Study of the Reaction of B, B2O3, and B4C with Na2CO3 in a

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Study of the Reaction of B, B2O3, and B4C with Na2CO3 in a Thermogravimetric Apparatus and of the Products by Transpiration Thermogravimetry, Thermal Ionization Mass Spectrometry, and Knudsen Effusion Mass Spectrometry T. S. Lakshmi Narasimhan, S. Nalini, P. Manikandan, R. Balasubramanian, and R. Viswanathan* Fuel Chemistry Division, Chemistry Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India

ABSTRACT: A thermogravimetric (TG) study of the reaction of sodium carbonate with elemental boron, B2O3, and B4C was conducted under flowing commercial argon. The reaction products (when initial B-to-Na atomic fraction ratio = 1) were further subjected to transpiration thermogravimetric (TTG) measurements. Some residues were subjected to Knudsen effusion mass spectrometry (KEMS) and thermal ionization mass spectrometry (TIMS) as well. The present study is a sequel to two experimental observations, related to (1) a steep fall in vacuum in TIMS as the filaments containing the mixture of Na2CO3 and B (or H3BO3 or B4C) were being heated to measure the boron isotope ratios and (2) the detection of Na2BO2+ as the only boron-containing positive ion in TIMS, at variance with the KEMS results on NaBO2 (s). KEMS showed NaBO2+ as the principal ion, originating from the major species NaBO2(g), and Na2BO2+ as a much less intense ion, originating from the minor species (NaBO2)2 (g). The thermograms recorded over (B + Na2CO3), (B2O3 + Na2CO3), and (B4C + Na2CO3) mixtures consistently revealed onset of chemical decomposition of Na2CO3 at temperatures much lower than the temperatures at which pure Na2CO3 would undergo thermal decomposition. The results of the TTG measurements were in accord with those reported for NaBO2, thereby confirming its formation and also indicating as a corollary that the oxidation of B and B4C (which are known to be kinetically hindered) proceeded through completion with Na2CO3 quickly reacting with the oxidation product. The mass spectrometric examination of the residues reaffirmed the predominance of NaBO2+ in KEMS and Na2BO2+ in TIMS, leading to the inference that, even in TIMS, it is only the monomeric species NaBO2(g) that yields Na+ as well as Na2BO2+ but through two processes that occur on the filament surface: NaBO2 → Na+ + (BO2)− and NaBO2 + Na+ → Na2BO2+.

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INTRODUCTION Determination of boron isotope ratios by thermal ionization mass spectrometry (TIMS) involves measurement of ion currents for A210BO2+ and A211BO2+, where A = Na,1 Rb,2 and Cs.3 In our laboratory, we have always used A = Na and observed Na+ and Na2BO2+ ions in positive TIMS mode (PTIMS) and BO2− in the negative TIMS mode (NTIMS). Nonobservation of NaBO2+ in PTIMS perplexed us a little since in the Knudsen effusion mass spectrometric (KEMS) studies of NaBO2(s),4,5 including the recent one by us,6 all three ions Na+, NaBO2+, and Na2BO2+ were observed in the mass spectrum of the equilibrium vapor with the ion intensities I+ being in such proportions that I(Na+) was 3 to 4 times higher than that of I(NaBO2+) and I(Na2BO2+) was 3 to 4 times lower than that of I(NaBO2+). The fact that during electron impact ionization (in KEMS) Na+ and Na2BO2+ were generated © 2013 American Chemical Society

only as fragment ions owing their origin to NaBO2(g) and (NaBO2)2(g), respectively, led us to infer whether, in TIMS, surface ionization reactions are as follows (NaBO2 )bulk → (NaBO2 )ad → (Na +)ad + (BO2 )−ad → (Na +)de + (BO2 )−de

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2(NaBO2 )bulk → 2(NaBO2)ad → (Na 2BO2+)ad + (BO2 )−ad → (Na 2BO2+)de + (BO2 )−de (2) Received: February 27, 2013 Accepted: April 20, 2013 Published: May 3, 2013 1792

dx.doi.org/10.1021/je400193h | J. Chem. Eng. Data 2013, 58, 1792−1805

Journal of Chemical & Engineering Data

Article

believed that we could obtain some evidence for and also some information on the solid state reaction that presumably occurs on TIMS filaments. Accordingly, we undertook a detailed TG study of reaction of elemental boron, B2O3, and B4C with Na2CO3. The initial studies were performed with samples that had the n(Na)/n(B) ≈ 1, and the later studies were performed on the samples that had n(Na)/n(B) ≠ 1, since in any TIMS work on samples of unknown B concentrations, the value of n(Na)/n(B) may be anywhere from 0.5 to 1.5. These TG studies gave a clear and consistent evidence that solid Na2CO3, which is otherwise practically resistant to thermal decomposition at t/°C < 800, begins to undergo chemical decomposition at much lower temperatures (t = (540 to 640) °C) when commercial argon is passed over the mixture of (B + Na2CO3) or (B2O3 + Na2CO3) or (B4C + Na2CO3). A very similar observation was made previously by Perlov and Roth9 who interpreted the thermogram for the mixture of B 2O3, Na2CO3, BaCO3 as giving evidence for the chemical decomposition of Na2CO3 starting at lower temperature (t/°C ≈ 560 °C) than that of BaCO3 (t/°C ≈ 700) even though the decomposition pressure of Na2CO3 at t/°C ≈ 700 is about 104 times lower. Lowering of decomposition temperature of Na 2CO3 was also observed when additives such as SiO2 or CaO were employed.10,11 Having already conducted a transpiration thermogravimetric (TTG) study of NaBO2,6 we believed that it would be prudent to subject the products of the reaction mixture with n(Na)/n(B) ≈ 1 to TTG examination under similar experimental conditions. The idea is that the reaction product expected is NaBO2 in which case TTG measurements could aid in confirmation of formation of NaBO2. The results with all three mixtures, being consistent with that obtained previously for NaBO2,6 indicated that the reaction in each case gave NaBO2 as the product. Reaction products in a few cases were also subjected to KEMS and TIMS examinations. The results reconfirmed the previously observed difference in the mass spectrum from TIMS and KEMS. While this paper is devoted to description of all these measurements, the thermogravimetric part forming the major part of the study and representing the work done for the first time, the results from TG and TTG are presented and discussed in greater detail than those from TIMS and KEMS.

or 2(NaBO2 )ad → (NaBO2 )2,ad → (Na 2BO2+)ad + (BO2 )−ad → (Na 2BO2+)de + (BO2 )−de

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The subscripts “bulk” and “ad” refer to the bulk film and adsorbed gaseous molecule or ion on the surface while the subscript “de” refers to the desorbed (or emitted) ionic gaseous species from the surface. The above inference clearly stems from our attempt to explain that the results of PTIMS are not inconsistent with those of KEMS, with appearance of Na+ attributed to (NaBO2)ad and that of Na2BO2+ to (NaBO2)2,ad. We further believed that it is because the thermal ionization sensitivity is very high compared to electron-impact ionization (about 5 to 6 orders of magnitude) that one gets to observe Na2BO2+ in PTIMS, even if the ratio of {2(NaBO2)ad/(NaBO2)ad} or of {(NaBO2)2,ad/(NaBO2)ad} were to be extremely small (say 0.96 0.99999 0.9998 > 0.999 > 0.97 > 0.99995



Article

n(Na)/n(B) ≈ 1), TG measurements were continued further with the objective of determining the rate of mass loss, dm/dt, at t/°C = 860, t/°C = 880, and t/°C = 900 and compare the results with those obtained by us previously over NaBO2 under “stopcock open” transpiration conditions. The aliquots from some bulk samples employed for the abovementioned reactions inside the TG apparatus and also the residues after the TG measurements in a few cases were subjected to TIMS examination. The TIMS instrument used was a newly acquired one from IsotopX, U.K. (model Isoprobe-T). With the KEMS study of solid NaBO26 showing Na2BO2+ as only a minor ion in comparison to the principal B-containing ion NaBO2+, a sample of sodium borate was also examined by TIMS. The KEMS measurements were performed mainly on those residues, the TG results of which were very similar to those obtained previously6 for NaBO2. The instrument used was from VG Micromass, U.K. (model 30 BK), the same one as that described in ref 6.

