1、ANDNASA TECHNICAL NOTE NASA TN D-7613(NASA-N-D-7613) HIGH-5EMPEr BATU: E NASS N74-20793SPECBCHETEFY - VAPOEIZAION CF GROUP 4-BkEJAL CA3BIDES (%AbA) Unclassified - unlimitedKnudsen effusion; Dissociation energies; Category 06Vaporization; Ti, Zr, Hf, and Th carbides;Ion counting19. Security Classif.
2、(of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price*Unclassified Unclassified 1 $3.75* For sale by the National Technical Information Service, Springfield, Virginia 22151Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-HIG
3、H-TEMPERATURE MASS SPECTROMETRY - VAPORIZATION OFGROUP IVB METAL CARBIDES*by Carl A. Stearns and Fred J. KohlLewis Research CenterSUMMARYThe high-temperature vaporization of the carbon-saturated carbides of titanium (Ti),zirconium (Zr), hafnium (Hf), and thorium (Th) was studied by the Knudsen effus
4、ion -mass spectrometric method. For each system the metal dicarbide and tetracarbidemolecular species were identified in the gas phase. Relative ion currents of the carbidesand metals were measured as a function of temperature. Second- and third-law methodswere used to determine enthalpies for the r
5、eactionsM(g) + 2C(s) = MC2(g)M(g) + 4C(s) + MC4(g)Experimentally determined reaction enthalpies were combined with published thermo-dynamic data to obtain the following dissociation energies (in kJ mol- ): Do (Ti-C256721, DO (Zr-C) = 57524, DO (Hf-C )= 668+28, DO (Th-C2) = 70522, DO (C2-Ti- C2) =0 0
6、 2 0 0 2 0 2 2121822, Do (C2-Zr-C2) = 128928, DO (C2-Hf-C2 ) = 134623, and D (C2-Th-C2)1403 23. In addition, maximum values have been established for the dissociated ener-gies of the metal monocarbide molecules TiC, ZrC, HfC, and ThC. Thermodynamicfunctions used in the calculations are discussed in
7、terms of assumed molecular struc-*An abbreviated account of this work was presented as a technical paper at theNineteenth Annual Conference on Mass Spectrometry and Allied Topics, sponsored bythe American Society of Mass Spectrometry and the American Society for Testing andMaterials Committee E-14,
8、Atlanta, Georgia, May 2-7, 1971, and printed as NASATM X-67844, 1971.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-tures and electronic contributions to the partition functions. The trends shown by thedissociation energies tf the carbides of Group
9、IVB are compared with those of neighbor-ing groups and discussed in relation to the corresponding oxides and chemical bonding.An appendix has been included to describe the high-temperature molecular beaminlet system and the double-focusing mass spectrometer used.INTRODUCTIONSome of the transition me
10、tal carbides are included in the small group of materialswhich may be useful for structural applications at temperatures above 2500 K. Theirmelting points are not the primary limitation on the usefulness of these carbides. Thus,other factors such as brittleness and/or vaporization will determine the
11、 major mode ofdeterioration. The present study is concerned with the vaporization of the carbides oftitanium (Ti), zirconium (Zr), hafnium (Hf), and thorium (Th). Some of our results forthe titanium-carbon system have been reported in earlier publications (refs. 1 and 2).In this report, we reconside
12、r the thermodynamics of the Ti-C molecules and comparethem with the zirconium, hafnium, and thorium carbide molecules.Vaporization studies of the Group IVB metal-carbon systems made prior to 1967have been summarized in an excellent review by Storms (ref. 3). At that time it wasgenerally believed tha
13、t the vapor phase in equilibrium with the condensed carbides wascomposed of metal atoms and polymeric carbon species except for the thorium-carbonsystem. In that system, ThC2 was shown to be the major gaseous thorium-containingspecies. Subsequent mass spectrometric vaporization studies of some trans
14、ition metal-carbon (refs. 4 to 6) and rare-earth - carbon systems (refs. 7 to 10 (and refs. citedtherein), 11 and 12) have established that the dicarbide (MC2) and tetracarbide (MC4)molecular species exist as stable molecules in the gas phase. The work reported here-in is an extension of such studie
15、s to the Group IVB metal-carbon systems.Vaporization studies of several rare-earth - carbon systems have demonstrated that-2the C2 group behaves as a “pseudo- oxide“ (refs. 7, and 10 to 14). Therefore, theexistence of stable molecules of the type M-C2 and C2-M-C2 was expected for thetransition metal
16、s on the basis of the existence of the stable transition-metal monoxidesand dioxides. The dissociation energies of the gaseous metal monocarbides of the elec-tropositive transition elements have been estimated by an empirical procedure (ref. 15),but none have been observed experimentally. In contras
17、t, for the noble metals platinum(Pt), iridium (Ir), rhodium (Rh), and ruthenium (Ru) the monocarbides are the majormolecular species (refs. 16 and 17). A metal tricarbide molecule (MC3) has beenpreviously reported for only the lanthanum-carbon (La-C) system (ref. 10).2Provided by IHSNot for ResaleNo
