Americium dioxide (AmO₂) is a black compound of

americium Americium is a synthetic element, synthetic chemical element; it has Chemical symbol, symbol Am and atomic number 95. It is radioactive and a transuranic member of the actinide series in the periodic table, located under the lanthanide element e ...

. In the solid state, AmO₂ adopts a

fluorite structure The fluorite structure refers to a common motif for compounds with the formula MX2. The X ions occupy the eight tetrahedral interstitial sites whereas M ions occupy the regular sites of a face-centered cubic In crystallography, the cubic (or is ...

(like CaF₂). It is used as a source of

alpha particle Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay but may also be produce ...

Historical context

The demand for americium dioxide stems from the difficulty of storing the element americium as a solution of americium(III) chloride because the alpha radiation and

hydrochloric acid Hydrochloric acid, also known as muriatic acid or spirits of salt, is an aqueous solution of hydrogen chloride (HCl). It is a colorless solution with a distinctive pungency, pungent smell. It is classified as a acid strength, strong acid. It is ...

decomposes storage containers over time. To solve the liquid storage problem, scientists at

Oak Ridge National Laboratory Oak Ridge National Laboratory (ORNL) is a federally funded research and development centers, federally funded research and development center in Oak Ridge, Tennessee, United States. Founded in 1943, the laboratory is sponsored by the United Sta ...

devised a synthesis to turn liquid americium–acid solution into a precipitated form of americium for safer handling and more efficient storage.

Synthesis

Synthesis of americium dioxide, as described by the Oak Ridge National Laboratory in 1960, starts by dissolving americium in hydrochloric acid, and then neutralizing the excess acid with

ammonium hydroxide Ammonia solution, also known as ammonia water, ammonium hydroxide, ammoniacal liquor, ammonia liquor, aqua ammonia, aqueous ammonia, or (inaccurately) ammonia, is a solution of ammonia in water. It can be denoted by the symbols NH3(aq). Although ...

(). Then, saturated

oxalic acid Oxalic acid is an organic acid with the systematic name ethanedioic acid and chemical formula , also written as or or . It is the simplest dicarboxylic acid. It is a white crystalline solid that forms a colorless solution in water. Its name i ...

solution () is added to the now neutralized solution to precipitate dull pink americium(III)

oxalate Oxalate (systematic IUPAC name: ethanedioate) is an anion with the chemical formula . This dianion is colorless. It occurs naturally, including in some foods. It forms a variety of salts, for example sodium oxalate (), and several esters such as ...

crystals; once complete precipitation is achieved, additional

is added to make a slurry. The slurry of americium oxalate and oxalic acid is next agitated before the americium oxalate is filtered out, washed with water, and partially dried in air. The americium oxalate is then calcinated in a

platinum Platinum is a chemical element; it has Symbol (chemistry), symbol Pt and atomic number 78. It is a density, dense, malleable, ductility, ductile, highly unreactive, precious metal, precious, silverish-white transition metal. Its name origina ...

boat. It is first dried in a furnace at and then heated to . When decomposition begins to occur, the oxalate will turn into the desired black americium dioxide; to ensure no oxalate remains in the newly forming dioxide, the oven temperature is increased and held at then slowly allowed to cool to room temperature.

Modern applications

Americium dioxide is the most widely used americium compound in ionising smoke detectors. The dioxide form is insoluble in water, making it relatively safe to handle in production. In the late 2010s, americium dioxide has been of interest to ESA as power source for

radioisotope thermoelectric generators A radioisotope thermoelectric generator (RTG, RITEG), or radioisotope power system (RPS), is a type of nuclear battery that uses an array of thermocouples to convert the heat released by the decay of a suitable radioactive material into electr ...

(RTGs) for deep space exploration spacecraft and

satellites A satellite or an artificial satellite is an object, typically a spacecraft, placed into orbit around a celestial body. They have a variety of uses, including communication relay, weather forecasting, navigation ( GPS), broadcasting, scientif ...

. A fully automated

chemical process In a scientific sense, a chemical process is a method or means of somehow changing one or more chemicals or chemical compounds. Such a chemical process can occur by itself or be caused by an outside force, and involves a chemical reaction of som ...

to produce americium dioxide was developed by nuclear researchers from the

University of Bristol The University of Bristol is a public university, public research university in Bristol, England. It received its royal charter in 1909, although it can trace its roots to a Merchant Venturers' school founded in 1595 and University College, Br ...

to be implemented on the

Sellafield Sellafield, formerly known as Windscale, is a large multi-function nuclear site close to Seascale on the coast of Cumbria, England. As of August 2022, primary activities are nuclear waste storage, nuclear waste processing and storage and nucle ...

nuclear site in

Cumbria Cumbria ( ) is a ceremonial county in North West England. It borders the Scottish council areas of Dumfries and Galloway and Scottish Borders to the north, Northumberland and County Durham to the east, North Yorkshire to the south-east, Lancash ...

, UK. It is based on the same principles as the historic production method developed at Oak Ridge National Laboratory.

Americium-aluminium alloys

Americium-aluminium alloys can be formed by melting americium dioxide with aluminium and an additional fluxing agent. The created alloy can undergo neutron irradiation to produce other

transuranic The transuranium (or transuranic) elements are the chemical elements with atomic number greater than 92, which is the atomic number of uranium. All of them are radioactively unstable and decay into other elements. Except for neptunium and pluton ...

nuclides Nuclides (or nucleides, from nucleus, also known as nuclear species) are a class of atoms characterized by their number of protons, ''Z'', their number of neutrons, ''N'', and their nuclear energy state. The word ''nuclide'' was coined by the Am ...

References

{{oxides Americium compounds Oxides Fluorite crystal structure