Isothermal calorimetric investigation of a reversible reaction in rapidly solidified Fe/sub 40/Fi/sub 40/B/sub 20/ Page: 3 of 7
This article is part of the collection entitled: Office of Scientific & Technical Information Technical Reports and was provided to Digital Library by the UNT Libraries Government Documents Department.
The following text was automatically extracted from the image on this page using optical character recognition software:
ISOTHERMAL CALORIMETRIC INVESTIGAT’ON
OF A REVERSIBLE REACTION IN RAPIDLY SOLIDIFIED Fe.0 Ni40 B10
J.R. Cost. R.O. Ellioti*. and J. T. Stanley6
Los Alamos National Laboratory. Los Alamos. N M. 87545. 'JSA
‘Ccnsuiiani. 'On itbbaticai ieav« from Arizona Sine University Tempo. AZ
A new method ii described for Isothermal calorimetric measurement
of time-dependent reversible reactions in which the rate oi heat flow Is
small compared to the noise and zero drift of the measurement. Results
using this method are presented which measure enthalpy changes as-
sociated with reversible phase changes In a rapidly solidified FeiC Ni,0
Studies of reversible phase changes induced by relatively small changes in temperature
provide an important method for furthering our understanding of kinetic phenomena in
rapidly solidified alleys. Typically, time-dependent phase changes are monitored
isothermally by measurement of some physical property such as electrical resistance, elastic
modulus, or density Isothermal measurements of the enthalpy changes concomitant with the
transformations have not been widely used tA-stndy the kinetics because calorimetric
techniques measure the rate (as opposed to the amount) of heat flow so that accurate
determinanon of enthalpy changes is not possible unless the reaction occurs relatively
rapidly. For example. Scott et al , In a study of reversible relaxation to a new state of
short-range order in “.morphous Fe4# Ni4a B)(l report isothermal calorimetric measurements
at 380°C and 330°C which show heat flow but. because the effect is so small, do not allow
cither the magnitude of the enthalpy change or the kinetics of the reaction to be determined.
The problem of making calorimetric measurements to monitor phase changes which
occur with relatively long time constants can best be understood by assuming an example of
simple exponential decay toward a new enthalpy state following a change in temperature at
aH(i) . AHI#I (l exp(-t/t)| (1)
where aHI0I is the total enthalpy chango and t is the time constant for the reaction. The rate
of this leaction. which is what is measured by the calorimeter, is
aAH(t)/dt - AHI0I exp(-t/t)/T . -2)
Because the rate of heat release is inversely proportional to the time constant t. then when the
latter beaomes lane, as it does at low temperature, the calorimeter signal beoomes
increasingly small until it becomes lost in the experimental noise of the measurement. This
inability to follow reactions with long time constants is unique for calorimetric measurments
compared to other physical properties. Because of this difficulty, special techniques must be
used if reaction kinetics are to be studied calorlmetrically. The primary purpose of this paper
is to describe and demonstrate such a special technique for the reversible reaction which we
have observed in as-crystallized Fe,a Ni4t B,a.
Here’s what’s next.
This article can be searched. Note: Results may vary based on the legibility of text within the document.
Tools / Downloads
Get a copy of this page or view the extracted text.
Citing and Sharing
Basic information for referencing this web page. We also provide extended guidance on usage rights, references, copying or embedding.
Reference the current page of this Article.
Cost, J.R.; Elliott, R.O. & Stanley, J.T. Isothermal calorimetric investigation of a reversible reaction in rapidly solidified Fe/sub 40/Fi/sub 40/B/sub 20/, article, January 1, 1983; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc1069992/m1/3/: accessed March 26, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.