18、 reproduction or networking permitted without license from IHS-,-,-The existence of molecular titanium dicarbide was first reported in 1967 inreference 18. A subsequent study (refs. 1 and 2) established the dissociation energy ofTi-C2 as 56821 kJ mol-1. This value was confirmed in another recent stu
19、dy (ref. 19).The authors of reference 20 used a mass spectrometer and observed the ZrC2(g)molecule at high concentrations relative to the metal (PZr /PZrC2 16 at 2660 K overthe ZrC-C-Ta system). They reported AG26 60 = -109 kJ mol-1 for the reactionZrC2(g) = Zr(g) + C2(g)In 1970 the results of a mas
20、s spectrometric study of the vaporization of zirconiumcarbide from 2000 to 2900 K were reported (ref. 21). The authors do not report theobservation of any molecular zirconium-carbon-containing species but do point out thatno ZrC(g) was observed. We are unaware of the observation of any hafnium-carbo
21、nmolecules prior to the present study.References 22 to 24 show that the saturated vapors over thorium dicarbide containatoms of Th and molecules of ThC2 in approximately equal amounts. References 22and 23 report the enthalpy for the reactionThC2(s) = ThC2(g)as 828.914.6 kJ mol-1 and the standard hea
22、t of formation of ThC2(s) asAH 2 98 K = -128. 4+15. 5 kJ mol- 1In the present study, the Knudsen effusion method coupled with the use of a sophisti-cated double-focusing mass spectrometer was employed to study the vaporization of therespective metal carbide - carbon saturated phases. The primary obj
23、ective of thisresearch was to obtain experimental thermochemical data which could be used to specifythe composition of the vapor phase for the metal carbides and to elucidate certain basicthermodynamic properties of simple gaseous molecules. These studies also help toestablish the nature of high-tem
24、perature chemical reactions and to provide vaporpressure data which are of potential engineering value. In addition to their usefulnessto materials engineers the data obtained can be beneficial in other disciplines. Oneexample is the field of astrochemistry. Tsuji (ref. 25) has employed thermodynami
25、cdata for molecular carbide molecules to demonstrate that these species are importantcomponents in the atmospheres of carbon-rich cool stars.3Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-DETAILS OF EXPERIMENTIn the course of our vaporization studi
26、es of various nitride, carbide, and oxidesystems, numerous modifications have been incorporated into our high-temperaturemass spectrometer facility as originally described in 1969 (ref. 26). Some of the detailsare contained in various reports (refs. 12, and 27 to 29), and some have never beenpublish
27、ed. Because the studies reported herein have had the advantages of most of theincorporated improvements, it is appropriate to describe the existing apparatus in somedetail. A complete description of our Knudsen cell assembly and mass spectrometer isgiven in the appendix.The procedure and experimenta
28、l details employed to obtain data for the titanium-carbon system have been described in detail in references 1 and 2. Only those aspectspertinent to the comparisons made in this report are repeated herein.For the experiments reported herein, an electron multiplier was used to measureion currents, wh
29、ich were taken as a measure of ion intensity. The gain of the electronmultiplier (electron output per ion input) for various ionic species was measured experi-mentally by ion-counting techniques with the SSR Instrument Company counting system(see appendix). Multiplier output current was measured by
30、direct-current methods witha vibrating reed electrometer. Load resistors of 10 , 108, and 109 ohms were used,and the integration time was between 10 and 20 seconds. Each ion intensity wasmeasured at least five consecutive times, and the average value was calculated. Thelowest detectable ion current
31、was approximately 2x10- 19 ampere at a signal-to-noiseratio of 3. 2.The carbides of titanium, zirconium, hafnium, and thorium were each prepared ineither tungsten (W) or tantalum (Ta) Knudsen cells with graphite (Ultra Carbon GradeUFS) liners by heating mixtures of each metal powder with excess grap
32、hite powder(Ultra Carbon Grade UFS-4) under high vacuum. This preparation was done in situ inthe mass spectrometer. At temperatures above 2000 K the metal and graphite reactedto form the carbon-saturated metal-carbide-condensed phases TiC + C, ZrC + C,HfC + C, and ThC2 + C. In general, the carbide p
33、hase is somewhat substoichiometricand varies slightly in composition with temperature (ref. 3). Formation of the carbidephase in each case was verified by X-ray diffraction analysis of the samples.PROCEDURE AND RESULTSVapor Species Identification and Appearance PotentialsThe mass spectrum of each me
