32135 Castle Ct, Suite 208, Evergreen, CO USA (open by appointment)
tel (303) 800-5707

Replacing Optical Glass with Transparent Zirconia

Description and Rationale

To obtain a high refractive index at high working temperatures, transparent ceramics offers a variety of materials but most are relatively difficult to process.  We present a material from the class of transparent ceramics that meets all the desired physical properties and furthermore is relatively easy to process.  This material is transparent zirconia, currently used in optics research as well as dental implants.

Composition

Transparent zirconia, also known as transparent cubic zirconium oxide, has the elemental formula ZrO2.  It is polycrystalline, having a cubic structure and requires doping additives (either Mg, Ca, or Y) to stabilize the cubic phase and give it added toughness and durability at higher temperatures.  Yttria-stabilization (using Y as the doping additive) is most common, added at 6 mol% to high purity zirconia though there are 3-8 mol% variants also in use.  This variant is more widely known as Y-TZP.

Properties

Transparent zirconia exceeds the required refractive index considerably, is hot workable and able to be colored with inexpensive materials at high temperature, is millable, has exceptional chemical durability, and can even be soldered using an ultrasonic soldering method.[i]

PropertyDesired from materialOffered by transparent zirconia
Refractive index At least 1.92.18[2] to
2.23[3]
Hot workableUp to 1100°C over 60
minutes
Optimal sintering temperature found to be 1100°C[4]
Shape stabilityShape must be stable at 1100°CWill keep pressed shape at 1100°C; after sintering, material is stable in oxidizing
environments at temperatures above 2000
°C[5]
Color and color stabilityTransparent or colored; color must be stable at 1100°C Transparent, transparent-colored by transition metal
oxides, or opaque-colored, all stable at this high temperature.
Mohs hardnessAt least 6Hardness of 7 on Mohs scale[6]
Chemical durabilityResistant to cleaning agents, weak alkalis, strong acids,
galvanic solutions, household washing and cleaning agents
Transparent zirconia will not break down in the presence
of these materials.
Compatible with soldering agentslead free soft solder, diluted
organic acids, alcohols, ketones, terpenes (cleaning after soldering)
Transparent zirconia will not
break down in the presence of these materials.  Like any glassy material, it can be
soldered using ultrasonic soldering.[7]
Ecologically SafeNeeds to be RoHS-compliantCan be made RoHS-compliant during manufacturing[8]

Zirconia can be colored through the addition of a relatively inexpensive transition metal oxide of iron, nickel, manganese, cobalt, chromium, copper, and/or vanadium in amounts ranging from 0.02-0.6 mol%, while yielding a final product that can still be highly transparent if porosity is controlled.[9]  Example colors that can be obtained are yellow (with iron oxide), light pink/peach (with nickel oxide), and purple (with cobalt oxide).  If transparency is not necessary when the material is colored, zirconia also supports a variety of colors prior to sintering where the color remains stable at sintering temperatures as high as 1450°C.[10]

Other material properties such as density (6.02 g/cm2), compressive strength (2500 MPa), thermal conductivity (2.2 W/m K) and dielectric strength (9.0 ac*kv/mm) are also available.[11]

Synthesis techniques for producing new material

Transparent zirconia, while appearing optically clear, actually consists of a fine microstructure composed of grains, grain boundaries, and pores.  Transparency is achieved when there are no pores and the crystal grains and grain boundaries are optically perfect so as to limit scattering and reflection.  These conditions in turn depend on the initial particle size and purity of the starting zirconia powder and the sintering temperature, time, atmosphere, and additives used.  Some sintering methods thus perform better than others in achieving a transparent material.

In this section, we will describe the conditions that have been shown to yield the best transparency.  Much of the process information that follows was obtained from one source, which should be consulted for further details if synthesis of transparent zirconia is to be attempted.[12]

Preparation

To make a new object from transparent zirconia, first the starting powder (of a grain size in the nanometer range) must be compacted (cold-pressed) into the desired shape of the object (“green body” formation).  The thinner the object, the greater its transparency will be.  The starting powder is mixed with a choice of binders, additives, dispersants, colorants, and/or pH controllers to form a stable colloidal suspension which is then consolidated through either a wet casting process (slip-casting) or a dry pressing process (via spray freeze drying).  It is then followed by debinding which removes the binders holding the particles together, and is ideally done with water and using binders that are water-soluble (i.e. polyethylene glycol).

