by introducing an additional parameter obtained by fitting to the empirical value of the adatom-surface migration energy. This surface-modified model is used to calculate (i) the self-desorption energy yielding a value in excellent agreement with experiment; (ii) optimized coefficients for truncated Fourier representations of the adatom-substrate interaction potential—shown to decay rapidly with harmonic order and (iii) force constants and frequencies for normal and lateral vibrations around the adsorption site. The results justify the implementation of Fourier series truncated at low order harmonics for the theoretical description of isolated adsorbates on crystal surfaces.

Diffuse Reflectance Spectroscopy as a Tool to Measure the Absorption Coefficient in Skin: South African Skin Phototypes

In any laser skin treatment, the optical properties (absorption and scattering coefficients) are important parameters. The melanin content of skin influences the absorption of light in the skin. The spread in the values of the absorption coefficients for the South African skin phototypes are not known. A diffuse reflectance probe consisting of a ring of six light delivery fibers and a central collecting fiber was used to measure the diffused reflected light from the arms of 30 volunteers with skin phototypes I–V (on the Fitzpatrick scale). The absorption coefficient was calculated from these measurements. This real‐time in vivo technique was used to determine the absorption coefficient of sun‐exposed and ‐protected areas on the arm. The range of typical absorption coefficients for the South African skin phototypes is reported. The values for the darker South African skin types were much higher than was previously reported for darker skin phototypes. In the analysis, the contributions of the eumelanin and pheomelanin were separated, which resulted in improved curve fitting for volunteers of southern Asian ethnicity without compromising the other groups.

The effect of a structured problem solving strategy on performance in physics in disadvantaged South African schools

Abstract A quasi-experimental study was undertaken to extend first-world research on physics problem solving into disadvantaged South African classrooms. Sixteen urban high schools, involving 189 learners, participated in the study, investigating the effect of a structured problem solving strategy on performance and conceptual understanding. This article focuses on the enhancement of problem solving performance in classroom tests and the midyear examination. The treatment group outperformed the control group by 8% in the midyear examination. Using ANOVA, this increased average score was statistically significant at the.001 level, indicating enhanced problem solving skills. Furthermore, it was demonstrated that the strategy was not transferred successfully to topics studied prior to implementation of the problem solving strategy. A theory is presented to explain the results in terms of the co-development of conceptual understanding and problem solving skill.

Two-dimensional graphite {0001} island on Pt{110} and its vicinal areas: Nucleation, growth mechanism and epitaxial orientation

Based on growth experiments with FEM and computational analysis of the interfacial energy, a growth model has been developed for graphite {0001} monolayers on rounded Pt single crystals: the final graphite layer orientation is initiated during its earliest growth stages in the vicinal area of Pt{110} with the formation of the smallest most stable nucleus (SMSN), which has the orientation graphite {0001}∥Pt{110} and graphite 〈1010〉 ∥Pt〈100〉. Initial layers, which have formed from the SMSNs separately on neighbouring {110} vicinal terraces, snap up and join at the layer edges to saturate graphite σ-bonds. Through this process, one continuous layer is created, the “dumbbell layer”. The dumbbell layer does not form in substrate areas where the depth of the terraces is smaller than the diameter of the SMSN. At sufficiently high supersaturations, the dumbbell layer will initiate further growth. Especially in the case of shorter terraces, the growing layer will make contact with step edges only and, thereby, create channels which can be accessed by diffusing surface particles. The preferred layer growth into surface areas of highest kink density suggests an interaction between strong Pt surface dipoles and the graphite layer.

Computerized pendulum experiment for the introductory physics laboratory

Abstract A Commodore 64 micro computer is used to control a simple pendulum experiment. The computer is interfaced to an interruptable light beam. It measures the pendulum parameters with good accuracy. The experiment is organized in three parts: (i) Tutorial: The student works through a tutorial based on an animated pendulum in preparation for the real experiment. (ii) Experiment (a): The student measures the pendulum parameters using conventional techniques. The computer constantly supervises the experiment and points out mistakes to the student. (iii) Experiment (b): The student uses the computer as an electronic measuring device to examine the relationship between period and amplitude.

Description of adsorption phenomena within the framework of the embedded atom method

We have calculated and compared the self-adsorption parameters (desorption energy, activation energy of adatom surface migration, vibration frequencies of adsorbed atoms, and truncated Fourier representation of the adatom-substrate interaction) for a W adatom on a W {110} surface using embedded-atom methods (EAM) introduced by Gollisch (G), by Finnis and Sinclair (FS) and by Johnson and Oh (JO), using the fitting parameters of the authors for bulk crystals. A small modification noticeably improved the results from the FS model. The equilibrium adsorbate-substrate binding energy, and height were mapped within a unit cell to generate the necessary data. The G and FS models both display double minima.

Theoretical Considerations on the Growth of Metallic Crystalline Superlattices

We deliberate on the conditions favorable to the growth of metallic crystalline superlattices (MCS) with (111) f.c.c./(110) b.c.c. interfaces. We use, with some motivation, equilibrium criteria (i) to justify the occurrence of the Kurdjumov-Sachs (KS) and Nishiyama-Wassermann (NW) orientations, and to show with analyses which also allow for elastic relaxation, that only the NW orientation that occurs at n.n. distance ratios in the interval 0.8 ≲ b f.c.c / a b.c.c ≲ 1.0 can yield the regular orientational relationships required for high quality MCS; ahd(ii) to show that, for the acquisition of the required smoothness of the interfaces, which is mainly determined by the growth mode - monolayer-by-monolayer (FM = Frank-van der Merwe) or island nucleation and growth (VW ≊ Volmer-Weber) mode - it is desirable to use material combinations with small surface free energy mismatch. Only then can VW growth (which inevitably occurs in each superlattice period) at relatively high supersaturation be FM-like and with low density of defects.

LESSON OBJECTIVES IN TERMS OF KNOWLEDGE AND COGNITION

A clearly stated lesson objective is considered an essential component of a well-planned lesson. Many teachers of Technology, a relatively new subject in South African schools, teach Technology with rather limited training both in content and methodological approaches. This study sought to investigate and classify lesson objectives framed or implied by teachers in their lesson plans, according to knowledge and cognitive process dimensions. The two-dimensional Taxonomy Table introduced by Krathwohl was adapted for Technology and formed the framework for this study. A mixed modal study was used to investigate sampled lesson objectives described by Technology teachers. Explicitly stated or inferred objectives were classified according to Krathwohl’s Taxonomy Table, after which the objectives in each cell were counted to establish the frequency of occurrence of objectives in each cell in the quantitative phase. In the qualitative phase, specific cases of explicit or inferred objectives were selected in order to examine and discuss lesson objectives in terms of knowledge and cognitive dimensions. It was found that most of the objectives lie on the lower level of the knowledge dimension and address mainly Factual and Conceptual knowledge. Lesson objectives were also positioned low in terms of cognitive levels, with Remember and Understand occurring most frequently in the cognitive process dimension. The paucity of lesson objectives that required complex knowledge and higher-order thinking is disappointing: teaching limited to less complex knowledge types and lower cognitive levels fails to develop learners’ higher-order thinking skills required for further study or independent practice in work environments. A recommendation flowing from the study is that, during pre-service training and in-service teacher support programmes, the importance of clear lesson objectives should be emphasised and assessments planned for such lessons should closely match the lesson objectives. Further research is also needed regarding the reasons for which low cognitive demands are made in the teaching of Technology.