142
Anal. Chem. 1982, 5 4 , 142-143
which remain constant for long periods of time.
LITERATURE CITED National Research Councll "Formaldehyde and Other Aldehydes"; National Academy Press: Washington, DC, 1981; Chapter 7. Gunby, Phll J. Am. Med. Assoc. 1980, 243, 1697-1703. Cares, Janet W. Am. Ind. Hyg.Assoc. J. 1988, 2 9 , 405-410. Klrn, Walter S.; Geraci, Charles L., Jr.; Kupel, Richard E. Am. Ind. Hyg.ASSOC.J. 1080, 41, 334-339. Beasley, Ronakl K.; Hoffman, Catherlne E.; Rueppel, Meivln L.; Worley, Jlrnrny W. Anal. Chem. 1080. 5 2 , 1110-1114. Colket, Meredith E.; Naegell, Davkl W.; Dryer, Frederick L.; Glassman, I.Envlron. Scl. Techno/. 1974, 8, 43-46. Godln, J.; Bouiey, G.; Boudene, C. Anal. Lett. 1978, AN, 319-326. Schnlzer, Arthur W.; Fisher, Gene J.; MacLean, Alexander F. J. Am, Chem. SOC. 1953, 7 5 , 4347-4348.
(9) Intersoclety Committee. "Methods of Air Sampling and Analysis"; Arnerlcan Public Health Assoclation: Washington, DC, 1977; pp 303-307. (10) "NIOSH Manual of Analytlcal Methods, 2nd ed."; DHEW (NIOSH) Publlcation No. 77-157A: Clncinnatl, OH, 1977; P and CAM 125. (11) Mlksch, Robert R.; Anthon, D. W.; Fanning, L. 2.; Hollowell, C. D.; Revzan, K.; Glanville, J. Anal. Chem. 1081, 53, 2118. (12) Nelson, Gary 0. "Controlled Test Atmospheres, Principles and Techniques"; Ann Arbor Science: Ann Arbor, MI, 1971; p 103.
RECEIVED for review May 26,1981. Accepted September 28, 1981, This work was supported by the Assistant Secretary for Conservation and Renewable Energy, Office of Buildings and Community Systems, Buildings Division of the U.S. Department of Energy, under Contract No. W-7405-ENG-48.
Pyroelectric Enthalpimetric Sensor Hamid Rahnamal' The Moore School of Electrical Engineering, Universlty of Pennsylvania, Philadelphia, Pennsylvania 19 104
Thermal methods for investigating material properties require a sensitive and reliable thermal sensor. The two types of thermal detectors generally used are resistance thermometers (hot wires) and thermocouples. The former has been widely employed in katharometry, gas chromatography ( I ) , anemometry (2) and turbulance (3),while thermocouples have been used in differential thermal analysis, calorimetry ( 4 ) , thermometric titrimetry, enthalpimetry (5),and radiometry (6). For thermal analysis a t the microcalorie level it is necessary to use more sensitive thermal detectors. Pyroelectric materials are suitable candidate materials for this purpose. The principal advantages of pyroelectrics over thermocouples are their higher temperature resolution (less than 0.1 "C), W/(HZ)'/~),shorter relower noise equivalent power ( sponse time (- lo4 s), generally lower sensitivity to ambient temperature change (3,and generally uniform sensitivity over their entire surface (8). Recently there has been growing interest in pyroelectric materials for these various applications (9, 10). One new application has been in anemometry (11). The pyroelectric anemometers have high gas flow rate accuracy ( l % , full scale, at flow rate from 0 to 100 cm3/min and near atmospheric pressure) and linear response and can be conveniently fabricated by standard integrated circuit technology. In the present study, an experiment has been conducted to detect heat exchange due to the melting of alloys. Two different low melting point alloy compositions were chosen for this experiment. Potential application of LiTa03 in analytical chemistry is briefly discussed.