Once cold-pressed zirconia is in a desired shape, it can be pre-sintered to put it in a form conducive to milling.[13]  At this stage the material will be chalk-like, not at all transparent.  After milling, additional processing may be done as desired, such as adding coloration and polishing.  Afterwards, the object must be sintered in order to harden and compactify the material (typically by around 25%), which will yield a transparent product.

Sintering

During sintering, the green body is heated at a high temperature to eliminate porosity and to develop the desired microstructure through densification (transport of matter from the grains into the pores, thus eliminating the pores).  However, coarsening (grain growth) also occurs during sintering and must be impeded through a two-step sintering cycle which has been shown to yield nanograin sizes.  In this method, the sample is heated to a high temperature until it is 75% as dense as it will finally be, and then it is rapidly cooled and held at a lower temperature until fully dense.

One study on the transparency of cubic yttria-stabilized zirconia found that the optimum sintering temperature within a range of 1000-1200°C was 1100°C. [14] Too high a temperature resulted in lower transparency due to the development of absorption and scattering defects within the material.

However, the outcome of conventional sintering is still a highly porous material, too porous for optical clarity.  To achieve this degree of transparency with conventional sintering, hot isostatic pressing (HIP) is necessary.  The sample is placed in a pressure vessel, the pressure is raised to an initial amount, and then the temperature is raised to the sintering temperature again.  During this heating time, the gas pressure is increased further, compressing the sample to remove the residual pores.

There exist alternatives to conventional sintering, namely hot-pressing and spark plasma sintering, which will more readily yield high transparency.  Hot-pressing requires high pressure to eliminate porosity, but it is difficult and costly to scale up large pressure vessels for a commercial operation.  Similarly, spark plasma sintering requires expensive equipment to generate the high electric currents necessary, and to achieve nanograin transparency, high pressure is also required.[15]  Thus we concentrate on a third alternative, microwave sintering, which has a number of advantages including lower cost.

Microwave Sintering

Like spark plasma sintering, this method allows for extremely short processing times, which in turn allow for the creation of microstructures that are unattainable by other sintering techniques.  This method is most favorable for the production of transparent zirconia due to the enhancement in sintered density of up to 46% over conventional sintering.[16]  Furthermore, the non-contact nature of microwave processing can eliminate energy costs needed to heat the walls of a large furnace or reactor.  A hybrid approach that combines a smaller gas or electric furnace with microwaves is recommended since zirconia will not absorb microwaves well until it reaches about 0.4-0.5 times its melting temperature.[17]

Alternatively, susceptor materials (those that readily absorb microwave radiation and can transfer the heat to less susceptible materials like zirconia) can be used to eliminate energy costs associated with heating the walls of a gas or electric furnace.  In one study, zirconia with 8% yttrium oxide (Y2O3) was sintered using susceptor rods made of silicon carbide (SiC) at 2.45 GHz.[18]  This is the standard microwave oven frequency, which also offers the benefit of lower equipment costs compared to microwave sintering at higher frequencies.  However, an optimal configuration of SiC rods was found to be necessary to avoid thermal runaway and cracks due to non-uniform heating.  Thus the hybrid approach of pre-heating the zirconia prior to employing microwaves may be the best approach.

Sintering Times

Microwave sintering can result in a five-fold reduction of sintering time, saving energy on heating costs per quantity of finished product.  Compared to conventional sintering which can take 10 hours with a maximum temperature of 1450°C for 2 hours, microwave sintering only requires 2 hours in the oven, with a maximum temperature of 1520°C for 35 minutes.[19]  However, this high temperature is unnecessary and as described previously, likely detrimental to achieving transparency.