The TG measurements are classified under nine series, four of which correspond to those on the starting chemicals and the rest on (B + Na2CO3), (B2O3 + Na2CO3), and (B4C + Na2CO3) mixtures. All chemicals were found to have moisture contained in them (B4C, the least), as evidenced by the occurrence of mass loss during the dynamic heating from t/°C = (25 to 120), the isothermal heating at t/°C = 120, the dynamic heating from t/°C = (120 to 300) and by the observation of practically no mass loss during much of the latter part of the isothermal heating at t/°C = 300. In each series, measurements were conducted on one or more aliquots, and in each aliquot, one or many runs were conducted, each run denoting one specific temperature program consisting of dynamic and isothermal heating steps. A platinum crucible was used to contain the samples. In some series, the Pt crucible was placed in an alumina crucible or placed over an alumina disc to avoid the Pt crucible getting stuck on the Pt-pan holder. The primary aim of our TG experiments was to obtain evidence for the occurrence of reaction within the constituents of (B + Na2CO3), (B2O3 + Na2CO3), and (B4C + Na2CO3) mixtures under flowing argon resulting in loss of CO2 and formation of sodium borates. In other words, we were looking for the occurrence essentially of a reaction, similar in nature that is presumed to occur in TIMS when Na2CO3 fusion is employed for isotope ratio measurements of B. In the case of samples that would give NaBO2 (that is, the samples having initial

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RESULTS AND DISCUSSION TG Measurements. Table 2 gives some details of our TG measurements on various samples, which essentially were performed to study the reaction between (a) B and Na2CO3, (b) B2O3 and Na2CO3, and (c) B4C and Na2CO3 under flowing commercial argon. The TG measurements performed on the

Table 2. Details of Thermogravimetric Measurements on Different Boron-Containing Samples under Flowing Commercial Argona m sample details c

1 : (B + Na2CO3) mixture with n(B)/n(Na) = 1.0

2: Na2CO3 3e: elemental boron 4f: (aliquot of sample 1 + Na2CO3) with n(B)/n(Na) = 0.82 5g: (B + Na2CO3) mixture with n(B)/n(Na) = 1.5 6c: B2O3 + Na2CO3 mixture with n(B)/n(Na) = 1.0

7h: B2O3 8c: (B4C + Na2CO3) mixture with n(B)/n(Na) = 1.0 9i: B4C

dates

aliquot

mg

n

from

to

1 2 3 4 5d 1 2 1 1

20.1 21.5 44.9 30.1 41.6 105.2 59.5 7.7 29.1

6 3 2 2 2 5 1 3 2

22 Aug 2011 27 Aug 2011 05 Sep 2011 09 Nov 2011 12 July 2012 10 Aug 2011 12 June 2012 25 Oct 2011 15 Nov 2011

25 Aug 2011 02 Sep 2011 08 Sep 2011 11 Nov 2011 14 July 2012 16 Aug 2011 13 June 2012 02 Nov 2011 17 Nov 2011

1 2 1 2 3 4 1 1 2 1

32.4 32.8 32.4 28.1 50.6 44.3 20.8 31.1 26.2 6.4

1 6 1 7 3 2 4 3 2 6

13 Sep 2011 07 Oct 2011 28 Nov 2011 02 Dec 2011 19 Dec 2011 04 Jan 2012 11 Jan 2012 22 Aug 2012 21 Sep 2012 02 Nov 2012

21 Oct 2011 08 Dec 2011 20 Dec 2011 06 Jan 2012 13 Jan 2012 24 Aug 2012 24 Sep 2012 14 Nov 2012

remarksb Figure 1, panels 1.1(a,b), Table 3 Figure 1, panels 1.2(a,b), Tables 3 and 5 Figure 1, panels 1.3(a,b), Tables 3 and 5 Figure 1, panels 1.4(a,b), Table 4 Figure 1, panels 1.5(a,b), Table 3 Figure 2, panels 2.1(a,b) Figure 2, panels 2.2(a,b) Figure 3, panels 3.1(a,b) and 3.2(a,b) Figure 4, Table 4 Figure 5, panels 5.1(a,b) Figure 5, panel 5.2 TG curve presumably affected by sample creep-out TG balance problem Figure 6, panels 6.1(a,b), Table 3 Figure 6, panels 6.2(a,b), Tables 3 and 5 Figure 7, panels 7.1(a,b) and 7.2(a,b) Figure 8, panels 8.1a,b Figure 8, panels 8.2a,b, Table 3 Figure 9, panels 9.1 and 9.2

a

m (column 3 heading) denotes the initial mass of the aliquot. n (column 4 heading) denotes total number of runs conducted with each aliquot. Every temperature program (comprising of isothermal and dynamic segments) constituted a separate run. Values of n(B)/n(Na) given in column 1 denote atomic fraction ratio of B and Na. bPertinent results are shown in figures and tables stated in this column. cTo obtain NaBO2 in the Na2O− B2O3 binary system with x(B2O3) = 0.50 where x(B2O3) denotes the mole fraction of B2O3. dThis is an aliquot from high purity B (Aldrich Chemicals) while other aliquots are of electro-deposited B of lesser purity (local). eRun 3 corresponded to measurements performed after adding Na2CO3 to the residue of run 2 to have n(B)/n(Na) = 1 and to obtain NaBO2 in the Na2O−B2O3 binary system with x(B2O3) = 0.50. fTo obtain a two phase mixture on the Na2O-rich side of NaBO2 in the Na2O−B2O3 binary system with x(B2O3) = 0.45: (Na4B2O5 + NaBO2) two-phase mixture. g To obtain a two phase mixture on the B2O3-rich side of NaBO2 in the Na2O−B2O3 binary system with x(B2O3) = 0.60: (NaBO2 + Na2B4O7) twophase mixture. hRun 3 corresponded to measurements performed after Na2CO3 was added to the residue of run 2 to have x(B2O3) = x(Na2CO3) = 0.5. iRuns 2 to 6 corresponded to measurements performed after Na2CO3 was added to the residue of run 1 to have n(B)/n(Na) = 1 and to obtain NaBO2 in the Na2O−B2O3 binary system with x(B2O3) = 0.50. 1794



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Figure 1. Thermogravimetric results with (B + Na2CO3) mixtures having Na-to-B atomic fraction ratio of ≈1. Panels 1.1a to 1.5a show temperature, mass vs time curves; panels 1.1b to 1.5b show corresponding mass vs temperature curves for the selected dynamic heating segments (600 to 700) or (500 to 700) °C.

