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Author: Subject: High-temperature flames.
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[*] posted on 6-8-2011 at 10:21


Ignorance is bliss

Outliers in life are modeled by chemical kinetics
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The WiZard is In
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[*] posted on 6-8-2011 at 12:06

Quote: Originally posted by niertap  

Why? Does it have some magic Hf?

Somewhere (I at the moment cannot put hands on it)
I came across a ref to 10 000o with O3 and Zr (?)
Hf (?) at 100 000 psi or some such.

Here is something to keep you busy.

The WiZ (Donald J Haarmann)

You have been chosen lead pyrotechnist for the first peopled mission to Dubious 7. Your mission, should you choose to accept, is: supply a pyrotechnic compound as a source of heat for cooking purposes. Because of the limited weight carrying capacity of the launch vehicle the reaction you choose must have the highest possible ratio of heat produced to weight of reactants, i.e., kilo calorie/gram (K/cal/gm) of reactants. There are no restrictions on the state of the reactants; they may be either solids, liquids or gases.

For those not familiar with the method of calculating the amount of heat produced by a reaction, the following explanation is provided.

The amount of heat in kilo calories is calculated by subtracting the heat(s) of formation [heat of formation is now called “standard enthalph of formation”] (Hf°) of the reactant(s) from the heat(s) of formation of the products. This value is then divided by the weight of the reactants to obtain heat per gram in kilo calories per gram.

Hf° prod (K/cal) Hfo react (K/cal) = Heat (K/cal)

gms of reactants = K/cal/gm

The heat of formation is the amount of heat required to form one gram MOLEcular weight (mole) of a compound from its constituent elements. If heat is liberated by the reaction the heat of formation has a negative value, i.e., the heat of formation of aluminium oxide Ai2O3 is 400.5 K/cal per mole (102 grams) of aluminium oxide. That is, if we were to react sufficient oxygen and aluminium to produce 102 grams of aluminium oxide we would also produce ( )400.5 K/cal of heat. Elements have a heat of formation of zero. Remember a negative value means heat has been produced.

Let us look at a simple reaction, such as the
burning of titanium in oxygen.

Ti + O2 = TiO2

One mole of titanium (47.9 gms) reacts with one mole of oxygen (32 gms) to produce one mole of titanium dioxide (79.9 gms). The heats of formation for titanium and oxygen are 0, while the Hf° of titanium dioxide is 225 k/cal mole.

47.9 gms + 32 gms = 79.9 gms

Ti + O2 = TiO2

0 k/cal 0 k/cal = 225 k/cal


Hf° products Hf° reactants
------------------------------------- = heat produced per gram weight of
weight of reactants

( 225) (0 + 0) = 225 K/cal
-------------------------------------------------------------------- = 2.82 k/cal per
(47.9 gms) + (32 gms) = 79.9 gms

A more complicated reaction:

3998.94 gms

1960.8 gms + 2038.14 gms

(16)(122.55gms) + (6)(339.69gms)

16KCIO3 + 6SbS3 --> 16KCI + 18SO2 + 3Sb2C4,

(16)( 93.5k/cal) (6)( 43.5) --> (16)( 51.6) + (18)( 71.0) + (3)(214)
( 1496 k/cal) + ( 261) --> ( 825.6) + ( 1278) + ( 642)

( 1757 K/cal) --> ( 2745.6 K/cal)
[reactants] [products]

988.6 k/cal

[products] [reactants]

( 2745.6 k/cal) ( 1757 k/cal)
-------------------------------------- = 0.25 k/cal gm
3999 gms of reactants

Now looking at these reactions and the way the results were obtained, two things will be obvious; first because the Hf° of the products is subtracted from the Hf° of the reactants it will be useful to produce products with high Hf° using reactants with the low Hf°. Indeed the best results will be obtained using reactants with either a low Hf° (small negative value), 0 (elements) or compounds with a positive ( + ) Hf°'s, of which there are not many. (Remember minus a minus is a plus.) There are only few inorganic compounds with positive values of Hf° and are some what exotic e.g., fluorine perchlo¬rate CIOF4, Hf° + 18.5, BrCI, + 14.6, N2O, + 19.49. Secondly, because the heat pro¬duced must be divided by the molecular weight of the reactants, it would be wise to use products with low molecular weights. The reaction between hydrogen and oxygen or fluorine comes to mind, however, these reactions produce only 3.2 K/cal/gm of heat despite their low molecular weights. These two reactions do, however, produce high specific impulse, which makes these reactions useful in rockets, and such.

Heats of formation can be found in the CRC Handbook among other references [my reaction was obtained using products in the CRC Handbook. Most libraries have a copy.

A final note; heat does not mean temperature, e.g., the reaction of oxygen with aluminium produces 3.9 K/cal/gm of heat vs. 3.6 for magnesium/oxygen, the magnesium reaction is capable of producing a temperature of 3600°C, whereas aluminium can only produce a temperature of 3000°C. Indeed the reaction between hydrogen and fluorine while only producing 3.21 K/cal/gm results in a temperature of 4030°C. In general the highest temperature produced by a reaction can never exceed the boiling temperature of the most refractory reaction product when the reaction products are solids. The highest value being approximately 4500°C (thorium oxide). The highest flame temperature for a reaction using gases is 4500°C for the reaction between cyanogen (C2N2) and oxygen. For comparison the flame temperature for oxy/acetylene is 3140°C.

Originally published in PGII Bulletin 21. May/June 1987

Scanned 21vi97

The best result I have been able to obtain is -- 6 K/cal per gram of reactants. Can you do as well or even better? Using boron and ozone

2B + 2O3  3B2O2

(6) (10.8gms) (2) (48gms) --> 160.8 gms

-906 - (+68) = -974 K/cal

-974 K/cal

= 6.05 K/cal

Added [Well… I see in the late Dr. Ellern’s book a better reaction! One that will yield -13.4 K/cal per gram!!]

[I have doubts about cyanogen and oxygen producing a 4500oC flame temperature. I believe this is base on an old value for the boiling point of carbon, e.g., 4500oC. 3600 oC is more likely.]

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[*] posted on 27-11-2017 at 20:12

Darn. Can't ask the WiZ what he found in Ellern now, and my copy is in storage 80 miles away.

In reference to the odd chemicals that were (or are being) offered at Skylighters going out of business sale, I've come across something interesting:

Attachment: Ti, Zr & Hf compounds as pyrotechnic fuels.pdf (1.1MB)
This file has been downloaded 31 times

Zirconium diboride, Hafnium carbide, etc. All are usually considered highly refractive ceramics for extreme environments... But they burn.

Merely mixed straight out of the jar from chemical supplier with stoichometric ammounts of Potassium nitrate, these ceramics BURN. Not quite as hot as the pure metals, but then, they don't go off from minor static discharges, bumps or mere dirty looks as the finely powdered metals do. I do not enjoy handling fine Zr powders in particular, I have had a couple of near misses.

From a quick look at the tables in the attached paper- Hafnium carbide has a density around 12 g/cc, and burns somewhere upwards of 3,000 C? Al is only around 2.7 g/cc. And it looks like the HfC is often supplied as particles verging on sub micron range? I can think of possible uses.

[Edited on 28-11-2017 by Bert]

[Edited on 28-11-2017 by Bert]

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