JPH0321067B2 - - Google Patents

Description

[Detailed description of the invention]

FIELD OF THE INVENTION The present invention relates to a lithium metal detection device, and more particularly, to a lithium metal detection device that detects thermally ionizable lithium particles or vapor.
or to an improved sensor for detecting mixtures of such lithium particles or vapors. PRIOR ART Alkali metal detection devices have been described in numerous publications, and more particularly in U.S. Pat.
No. 4,095,171 entitled “Alkali Metal Ionization Detector” dated June 13,
No. 4,117,396 entitled "Thermal Ionizing Particles and/or Vapors" dated September 26, 1978. Prior art alkali metal detection devices have an electrically heated filament electrode, typically a MoSi ₂ -cermet wire coated with SiO ₂ , and a collector electrode, which is typically plate-shaped. The filament electrode is heated to about 900° C. by a first electrical circuit and exposed to an atmosphere or carrier gas in which the alkali particles of interest are believed to be present. Contact of the particles with the filament electrode surface causes the particles to thermally dissociate (if the particles are an alkaline compound), become positively ionized and evaporate from the filament electrode surface, producing positive ions.
An electric field created between the filament electrode and the collector electrode by the second electrical circuit moves the ions to the collector electrode, producing an ionic current detectable with a sensitive ammeter. The magnitude of the ionic current indicates the presence and concentration of alkali metal in the gas atmosphere to which the heated filament electrode is exposed. Alkali metal detectors are mainly used in fast neutron breeder reactors to monitor alarms when sodium leaks into the air. If sodium leaks, it reacts with air to form sodium and sodium compounds like airborne particles (smoke or other aerosols). To detect such leaks, a highly sensitive alkali metal detector (with a sensitivity of about 3 x 10 ^-12 g of sodium per c.c. of detected gas) was developed by Westinghouse Electric Company. , manufactured and sold. Recently, new applications for alkali detection devices have been established. Fusion reactors and fusion test systems incorporate systems containing large amounts of high temperature liquid lithium. The detection device, which is essentially identical in its relevant features to the commercially available device described in the above-mentioned patent specification, detects the presence of lithium in the air, since lithium, like sodium, burns in the air and produces smoke. Safe monitoring of leaks is desirable. LiOH, _Li2O
In experiments using Li ₂ CO 3 and Li 2 CO ₃ aerosols, the detection device did not respond, unlike similar experiments using sodium compound aerosols. The authors investigated the cause of the lack of responsiveness of the detection device to lithium and improved the detection device, making it possible to detect lithium. DISCLOSURE OF THE INVENTION It is therefore an object of the present invention to overcome the drawbacks of the prior art and to provide an improved lithium metal detection device. The present invention provides a filament electrode for responding to collisions of lithium atoms and lithium compounds in a carrier gas by ionizing the lithium atoms and lithium compounds on a heated surface to produce lithium cations; and a first electric circuit means for adjusting and maintaining the temperature of the filament electrode at or above 1100° C. for ionizing the lithium compound, a collector electrode, and a lithium in a carrier gas from the filament electrode to the collector electrode. and second electrical circuit means for generating and detecting a flow of lithium cations indicative of the presence and concentration of lithium metal. The alkali metal detector described above has been modified so that the detector filament operates at a significantly higher temperature than normal. The detector filament in the detection of lithium must operate at temperatures above 1100 °C. The invention will be explained in more detail below on the basis of exemplary embodiments and figures. The alkali metal detection device effectively detects lithium when its filament temperature is operated at or above 1100°C. For this purpose, a filament ammeter can be used as the first electric circuit means of the filament, and a current regulating means such as a variable voltage power supply can be used in combination to control the filament current to a predetermined value to obtain a desired filament temperature. The relationship between filament temperature and filament current can be determined by using an optical pyrometer and referring to a calibration curve obtained under operating conditions where other parameters such as atmospheric gas flow are kept constant, even if they are present. . FIG. 1 (prior art) is a diagram showing the response curve of a standard Westinghouse alkali metal detection device that has been developed. FIG. 1 shows the relationship between the net response value (the signal minus the background) of the detection device on the vertical axis 1 and the temperature on the horizontal axis 2 for sodium and potassium compounds. It is clear from this figure
The net response value 4 of the detector for aerosols containing NaOH and KOH is about 10 times the background 3 over all measured temperature ranges. This is because sodium and potassium compounds dissociate at the low temperatures at which the detection device normally operates (approximately 900°C).
It also shows that the temperature is sufficient to ionize the generated alkali metal atoms. Although the net response value of the sensing device increases somewhat by operating at higher temperatures, it is believed that operating at such higher temperatures shortens the life of the filament. Since the response value of the detection device at 900°C is sufficiently larger than the background, operation at 900°C is preferred. FIG. 2 shows curves similar to FIG. 1 for the lithium compounds LiOH and Li ₂ CO ₃ . These behaviors are markedly different from those for sodium and potassium. The net response value 4 of the detection device at a filament temperature of 900 DEG C. is at most one-tenth less than the background (the response value is essentially zero). At 1000 DEG C. to 1100 DEG C., as shown in FIG. 2, the net response value of the detection device exceeds the background signal, making it possible to detect lithium.
Only at high filament temperatures (1100° C.) the behavior of the response values for lithium is similar to that for sodium and potassium, ie at least an order of magnitude higher than the background. Obviously, a filament operating temperature of around 1100°C is suitable for lithium detection. A possible theoretical explanation for these experimental results is that lithium compounds are very stable compared to sodium and potassium compounds. Table 1 lists the dissociated erthalpy of the related compounds. Sodium hydroxide and potassium hydroxide have almost the same values, while lithium compounds require more energy to dissociate. This is almost qualitatively consistent with the filament temperature relationship, but in reality these compounds dissociate (sometimes with catalytic reactions) at surfaces consisting essentially of SiO ₂ , so that the applicable net The dissociation enthalpy of is unknown. Since the thermal ionization reaction is considered a catalytic reaction,
Not only the filament temperature but also the properties of the filament electrode are important. The response values of the detection device shown in FIG. 2 were obtained using a MoSi ₂ -cermet wire coated with SiO ₂ .

