Sodium–potassium alloy

Key takeaways

Sodium–potassium alloy belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Sodium–potassium alloy to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Sodium–potassium alloy from memory before moving on to harder problems.

Reference excerpt

Sodium–potassium alloy, colloquially called NaK (commonly pronounced ), is an alloy of two alkali metals: sodium (Na, atomic number 11) and potassium (K, atomic number 19). NaK is normally liquid at room temperature. Various commercial grades are available. Like its constituent elements, NaK is highly reactive with water and may catch fire when exposed to air, so it must be handled with special precautions.

Properties

Physical properties NaK containing 40% to 90% potassium by mass is liquid at room temperature. The eutectic mixture consists of 77% potassium and 23% sodium by mass (NaK-77), and it is a liquid from −12.6 to 785 °C (9.3 to 1,445.0 °F), and has a density of 0.866 g/cm3 at 21 °C (70 °F) and 0.855 g/cm3 at 100 °C (212 °F), making it less dense than water. It is highly reactive with water and is stored usually under hexane or other hydrocarbons, or under an inert gas (usually dry nitrogen or argon) if high purity and low levels of oxidation are required. A solid compound, Na2K, exists at low temperatures, containing 46 percent potassium by mass. NaK has a very high surface tension, which makes large amounts of it pull into a bun-like shape. Its specific heat capacity is 982 J/(kg⋅K), which is roughly one quarter of that for water, but heat transfer is higher over a temperature gradient due to higher thermal conductivity.

Chemical properties

When stored in air, it forms a flammable potassium superoxide coating which reacts explosively with water and organics. NaK is not dense enough to sink in most hydrocarbons, but will sink in lighter mineral oil. It is unsafe to store in this manner if the superoxide has formed. A large explosion took place at the Oak Ridge Y-12 facility on December 8, 1999, when NaK was cleaned up after an accidental spill, inappropriately treated with mineral oil, and then scratched with a metal tool. The liquid alloy also attacks PTFE ("Teflon"). Sodium–potassium alloy polymerizes dimethyldichlorosilane into polysilanes with a Si-Si backbone and methyl radicals, primarily dodecamethylcyclohexasilane: 6 ( CH 3 ) 2 SiCl 2 + 12 M ⟶ [ ( CH 3 ) 2 Si ] 6 + 12 MCl ( M = Na , K ) {\displaystyle {\ce {6 (CH3)2SiCl2 + 12 M -> [(CH3)2Si]6 + 12 MCl}}\ {\ce {(M = Na, K)}}}

Further alloys with low melting points Further alloys with low melting points are Cs77K23 at −37.5 °C (−35.5 °F), Cs19Na at −30 °C (−22 °F) and Na2Rb23 at −5 °C (23 °F). The alloy consisting of 40.8% caesium, 11.8% sodium and 47.4% potassium has a melting point of −79.4 °C (−110.9 °F).

Usage

Coolant NaK has been used as the coolant in experimental fast neutron nuclear reactors. Unlike commercial plants, these are frequently shut down and defuelled. The use of lead or pure sodium, the other materials used in practical reactors, would require continual heating to maintain the coolant as a liquid. Use of NaK overcomes this. The Dounreay Fast Reactor was an example; the NaK used in the reactor was destroyed in 2012. The United States' experimental SNAP-10A satellite, which was the first nuclear reactor in space, used NaK as coolant. The NaK was circulated through the core and thermoelectric converters by a liquid metal direct current conduction-type pump. The satellite was launched in 1965, and as of 2022 is the only fission reactor power system launched into space by the United States. The Soviet RORSAT radar satellites were powered by a BES-5 reactor, which was cooled with NaK. In addition to the wide liquid temperature range, NaK has a very low vapor pressure, which is important in the vacuum of space. An unintended consequence of the usage as a coolant on orbiting satellites has been the creation of additional space debris. NaK coolant has leaked from a number of satellites, including Kosmos 1818 and Kosmos 1867. The coolant self-forms into droplets of sodium–potassium of up to several centimeters in size. These objects are space debris. The Danamics LMX Superleggera CPU cooler uses NaK to transport heat from the CPU to its cooling fins.

Desiccant In contact with water, hydrogen is created. Because of this property, sodium–potassium alloys are used as desiccants in drying solvents prior to distillation.

Hydraulic fluid

Eutectic NaK (NaK-77, an alloy of 77% potassium and 23% sodium by mass) can be used as a hydraulic fluid in high-temperature and high-radiation environments, for temperature ranges of −12 to 760 °C (10 to 1,400 °F). Its bulk modulus at 538 °C (1,000 °F) is 2.14 GPa, higher than of a hydraulic oil at room temperature. Its lubricity is poor, so positive-displacement pumps are unsuitable and centrifugal pumps have to be used. Addition of caesium shifts the useful temperature range to −71 to 704 °C (−96 to 1,299 °F). NaK-77 was tested in hydraulic and fluidic systems for the Supersonic Low Altitude Missile. NaK may also be used to transmit forces inside high temperature pressure transducers as an alternative to mercury.

Chemical methods NaK can be used as catalyst in some reactions, such as isobutylbenzene, a precursor to ibuprofen.

Synthesis and production Industrially, NaK is produced in a reactive distillation.

Illustrations

Worked examples

Example 1 — a first encounter with Sodium–potassium alloy

Start with the simplest possible case. Write down what Sodium–potassium alloy claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Sodium–potassium alloy before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Sodium–potassium alloy ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Sodium–potassium alloy

In research: Sodium–potassium alloy appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Sodium–potassium alloy in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Sodium–potassium alloy is common in secondary-school and first-year university syllabi. It links to neighbouring topics Coolants, Desiccants, Fusible alloys, so understanding it makes those chapters shorter.
In everyday life: Look for Sodium–potassium alloy outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Sodium–potassium alloy in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Sodium–potassium alloy means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Sodium–potassium alloy out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Sodium–potassium alloy in simple terms?

Sodium–potassium alloy, colloquially called NaK (commonly pronounced ), is an alloy of two alkali metals: sodium (Na, atomic number 11) and potassium (K, atomic number 19). NaK is normally liquid at room temperature.

Why does Sodium–potassium alloy matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Sodium–potassium alloy?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Sodium–potassium alloy.