additional source of oxygen (as according to 4B +2Na2CO3 + 3O2 → 4NaBO2 + 2CO2 and B4C + 2Na2CO3 + 4O2 → 4NaBO2 + 3CO2). That the oxygen impurity in the commercial argon which we use as carrier gas can serve as an effective source of oxygen became known to us when we were attempting to perform transpiration thermogavimetric (TTG) measurements on ruthenium tellurides. As mentioned in the introduction of our recent paper on the study of CaCO3,15 the TTG measurements on ruthenium tellurides showed actually a mass gain (instead of the mass loss

constituents of the three mixtures, that is, B, B2O3, B4C, and Na2CO3 provided the means of detecting the occurrence of reaction between Na2CO3 and B as well as with B2O3 and B4C (through a comparison of thermograms). One may readily comprehend the feasibility of reaction between B2O3 and Na2CO3 (as according to B2O3 + Na2CO3 → 2NaBO2 + CO2) leading to the formation of NaBO2 phase (an equi molar composition in the Na2O−B2O3 phase diagram12−14) and not that between B and Na2CO3 or between B4C and Na2CO3, unless there is some 1795



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Figure 2. Thermogravimetric results with pure Na2CO3. Panels 2.1a and 2.2a show temperature, mass vs time curves; panels 2.1b and 2.2b show corresponding mass vs temperature curves in the dynamic heating segment (600 to 700) °C.

by a slow rate of mass gain in the (400 to 500) °C dynamic segment, relatively steeper rate of mass gain in the (500 to 600) °C dynamic segment to slow down at the end. The (600 to 700) °C dynamic segments (Figure 1, panels 1.1b to 1.4b) or the (500 to 700) °C dynamic segment (Figure 1, panel 1.5b) present the most interesting feature − first an onset of continuous decrease in mass only to turn around to resume mass gain. Since the effect of oxidation of elemental boron would cause mass-gain and the effect of reaction between the oxidized boron and Na2CO3 would cause mass-loss, an obvious inference one can make from the features seen in Figure 1, panels 1.1b to 1.5b, is that the net mass gain or mass loss observed in any region is governed by which of these two effects is more overwhelming. This inference is especially valid when the vapor pressures are very low for the starting materials, Na2CO316 or B17 or for the intermediate product B2O318 or for the final envisaged product, NaBO2,6 and therefore, the mass loss incurred cannot be due to vaporization of these four substances. The first indirect evidence of completion of reaction was obtained from the change in the color of the (B + Na2CO3) mixture from black to white. There was also a net mass gain, reasonably consistent with calculation of mass gain expected for complete conversion of B to B2O3 and mass loss expected for complete loss of CO2 through decomposition of Na2CO3. What was put as powder in the platinum crucible (to about half or three-fourth of its depth which is 5 mm) got sintered to a disc of just about 1 mm thick, shrunk from all sides, but not hard. The disc was turned upside down and the TG experiments continued to ensure that the reaction is complete. In one of the experiments,

which was expected to occur owing to their vaporization as dimeric tellurium vapor species), which we believe occurred due to the propensity of Ru to pick up oxygen from the carrier gas. Thus, we presumed that the commercial argon carrier gas itself could meet the requirement of oxygen for the reaction to occur between B and Na2CO3 or between B4C and Na2CO3. The results obtained by us on five aliquots of (B + Na2CO3) mixture and on two aliquots of (B4C + Na2CO3) with n(Na)/n(B) ≈ 1 showed that the reaction did occur as in the case of (B2O3 + Na2CO3). Measurements on (B + Na2CO3) Mixture (n(Na)/n(B) ≈ 1) and on Na2CO3. Figure 1, panels 1.1a to 1.5a, shows the thermograms, temperature and mass vs time plots, obtained over five aliquots of (B + Na2CO3). All these plots correspond to runs in which the temperature range t/°C = (300 to 800) was accessed first. This temperature range often encompassed both isothermal and dynamic heating segments. The results for one dynamic heating segment in the temperature range from t/°C = (600 to 700) (or from t/°C = (500 to 700)) are shown in Figure 1, panels 1.1b to 1.5b, as mass vs temperature plots, adjacent to panels 1.1a to 1.5a. Figure 2, panels 2.1a, 2.1b, 2.2a, and 2.2b, shows similar plots for the two aliquots of Na2CO3. The thermograms for Na2CO3 shown in Figure 2, panels 2.1(a and b) and 2.2(a and b), are straight lines in nature, parallel to X axis. That is, there was practically no mass-loss in the temperature ranges covered. This is not altogether unexpected because decomposition pressure of Na2CO3 even at t/°C = 800 is very low (about 6 × 10−4 Pa).16 On the other hand, the thermograms for (B + Na2CO3) mixtures, in general, are all characterized 1796



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Table 3. Results of Transpiration Thermogravimetric Measurements under Flowing Commercial Argona on the Reaction Products, the Starting Mixtures Being (B + Na2CO3), (B2O3 + Na2CO3), or (B4C + Na2CO3), all with n(Na)/n(B) ≈ 1b (B + Na2CO3)

(B2O3 + Na2CO3)

(B4C + Na2CO3)

t

dm/dt

t

dm/dt

t

dm/dt

t

dm/dt

t

dm/dt

t

dm/dt

°C

mg·s−1

°C

mg·s−1

°C

mg·s−1

°C

mg·s−1

°C

mg·s−1

°C

mg·s−1

aliquot 1, run 6, 25 Aug 2011 900 2.02 × 10−5 880 9.60 × 10−6 860 4.58 × 10−6 900 1.26 × 10−5 880 7.14 × 10−6 aliquot 2, run 3, 02 Sep 2011 900 880 860 900 880 860 880

1.56 × 10−5 8.08 × 10−6 4.73 × 10−6 1.29 × 10−5 7.60 × 10−6 4.06 × 10−6 6.65 × 10−6

aliquot 3, run 3, 08 Sep 2011 900 2.09 × 10−5 880 1.05 × 10−5 860 5.66 × 10−6 880 6.77 × 10−6 900 1.73 × 10−5 880 7.73 × 10−6 860 5.01 × 10−6 880 8.86 × 10−6 900 1.36 × 10−5 860 4.73 × 10−6

aliquot 4, run 2, 14 July 2012 900 2.02 × 10−5 880 4.67 × 10−6 860 3.73 × 10−6 900 7.10 × 10−6 880 3.67 × 10−6 860 1.33 × 10−6 880 4.36 × 10−7 900 7.08 × 10−6 880 3.86 × 10−6 860 1.11 × 10−6

aliquot 3, run 3, 20 Dec 2011 900 3.29 × 10−5 880 1.10 × 10−5 860 3.47 × 10−6 880 1.15 × 10−5 860 3.34 × 10−6 880 1.26 × 10−5 900 2.29 × 10−5 880 1.11 × 10−5 860 3.34 × 10−6 880 1.23 × 10−5 900 2.31 × 10−5 880 1.08 × 10−5 860 5.57 × 10−6 880 9.24 × 10−6 900 1.26 × 10−5

aliquot 4, run 2, 06 Jan 2012 900 2.19 × 10−5 880 9.84 × 10−6 860 5.23 × 10−6 880 8.23 × 10−6 900 1.29 × 10−5 880 7.21 × 10−6 860 4.01 × 10−6 880 5.16 × 10−6 900 7.39 × 10−6 880 3.80 × 10−6 860 2.18 × 10−6 880 3.87 × 10−6 900 7.98 × 10−6 880 3.59 × 10−6 860 1.93 × 10−6

aliquot 2, run 1, 24 Sep 2012 900 2.11 × 10−5 880 9.40 × 10−6 860 5.10 × 10−6 900 1.79 × 10−5 880 9.59 × 10−6

