The role of tunnel K⁺ ions on the growth and stability of tunnel-structured cryptomelane-type MnO₂ nanofibers (denoted as cryptomelane nanofibers hereafter) has been discussed by means of X-ray diffraction and electron microscopy. Cryptomelane nanofibers with typical diameters of 20–80 nm and lengths of 1–6 μm have been synthesized by means of a simple hydrothermal reaction of KMnO₄ and MnSO₄ aqueous solutions at 140 °C. The growth of cryptomelane nanofibers under hydrothermal conditions follows a dissolution–recrystallization process and involves a morphological transformation from a layered precursor to the tunnel-structured cryptomelane, in which the K⁺ ions play important roles in templating and stabilizing the tunneled framework. The presence of tunnel K⁺ ions also enhances the frame stability of the cryptomelane nanofibers at elevated temperatures. The formation of a layered K_xMn₂O₄ (x ≈ 0.26) with a hexagonal phase structure has been observed at about 900 °C. The transformation from tunneled cryptomelane to layered K_xMn₂O₄ also follows the dissolution–recrystallization growth mechanism, in which the diffusion of K⁺ ions at high temperatures represents a critical process. The topological correlation between the tunneled and layered MnO₂ materials might provide useful information for the synthesis of MnO₂ nanomaterials with controlled microstructures for different applications.

Freeze protection in ventilation systems is important to avoid freeze damages and increase in maintenance costs. Freezing in a ventilation system can appear in construction and operation phase. The aim of this study was to test a new method for freeze protection in ventilation system. This method implies use of an additional heat exchanger. The method used two hydronic circuits: the first one with water on energy supply side, and second one with mixture of glycol and water at the secondary side. The mixture transfers heat from the energy source via an additional heat exchanger to the coil in the air handling unit (AHU). AHU with freeze protection from a manufacturer was tested in the laboratory. Since it was not possible to obtain a supply air temperature of -20 oC in the laboratory conditions, a model was developed on the MATLAB\Simulink platform. Several operation scenarios were tested on the Simulink model and they included use of three types of energy sources: boiler, heat pump, and district heating with outdoor temperature compensation. Results showed that even new method with mixture of glycol and water could tolerate low air temperature, the AHU would require known sequence control for freezing protection, where the circulation pump between energy source and heat exchanger starts first. This sequence control is necessary regardless of energy source. Finally, the article gave recommendations on how to define sequence control to avoid freezing in AHU.