Boron Nitride—The Perfect Radiative Cooling Material

In addition to being used to prepare thermal conductive materials, boron nitride is also an ideal material for radiative cooling.

What is a radiative cooling material?

A radiative cooling material is a novel type of material capable of achieving cooling by passively radiating heat into outer space. It requires no electrical input and relies entirely on the material's inherent physical properties.

Its core principles are based on two fundamental physical processes:

1. High solar reflectivity: It reflects as much sunlight as possible (primarily in the wavelength range of 0.3–2.5 µm) to reduce the absorption of solar heat.

2. High infrared emissivity in the atmospheric window: Within the "atmospheric window" (8–13 µm), where Earth's atmosphere is highly transparent to specific infrared bands, the material exhibits strong thermal radiation capability, directly emitting heat in the form of infrared rays into the cold outer space (approximately 3K).

Simply put, this material functions like a "one-way heat dissipation channel": it blocks solar heat from entering while "dumping" the object's own heat into the universe.

Hexagonal boron nitride (h-BN) possesses the perfect characteristics for radiative cooling.

Core Reflective Properties:

• High solar reflectance: h-BN exhibits very high reflectivity in the visible to near-infrared wavelength range. This gives it a bright white appearance, which is determined by its wide bandgap and specific electronic structure.

• High infrared emissivity: In the mid-infrared range, particularly within the 8–13 µm atmospheric window, h-BN is a strong absorber. According to Kirchhoff's law of thermal radiation, it also exhibits high emissivity in this band.

With current technological capabilities, it is entirely feasible to produce highly flexible boron nitride films with high content for radiative cooling applications.

Application Scenarios

1. Building Energy Efficiency:

   • Used as exterior wall coatings or roof coatings, significantly reducing surface and interior temperatures of buildings, thereby saving 20%–50% of air conditioning energy consumption.

   • Made into cooling films and applied to building glass or exterior walls.

2. Heat Dissipation for Power Electronic Devices:

   • Used for cooling outdoor communication base stations, data center servers, photovoltaic power stations, etc., improving equipment lifespan and operational efficiency.

   • Providing passive cooling solutions for wearable electronic devices.

3. Water Resource Collection:

   • Utilizing radiative cooling surfaces to condense water vapor from the air, enabling "atmospheric water harvesting" in arid regions.

4. Personal Thermal Management:

   • Made into textile fibers or fabrics for producing "cooling clothing," providing a comfortable microclimate for outdoor workers in high-temperature environments.

5. Mitigating Urban Heat Island Effect:

   • Large-scale application on urban roads and rooftops can effectively reduce the overall environmental temperature of cities.

Challenges and Future Directions

Despite its promising prospects, radiative cooling materials still face several challenges:

• Environmental Adaptability: In humid, cloudy, or rainy conditions, the transparency of the atmospheric window decreases, significantly reducing cooling performance.

• Durability and Pollution: Surface dust and contaminants can severely affect reflective and radiative performance, necessitating self-cleaning properties (e.g., superhydrophobic surfaces).

• Cost and Scalability: The cost of high-performance photonic structural materials remains relatively high. Achieving low-cost, large-scale production is key to commercialization.

• Spectral Regulation: A future research focus is developing "smart" radiative cooling materials that can dynamically adjust their radiative properties based on ambient temperature, achieving the effect of "warm in winter and cool in summer."