The exit stopcock fully opened and the flow was ∼1.1 mL/s. The values of {(dm/dt)/(mg/s)} from previous transpiration thermogravimetric experiments on NaBO26 under nearly similar conditions of carrier gas flow are 8.7 × 10−6 at t/°C = 900; 4.9 × 10−6 at t/°C = 880; 2.8 × 10−6 at t/°C = 860. The standard uncertainties u are u(t) = 2 °C, u(dm/dt) < 0.05·(dm/dt) bn(Na)/n(B) is the atomic fraction ratio of Na and B. a

the cycle of reaction between B2O3 and Na2CO3 and oxidation of B to B2O3 apparently continues to effect complete oxidation of B as well as complete decomposition of Na2CO3, and finally yield the phase or phase mixture appropriate for the Na-to-B ratio in the Na2O−B2O3 phase diagram. Circumstantial evidence for these inferences came from a TG experiment conducted after adding Na2CO3 to the residue of the above run on elemental boron (which led to minimal formation of B2O3). Continuous mass loss was found to occur even at low temperatures, while rapid rate of mass loss was found to occur from t/°C = 650. A long isothermal segment at t/°C = 700 (for 50 h) was also conducted. The t/°C = 700 isothermal segment showed a clear turn-around in mass vs time, a feature similar to those shown in Figure 1, panels 1.1a to 1.5a or 1.1b to 1.5b. Figure 3, panels 3.1 and 3.2, show the results described above, Figure 3, panel 3.1, depicting the effect on elemental boron and Figure 3, panel 3.2, depicting the effect after adding Na2CO3 to the partly oxidized boron. We attribute that the shift in turn-around in mass vs time curve to a relatively higher temperature (Figure 3, panel 3.2a) occurred in this case to the sample being a mixture (B covered with a layer of B2O3 + Na2CO3). While the reaction between B2O3 layer and Na2CO3 did induce the mass-loss in the t/°C = (600 to 700) dynamic segment (Figure 3, panels 3.2a and 3.2b), the breach of the B2O3 layer as a result of this reaction occurred at some point of time during the t/°C = 700 isothermal segment leading to the exposure of B for oxidation and hence a turnaround shown in Figure 3, panel 3.2a. Measurements on (B+Na2CO3) Mixtures with 1 < n(Na)/ n(B) < 1. Our next interest was to study (B+Na2CO3) mixtures with Na-to-B ratio ≠ 1 because in TIMS studies, if one does not know the initial concentration of B in the sample, addition of Na2CO3 could result in having overall Na/B ratio less or greater than one. Furthermore, we were also curious as to how the thermograms of such samples would look like. With the sample having n(Na)/n(B) about 1.2, we restricted the maximum t/°C = 638 since there exists an eutectic around t/°C = 640 in the

the disc was crushed, put back, and the TG measurements continued to have at least one set of isothermal segments at three higher temperatures, t/°C = 900, t/°C = 880, and t/°C = 860. These measurements will be referred to as transpiration thermogravimetric (TTG) measurements because the objective was to examine whether the values of rate of mass loss of the reaction products agree with those obtained by us previously in the TTG study of NaBO26 under similar conditions of carrier gas flow. Table 3 shows the results for the first three aliquots and the fifth aliquot. The results of the fourth aliquot were characterized by very high scatter of mass values which imparted great uncertainty to the values of dm/dt from them, and hence rejected. The values of dm/dt corresponding to the first set of isothermal segments generally were higher by a factor of 2 compared to those obtained by us previously over NaBO2,6 but in the second or third set of isothermal segments, the values became reasonably consistent with those for NaBO2. Measurements on Elemental Boron. A TG experiment on elemental boron was conducted to examine the rates of mass gain at different temperatures, in the absence of Na2CO3. It turned out that even when the sample was heated for a total of about 76 h at t/°C = 700 (in two runs), the rate of mass gain decreased monotonically from (1.4 × 10−4 to 8.3 × 10−6) mg·s−1 in 24 h (in the first run) and to 4.0 × 10−6 mg/s (in the second run). If the mass gain is an indication of conversion to B2O3, as evidenced by observations of globules at the end of the first run, it was clear that oxidation had practically stopped with only a small fraction of B actually converted to B2O3. The recent study of Jain et al.19 confirmed that oxidation of boron even in a steam of oxygen becomes sluggish at T > 900 K, and stated that the formation of glassy layer of boron oxide on the surface of boron powder prevented complete oxidation. Our observation on elemental boron when viewed with that in the experiments on the (B+Na2CO3) mixtures leads us to infer that in the case of the latter, reaction with Na2CO3 of whatever little B2O3 formed initially creates a favorable condition for further oxidation of B; and furthermore, 1797



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Figure 3. Thermogravimetric results with elemental boron (panels 3.1a,b), and with the mixture containing partially oxidized boron residue and Na2CO3 such that the overall Na-to-B atomic fraction ratio is ≈1 (panels 3.2a,b). Panels 3.1a and 3.2a show temperature, mass vs time curves; panels 3.1b and 3.2b show corresponding mass vs temperature curves in the dynamic heating segment (600 to 700) °C.

per minute and isothermal heating at t/°C = (620 or 660) for one hour before effecting the dynamic heating in either direction. The idea was to seek evidence for the eutectic at t/°C ≈ 640 in the SDTA mode, but in vain, perhaps because the amount of the sample was too low to reveal the endothermic or exothermic effect. The residue looked good, but the crucible got stuck with the pan, probably due to creeping that might have occurred upon melting. A TG experiment with the sample having n(Na)/n(B) < 1 yielded a thermogram (see figure 5.1) having very similar features as shown in figures 1.1 to 1.4 for samples with n(Na)/n(B) = 1: steep fall in mass and a turn around in the dynamic heating segment of t/°C = (600 to 700). Visual examination at the end of this experiment gave the surprise of no sample remaining inside the crucible and the crucible itself having got blackened. Since the reaction product in this case was to be the biphasic region (NaBO2 + Na2B4O7) having an eutectic at t/°C ≈ 744 °C, our next experiment on a fresh aliquot of this sample was conducted as 6 runs, the first three having a maximum t/°C = 650 and the next three having a maximum t/°C = 700. Visual examination of the residue was performed at the end of each run. Figure 5, panel 5.2, shows the thermogram for the run in which the sample was taken to t/°C = 650 for the first time. The feature is similar to that for the sample with n(Na)/n(B) > 1 (see Figure 4): continuous mass gain in the isothermal segment of t/°C = 600 and a turnaround to mass-loss in the dynamic segment of t/°C = (600 to 650), and a continuous mass loss in the isothermal segment of t/°C = 650. The salient results from the subsequent runs with this aliquot are: during an isothermal segment of t/°C = 650, there was a turn-around (in the mass vs time curve) after which the

composition range which the reaction product is likely to end up in - that corresponding to the biphasic region (NaBO2 + Na4B2O5). Figure 4 gives the thermogram mass and temperature

