Vanadium dioxide single-crystal precipitates with controlled particle sizes were produced in an amorphous, fused SiO2 host by the stoichiometric coimplantation of vanadium and oxygen ions and subsequent thermal processing. The effects of the vanadium dioxide nanocrystal size, nanocrystal morphology, and particle/host interactions on the VO2 semiconductor-to-metal phase transition were characterized. VO2 nanoparticles embedded in amorphous SiO2 exhibit a sharp phase transition with a hysteresis that is up to 50 °C in width—one of the largest values ever reported for this transition. The relative decrease in the optical transmission in the near-infrared region in going from the semiconducting to the metallic phase of VO2 ranges from 20% to 35%. Both the hysteresis width and the transition temperature are correlated with the size of the precipitates. Doping the embedded VO2 particles with ions such as titanium alters the characteristics of the phase transition, pointing the way to control the hysteresis beha...

Synthesis and characterization of size-controlled vanadium dioxide nanocrystals in a fused silica matrix

Pure single-crystal rutile-type VO2 has been synthesized via the hydrothermal reaction of V2O5, oxalic acid, and tungstenic acid at low temperatures. The products have been characterized by means of X-ray diffraction, transmission electron microscopy, scanning electron microscopy, selected area electron diffraction, differential scanning calorimetry, and X-ray photoelectron spectroscopy. It is shown that a part of VO2 powders have a “snowflake” morphology assembled by VO2 rods which have a monoclinic structure with a preferential orientation along 110 direction. The adulterating agent tungstenic acid plays a key role in the synthesis of rutile phase. The DSC results indicated that the metal-semiconductor transition temperature of VO2 powders was reduced to room temperature or below. The reaction details and mechanisms are described and discussed. The substitution of a part of V atoms with larger W atoms induces the distortion of the VO6 octahedra, promoting the transition from VO2 (B) to VO2 (R).

Pure Single-Crystal Rutile Vanadium Dioxide Powders: Synthesis, Mechanism and Phase-Transformation Property

Molybdenum-doped vanadium dioxide coatings on glass produced by the aqueous sol-gel method

The peculiarities in the electronic structure of the seemingly simple binary vanadium oxide VO2, as manifested in a pronounced metal−insulator phase transition in proximity to room temperature, have made it the subject of extensive theoretical and experimental investigations over the last several decades. We review some recent advances in theoretical treatments of strongly correlated systems along with ultrafast measurements of VO2 samples that provide unprecedented mechanistic insight into the nature of the phase transition. Scaling VO2 to nanoscale dimensions has recently been possible and has allowed well-defined VO2 nanostructures to serve as model systems for measurements of intrinsic properties without obscuration from grain boundary connectivities and domain dynamics. Geometric confinement, substrate interactions, and varying defect densities of VO2 nanostructures give rise to an electronic and structural phase diagram that is substantially altered from the bulk. We postulate that design principles...

Microscopic and Nanoscale Perspective of the Metal−Insulator Phase Transitions of VO2: Some New Twists to an Old Tale

The atmospheric pressure chemical vapour deposition reaction of vanadyl acetylacetonate and tungsten hexachloride with oxygen led to the production of thin films of tungsten doped monoclinic vanadium dioxide on glass substrates. Scanning electron microscopy and X-ray diffraction indicated that the films had different morphologies and crystallinities depending on the deposition conditions used. Transmission and reflectance measurements showed a significant change in properties in the near infra-red either side of the metal to semiconductor transition. Variable temperature transmission studies show that the metal to semiconductor transition was lowered by tungsten doping. The effect of film thickness was studied with un-doped and doped films. It was found that film thickness limited the intensity of light passing through the film and the extent of the thermochromic transition but was found not to influence the hysteresis width or temperature of transition. Different film growth conditions led to a range of film morphologies which profoundly affected the resulting optical properties of the films. It was found that film morphology and preferred crystallographic orientation had a marked influence on the width and switching temperature of the thermochromic transition.

