Zirconium dioxide (ZrO2, also known as zirconia) is a versatile material in the ceramic industry, serving many uses due to its high hardness, chemical resistance, and unique electronic and optical properties. Compared to other ceramics, zirconia has higher compressive and flexural strength (see Table 1). These features are desirable for the foundry, refractories and electronics industries, among others, with a diverse set of end uses.
As a transparent inorganic material, zirconia is also notable for its high refractive index (RI) of 2.10, which is bested only by titania and diamond (see Tables 2 and 3). RI is a measure of the velocity of light traveling through a medium relative to its velocity through a vacuum. High-RI materials are important for applications including antireflective coatings, light extraction, lithography and optics.
While it seems counterintuitive that refractive index properties can create antireflective materials and light extraction materials, the difference stems from the order of high and low index materials. Fresnel reflection loss occurs when there is a change in refractive index. By alternating the RI of materials abruptly, an antireflective structure is created; materials that ease the RI change from the inside of a device to the outside reduce Fresnel loss and increase efficiency.
Metal oxides like zirconia have many of the qualities that are desirable for optoelectronic applications but are only manufacturable at high temperature. Therefore, lighting and display technologies are adopting ceramic-polymer nanocomposites to improve device performance. Polymers are the material of choice for creating optical components and films because of their durability, clarity, and ability to create complex shapes, but they have lower refractive index than many components within light emitting devices. Ceramic nanocrystals can also be used to tune properties other than RI, including dielectric constant, hardness, toughness, transparency, thermal conductivity, coefficient of thermal expansion and chemical resistance.
Zirconia powder has a poor dispersibility in solvents and polymers due to the tendency of particles to agglomerate. Improving dispersibility of the native ceramic is a necessary first step toward using ZrO2 in these applications.3 Good dispersibility is a function of the surface properties and size of the particles in a medium. New technologies have been developed encompassing a proprietary material synthesis, coating technology, application engineering and high-volume manufacturing in order to achieve the best dispersibility and perfectly clear zirconia nanocomposites.* This approach is effective from a scale and cost perspective, and the resulting particle quality gives end users flexibility for process integration. The disruptive technology enables high-quality ceramics to be incorporated into polymer systems for a variety of applications. With a narrow size distribution (under 10 nm), the nanocrystals maintain good dispersibility while reducing the tendency to agglomerate.
In effect, this allows the optical clarity, RI, hardness and dielectric properties of ZrO2 to be integrated into polymer processes while maintaining the current manufacturing techniques. This powerful combination has allowed these nanocrystal materials to find applications in numerous technology areas.
The introduction of zirconia nanocrystals into silicone has been shown to increase the overall RI of the resulting cured nanocomposite. This represents an increase from 1.41 to 1.55 for methyl silicones and from 1.54 to 1.64 for methyl phenyl silicones at 450 nm, depending on the zirconia loading. These gains have been greatly sought after in the LED industry to extend the RI of methyl silicone without needing to increase the molar content of aromatic functional groups, such as phenyl, in the overall polymer. Methyl phenyl silicones have a higher RI than dimethyl silicones, but often show yellowing during aging, which both reduces the light output and shifts the color point of the white light. Nanocomposites without phenyl functionality could provide better stability under photo-thermal aging with equivalent RI values to methyl phenyl silicones. In addition, the incorporation of zirconia nanocrystals to methyl phenyl silicones results in materials with the highest RIs in traditional silicone systems.
When zirconia nanocrystals are added to dimethyl silicone systems such that the RI is the same as a methyl phenyl system, equivalent lumen output has been demonstrated compared to the commercial methyl phenyl systems. Figure 1 compares the efficiency of blue LEDs with both silicone types and a 1.52 RI solvent-free dimethyl nanocomposite. This novel encapsulant was shown to increase the efficiency of the blue LEDs up to 7% compared with dimethyl silicone encapsulants.
OLED Lighting and Display
Two additional important applications include OLED lighting and OLED display. Although both applications are based on similar OLED emissive stacks, the requirements for successful light extraction are very different. In both cases, creating solution-processable materials with high RI is key to improving efficiency while simultaneously being compatible with the OLED manufacturing process. Extraction layers for OLED lighting benefit from the addition of scatterers or other microstructures. When using zirconia nanocrystals, such approaches have doubled the light output compared to state-of-the-art OLED lighting panels.5
OLED display offers numerous areas for high hardness and high RI materials, including exterior hard coatings, antireflective layers, optically clear adhesives, and microlens extraction structures. Figure 2 demonstrates one potential structure of an OLED display device with microlenses made from zirconia nanocrystal products for improved light output.
Optical Films, Optics and Printed Electronics
In optical devices, printed electronics and optical film applications, the high transparency and processability of zirconia nanocrystal-enabled nanocomposites is further augmented by improvements in dielectric constant and hardness and minimal effect on Abbe number. Materials that have a constant RI over the visible spectrum have a higher Abbe number. This measure of chromatic aberration is used to design optics and optoelectronics. Mixing ZrO2 nanocrystals and polymers has a minimal effect on Abbe number, which normally decreases with high RI materials.6
The addition of zirconia nanocrystals to polymer systems also increases the hardness and scratch resistance of the polymer nanocomposite. At 20% ZrO2 loading, hardness and scratch resistance roughly double for model epoxy systems.
With a high dielectric constant, the zirconia material can be used to tune the electronic properties of polymers to exact specifications. This quality has found applications in printed electronics and can be used to create flexible insulating layers. Figure 3 shows how dielectric constant can be increased in acrylic polymers with the addition of zirconia nanocrystals, achieving a more than four-time improvement at 80 wt% zirconia loading.7
Materials and Manufacturing
Over the past four years, the liquid synthesis process for zirconia nanodispersions has been scaled up from laboratory gram-scale quantities to metric ton production-scale volumes. The scale-up of nanomaterials has been a significant challenge to the market adoption of nanotechnology. Careful control of processes during all aspects of production enables these cost-competitive nanocrystals to be produced at very high quality. Scaled materials are statistically in-control using a number of manufacturing quality parameters.8
A library of dispersions and formulations has been developed that enables highly stable transparent dispersions and films with a variety of polymer systems. Extensive work has been done in acrylics, epoxies, siloxanes and silicones. For example, dispersions are regularly tested for optical clarity with common acrylic oligomers used in industry, and for specific unique customer polymer systems.9 In addition, the general synthetic approach for zirconia nanocrystals is applicable to a variety of other transition metal and main group elements, including hafnia nanocrystals, titania, alumina, and yttria systems.
The introduction of zirconia nanocrystals to polymers has provided numerous benefits to applications ranging from LED encapsulation, hard coatings, and ITO refractive index matching films to OLED lighting and displays. Well-engineered composite systems can realize the best qualities of each component material, and the manufacturing flexibility afforded by polymers can be further improved by the inherent strength, thermal, optical, and electronic properties of ceramics.
By Michael A. Weinstein and Matthew D. Healy and originally published in Ceramic Industry Magazine.