Nitinol Heat Engine: Thermal to Mechanical Energy

A Nitinol heat engine converts thermal energy into mechanical work through a solid-state phase transition. As the wire enters hot water, it forcefully snaps back to its trained straight shape, generating torque that continuously spins the pulleys.

Advanced ⏱ 45 min ⚗️ Class 9-12 🏠 4 of 7 things are at home 🧑‍🔬 Do it with an adult

Time: 45 min
Difficulty: Advanced
Best for: Class 9-12
Subject: Physics
Things needed: 7, 4 at home
Steps: 11
Cost: Low, a few things to find
You will learn: Shape Memory Alloys and Thermodynamic Heat Engines

Nitinol Heat Engine: Thermal to Mechanical Energy, Experiment · Class 11-12 | jijnas.com

Before you start

What you need

0 of 7 ready

Tap each thing once you have it in front of you. Items with the house sign are already somewhere in your home.

Trained Nitinol wire loop (0.5 mm thickness, 50-70 degree Celsius transition) 1 loop
Plastic pulleys with ball bearings (one small ~3 cm, one large ~7 cm) 2 pieces No luck? Use toy car wheels or spinny craft pulleys mounted on smooth axle pins.
🏠 Support frame base (acrylic sheet or wooden plank with stand rods) 1 set No luck? Use meccano strips or wooden ice-cream sticks glued together.
🏠 Heat-resistant steel bowl or glass beaker (250 ml) 1 No luck? Use small kitchen patila or steel cup.
Laboratory thermometer (0-100 degree Celsius) 1 No luck? Use digital cooking thermometer.
🏠 Hot water source (50-90 degree Celsius) 500 ml
🏠 Kitchen tongs or heat-resistant gloves 1 pair

💬 Send to a parent

Think first

What do you think will happen?

Real scientists guess before they look. Write one line now. After the steps, the page will show it next to what you should see.

Safety first, an adult should be with you.

Scalding hazard from hot water bath (50-90 degrees Celsius). Handle hot water with caution and use heat-resistant gloves.

Do it

The procedure

Tap a step when it is done. Lab mode shows one step at a time in big letters, reads it aloud if you want, and keeps the timer.

Set up your support frame on a flat, stable surface.
Mount the two plastic pulleys onto the frame rods so both wheels rotate freely on their bearings.
Stretch the continuous Nitinol wire loop around the track of both pulleys.
Put on heat-resistant gloves before working near hot liquids.Wear heat-resistant gloves to protect your hands from scalding injuries.
Heat 500 ml of water to a temperature between 50-90 degrees Celsius.Handle the hot water source with care to avoid accidental spills or burns.
Measure the liquid temperature using your laboratory thermometer to confirm it is within 50-90 degrees Celsius.
Position the heat-resistant steel bowl directly beneath the smaller pulley.
Pour the hot water into the bowl until the lower edge of the smaller pulley is partially submerged.Pour hot water slowly to prevent splashes and scalding.
Confirm that the larger pulley sits completely above the water bath in the cool air.
Flick the larger wheel gently with your finger to initiate the rotation of the wire loop.
Observe the continuous rotation of both wheels driven by the heat-induced shape memory effect.

01 · The idea

The Concept

Solid-state heat engines do not rely on expanding gases or steam to drive mechanical movement. Instead, they use smart materials like Nitinol (a nickel-titanium alloy) that change their internal crystal structure when heated. By passing a loop of Nitinol wire between hot and cold environments, thermal energy turns directly into continuous rotational motion.

02 · What you should see

Observation

When a continuous loop of Nitinol wire is mounted around two pulleys and the smaller pulley is dipped into a hot water bath (typically above 50-70 degrees Celsius), a gentle manual flick sets the engine in motion. The wheels begin to rotate rapidly and continuously, reaching speeds of several hundred RPM. The motion continues uninterrupted as long as the hot water remains warm and the surrounding air or cold bath stays sufficiently cool.