Analysis

Durability Test Data

Much of the available test data is from dental use of transparent zirconia, where optical transparency is not necessary and thus the material is often translucent.  The only difference between the transparent and translucent forms is the size and quantity of micropores in the material.  Thus it is believed that these test data are representative of transparent zirconia as well.

Additionally, dental use requires that this material be subjected to daily shear and compressive stresses of much greater magnitude than the same material would for optical use.  The durability over periods of years indicated by the dental use data below suggest that this material would have an exceptional durability for lighter-duty applications such as optics.

1) Fracture toughness of yttrium stabilized zirconia sintered in conventional and microwave ovens[20]

Summary: Comparison of the strength and toughness of conventionally sintered vs. microwave sintered zirconia.

Conclusion: Microwave sintering yields a product with a strength comparable to or exceeding that of conventional sintering.  Durability has been tested to be 10-13 years under conditions of heavy wear, with over 20 years possible.

Additional findings
Flexural strength (MPa): 900-1200
(page 15)

Fracture toughness (MPa*sqrt(m))
9-10 (page 15)

Study 1: Mean fracture toughness of 5 conventionally
sintered and 5 microwave sintered ZrO2 specimens: 4.48
(conventional), 4.63 (microwave) (page 36)
Study 2: Mean fracture toughness
by sintering type, 48 specimens each: 5.30 (conventional), 5.36 (microwave)
(page 51)
Crack propagation after veneering and fatique loading in
water: heat treatment and/or veneering reduced the fracture resistance but
there was no significant effect of fatigue loading on fracture
resistance.  Functionality of the
veneer could still be possible for beyond 20 years (page 23-24).
Veneer restorations with greater
than 90% survival rate over a 10-13 year time frame (page 25)

2) Furnace cyclic behavior of plasma-sprayed zirconia-yttria and multicomponent rare earrth oxide doped thermal barrier coatings[21]

Summary: NASA study of the thermal cycling of a very thin (50 micrometer) layer of yttria-stabilized zirconia exposed to repeated heatings to 1163°C for 45 min and cooldowns to 120°C over either 15 minutes or 3 hours.

Conclusion: This very thin layer can endure a harsh degree of thermal cycling for hundreds of cycles without delamination or spallation.  This indicates that thicker layers or much larger objects made of this material should be better able to withstand such conditions.

Cost and Availability

Zirconium, ZrSiO4, is the mineral used as the raw base material for zirconium dioxide production.[22]  Zirconium dioxide itself is available from retail prices of $498/kg for 99.95% purity, 20 nm particle sizes[23] to bulk prices of $14.99/kg (99.95% purity, 20-180 nm particle sizes).[24]

The yttria needed to stabilize the zirconia is derived from yttrium oxide, Y2O3.  It is more expensive than zirconium but is only needed at concentrations of 6 mol%.[25]  Yttrium oxide at 99.99% purity is available at retail prices of $1024/kg[26] ($1392/kg for a 30-45 nm nanopowder[27]), or in bulk prices of $10.40/kg.[28]  If buying at retail for process testing purposes,  99.95% zirconia-8% yttria nanopowder is available that is $588/kg for 20 nm particle sizes.[29]

For microwave sintering, one ready to use product is the MicroSinterWave A1614 from Micro Sintering Solutions.[30]  It operates in batch mode, suitable for smaller commercial applications.  For larger-scale commercial applications, two options were found that had flexible settings required to fine-tune the process of transparent zirconia manufacturing.  The first, the Spheric/SynoTherm AMPS continuous microwave furnace, allows for pre-programmed sintering times and temperatures, and is meant for industrial-scale, large throughput microwave sintering of zirconia-based parts.[31]  However, further research into the company indicated they may no longer be in operation or only intermittently so.  An equally capable replacement would be either the Enerzi Microheat (batch) or Microheat-C (continuous) sintering furnaces, both meant for true industrial use.[32]

Obtaining a Sample

There are a variety of companies that manufacture zirconia ceramics,[33] though most will provide white or translucent samples which not exhibit the desired optical quality.  Transparent zirconia samples have been manufactured for research use, and samples can be found through inquiry at the following universities.  These pages also provide visible samples showcasing the degree of achievable optical transparency.