Figure 4. Temperature, mass vs time curve with (B + Na2CO3) mixture having Na-to-B atomic fraction ratio ≈ 1.2.

vs time plot corresponding to the isothermal segment t/°C = 600, dynamic segment t/°C = (600 to 638), and isothermal segment at t/°C = 638. The feature shown in the mass vs time curve yields similar information as that for the sample with n(Na)/n(B) = 1: that onset of chemical decomposition of Na2CO3 does start at low temperature, in this case at t/°C ≈ 634. The residue appeared powdery with very few black spots. The residue was subsequently subjected to a few runs across t/°C = 620 and t/°C = 660 in both increasing and decreasing temperature directions, consisting of dynamic heating of (+2 or −2) °C 1798



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pan and the crucible with Millipore water that the crucible could be removed); the experiment with second aliquot was marred by the balance pan holder often touching the furnace and causing highly unstable mass values. The thermograms corresponding to the third and fourth aliquots are shown in Figure 6, panels 6.1 and 6.2. Understandably, the mass gain portion, seen in Figure 1, panels 1.1b to 1.5b, for the (B + Na2CO3) mixtures is missing, but, evidence for the onset of chemical decomposition of Na2CO3 at t/°C ≈ 650 is once again clearly obtained. The TG measurements over these two aliquots were continued to obtain dm/dt values at three higher temperatures, t = (900, 880, and 860) °C. Table 3 includes the results obtained on many isothermal segments (columns 7 to 10). The values are reasonably consistent with that obtained by us over NaBO2 previously.6 Measurements on B2O3. The thermogram for a B2O3 sample is shown in Figure 7, panel 7.1, whereas that after the addition of Na2CO3 is shown in Figure 7, panel 7.2. Figure 7, panel 7.1, shows absence of mass loss as expected for the low vapor pressure of B2O3,18 whereas the feature of Figure 7, panel 7.2b, is very similar to that of Figure 6, panels 6.1b or 6.2b, characteristic of reaction between B2O3 and Na2CO3. Measurements on (B4C + Na2CO3) Mixtures with n(Na)/n(B) = 1 and on B4C. Encouraged by the interesting results from (B + Na2CO3) and (B2O3 + Na2CO3) mixtures, we proceeded to investigate the (B4C + Na2CO3) mixture with n(Na)/n(B) ≈ 1. Thermogram patterns obtained on two aliquots (Figure 8, panels 8.1(a,b) and 8.2(a,b)) were similar to those obtained for the (B + Na2CO3) mixtures, the temperature onset for chemical decomposition hovering around t/°C = 640 °C. The (dm/dt) values at t/°C = 900 °C, t/°C = 880 °C, and t/°C = 860 obtained for the second aliquot (Table 3) were in reasonable accord with that for NaBO2. The residue, however, was a thin film like that obtained over (B2O3 + Na2CO3) mixture, instead of a cake as that obtained over (B + Na2CO3). The TG experiment with pure B4C gave results that indicated extremely slow progress of its oxidation (see Figure 9, panel 9.1), similar to the situation with elemental boron. The rate of mass gain during the t/°C = 800 °C isothermal heating segment decreased from 5.7 × 10−6 mg·s−1 (at the first hour) to 2.2 × 10−6 mg·s−1 (at the last hour). The appearance of the residue after this TG experiment (run 1) on B4C was practically the same as the starting sample which consisted of four small black grains of B4C. The thermogram obtained after the addition of Na2CO3 to this residue (see Figure 9, panel 9.2) revealed that B4C which showed great resistance to oxidation readily induces the chemical decomposition of Na2CO3 at about t/°C = 650 °C. Subsequent runs, performed with the objective of obtaining TTG results, yielded dm/dt values which were decreasing from run to run and also were a factor of 2 lower than those known for NaBO2. The residue was a thin whitish transparent glassy film sticking at the bottom of the crucible. Insights into Decomposition of Na2CO3 and Oxidation of B and B4C. We summarize below the main inferences from the TG measurements performed to study the reaction of Na2CO3 with B (with Na-to-B ratios = 2/3, 1, and 11/9), with B2O3 as well as B4C (with n(Na)/n(B) = 1) under flowing commercial argon: (1) Formation of sodium borates occur in all cases at t/°C ≤ 700, evidently facilitated by the onset of chemical decomposition of Na2CO3 at temperatures where the normal decomposition pressure of Na2CO3 is ≤2 × 10−5 Pa; (2) the reaction involving boron (be it as element or B4C) at different stages is dictated by the magnitude of the effects of two reactions, one involving oxidation of B or B4C and the other involving chemical

Figure 5. Thermogravimetric results with two aliquots of the (B + Na2CO3) mixture having Na-to-B atomic fraction ratio ≈ 0.67. Panel 5.1a shows temperature, mass vs time curves for aliquot 1. Panel 5.1b shows corresponding mass vs temperature part in the selected dynamic segment (600 to 700)°C. Panel 5.2 shows temperature, mass vs time curve for aliquot 2.

mass gain continued through out the remaining time (about 18 h); the mass gain continued in the t/°C = 700 isothermal segment (for about 20 h) also; and finally there was no mass change with time for duration as long as 10 h in the t/°C = 700 isothermal segment. The last result was taken to indicate that reaction was complete and that the vapor pressure of the reaction product (which should correspond to the biphasic region (NaBO2 + Na2B4O7) when Na/B = 0.67) is too low to cause any mass loss at t/°C = 700. Measurements with (B2O3 + Na2CO3) Mixtures with n(Na)/ n(B) = 1. Having consistently seen a V shaped feature in thermograms of all (B+Na2CO3) mixtures (what we termed sometimes as turn-around), determined by the combination of two reactions, one the oxidation of B and the other the chemical decomposition of Na2CO3, we were interested in studying (B2O3 + Na2CO3) mixture, with Na-to-B ratio = 1. TG experiments with four aliquots were performed, but those with the first two aliquots ran into problems: the TG curve of the first aliquot showing at one point of time a mass loss of 50 mg while the total initial mass of the sample was only 32 mg, and the crucible getting strongly stuck at the bottom of the pan (and only by spraying the 1799



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Figure 6. Thermogravimetric results with two aliquots of the (B2O3 + Na2CO3) mixture having Na-to-B atomic fraction ratio of ≈ 1. Panels 6.1a and 6.2a show temperature, mass vs time curves; panels 6.1b and 6.2b show corresponding mass vs temperature parts in the selected dynamic heating segment (600 to 700) °C.

