Paul E. Thoma

Effect of composition on the amount of second phase and transformation temperatures of NixTi90−xHf10 shape memory alloys

Abstract The influence of Ni and Ti content of Ni x Ti 90− x Hf 10 shape memory alloys on the presence of a second phase [Ni(Ti+Hf) 2 ] and transformation temperatures (TTs) is determined. The Ni content of the alloys in this investigation ranges from 49.0 to 50.2 at.% in increments of 0.2 at.%. The presence of second phase is investigated by metallographic examination and image analysis of homogenized alloys (900°C for 360 ks), and their TTs are determined by differential scanning calorimetry. The amount of second phase decreases with increasing Ni content and is least with a Ni content of 50.2 at.%. The martensite to austenite and austenite to martensite TTs remain constant as the Ni content increases from 49.0 to 50.0 at.%. A rapid decrease in the TTs is observed when the Ni content of the Ni x Ti 90− x Hf 10 alloy exceeds 50.0 at.%.

A model for the relative environmental stability of a series of polyimide capacitance humidity sensors

A capacitance humidity sensor is used as a test device to characterize the performance of 13 polyimide films in relative humidity sensing applications. This sensor has a multilayer, free-standing film construction. It consists of a humidity sensitive polyimide dielectric core and conductive layers consisting of carbon filled polysulfone on each side of the polyimide film to form a capacitor. Thirteen polyimide films, including commercial polyimides and films of novel chemistry, are investigated to determine the long term stability of sensors using the films exposed to 85°C/85% RH for a total of 28 days. Differences in film chemistry are used to interpret trends in the environmental stability of the films.

Electret and rhodium gas sensors

Abstract The organic gas-sensing characteristics of an electret and a rhodium probe are presented and discussed. The electret consists of a negatively charged PFA polymer layer bonded to a metal back. The electret detects changes in organic gas concentration as a result of dipole-charge and induced dipole-charge interactions between the adsorbed gas and negative charge in the PFA polymer layer. The rhodium-plated probe senses changes in organic gas concentration due to electron donor/acceptor interactions between the adsorbed gas and surface of the probe. The rhodium probe is able to distinguish between strong and weak electron donor gases by the polarity of the response signal.

A Mechanism for Supercooling in Organic Liquids

In this investigation, a mechanism for supercooling in organic liquids is formulated. By comparing the melting temperature and spontaneous freezing temperature of the chemicals evaluated with their molecular characteristics, the factors promoting supercooling are developed. The results obtained indicate that the following molecular characteristics promote supercooling in organic liquids: sharing of electrons between the atoms of a molecule; A three-dimensional chemical structure; A permanent, three-dimensional, and partially charged pocket within the chemical structure; A partially charged projection having a charge opposite that of the pocket and located on the side of the molecule opposite that of the pocket.

The Influence of Hafnium Content, Cold Work, and Heat Treatment on the R-Phase Transformation of Niti Based Shape Memory Alloys

The R -phase transformation of a Ti-rich NiTi shape memory alloy (SMA) and two ternary SMAs having the compositions Ni 49 Ti 51-X Hf x with lat% and 3at% Hf, has been investigated. The influence of cold work (CW) and heat treatment (HT) on the R -phase transformation is analyzed thermally using Differential Scanning Calorimetry (DSC). Results show that the R -phase transformation depends on the SMA composition as well as the CW and HT conditions in a complex manner. For example, the formation of R -phase upon cooling from austenite ( A ) is increasingly suppressed with the substitution of Hf for Ti. For the ternary SMA with 3at% Hf, the A→R and R→A transformations are observed only at relatively large amounts of CW (above approximately 40%) and at a high HT temperature (450°C). DSC results also show that for the Ti-rich NiTi and the ternary SMA containing lat% Hf, the A→R and R→A transformation temperatures (TTs) are insensitive to cold work at a HT temperature of 450°C. However, at a lower HT temperature of 350°C, the TTs are found to decrease with increasing CW. For a given CW, the A→R and R→A transformations decrease with decreasing HT temperature and the effect is greatest at high CW (>50%) conditions. An effort is made to identify the factors responsible for the observed behavior in the R -phase transformation.

Development of the Glassy State of Benzophenone and Effect of Heating Rate from the Glassy State on Solidification

Benzophenone supercools to a glass when cooled to {minus}100 C. In fact, it is difficult to freeze benzophenone on cooling. In this investigation, the effect of cooling rate and the minimum cooling rate to obtain benzophenone as a glass is determined. From the glassy state, the influence of heating rate on the solidification temperature of benzophenone is determined. When heated at 3 C/min., solidification starts at about {minus}29 C. Upon additional heating, melting usually starts at about +24 C, which is 23 C lower than the solid equilibrium structure melting temperature of 47 C. Occasionally the solid that forms at about {minus}29 C undergoes a solid state phase transformation at about +22 C, when heated at 3 C/min. If this solid state phase transformation occurs, then the solid benzophenone starts to melt at 47 C. When solid benzophenone with the equilibrium structure is cooled to {minus}100 C, no solid state phase transformation occurs. It appears that the structure that solidified at {minus}29 C is metastable.

The Interactive Response of Sma Springs which Control the Position of Dampers in Personal Environments

Personal Environments ™ is a Johnson Controls, Inc. system for the control of indoor office environments. In this system, four SMA coiled extension springs interact to control the position of two dampers. Two opposing SMA springs connected to one damper control the percentage of primary conditioned air entering the system, and two opposing SMA springs connected to the second damper control the percentage of room air reentering the system. The interactive operation of the SMA springs is discussed and a model is presented on the interactive response of the SMA springs. The SMA spring transformation temperatures, force - strain relationship, and the experimentally measured interactive response to temperature are presented and discussed.

The Effect of Thermal Processing in Vacuum on the Water Adsorption Characteristics of Carbon Black

Commercially available carbon black contains oxygen complexes on its surface that affect the surface properties of the carbon. Water adsorption on the surface of carbon black is influenced by the amount and type of oxygen complexes present. When carbon black is heated in vacuum at sufficiently high temperatures, removal of the oxygen complexes occurs and the surface of the carbon particles is modified. The amount of water adsorbed by the carbon is dependent on the vacuum heat treatment temperature. As the heat treatment temperature increases, water adsorption on the carbon decreases. Commercially available electrically conductive carbon black adsorbs from 1.25% to 2.50% water when exposed to 50% relative humidity for 24 hours at 25°C. This variation in water adsorption is due to a difference in the amount of oxygen complexes on the surface of the carbon. The carbon with more oxygen complexes adsorbs more water. However, when this carbon black is heat treated at 1200°C for 4 hours in a vacuum of 1 × 10 −5 torr or better, the water adsorbed by the carbon is 0.4% when exposed to 50% relative humidity. Data showing the dependence of water adsorption on vacuum thermal processing are presented and discussed.

Anisotropy of tensite recovery stress and two way memory effect of cold rolled TiNi sheet

The texture of cold rolled and heat treated TiNi sheet has been measured and designated as {110} p . The.anisotropy of the tensile recovery stress, the stress dependence of the transformation temperatures, and the development of the two-way memory effect during thermal fatigue have been measured. The resulting changes of texture after thermal fatigue are presented and explained. The observed effects are explained on the basis of the parent texture and the specific martensite variants which are formed according to the lattice correspondence between austenite and martensite.