SELF-PROPAGATING LOW-TEMPERATURE SYNTHESIS AND PRE-TREATMENT OF CHALCOGENIDES FOR SPARK PLASMA SINTERING

Abstract

A method is provided for producing an article which is transparent to IR wavelength in the region of 4 μm to 9 μm. The method includes the steps of (a) Producing ultra-fine powders of ZnS, (b) followed by pretreatment of the ultra-fine powders under reduced gas conditions including H2, H2S, N2, Ar and mixtures there of (c) followed by vacuum (3×10.sup.−6 torr) treatment to remove oxygen and sulfates adsorbed to the surface disposing a plurality of nano-particles on a substrate, wherein said nanoparticles comprise ZnS with ultra-high purity of cubic phase; (b) subjecting the nano-particles to spark plasma sintering thereby producing a sintered ZnS product with IR transmission reaching 75% in the wavelength range of 4 μm to 9 μm.

Claims

1. A process for producing an article which is transparent to Infra-red in the wavelength range of 3 μm to 12 μm, comprising: TABLE-US-00001 synthesizing ultra-fine powders utilizing oxidizer including metal acetates, metal nitrates with fuels glycine, thiourea, and thioacetamide, through Self-propagating low temperature synthesis and generation of population of ZnS nano- particles; pre-treating the said ZnS nano-particles with gases H.sub.2S, H.sub.2, N.sub.2, Ar gases or mixtures of gases there of; vacuum the said ZnS nano-particles to remove moisture, oxygen treating and sulfates; filling the said ZnS nano-particles inside a die-set cavity and applying pressure on the nano-particles; subjecting the said ZnS nano-particles to programmed pulsed spark plasma treatment resulting in heating of nano-particles and sintering of the said nano-particles to form disk of ZnS; subjecting the said ZnS disk to hot-isostatic press and thus obtaining the final product that is transparent to visible, mid IR and Long wave Infra-red wavelength.

2. The process according to claim 1 wherein the pre-treatment of said ZnS nano-particles is done in a mixture of H.sub.2S and H.sub.2 with volume ratio 1:(4, 9), at a temperature in the range of 450° C. to 700° C. and duration in the range of 2 to 4 hours.

3. The process according to claim 1 wherein the pre-treatment of said ZnS nano-particles is done in a mixture of, H.sub.2S and N.sub.2 with volume ratio 1:(4,9), at a temperature in the range of 450° C. to 700° C. and duration in the range of 2 to 4 hours

4. The process according to claim 1 wherein the pre-treatment of said ZnS nano-particles is done in a mixture of H.sub.2S and Ar with volume ratio 1:(4, 9), at a temperature in the range of 450° C. to 700° C. and duration in the range of 2 to 4 hours

5. The process according to claim 1 wherein the said vacuum treatment of ZnS nano-particles is done in the range of temperatures less than 600° C. and in the range of vacuum of 1×10.sup.−3 Torr to 3×10.sup.−6 Torr for a time duration of 3-6 hr.

6. The process according to claim 1 wherein the said pulsed spark plasma treatment can be replaced by Laser sintering or Micro-wave sintering or vacuum sintering or cold-Isostatic pressing or combinations of the sequence of these sintering methods.

7. The process according to claim 1 wherein the said mold is passivated with protective coating to prevent contamination entering the finished product.

8. The process according to claim 1 wherein the said ZnS disk having an Infra-red transmission in the range of 70% and 75% in the wavelength range of 4 μm to 9 μm.

9. The process according to claim 1 wherein the said ZnS disk having an Infra-red transmission is greater than or equal to 65% in the wavelength between 3 μm to 12 μm.

10. A process for producing an article which is transparent to Infra-red in the wavelength range of 3 μm to 12 μm, comprising: TABLE-US-00002 synthesizing ultra-fine powders utilizing oxidizer including metal acetates and metal nitrates with fuels thiourea, thioacetamide, SeS.sub.2 or glycine through Self-propagating low temperature synthesis and generation of population of ZnSe nano-particles; pre-treating the said ZnSe nano-particles with gases H.sub.2S, H.sub.2, N.sub.2, Ar gases or mixtures of these gases; vacuum the said ZnSe nano-particles to remove sulfates, oxygen treating and moisture; filling the said ZnSe nano-particles inside a die-set cavity and applying pressure on the nano-particles; subjecting the said ZnSe nano-particles to programmed pulsed spark plasma treatment resulting in heating of nano- particles and sintering of the said nano-particles to form disk of ZnSe; subjecting the said ZnSe disk to hot isostatic press and thus obtaining the final product that is transparent to Infra-red wavelength.

11. The process according to claim 10 wherein the said ZnSe disk having an Infra-red transmission in the range of 70% and 75% in the wavelength range of 4 μm to 9 μm.

12. The process according to claim 1 wherein the said ZnSe disk having an Infra-red transmission is greater than or equal to 65% in the wavelength between 3 μm to 12 μm.