mixtures was that the onset of decomposition of Na2CO3 occurs when a liquid layer is formed (due to an eutectic in the Na2O− SiO2) at the Na2CO3/SiO2 interface and the reaction product is Na2SiO3. This inference was based on the evidence from the results of the electron probe micro analysis (EPMA) of the sample quenched in the middle of the decomposition process. In the present study, the TG experiments with (B2O3 + Na2CO3) and (B4C + Na2CO3) mixtures gave residues which were glassy film like (and strongly adherent to the base of the platinum cup) which make us suspect whether Na2CO3 decomposition in these cases involved formation of a liquid phase at some stage during the eventual conversion to NaBO2. Since the phase diagram of Na2O− B2O3 shows that while B2O3 melts at about t/°C = 450, the biphasic mixtures on either side of NaBO2 involve generation of liquid phases at t/°C = 640 (on Na2O-rich side) and at t/°C = 744 (on B2O3-rich side), it is possible that any one of these factors could have imparted strong adhesion of the residue to the cellbottom. Mass Spectrometric Measurements. Our basic objective of performing TIMS and KEMS measurements, reported in this paper, was to understand whether observation of different kinds of ions in TIMS and KEMS resulted because of the difference in the nature of ionization processes or because of the difference in the nature of the samples used in TIMS and KEMS. TIMS Measurements. Table 4 gives some salient results obtained in TIMS examination of various samples. A general comment that can be made on the results given in Table 4 is that irrespective of the nature of the sample, that is, whether the sample loaded was already a sodium borate (samples 2 and 3), or

decomposition of Na2CO3 presumably by B2O3 which has its onset at t/°C = (600 ± 50), and the two reactions act in tandem toward the eventual formation of sodium borates; and finally (3) if formation of sodium borates in the case of (B + Na2CO3) and (B4C + Na2CO3) mixtures would have to necessarily involve reaction between Na2CO3 and B2O3, then it may be stated that while complete oxidation of B or B4C was not feasible even under high oxygen environment,19,20 it became feasible in a very low oxygen environment, but in the presence of Na2CO3, ready to interact with the generated oxidation product to yield a phase or a phase mixture along the Na2O−B2O3 binary system. The proposal put forward by us that it is ultimately the direct reaction of B2O3 with Na2CO3 that facilitated the lowering of decomposition temperature of Na2CO3 (even in the case of (B + Na2CO3) and (B4C + Na2CO3) mixtures), is somewhat similar to that proposed by Perlov and Roth in 1993, in whose study,9 TG experiments have been reportedly made on the mixture of B2O3 with BaCO3 and Na2CO3. The mechanisms proposed by Siriwardane et al.11 for the lowering of the decomposition temperature of Na2CO3 by CaO was through the formation of an intermediate complex Na2···CO3−Ca···O and its subsequent split as (Na2O + CaO + CO2). This inference was based on the evidence they obtained for formation of CaCO3- type species at some intermediate temperatures in their in situ high temperature X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurements. Although these authors observed that Ca(OH)2 induced an even more pronounced effect on the decomposition temperature of Na2CO3, CaCO3 induced no effect. The mechanism proposed by Kim et al.10 in the case of (SiO2 + Na2CO3) 1800



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Figure 7. Thermogravimetric results with B2O3 (panel 7.1a,b) and with the mixture containing the residue and Na2CO3 such that the overall Na-to-B atomic fraction ratio is ≈ 1 (panel 7.2a,b). Panels 7.1a and 7.2a show temperature, mass vs time curves; panels 7.1b and 7.2b show corresponding mass vs temperature curves in the dynamic heating segment (600 to 700) °C.

It is known that the boron containing ion in negative thermal ionization (NTIMS) mode is BO2− and that its ionization efficiency is much higher than that for M2BO2+ (M = Na, Cs) formed in PTIMS mode. If one could switch from PTIMS to NTIMS and vice versa and make measurements of I(Na+) and I(BO2−) with time at constant filament current, one might be able to obtain a more direct confirmation of whether the dissociation of (NaBO2)ad,g to (Na+)ad,g and (BO2−)ad,g is the principal ionization process, in which case the ratio of [I(Na+)/ I(BO2−)] would be expected to remain reasonably constant. Furthermore, it might also be interesting to examine whether we could detect emission of [Na(BO2)2]− in NTIMS mode in case this ion is formed by attachment of (BO2−)ad,g with (NaBO2)ad,g, just as in PTIMS mode the emission of Na2BO2+ is proposed to occur by attachment (Na+)ad,g with (NaBO2)ad,g. If one is successful in detecting [Na(BO2)2]− then it would also be interesting to examine whether the ratio {I(BO2−)/I[Na(BO2)2]−} has any relation to the ratio [I(Na+)/I(Na2BO2+)] in PTIMS. Although we met with success in our attempt to measure boron isotope ratios in NTIMS mode using BO2−, since the subsequent switching from NTIMS to PTIMS mode (even under filament off conditions) resulted in the breakdown of the acceleration voltage unit, we could not perform the switch-over experiments which might have given some useful information on ionization aspects in TIMS. KEMS Measurements. Residues from three TG experiments were taken in Pt crucibles for KEMS measurements. They were from aliquots 2 and 3 of (B + Na2CO3) mixture and aliquot 4 of (B2O3 + Na2CO3) mixture, both mixtures having Na-to-B ratio ≈1. In the case of the residue from (B2O3 + Na2CO3) mixture, the residue which was just like a small island of film at the bottom of the crucible, was not scrapped off, but the crucible itself has

it was a residue from a TG experiment (which gave circumstantial evidence that a sodium borate has been formed) or only a physical mixture (B + Na2CO3), (B2O3 + Na2CO3), and (B4C + Na2CO3), Na+ was the dominating ion with Na2BO2+ also appearing in all cases except in the case of sample no. 4, which was the residue from the TG experiment on (B + Na2CO3) mixture with Na-to-B ratio = (0.55/0.45). Since we observed Na2BO2+ even in the case of sample no. 6 which was a mixture of (B + Na2CO3) with Na-to-B ratio = (0.6/0.4), we reason that the failure to detect Na2BO2+ in sample no. 4 is not to be taken to indicate that the higher the n(Na)/n(B) (that is >1), the lower will be the possibility to detect Na2BO2+. That Na2CO3 loaded on to a single filament configuration did not show vacuum fall while the filament was being heated, and nor did it yield Na+ even at a filament current of 1.9 A can be taken as proof that Na+ in all our B-containing samples owed its origin to formation of a sodium borate (or a mixture of sodium borates) on the filament. Loading of NaBO2 on a triple filament configuration (sample no. 2 in Table 4) initially showed some Na2BO2+ but soon disappeared with Na+ fast rising to saturation. For instance, I(Na+)/I(Na2BO2+) ratio at filament currents of 2.12 A (side vaporization filament) and 2.15 A (center ionization filament) was about 950, but upon increase of the ionization filament current further, Na2BO2+ vanished while I(Na+) attained the saturation value of 10 V. Thus, with all our sample loadings (performed without using graphite, a layer of which could facilitate better recombination of the dissociation products Na+ and BO2−),8 it appears more likely that in our TIMS studies, the emission of Na2BO2+ occurred as a result of attachment of Na+ ion (the dissociation product represented by eq 1) with the molecular species NaBO2, both (Na+)ad,g and (NaBO2)ad,g remaining mobile on the heated filament surface. 1801



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Figure 8. Thermogravimetric results with (B4C + Na2CO3) mixture having Na-to-B atomic fraction ratio of ≈ 1. Panels 8.1a and 8.2a show temperature, mass vs time curves; panels 8.1b and 8.2b show corresponding mass vs temperature curves for the selected dynamic heating segment (500 to 800) °C.