[Table] Figure 3 is a schematic diagram of a Westinghouse alkali metal detector modified to operate at temperatures of 1100°C or higher for lithium detection. The detector housing 5 includes a filament electrode 7 and a collector electrode 8. A carrier gas stream 6 passes through the detector housing 5 and is in fluid contact with the filament electrode 7 . The carrier gas stream 6 may contain lithium ions 11 and an aerosol-like lithium-air reaction product 12. The filament electrode 7 is equipped with a variable voltage power supply 15, a calibration curve similar to that illustrated in FIG. 4, and an ammeter 10.
Heat to 1100°C or higher if desired. The lithium-air reaction product 12 comes into contact with the filament electrode 7 to form lithium ions 11.
, which is attracted to the collector electrode 8 by the electric field generated by the bias power supply 14.
The ion current is read by an ammeter 9 or an alarm is issued. Tests using a model of the detection device with a filament electrode temperature of 1100°C were successful, and lithium was detected. Although the general apparatus according to the invention has been described, it should be understood that various modifications can be made without departing from the true spirit and scope of the invention. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a detection response curve by a prior art detection device, FIG. 2 is a diagram showing a detection response curve by a detection device of the present invention, and FIG. 3 is a diagram showing a Westinghouse sodium detection device improved for lithium detection. FIG. 4 is a diagram showing a typical filament temperature calibration curve. In the figure, 1...Vertical axis, 2...Horizontal axis, 3...Background, 4...Net response value of the detection device, 5...Detection device housing, 6...Carrier gas flow, 7
... Filament electrode, 8 ... Collector electrode,
9,10... Ammeter, 11... Lithium ion,
12... Air reaction product, 14... Bias power supply, 15... Variable voltage power supply.

Claims (1)

Claims: 1. A filament electrode for responding to collisions of lithium atoms and lithium compounds in a carrier gas by ionizing the lithium atoms and lithium compounds at a heated surface to produce lithium cations; a first electric circuit means for adjusting and maintaining the temperature of the filament electrode at 1100° C. or higher to ionize the lithium atoms and lithium compounds; a collector electrode; and a carrier gas from the filament electrode to the collector electrode. and second electrical circuit means for generating and detecting a flow of lithium cations indicative of the presence and concentration of lithium in the metal. 2 MoSi ₂ − with filament electrode coated with SiO ₂
The lithium metal detection device according to claim 1, which is a cermet wire. 3. A lithium metal detection device according to claim 1 or 2, wherein the first electric circuit means comprises a variable voltage power supply.