Advanced Material Processing and Synthesis (AMPS) Laboratory

Contact: Dr. Javier. E. Garay, phone 951-827-2449, email: jegaray@engr.ucr.edu

http://www.engr.ucr.edu/~jegaray/Research.html

http://www.me.ucr.edu/mepeople/faculty/garay.html

Fraunhofer Institute for Ceramic Technologies and Systems

Contact: Dr. Andreas Krell, phone +49 351 2553-7538, email through URL below

http://www.ikts.fraunhofer.de/en/research_fields/materials/oxide_ceramics/ceramicsonbasezirconiumoxide/polycristallinetrnsp.html

References

[i] http://www.assemblymag.com/articles/85409-soldering-the-unsolderable
[2] Polycrystalline ZrO2: new transparent ceramics with high refractive index.
http://www.ikts.fraunhofer.de/en/research_fields/materials/oxide_ceramics/ceramicsonbasezirconiumoxide/polycristallinetrnsp.html
[3] Yamashita et al. Development of highly transparent zirconia ceramics. http://www.tosoh.co.jp/technology/report/pdfs/2012_02_02.pdf
[4] http://iopscience.iop.org/1468-6996/12/5/055003/pdf/1468-6996_12_5_055003.pdf, page 1
[5] http://www.google.com/patents/US3432314
[6] http://www.google.com/patents/US5472795
[7] http://www.assemblymag.com/articles/85409-soldering-the-unsolderable
[8] http://www.coorstek.com/resource-library/library/8510-1277_clean_ceramics.pdf
[9] http://www.google.com/patents/US20130217562
[10] http://www.zirkonzahn.com/us/prettau-zirconia/prettau-anterior
[11] http://www.coorstek.com/materials/ceramics/zirconia_YTZP_Sintered.php
[12] http://digital.csic.es/bitstream/10261/83021/1/Sintering%20to%20transparency.pdf
[13] http://www.dentalaegis.com/idt/2013/01/an-introduction-to-millable-dental-materials
[14] http://iopscience.iop.org/1468-6996/12/5/055003/pdf/1468-6996_12_5_055003.pdf, page 1
[15] http://iopscience.iop.org/1468-6996/12/5/055003/pdf/1468-6996_12_5_055003.pdf, page 1
[16] http://www.mri.psu.edu/faculty/agrawal/media/023.pdf
[17] http://cdn.intechopen.com/pdfs-wm/29775.pdf, page 9
[18] http://cdn.intechopen.com/pdfs-wm/29775.pdf, page 16
[19] http://ir.uiowa.edu/cgi/viewcontent.cgi?article=2402&context=etd, page 42
[20] http://ir.uiowa.edu/cgi/viewcontent.cgi?article=2402&context=etd
[21] http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20020062708.pdf
[22] http://www.vdv-dentalkeramiek.be/pdf/Informatie%20PRETTAU%20tandartsen.pdf, page 5
[23] http://www.us-nano.com/inc/sdetail/7713
[24] http://www.alibaba.com/product-detail/Ultra-fine-cubic-zirconium-oxide-99_699949883.html
[25] http://www.google.com/patents/US3432314
[26] http://www.sigmaaldrich.com/materials-science/material-science-products.html?TablePage=19922550
[27] http://www.us-nano.com/inc/sdetail/748
[28] One price offering is here: http://www.metal-pages.com/marketplace/proposal/id/7287/; more recent price offerings may be available here under Marketplace, Sellers: http://www.metal-pages.com/metals/yttrium/metal-prices-news-information/
[29] http://www.us-nano.com/inc/sdetail/9448
[30] http://www.dentalzirconia.com/zirconia-sintering-furnaces/
[31] http://www.ceramicindustry.com/articles/91570-industrial-scale-microwave-sintering
[32] http://www.enerzi.co/sinteringfurnaces.aspx
[33] http://www.thomasnet.com/products/zirconia-ceramics-95996690-1.html