Figure 9. Thermogravimetric results with B4C (panels 9.1a,b) and with the mixture containing the residue and Na2CO3 such that the overall Na-to-B atomic fraction ratio is ≈ 1 (panels 9.2a,b). Panels 9.1a and 9.2a show temperature, mass vs time curves; panels 9.1b and 9.2b show corresponding mass vs temperature curves for the selected dynamic heating segment (500 to 800) °C. 1802



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Table 4. Salient Features of TIMS Examinationa of Some B-Containing Samples in Positive Thermal Ionization Mode no. c

1

sample details

filament

physical mixture of (B+ Na2CO3) with rhenium n(Na)/n(B) = 1.5 to 2

2d solid NaBO2, obtained by dehydration tantalum of NaBO2·4H2O in the TG apparatus

3

solid NaBO2·4H2O

tantalum

bead filament no. date of analysis current/A 2 8 12

06 April 2011 06 April 2011 10 May 2011

1.83 1.94 1.70

1 2 3 17

08 June 2011 08 June 2011 09 June 2011 25 Nov 2010

14 3

26 Nov 2010 10 Dec 2010

1.61 1.50 1.67 1.66 1.58 1.52 1.69

19 12 18

27 Jan 2012 20 June 2011 20 June 2011

1.72 1.72 1.60 1.62

4 5 6

7

8c

residue from TGA (aliquot 4 of sample tantalum 1; n(Na)/n(B) = (0.5/0.5) residue from TGA (aliquot 1 of sample tantalum 4; n(Na)/n(B) = (0.55/0.45) physical mixture of (B+ Na2CO3) with tantalum n(Na)/n(B) = (0.6/0.4)

physical mixture of (B2O3+ Na2CO3) with n(Na)/n(B) = (0.5/0.5)

physical mixture of (B4C + Na2CO3) with n(Na)/n(B) = (0.5/0.5)

18

27 Jan 2012

1.95

13

31 Jan 2012

1.88

11 8 11

01 Feb 2012 07 Feb 2012 08 Feb 2012

1.94 1.94 1.94

tantalum

14

02 Feb 2012

1.90 1.91

rhenium

12 15

17 Oct 2012

rhenium

6

22 Oct 2012

1.67 1.68 1.69 1.72 1.81 1.84 1.90 1.94

[I(Na+)/I(Na2BO2+)]b 11 46 17 34 12 8 12 6 39 6 50 79 450 267 increase from 1000 to 3000 in 12 min with decrease in intensities of both ions decreased from 900 to 300 in 30 min with nearly stable Na2BO2+ but decreasing Na+ increased from 83 to 225 Na2BO2+ not detected even while I(Na+) was as high as that (1.4 V) obtained in the above residue increased from 5 to 100 as I(Na+) increased from 3 mV to 1.1 V ∼19 with I(Na+) ≈ 2.5 V; at lower filament currents also, the ratio was ∼20 ∼154 at I(Na+) ≈ 2.4 V. The ratio increased to 1425 with I(Na+) increasing to 9 V, but I(Na2BO2+) decreasing to ∼6 mV from 21 mV ∼15 with I(Na+) only ∼25 mV high ∼750 to 1750 with I(Na+) increasing first to the saturation value of 10 V and then decreasing to 2.4 V and further down very fast. I(Na+) = saturation value of 10 V while I(Na2BO2)+ was hardly 1 mV 463 436 384 at I(Na+) = 7.2 V increased from 16 to 30 with I(Na+) stable at 150 mV and I(Na2BO2+) decreasing. increased from 34 to 72 with I(Na+) stable at 430 mV and I(Na2BO2+) decreasing increased from 139 to 344 with I(Na+) decreasing from (3.3 to 2.2) V and I(Na2BO2+) decreasing from (16 to 4) mV increased from 406 to 1162 with I(Na+) decreasing from (6.6 to 5.4) V and I(Na2BO2+) decreasing from 11 to 3 mV

Ions observed in the case of all samples were Na+, Na2BO2+. Reaction occurred in situ during filament heating. n(Na)/n(B) denotes the atomic fraction ratio of Na and B. bI(Na2BO2+) is the total ion intensity corresponding to both 10B and 11B. That is, I(Na2BO2+) = I(Na210BO2+) + I(Na2 11 BO2+). cBoron was enriched in 10B (10B/11B ≈ 2.1 to 2.2); In all other samples, boron had natural abundance (10B/11B ≈ 0.25). dAnother aliquot of the same sample loaded onto a side filament of a triple assembly (bead no. 12; all tantalum filaments). a

NaBO2+ and (303 ± 35) kJ·mol−1 for Na2BO2+, which showed clogging of the Knudsen cell orifice. An aliquot of (B + Na2CO3) physical mixture with n(Na)/n(B) ≈ 1 was subjected to KEMS investigation in order to examine whether reaction that has occurred under TG and TIMS conditions will take place under KEMS condition also. Evolution of CO2 was noticed at temperature as low as 765 K, but at each increasing temperature, I(CO2+) decreased with time and at T > 1050 K, it was very low. From T = 1074 K, Na+ and NaBO 2+ were also detected and were shutterabale, but the intensities decreased considerably. Subsequently, only from T = 1180 K were they measurable, and the ratio I(Na+)/ I(NaBO2+) was about 4. These results suggested that conversion to NaBO2 occurred under KEMS conditions also, perhaps mainly aided by the moisture present in the mixture. Insight into Vaporization and Ionization of NaBO2 in TIMS. Summing up the information obtained in KEMS and TIMS

been placed in an outer Pt crucible for KEMS. Table 5 gives the salient results. The observation of same ions, Na+ and NaBO2+, and Na2BO2+ in these samples (as over NaBO26) were taken as the first qualitative evidence that reaction of (B + Na2CO3) or (B2O3 + Na2CO3) did occur under flowing argon in the TG apparatus to yield NaBO2. However, evidence in terms of I(Na +)/ I(NaBO 2+) and I(NaBO2+)/I(Na2BO2+) ratios or temperature dependence of intensities of NaBO2+ and Na2BO2+ was rather scratchy. Nevertheless, we seek to derive comfort from the fact that even in our previous study of NaBO2,6 the results from series 1 and 2 were rejected as suspect due to large scatter in apparent enthalpies of sublimation of NaBO2+ and Na2BO2+. The apparent enthalpy values obtained in this study on one single day of measurement were low (see run no. 2 in Table 5) but so were also the values over the samples in the first two series of our previous6 measurements on NaBO2: (267 ± 16) kJ·mol−1 for 1803



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Table 5. Salient Results from KEMS Measurements on Residues from TG Measurements

run

date

1

02 Jan 2012

2

03 Jan 2012

3

04 Jan 2012b

4

11 Jan 2012

5 6

12 Jan 2012 13 Jan 2012c

7

18 Jan 2012

8

19 Jan 2012

n1a

T

I(Na+)/I(NaBO2+)

