Four-point probe measurements using current probes with voltage feedback to measure electric potentials

We present a four-point probe resistance measurement technique which uses four equivalent current measuring units, resulting in minimal hardware requirements and corresponding sources of noise. Local sample potentials are measured by a software feedback loop which adjusts the corresponding tip voltage such that no current flows to the sample. The resulting tip voltage is then equivalent to the sample potential at the tip position. We implement this measurement method into a multi-tip scanning tunneling microscope setup such that potentials can also be measured in tunneling contact, allowing in principle truly non-invasive four-probe measurements. The resulting measurement capabilities are demonstrated for BiSbTe$_3$ and Si$(111)-(7\times7)$ samples

Breakdown behavior of electronics at variable pulse repetition rates

The breakdown behavior of electronics exposed to single transient electromagnetic pulses is subject of investigations for several years. State-of-the-art pulse generators additionally provide the possibility to generate pulse sequences with variable pulse repetition rate. In this article the influence of this repetition rate variation on the breakdown behavior of electronic systems is described. For this purpose microcontroller systems are examined during line-led exposure to pulses with repetition rates between 1 KHz and 100 KHz. Special attention is given to breakdown thresholds and breakdown probabilities of the electronic devices

On Extracting Mechanical Properties from Nanoindentation at Temperatures up to 1000∘^{\circ}C

Alloyed MCrAlY bond coats, where M is usually cobalt and/or nickel, are essential parts of modern turbine blades, imparting environmental resistance while mediating thermal expansivity differences. Nanoindentation allows the determination of their properties without the complexities of traditional mechanical tests, but was not previously possible near turbine operating temperatures. Here, we determine the hardness and modulus of CMSX-4 and an Amdry-386 bond coat by nanoindentation up to 1000$^{\circ}$C. Both materials exhibit a constant hardness until 400$^{\circ}$C followed by considerable softening, which in CMSX-4 is attributed to the multiple slip systems operating underneath a Berkovich indenter. The creep behaviour has been investigated via the nanoindentation hold segments. Above 700$^{\circ}$C, the observed creep exponents match the temperature-dependence of literature values in CMSX-4. In Amdry-386, nanoindentation produces creep exponents very close to literature data, implying high-temperature nanoindentation may be powerful in characterising these coatings and providing inputs for material, model and process optimisations

Fabrication of a thin silver nanowire composite film and investigation of a patterning technique.

Electrically conductive polymers encompass an exciting field of research for applications in dye-sensitized solar cells (DSSC’s). DSSC’s possess several advantages over other types of solar cells. They offer the potential for high quantum efficiency, solar conversion efficiency approaching that of traditional silicon panels, rapid charge transfer kinetics for photo-excited electrons, mechanical flexibility, and cost efficient manufacturing processes. However, key drawbacks to their large scale production and performance lifetime lie in their reliance on costly indium tin oxide (ITO), fluorinated tin oxide (FTO), and platinum for electrode materials, and the mechanical fragility inherent to a liquid electrolyte layer component. Much of the current research in the field concerns identifying an effective solid material to replace the liquid electrolyte presently used in DSSC’s. Within this field of research, electrically conductive polymers (ECP’s) have attracted much interest. One such ECP, PEDOT:PSS [poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)] has a combination of high flexibility, electrical conductivity, and optical transmittance, making it a viable candidate. Another subset of this research field involves qualifying a cheaper candidate for replacing the costly platinum electrode of DSSC’s. Metallic nanoparticles, whose characteristic dimension is on the order of one to hundreds of nanometers, possess unique physical properties emergent at their size scale. Particularly, nanowires of highly conductive metals such as silver deposited on the surface of ECP’s raises the possibility of an electrode made using simple bulk processing techniques and using drastically lower quantities of expensive materials for the electrode than typical for DSSC’s while offering the same level of conductivity. Furthermore, an interesting challenge lies in patterning the nanowire network using a simple bulk driving force. Achieving facile nanowire alignment, and thus anisotropic electrical conductivity, opens the door to a variety of applications extending beyond solar cell electrodes, such as flexible nanoscale circuitry. The present thesis describes and evaluates the physical properties of composite thin films of PEDOT:PSS with silver nanowire networks. A simple laboratory scale method for creating a randomly aligned silver nanowire network on PEDOT:PSS is first studied, revealing impressive conductivity increases on the order of 120 versus the bare PEDOT:PSS film. Sheet resistances of the composite films average 37 Ω/□ with average ultraviolet and visible light (UV-Vis) transmittances of 63%. UV-Vis transmittance correlates inversely with the surface concentration of nanowires as expected, with a power regression fit to the data (R2 = 0.96). Of note, no strong correlation is detected between sheet resistance and nanowire surface concentration within the range tested. Additionally, a method for aligning silver nanowires on PEDOT:PSS using a magnetic field is explored. Unfortunately, no significant anisotropy in conductivity is measured using the conditions outlined in these experiments, and explanations are discussed leading into recommendations for future work

