This video details an experimental attempt to create permanent magnets from abundant and inexpensive materials: iron and nitrogen, forming iron nitride (specifically the alpha double-prime phase). The process, derived from academic research, involves high-energy ball milling of iron powder and ammonium nitrate, followed by a low-temperature annealing step. The creator constructs a custom planetary ball mill capable of intense grinding, highlighting its power and the pyrophoric nature of the resulting powder. To characterize the magnetic properties, a custom vibrating sample magnetometer (VSM) is designed and built, drawing parallels to a guitar pickup. While the synthesized iron nitride exhibits some magnetic retentivity and coercivity, the resulting magnets are currently very weak, attributed potentially to oxidation during handling. The video demonstrates the VSM's functionality in plotting the BH curve and discusses future steps for process improvement and material characterization.
Summarizing the Creation and Testing of Iron Nitride Magnets
0:02 The Quest for Novel Permanent Magnets: The video introduces the goal of making permanent magnets from iron and nitrogen (iron nitride), highlighting its potential to be cheaper and even better than neodymium, despite no one successfully achieving this on a practical scale yet.
0:40 Iron Nitride Powder Synthesis: The process begins with energetically mixing ammonium nitrate and iron powder in a ball mill. This vigorous pounding decomposes the ammonium nitrate, causing nitrogen atoms to diffuse into the iron. The mixture is then cooked at around 200°C for an extended period to grow the desired iron nitride phase.
1:12 The Grinding Challenge: Nitrogen diffusion into solid iron is difficult, necessitating grinding the iron particles down to the nanometer scale, even when starting with fine powder. This high-energy mixing and grinding make a ball mill an ideal tool.
2:51 Limitations of Standard Ball Mills: Regular rock tumblers are limited by gravity in the energy they can impart; spinning them too fast causes material to stick to the walls, stopping grinding.
2:51 Introduction to the Planetary Ball Mill: This specialized mill overcomes gravity limitations by simulating a stronger gravitational field through rapid rotation and revolution, allowing for arbitrarily high energy grinding.
3:45 Building a DIY Planetary Ball Mill: Due to the high cost, size, and vibration of commercial units, the creator built his own open-source planetary ball mill (parts ~$1000). It's an "insanely powerful grinder," capable of turning microscope slides into powder in seconds, and concentrates 100 watts of mechanical energy per chamber.
6:00 Handling Pyrophoric Powder: The grinding chambers are hermetically sealed because the resulting iron nitride powder is pyrophoric (ignites spontaneously in air). The creator recounts incidents of ignition and plastic bag melting upon exposure to air, emphasizing the need for an inert atmosphere during handling.
7:34 Critical Annealing Process: The powder is annealed in a kiln at 180-190°C for about a day. Maintaining this precise low temperature is crucial to ensure the growth of the specific magnetic alpha double-prime iron nitride crystal structure; higher temperatures can ruin it.
8:09 Forming the Magnets: Sintering (heating to fuse particles) is not possible as it would destroy the delicate crystal structure. Instead, the powder is cast into epoxy (approximately 50% by mass), with some samples cured in a magnetic field to test for grain orientation effects.
9:30 Weak but Present Magnetism: The cast iron nitride samples are confirmed to be magnets, though very weak ones, unable to hold themselves against an iron surface. This weakness is possibly due to oxidation during handling, as the nitrogen content could not be immediately analyzed.
10:05 Introduction to the Vibrating Sample Magnetometer (VSM): A VSM measures magnetic properties by vibrating a sample in a magnetic field and detecting induced voltages in pickup coils, similar in principle to an electric guitar pickup.
10:36 VSM Components and Design: The custom VSM features a loudspeaker for vibration, a linear potentiometer for precise sample position sensing, a water-cooled electromagnet (1200W peak, 1.6 Tesla) for applying a controlled magnetic field, a commercial magnetic probe, and humbucker-designed pickup coils (flat coils for space efficiency).
15:12 Coil Design Principles: The video explains that for optimal magnetic field generation, only the total mass of copper in the coil matters, not the number of turns or current, as long as the power supply is maximized. Similarly, for pickup coils, the cross-sectional area of the coil is the primary factor for signal power, not just voltage.
17:36 Lock-in Amplifier for Sensitivity: A lock-in amplifier is used to amplify only signals in phase with the sample's vibration (reference signal from the potentiometer), providing extremely high sensitivity (down to nanovolt range) for precise measurements.
19:38 BH Curve Measurement: The VSM setup plots the BH curve (magnetic signal vs. applied magnetic field), which indicates how easily a material is magnetized and how well it retains magnetism.
22:00 VSM Results Confirmation: The iron nitride sample exhibits a small but distinct hysteresis loop on the BH curve, confirming it possesses both retentivity (retains magnetism) and coercivity (resists demagnetization), thus verifying it as a permanent magnet, albeit a very poor one compared to commercial options.
22:36 VSM Utility and Calibration: The VSM is crucial for accurately characterizing samples to improve the process. It can be calibrated using a standard (e.g., 100mg of pure nickel) to compare results across different research settings.
23:56 Future Research Directions: The creator plans follow-up work focusing on better atmosphere control during powder handling to prevent oxidation and conducting measurements to confirm the nitrogen content in the final solid phase, aiming to improve the magnetic properties.