DEVICE STRUCTURE AND MEASUREMENT Czochralski grown, z-cut LiTa03 (lithiumtantalate) plates, one side polished, were used to carry out the experiment. Gold films 5 pm thick were evaporated on the polished side to provide two metal electrodes (the sample electrode and the reference electrode) and 0.5 pm thick Au was evaporated on the back side for the ground contact. The pyroelectric plate used was 6 X 6 mm2 and 0.129 mm thick. The finished structure was then mounted on two parallel support wires. These wires were attached to the edges of the back face of the sample using air-drying silver paste. Silver paste was also used to make electric contacts between fine aluminum wires and the two electrodes. A Unitron high-intensity lamp was used to provide roughly uniform irradiation on the back side of the device. The irradiated area was slightly larger than 'Present address: INTELSAT, 490 L'Enfant Plaza, S.W., Washington, DC 20024. 0003-2700/82/0354-0 142$01.25/0
the sample area. A 160 Keithley digital multimeter with high input impedance (- 100 MQ)was used to measure the pyroelectric response. The resistance of the device was greater than 10 MQ (the resistivity of LiTaOs is about 1013Q cm). The output of this meter was connected to a chart recorder. Two different low melting point alloy compositions: Wood's metal (Bi (50%), Pb (25%), Sn (12.5%), Cd (12.5%) with a melting point of 70 "C) and a ternary eutectic (In (51%),Bi (32.5%),Sn (16.5%)with a melting point of 60.5 "C) with weights of 30 and 17.0 mg, respectively, were placed on the sample electrode. Figure 1shows schematically (a) the top view and (b) the cross sectional view of the sample.
RESULTS AND DISCUSSION The pyroelectric voltage response of a pyroelectric material is proportional to the time derivative of the temperature (9). The differential signal of the device is therefore a measure of the average temperature change difference between the two electrodes. Figure 2 shows the variation of the differential pyroelectric signal vs. time when the light is on (for heating) and off (for cooling). The dashed line is for the case when there was no alloy on the sample electrode. At point "a" on this figure, the sample was exposed to high-intensity light. Consequently the signal rises rapidly from 0 to 0.94 V. A large pyroelectric signal from each electrode upon the sudden exposure of the sample to the light was expected since the pyroelectric response is proportional to the time rate of temperature change. However, the nonzero differential signal during the initial stage of both heating and cooling (see points a and b in Figure 2) is attributed to the different heat capacities of the sample and reference "electrodes" and to the nonuniformities in the incident light. As the sample warms up, two peaks (cy and (3) appear, which designate the melting of each alloy (with 10 "C temperature difference). About 12 s from the starting point, the sample is at steady state and therefore the electrode response and differential signal vanish. No stationary short circuit current or photovoltaic emf was observed under light illumination. This indicates that there was no anomalous photovoltage in the LiTaO, used in this study (12). When the light is turned off, the sample cools down. The two peaks (aand (3) appearing in the cooling curve are due to the solidification of the alloys. For performance evaluation of the device and for comparison with those of conventional DTA/DSC instruments, quantitative information on the fusion process is needed. This has been left to be determined in further study. However, 0 1981 Arnerlcan Chemical Society
143
Anal. Chem. 1982, 5 4 , 143-146 -.6rnm+
-.-Reference
eiectrode
r=-\- -~
:>. Ailay
,_Contact wire
Ibl
-. LiTa03
=3-'IiIIflI --
Radiation
Flgure 1. (a) Top view aind (b) cross sectionial view of the sample.
4 OAV-+-&.Sec.+
I
resistor can be readily made. Third, the measured NEP in LiTaO, is 1.3 X 10-l" W/(Hz)'12 (14). Consequently, in principle, detection of heat exchange as low as a nanocalorie would be possible. Finally, in contrast with other thermal detectors, the pyroelectric current response depends on the rate of change of temperature rather than on the temperature itself. For this reason the maximum response is achieved at times shorter than the thermal relaxation time of the element, so that pyroelectrics are basically higher-frequency devices than other thermal detectors. The disadvantage of LiTaO, for DTA applications appears to be the low operational temperature, limited by Curie temperature (618 "C ( 7 ) ) . Yet, some pyroelectrics have been shown to have a higher Curie temperature (LiNbO,, for example, has a Curie temperature 1210 O C ( 7 ) ) .