03 · Why it happens

Why It Happens

Nitinol is a smart shape memory alloy made of nickel and titanium that changes its internal structural rigidity with temperature. In cool air, the wire stays in a soft, bendable state called martensite, allowing it to curve easily around the pulleys. When a section of the wire enters the hot water, it instantly shifts to a rigid, strong state called austenite, trying forcefully to snap back into its trained straight shape.

This sudden straightening force on one side of the pulley creates an unbalanced torque, pulling the next cool section of wire into the hot water while moving the heated section out to cool down. Think of it like a thermal muscle that continuously contracts and stiffens when dipped in heat and relaxes when cooled, generating continuous rotary motion.

04 · The science

The Science Behind It

At the microscopic level, Nitinol undergoes a solid-state phase transition. Unlike water freezing into ice, this transition occurs entirely within a solid crystal lattice. At lower temperatures, the martensite phase features a 'twinned' structure that deforms easily under stress. Upon absorbing thermal energy above its transformation temperature, the crystal lattice undergoes a shear-like transformation into a rigid, highly ordered austenite phase, restoring its original geometry with significant output force.

From an NCERT Class 11 Thermodynamics perspective, this apparatus operates as a cyclic heat engine working between a hot reservoir (T_H) and a cold reservoir (T_C). Thermal energy (Q_H) absorbed from the hot bath drives the endothermic phase change, producing useful mechanical work (W). The wire then rejects heat (Q_C) into the cool air, undergoing an exothermic reverse phase change back to martensite. In agreement with the Second Law of Thermodynamics, net work output is possible only because a temperature differential exists to sustain heat flow across the working medium.

05 · Take it further

Make It a Project

Here are ways to expand this setup into a complete science fair investigation:

Independent Variable: Temperature of the hot water bath (ranging from 50 to 90 degrees Celsius) or mechanical load attached to the axle.
Dependent Variable: Rotational speed of the heat engine in revolutions per minute (RPM) and calculated mechanical power output.
Controlled Variables: Ambient air temperature, Nitinol wire diameter, pulley axle friction, submerged length of wire, and total wire loop length.

Data to Record: Record hot and cold reservoir temperatures, compute Δ T, and measure rotational speed using a tachometer. Calculate output power by measuring how quickly the axle can lift a known small weight attached to a thread.

Follow-up Challenge: Calculate the theoretical Carnot efficiency η_Carnot = 1 - (T_C / T_H) using absolute temperatures in Kelvin, then compare it to the actual mechanical efficiency derived from your power measurement.

06 · Questions students ask

FAQs

Why does the Nitinol engine need to be started with a gentle spin?

Nitinol engines require an initial flick because the temperature differential across the loop needs a brief kick to establish momentum. Until the loop moves, the wire in the hot bath stays uniform and cannot generate an unbalanced force. Giving it a manual spin pushes fresh, cool wire into the hot water, starting the continuous torque cycle.

What is the difference between martensite and austenite states?

Martensite is the cool, flexible phase of Nitinol where the crystal lattice deforms easily under light bending forces. Austenite is the hot, rigid phase that forms when heated above its transition temperature. In the austenite state, the atoms rearrange into a tight lattice, forcing the wire back into its original trained shape.

How does this experiment relate to the Second Law of Thermodynamics?

The Second Law states that no heat engine can convert all absorbed heat into mechanical work without rejecting some to a colder body. In this setup, heat moves from the hot water bath to the cooler air surrounding the top pulley. The engine stops spinning if the ambient air becomes as hot as the bath.

Why does the engine stop spinning after a few minutes?

The engine slows down and stops when the hot water bath cools down below the critical transition temperature of Nitinol, usually around 50-70 degrees Celsius. It can also stop if the surrounding air heats up too much, reducing the cooling rate needed for the wire to relax back into its flexible martensite state.

How do you train Nitinol wire to remember a specific shape?

You train Nitinol by clamping it into your desired shape, such as a straight line, and heating it to roughly 500 degrees Celsius for several minutes. After rapid quenching in cold water, this shape becomes permanently set in its high temperature austenite memory. Any future heating will force it back into this trained shape.

Write it up

Your lab notebook

Two minutes here and your lab record is done. Everything stays on this device until you copy it out.

Did it work?

What I actually saw What I would change next time

💼 Same science, as a real job

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