|Image from AmazingRust.com of a simple thermite reaction involving iron oxide and aluminum. This video shows thermite melting through a car.|
2 Al + Fe2O3 → Al2O3 + 2 Fe
|The canonical thermite reaction is simple, lacks the aromatic hydrocarbons and nitrogen found in conventional high explosives, and is highly exothermic.|
|ABOVE: Relationship of particle size to reaction rate in thermites|
BELOW: General relationship of reaction rate to the form of energy released in compositions that have the capacity to be high explosives
(Al + Fe2O3)
(Al + CuO)
The mixing [of ultra fine grain (UFG) aluminum and UFG metal oxides] is accomplished by adding these reactants to a liquid solution where they form what are called "sols", and then adding a gelling agent that captures these tiny reactive combinations in their intimately mixed state (LLNL 2000). The resulting "sol-gel" is then dried to form a porous reactive material that can be ignited in a number of ways. 
|Graphic from a DTIC (Defense Technical Information Center) Review publication on advanced energetic materials.|
|Two images of iron-rich spheroids from the USGS Particle Atlas of World Trade Center Dust. |
|Illustration from a damage assessment report prepared for Deutsche Bank, the owners of a skyscraper severely damaged by projectiles from the South Tower. The report was commissioned, in part, to determine the nature and extent of contamination of the building, which is slated for demolition.|
|Dr. Steven E. Jones describing molten metal seen at Ground Zero.|
As usual, we search for possible prosaic explanations for these metallic spherules in the WTC dust. The most obvious possible source is the melting of large quantities of steel in the buildings followed somehow by formation of tiny droplets of molten steel. As discussed above, however, steel melts at about 1538ºC (2800ºF) – and the temperatures in the buildings were no where near [sic] hot enough to melt steel, and certainly not in large quantities required for the amounts seen in the dust (and pouring out of the South Tower before collapse). Furthermore, we have looked at the chemical compositions of a number of iron-rich spherules as well as that of steel, and the compositions are not the same at all. It should not be surprising, however, as we analyze more spherules to find some that are steel-like in composition, assuming that thermite cutter-charges were in fact used to cut through steel. We should then find both steel- and thermite-residue spherules.
Could these droplets be due to molten aluminum alloy (from the jets) striking rusty steel and/or other office materials to somehow generate the iron-rich spheres? We performed experiments with molten aluminum poured onto rusty steel, then onto crushed gypsum and concrete (on the rusty steel) – and observed no formation of iron-rich droplets at all nor any sign of vigorous chemical reactions.
One can estimate the implied amount of thermite needed to generate so many iron-rich spheres in the WTC dust. In a sample of 32.1 grams of WTC dust, I observed with the unaided eye two metallic-looking spheres, in addition to the micron-sized spherules collected using a magnet. The mm-size spheres proved to be iron-aluminum rich. The mass of these two larger spheres (0.012g) found in this sample can be used to provide a crude estimate of the fraction of iron-rich spheres in the dust: 0.012g/32.1g = 0.04%. If the mass of the WTC dust was about 30,000 tons, then the iron-rich spherule content would be of the order of ten tons. This is a very rough estimate based on one small sample, and is only provided to give an idea of the amount of thermite-type reactants and products which may be involved here. An investigation well beyond the scope of this paper would look for purchases of aluminum and iron-oxide powders (and sulfur) in multi-ton-quantities prior to 9/11/2001.
|Fig. 2 from Active Thermitic Material Discoveredshowing chips from the four different dust samples.|
|Map of Lower Manhattan showing locations of the four samples (blue points) and the Twin Towers (red points).|
|Portions of Fig. 4 and Fig. 5: Two scanning electron microscope images of bi-layered chips.|
|Fig. 9, showing a highly magnified view of the red layer. Note the hexagonal plate-like particles, and the smaller faceted particles, both lighter in color than the porous matrix.|
|Fig. 7: "XEDS spectra obtained from the gray layers from each of the four WTC dust samples ..."||Fig. 6: "XEDS spectra obtained from the red layers from each of the four WTC dust samples ..."|
|Fig. 10, showing the BSE image and accompanying XEDS maps for Fe, Al, O, Si, and C of a portion of an untreated red layer.|
|Fig. 15, showing the BSE image and accompanying XEDS maps of Fe, Al, O, Si, and C for a red-layer sample soaked in MEK.|
|Fig. 19 compares the DSC traces of a chip from each of the four samples.|
|Fig. 29, labeled "DSC trace of sample 1 (blue line) compared with DSC of xerogel Fe2O3/UFG Al nanocomposite (from Tillotson et al. ). Both DSC traces show completion of reaction at temperatures below 560ºC".|
We have discovered distinctive red/gray chips in significant numbers in dust associated with the World Trade Center destruction. We have applied SEM/XEDS and other methods to characterize the small-scale structure and chemical signature of these chips, especially of their red component. The red material is most interesting and has the following characteristics:
- It is composed of intimately mixed aluminum, iron, oxygen, silicon and carbon. Lesser amounts of other potentially reactive elements are sometimes present, such as potassium, sulfur, barium, lead and copper. [4,6]
- The primary elements (Al, Fe, O, Si, C) are typically all present in particles at the scale of tens to hundreds of nanometers, and detailed XEDS mapping shows intimate mixing.
- On treatment with methyl-ethyl ketone solvent, some segregation of components was observed. Elemental aluminum became sufficiently concentrated to be clearly identified in the pre-ignition material.
- Iron oxide appears in faceted grains roughly 100 nm across whereas the aluminum appears in plate-like structures. The small size of the iron oxide particles qualifies the material to be characterized as nano-thermite or super-thermite. Analysis shows that iron and oxygen are present in a ratio consistent with Fe2O3. The red material in all four WTC dust samples was similar in this way. Iron oxide was found in the pre-ignition material whereas elemental iron was not.
- From the presence of elemental aluminum and iron oxide in the red material, we conclude that it contains the ingredients of thermite.
- As measured using DSC, the material ignites and reacts vigorously at a temperature of approximately 430ºC, with a rather narrow exotherm, matching fairly closely an independent observation on a known super-thermite sample. The low temperature of ignition and the presence of iron-oxide grains less than 120 nm show that the material is not conventional thermite (which ignites at temperatures above 900ºC) but very likely a form of super-thermite.
- After igniting several red/gray chips in a differential scanning calorimeter run to 700ºC, we found numerous iron-rich spheres and spheroids in the residue, indicating that a very high-temperature reaction had occurred, since the iron-rich product clearly must have been molten to form these shapes. In several spheres, elemental iron was verified since the iron content significantly exceeded the oxygen content. We conclude that a high-temperature reduction-oxidation reaction has occurred in the heated chips, namely, the thermite reaction.
- The spheroids produced by the DSC tests and by the flame test have an XEDS signature (Al, Fe, O, Si, C) which is depleted in carbon and aluminum relative to the original red material. This chemical signature strikingly matches the chemical signature of the spheroids produced by igniting commercial thermite, and of many of the micro-spheres found in the WTC dust. 
- The presence of an organic substance in the red material is expected for super-thermite formulations in order to produce high gas pressures upon ignition and thus make them explosive. The nature of this organic material in these chips merits further exploration. We note that it is likely also an energetic material, in that the total energy release sometimes observed in DSC tests exceeds the theoretical maximum energy of the classic thermite reaction.
|An electron microscope equipped with an EDAX GENESIS 2000 X-Ray Microanalysis System.|
|EDS spectrum of a yellow paint sample, from ModernMicroscopy.com. EDS spectra allow the easy identification of the most abundant elements in a sample, while requiring some analysis to estimate relative quantities.|