34、tal carbide - carbon system was examined in detailat mass-to-charge ratios m/e to about 300. In all cases the observed ions were4Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-identified by mass-to-charge ratio and isotopic abundance. In addition to
35、 the metal ion(M+) and various carbon polymer (C1+ , C2 , C3 + , C4 , C5 +) ions, the dicarbide(MC2 ) and tetracarbide (MC4 +) ions were positively identified for each system. Themetal monocarbide (MC+) and tricarbide (MC3+) ions were identified at low intensity forthe Zr, Hf, and Th systems but not
36、 for the Ti system. In the Hf experiment, Ta+ ,TaC+, and TaC2+ ions were observed, along with the analogous Hf- and C-containingspecies. These ions were assumed to result from the interaction of the graphite linerwith the tantalum Knudsen cell. Shutter measurements were used to establish that allion
37、 species of interest had neutral precursors originating from the Knudsen cell. Noneof the metals and carbide species were detected with the shutter closed.The parent molecular species were identified by measurement of appearance poten-tials and ionization efficiency curves when ion intensities were
38、sufficiently high.Measured values of the appearance potentials are listed in table 1. The low values ofthe appearance potentials for TiC2+ , ZrC2 , ThC2+, TiC4 , and ThC4+ indicate thatthey are parent ions formed directly by electron impact of the respective molecules. Itwas assumed by analogy that
39、the HfC2+, ZrC4+ , and HfC4 ions are also parent ionspecies even though their appearance potentials were not determined.TABLE 1. - APPEARANCE POTENTIALSIon Ti Zr Hf ThAppearance potential, eVM+ a6.8 2 a6. 8 4 a7. 0 a6. 95MC+ (b) (c) (c) 8. 0+1.0d1 7.5+1.0MC2+ 8. 7i0. 5 7. 5-0. 5 (c) 6. 50. 3MC3+ (b)
40、 (c) (c) 9.2+1.0MC4+ 9.0+1. 0 (c) (c) 10.0+i. 0aused as an internal standard for calibration of the electronenergy scale (ref. 60).blon not identified.cInsufficient intensity for appearance potential measurement.dPosition of sharp upward break in ionization efficiency curve.5Provided by IHSNot for R
41、esaleNo reproduction or networking permitted without license from IHS-,-,-For the thorium system both ThC+ and ThC3+ are probably parent ion speciesbecause their appearance potentials are low. The values estimated for the appearancepotentials of ThC+ and ThC3+ as fragments from ThC2 and ThC4, respec
42、tively, areAP(ThC+ f r a g m e n t ) = AP(ThC2+ ) + D (C2) = 6. 5 eV + 6.2 eV = 12.7 eVandAP(ThC3+ fragment) = AP(ThC4+) + D O (C2) = 10. 0 eV + 6.2 eV = 16.2 eVBecause these estimates are considerably higher than the experimental values, weconcluded that these species are parent ions. The ionizatio
43、n efficiency curve for ThC+had a sharp break at 17. 5+1.0 eV, which probably corresponds to the onset of a frag-mentation reaction such asThC2 + e- = ThC + C + 2eThe ThC+ fraction which had an appearance potential of 8. 0+1. 0 eV was estimated to beabout 10 percent of the total ThC+ ion current.No d
44、efinite assignment was made for the low-intensity ZrC+ , ZrC3 +, HfC+, orHfC3 , which may be either parent or fragment ions.Ion Intensity MeasurementsIon currents were measured for the species M+, MC2 , and MC4+ as a function oftemperature for each of the metal-carbon systems. In addition, the inten
45、sities of MC+and MC3+ were measured at one or two temperatures for each system. The low tem-perature for each experiment is the temperature below which reliable measurements ofthe intensity could not be made for the least-abundant carbide species. The upper tem-perature of each experiment was set by
46、 experimental conditions involving the Knudsencell. Both the tungsten and tantalum cells were partially carbided during the duration ofthe various experiments. Tungsten cells which had partially carbided tended to meltabove 2875 K. Tantalum cells proved more useful, and the upper limit of 3100 K was
47、caused by failure of the tungsten cell supports.For at least one experiment on each metal-carbon system, a determination of thesensitivity constant relating pressure to ion intensity was made by calibration of thesystem using either gold (Ti experiment, refs. 1 and 2) or silver (Zr, Hf, and Th6Provi
48、ded by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-experiments). For the silver calibration, a 0. 005-millimeter-thick silver foil with adiameter equal to that of the interior of the Knudsen cell was placed over the sample inthe Knudsen cell. The ion intensi
49、ty of 107Ag+ was measured at several temperaturesbelow the melting point. Subsequently, the silver was melted and vaporized. Frommeasurement of 10 7Ag+ intensities and the known vapor pressure (ref. 30) of silvermetal, the sensitivity constant was determined.Titanium-carbon system. - Table 2 presents data for ion intensities of Ti+ , TiC2 ,TABLE 2. - ION INTENSITIES FOR THE TiC + C(