Doped and un-doped vanadium dioxide thin films prepared by atmospheric pressure chemical vapour deposition from vanadyl acetylacetonate and tungsten hexachloride: the effects of thickness and crystallographic orientation on thermochromic properties

Comparison between vanadium dioxide coatings on glass produced by sputtering, alkoxide and aqueous sol–gel methods

The continued proliferation of imaging infrared (I2R) missile systems has created the need for fully representative infrared scene generators. In order to test these I2R systems correctly a very large dynamic range is required of the scene generator. This cannot easily be produced by the standard approach of an array of suspended resistor heater elements. This paper therefore describes the fabrication of infrared optical modulators for use in high dynamic range and high frame rate scene generation. Modulation is produced by the heating of a variable reflectivity coating. This is achieved by coating a suitable substrate with a thin film of vanadium dioxide (VO2) via a reactive sputtering process. VO2 undergoes a semiconductor to metal phase transition at approximately 68°C and the associated change in reflectivity is exploited to create the modulator. The correct stoichiometry of the thin film of VO2 is critical in producing high frame rate devices. Both transmissive and reflective modulators are described.

Infrared optical modulators for missile testing

The laser is an integrated part of many weapon systems, such as laser guided bombs, laser guided missiles and laser 
beam-riding missiles. These systems pose a significant threat to military assets on the modern battlefield. The lasers 
used in beam-riding missiles are particularly hard to detect as they typically use relatively low power lasers. Beamriders 
are also particularly difficult to defeat as current countermeasure systems have not been optimized against this threat. 
Some recent field trails conducted in the United Arab Emirates desert have demonstrated poor performance of both laser 
beam-riding systems and the LWRs designed to detect them. The aim of this research is to build a complete evaluation 
tool capable of assessing all the phases of an engagement of a main battle tank or armoured fighting vehicle with a laser 
based guided weapon. To this end a software model has been produced using Matlab & Simulink. This complete model 
has been verified using lab based experimentation and by comparison to the result of the mentioned field trials. This 
project will enable both the evaluation and design of any generic laser warning receiver or missile seeker and specific 
systems if various parameters are known. Moreover, this model will be used as a guide to the development of reliable 
countermeasures for laser beam-riding missiles.

The simulation of laser-based guided weapon engagements

An IR reflective optical modulator has been fabricated. This has been achieved by coating a thick film resistor network with a thin film of vanadium dioxide via a reactive sputtering process. Vanadium dioxide undergoes a semiconductor to metal phase transition at approximately 68 degrees C, therefore to switch the reflective optical modulator the thick film resistors in the network are driven electrically. As the resistors heat up to beyond the transition temperature, the vanadium dioxide undergoes its transition from a transparent semiconductor state to a highly reflective metallic state. Provided the thick film heater network, underneath the vanadium dioxide, is non- reflective, then a significant change in reflectivity is observed upon undergoing the transition, and hence the modulation of reflected IR radiation is achieved. The useful waveband of operation of the device encompasses the region 2-25 micrometers , this is primarily limited by the transparency of the semiconductor state of the vanadium dioxide. Correct stoichiometry of the thin film of vanadium dioxide is critical in producing good modulation depth in the reflective mode. Several devices have been fabricated and tested. They show a reflectivity increase of approximately 13:1 upon switching. The devices to date have demonstrated switch on speeds of 0.1 s and switch off speeds of 0.2 s. This has been achieved without any form of substrate temperature control apart from that produced by the electrical drive. Very slight changes in the stoichiometry of the vanadium dioxide thin film can greatly increase the temperature range and hysteresis of the semiconductor to metal phase transition. This has been utilized to allow partial phase transitions to occur, yielding partial increases in reflectivity, and hence the ability to generate grey levels in the reflected IR radiation.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Infrared reflective optical modulator

The diffuse and specular reflections of four representative scatterable anti-personnel landmines have been measured in the UV, visible and IR regions of the electromagnetic spectrum. These results are presented here and are compared to potential backgrounds in which such anti-personnel mines are likely to be sown.

John A. Coath

Papers

Comparison between vanadium dioxide coatings on glass produced by sputtering, alkoxide and aqueous sol–gel methods

Infrared optical modulators for missile testing

The simulation of laser-based guided weapon engagements

Infrared reflective optical modulator

Regions of high contrast for the detection of scatterable land mines