K

n2a

mean ratio

I(Na2BO2+)/ I(NaBO2+) n3a

mean ratio

remarks

residue after TG measurements on (B + Na2CO3) with Na/B ≈ 1; aliquot 3 of sample 1 in Table 2 1045 to 1098 1 1.6 only two temperatures accessed. In 3.5 h at T = 1098 K, I(NaBO2+) decreased by a factor of 1.4 12 1060 to 1185 3 1.7 ± 0.5 14 0.25 ± 0.08 ΔsubHapp Na11BO2+ = 247 kJ·mol−1 ΔsubHapp Na211BO2+ = 343 kJ·mol−1 9 1074 to 1175 1 1.4 2 0.33 ± 0.01 I(NaBO2+) decreased by a factor of 4 at T = 1175 K in 5.5 h residue after TG measurements on (B + Na2CO3) with Na/B ≈ 1; aliquot 2 of sample 1 in Table 2 5 1154 to 1168 1 0.7 1 0.12 I(NaBO2+) decreased by a factor of 1.6 at T = 1160 K in 4 h 1 1160 I(NaBO2+) decreased by a factor of 3 at T = 1160 K in 8 h 5 1160 to 1244 2 0.6 ± 0.2 decrease in both I(Na+) and I(NaBO2+) prompted us to increased the T in steps to 1244 K, 5 K above melting temperature of NaBO2. I(NaBO2+) was still very low and Na+ became nondetectable residue after TG measurements on (B2O3 + Na2CO3) with Na/B ≈ 1; aliquot 4 of sample 5 in Table 2d 2 1160 to 1175 5 I(NaBO2+) was very low compared to previous samples (8 to 10 times) and it reduced further upon increase in T I(Na+)/I(NaBO2+) decreased from 9.4 to 0.7 in 3.5 h 1 1195 3 0.5 ± 0.3 I(NaBO2+) at T = 1195 K is same as that at T = 1175 K on 18 Jan 2012 2

a n1 denotes the number of temperature points accessed; n2 denotes the number of times I(Na+)/I(NaBO2+) was measured; n3 denotes the number of times I(Na2BO2+)/I(NaBO2+) was measured. bThere was practically no sample left inside the Knudsen cell (except a tiny black dot); The Knudsen cell lid had a black deposit. cThe orifice was found to be fully clogged with a transparent and glassy substance. The Knudsen cell had a tiny black particle. dThe residue itself was only a thin film at the bottom of the crucible.

confirmed that, although the oxidation of B (or B4C) is severely hampered by slow kinetics, the presence of Na2CO3 facilitates fast conversion of B (or B4C) to sodium borates and thus indirectly the oxidation of B (or B4C), too. The transpiration thermogravimetric measurements provided the evidence that when the n(Na)/n(B) in the reaction mixtures is about 1, the rate of mass-loss of the product of the reaction is in accord with that expected for NaBO2, congruent vaporization of which was confirmed previously by us.6 Mass spectrometric measurements performed on some residues of TG measurements not only provided further evidence for formation of sodium borates in the TG apparatus but also led us to gain some insight to certain aspects related to vaporization-ionization processes in KEMS and TIMS.

studies and assuming that vaporization coefficient for the monomeric NaBO2 vapor species will not be so low to prevent it from coming out under nonequilibrium conditions such as that which could exist in TIMS, we infer that while the surface ionization cross section for the dissociation NaBO2(g) to Na+ and BO2− is high, it is negligibly low for the formation of NaBO2+ from NaBO2(g). A question may arise as to whether detection of Na2BO2+ in TIMS can be similarly attributed to dissociation of dimer on the filament surface, that is, (NaBO2)2(g) to Na2BO2+ and BO2−. Observation of huge variation in [I(Na+)/I(Na2BO2+)] ratios even at same filament currents (see Table 4) negates such a possibility although one should strictly consider correlating [I(Na+)/I(Na2BO2+)] with monomer-to-dimer ratios only under equilibrium conditions. Besides, even under equilibrium conditions, the ratio remaining constant or varying with time, is dictated by how the vaporization proceeds and according to the phase rule (that is, by the variance of the system at every instant). Based on all these considerations, we tend to conclude that in TIMS both Na+ and Na2BO2+ owe their origin to NaBO2(g), through two different steps though. Furthermore, the flux of the two ions are possibly governed by various surface-related factors including the changes in surface area or morphology of the sample, and therefore, the values of [I(Na+)/I(Na2BO2+)] show large scatter.

■

AUTHOR INFORMATION

Corresponding Author

*Tel: 91 44 2748 0098. Fax: 91 44 27480065. E-mail: rvis@igcar. gov.in; [email protected]. Notes

The authors declare no competing financial interest.

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■

REFERENCES

(1) Rao, R. M.; Parab, A. R.; Sasibhushan, K.; Aggarwal, S. K. Int. J. Mass Spectrom. 2009, 285, 120−125. (2) Rao, R. M.; Parab, A. R.; Sasibhushan, K.; Aggarwal, S. K. Anal. Methods 2011, 3, 322−327. (3) Ishikawa, T.; Nagaishi, K. J. Anal. At. Spectrom. 2011, 26, 359−365. (4) Büchler, A.; Berkowitz-Mattuck, J. B. J. Chem. Phys. 1963, 39, 286− 291. (5) Gorokhov, L. N.; Gusarov, A. V.; Makarov, A. V.; Nikitin, O. T. Teplofizika Vysokikh Temperatur 1971, 9, 1173−1176 (English translation pp 1082−1084).. (6) Lakshmi Narasimhan, T. S.; Viswanathan, R.; Nalini, S. J. Phys. Chem. B 2011, 115, 13261−13270.

CONCLUSIONS The TG measurements on (B + Na2CO3), (B2O3 + Na2CO3), and (B4C + Na2CO3) mixtures under flowing commercial argon were performed essentially to understand how the reactions between B-containing samples and Na2CO3 take place on the surface of TIMS filaments. The results showed that sodium borates are formed through chemical decomposition of Na2CO3, occurring at temperatures much lower than that at which thermal decomposition of Na2CO3 is known to occur. The results also 1804



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(7) Lev. S. Kudin, Dean of the Inorganic Chemistry and Technology Faculty, Ivanovo State University of Chemical Technology (ISUCT), Russia. E-mail correspondence. (8) Wei, H. Z.; Jiang, S. Y.; Hemming, G. N.; Yang, J. H.; Xiao, Y. K.; Yang, T.; Yan, X.; Yan, Y. J. Phys. Chem. A 2011, 115, 14570−14580. (9) Perlov, D.; Roth, M. J. Cryst. Growth 1993, 130, 686−689. (10) Kim, J. W.; Lee, Y. D.; Lee, H. G. ISIJ Int. 2001, 41, 116−123. (11) Siriwardane, R. V.; Poston, J. A., Jr.; Robinson, C.; Simonyi, T. Energy Fuels 2011, 25, 1284−1293. (12) Milman, T.; Bouaziz, R. Ann. Chim (Paris) 1968, 3, 311−321. (13) Spear, K. E.; Besmann, T. M.; Beahm, E. C. MRS Bull. 1999, 24, 37−44. (14) Wang, C.; Yu, H.; Liu, H.; Jin, Z. J. Phase Equilib. 2003, 24, 12−20. (15) Viswanathan, R.; Lakshmi Narasimhan, T. S.; Nalini, S. J. Chem. Eng. Data 2010, 55, 3779−3785. (16) Stern, K. H; Wise, E. L. High Temperature Properties and Decomposition of Inorganic Salts. Part 2. Carbonates. NSRDS-NBS 30, Nat. Stand. Ref. Data Ser., Nat. Bur. Stand. (US). 1969. (17) Honig, R. E. Vapor Pressure Data for the Elements. In The Characterization of High-Temperature Vapors; Margrave, J. L., Ed.; Wiley: New York, 1967; p 475. (18) Jacobson, N. S.; Myers, D. L. J. Phys. Chem. B 2011, 115, 13253− 13260. (19) Jain, A.; Joseph, K.; Anthonysamy, S.; Gupta, G. S. Thermochim. Acta 2011, 514, 67−73. (20) Lavrenko, V. A.; Pomytkin, A. P.; Kyslyj, P. S.; Grabchuk, B. L. Oxidat. Metals 1976, 10, 85−95.

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Study of the Reaction of B, B2O3, and B4C with Na2CO3 in a

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