Etched graphene quantum dots on hexagonal boron nitride

We report on the fabrication and characterization of etched graphene quantum dots (QDs) on hexagonal boron nitride (hBN) and SiO2 with different island diameters. We perform a statistical analysis of Coulomb peak spacings over a wide energy range. For graphene QDs on hBN, the standard deviation of the normalized peak spacing distribution decreases with increasing QD diameter, whereas for QDs on SiO2 no diameter dependency is observed. In addition, QDs on hBN are more stable under the influence of perpendicular magnetic fields up to 9T. Both results indicate a substantially reduced substrate induced disorder potential in graphene QDs on hBN

Detection of transient disturbing signals on PC boards

This paper shows a possibility to visualize signal propagation in electronic circuits. Instead of using various galvanic measurement points all over the circuit, a test method is shown which measures the radiated field of the printed circuit board. By use of a 2-dimensional positionable field probe it is possible to get an overview over the signals running on the different parts of the PCB. In order to measure transient disturbing signals and distinguish them from normal device operation, problems of probe design and triggering need to be discussed

The influence of social support on teacher self-efficacy in novice agricultural education teachers /

Teacher self-efficacy affects student achievement, job satisfaction, and teacher retention. Although the benefits of social support have been extensively studied in medicine and psychology, limited research has been completed in education to evaluate the ways in which social support influences teacher self-efficacy. The purpose of this descriptive-relational study was to determine the influence of sources and types of support on teacher self-efficacy in novice agricultural education teachers. The target population was novice teachers of agriculture from Illinois (n = 192) and Indiana (n = 104). Teachers' perceptions of support from three non-school sources (e.g., spouse or partner, family, friends) and six school sources (e.g., administrators, teachers at school, teachers in FFA section or district, students, parents, community) of support within three support constructs were used to predict the contribution of social support on teacher self-efficacy. Novice agricultural education teachers' perceptions of support from school sources -- predominantly students and community -- explained 25.3 percent of the variance in teacher self-efficacy. Whereas mastery experiences are widely recognized as the primary source of self-efficacy, the results from this study imply the support (i.e., verbal or social persuasion) novice agricultural education teachers perceive from students and community are the most significant predictors of teacher self-efficacy. These findings advocate the need for novice teachers of agriculture to develop quality relationships with students and community members to increase teacher self-efficacy and potentially improve teacher retention.Dr. Jon Simonsen, Dissertation Supervisor.Includes bibliographical references (pages 205-221)

Purification of prostaglandin E2-9-oxoreductase from human decidua vera

AbstractProstaglandin E2-9-oxoreductase (PGE2-9-OR), the enzyme which converts prostaglandin E2 (PGE2) to prostaglandin F2α (PGF2α), has been detected in human decidua vera. A 105-fold purification was achieved when the centrifuged homogenate was fractionated sequentially by DEAE—Trisacryl, hydroxyapatite—agarose gel, ultrogel AcA 44 and Matrex gel blue A gel chromatographies. The following kinetic constants for PGE2-9-OR have been obtained. The equilibrium constant with respect to PGE2 is 83 μM, the Michaelis constant, Km, for PGE2 is 80 μM, for NADPH 1.6 μM. The maximal velocity for the forward reaction is V1 = .203 pmol/min. The enzyme was inhibited by progesterone, oestradiol-17β, cortisol and pharmaceutical drugs. An activating effect could be demonstrated with Ca2+ and oxytocin. The occurrence of PGE2-9-OR in the decidua vera suggests that this enzyme may be responsible for the transformation of PGE2 to PGF2α in these tissues. This may be an important mechanism for the initiation and maintenance of uterine contractions