CONCLUSION
n
The structure described in this study can be used to investigate phase transformations involving small amounts of material. Similar structures can be employed in various disciplines for thermal analysis and calorimetric measurement.
ACKNOWLEDGMENT The author thanks J. N. Zemel for useful discussion and for reading the original manuscript. LITERATURE CITED
1
heating
Flgure 2. Pyroelectric differential signal vs. time. Two low meting polnt alloy compositions were lplaced on the sample electrode.
there are several significant features of LiTaO, which should be mentioned at this point. First, LiiTaOs has favorable mechanical properties; it can be polished and cleaved into wafers, each wafer can be scribed and broken into small pieces as is commonly done with silicon wafers. Therefore, by chosing suitable dimensions for a specific purpose in thermal analysis, a desired heat transfer condition can be easily maintained. Moreover, the designed1 device can be mass produced. Second, the fabrication of thin metal film geometries using photolithography is a standard component of current integrated circuit technology. With this technique, (evaporatedNichrome and gold thin films on LiTaO, wafer are routinely etched into small elements to produce heater resistor and electrical contact, respectively (13). Thus, for DTA applications, the photolithographically defined electrode's areas and heater
Keulemans, A. I.M. "Gas Chromatography", 2nd ed.; Verver, C. G., Ed.; Reinholt: New York, 1969; p 91. Sandborn, W. A. "Resistance Temperature Transducers"; Metrology Press: Fort Collins, CO, 1972. Frost, W., Nlaulden, T. H., Eds. "Handbook of Turbulence, Vol. 7, Fundamentals and Applications"; Plenum Press: New York and London, 1977; p 315. Gorn, P. D. "Thermoanalytical Methods of Investlgation"; Academic Press: New York, 1965. Barthel, J. "Thermometric Titrations"; Wiley: New York, 1975; Chapter 9, p 159. Stevens, N. 8. I n "Semlconductor and Semimetals, Vol. 5, Infrared Detectors"; Wlllardson, R. D., Bear, A. G., Eds.; 1970; "Radiation Thermopiles", Chapter 7, p 287. Beerman, H. P. hfraredfhys. 1975, 15, 225. Landa, I.; Kremen, J. C. Anal. Chem. 1974, 4 6 , 1964. Llnes, M. E.: Glass, A. M. "Principles and Applications of Ferroelectrics and Related Materlals"; Clarendon Press: Oxford, 1977; Chapter 16, p 561. Lang, S. B. "XI1 Literature Guide to Pyroelectriclty 1980"; Ferroelectrics, 1961, 34, 71. Rahnamai, H.; Zemel, J. N., "Pyroelectric Anemometers-Preparation and Flow Vuiocity Measurements", Sensors and Actuators, Vol. 2, No. 1, Aug 198'1. Part of thls work is also published in the Proceedings of IEEE International Meetlng on Electron Devlces, Dec 1980, p 680. Brody, P. S. J. Solid State Chem. 1975, 12, 193. Young, J. C., Frederick, R., University of Pennsylvania, prlvate communication. Roundy, C. B.; Byer, R . L. J. Appl. Phys. 1973, 4 4 , 929.
RECEIVED for review May 29,1981. Accepted October 9,1981.
Sample Introduction System for Atmospheric Pressure Ionization Mass Spectrometry of Nonvolatile Compounds Hidekl Kambara central Research Laboratoty, Hltachi, Ltd., Kokubunll, Tokyo 785, Japan
Atmospheric pressure ionization (API) mass spectrometry is known to be a sensitive analytical method. Although it has been successfully applied to various analytical problems (1--7), the subjects for analysis have been restricted to volatile gaseous samples. Thiia is because samples are required to be
in a gaseous phase before ionization at atmospheric pressure. Thermal evaporation is frequently used to change samples from a liquid or solid state to a gaseous one (8). However, this method cannot be applied to nonvolatile thermolabile compounds, the analysis of which is one of the major subjects
0003-2700/62/0354-0143$01.25/0 0 1961 Amerlcan Chemical Society