13. A process for producing an article which is transparent to Infra-red in the wavelength range of 3 μm to 12 μm, comprising: TABLE-US-00003 synthesizing ultra-fine powders utilizing oxidizer including metal acetates, metal nitrates with fuels thiourea, thioacetamide, or glycine through Self-propagating low temperature synthesis and generation of population of CdS nano- particles; pre-treating the said CdS nano-particles with gases H.sub.2S, H.sub.2, N.sub.2, Ar gases or mixtures of these gases; vacuum the said CdS nano-particles to remove moisture, sulfates treating and oxygen; filling the said CdS nano-particles inside a die-set cavity and applying pressure on the nano-particles; subjecting the said CdS nano-particles to programmed pulsed spark plasma treatment resulting in heating of nano-particles and sintering of the said nano-particles to form disk of CdS; subjecting the said CdS disk to hot isostatic press and thus obtaining the final product that is transparent to Infra-red wavelength.

14. The process according to claim 10 wherein the said CdS disk having an Infra-red transmission in the range of 70% and 75% in the wavelength range of 4 μm to 9 μm.

15. The process according to claim 1 wherein the said CdS disk having an Infra-red transmission is greater than or equal to 65% in the wavelength between 3 μm to 12 μm.

16. The knoop hardness for the nano-grained ZnS samples were measured and found to be in the range of 290-320 kg/mm.sup.2.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] FIG. 1 is a flow chart depicting an embodiment of a method for producing sintered articles in accordance with the teachings herein.

[0042] FIG. 2 is a powder X-ray diffraction of a zinc sulphide nano-powder sample made in accordance with a methodology disclosed herein.

[0043] FIG. 3 is an illustration of voltage-current vs Time during spark plasma dwell time.

[0044] FIG. 4 is an FTIR spectrum of a ZnS article made in accordance with the teachings herein.

[0045] FIG. 5 is an SEM image of a sintered product made in accordance with the teachings herein.

[0046] FIG. 6. ZnS sintered disk via SPS method show X-ray diffraction of a zinc sulphide sample made in accordance with a methodology disclosed herein.

DETAILED DESCRIPTION

[0047] FIG. 1 shows the process flow chart of the innovative integrated process of the current invention. The process commences with SPLTS. SPLTS involves an exothermic reaction between metal acetates, metal nitrates and a fuel at low temperature <500° C. SPLTS synthesis is an important powder processing technique generally used to produce complex oxide ceramics such as aluminates. The process involves the exothermic reaction of an oxidizer such as metal nitrates, metal acetates, ammonium nitrate, and ammonium perchlorate and an organic fuel, typically thiourea (CH4N2O), Thioacetamide, carbohydrazide (CH6N4O), or glycine (C2H5NO2). The combustion reaction is initiated in a box furnace or on a hot plate at temperatures of 500° C. or less; much lower than the phase transition of the target material.

[0048] In a typical reaction, the precursor mixture of water, including metal acetates, metal nitrates, and fuel including glycerol, thiourea and thioacetamides decomposes, dehydrates, and ruptures into a flame after about 3-5 min. The resultant product is a voluminous, foamy powder which occupies the entire volume of the reaction vessel. The chemical energy released from the exothermic reaction between the metal nitrates and fuel can rapidly heat the system to without an external heat source. SPLTS synthesized powders are generally more homogeneous, have fewer impurities, and have higher surface areas than powders prepared by regular conventional solid-state methods.

[0049] CdS, ZnS and ZnSe nano-powders were produced via SPLTS using respective Nitrates including Cadmium Nitrates, Zinc Nitrates and acetates including cadmium acetates and Zinc acetates. Sulfur sources including thiourea, thioacetamides and Selenium disulfides. The current invention synthesizes ZnS through SPLTS. FIG. 2 shows the x-ray diffraction analysis of cubic crystalline phase of ZnS

[0050] FIG. 2 shows a high resolution powder X-ray diffraction of ultra-high purity (99.99%) cubic zinc sulphide nano-powder. The diffraction peak 20 corresponds to the (1, 1, 1) plane of the cubic crystalline ZnS material, and similarly, the peak 21 corresponds to the (2, 0, 0) plane, the peak 22 corresponds to the (2, 2, 0) plane, the peak 23 corresponds to the (3,1,1) plane, the peak 24 corresponds to the (2,2,2) plane, the peak 25 corresponds to the (4,0,0) plane and peak 26 corresponds to the (3,3,1) plane of the cubic crystalline ZnS material. As can be seen from the FIG. 2, there are only cubic phases and these are the ones contributing to the increase in IR transmission in the finished product.

[0051] FIG. 3 shows the voltage-current vs time during the spark plasma dwell time of two minutes. During this dwell time the vacuum was held at 10 Pascal and the mold temperature was around 750°-820° C. with punch pressure around 100-120 MPa. It can be seen that the current initially starts with high 31 value of around 675 Amps and starts decreasing and fluctuating 32 around 630 Amps thus indicating the grain-welding process the evaporation of material and fusion of inter-grain region. The voltage plot shown is the variation of voltage across the electrodes (top and bottom punches) and the voltage is also fluctuating depending on the current. Note that the voltage plotted is not the supply voltage that is in the form of regularly shaped pulses.

[0052] The ZnS ceramic specimens are preferably polished in three different steps such as grinding, polishing, and fine polishing. Initial grinding removes any saw marks and cleans the specimen surface. This is accomplished manually on a dry 240 grit Si.sub.3N.sub.4 sand paper. The Si.sub.3N.sub.4 abrasive particles are bonded to the paper for fast stock removal. The polishing and fine polishing removes the artifacts of grinding. During polishing, a COTLAP™ Polish Cloth was used with 3 μm diamond powder. A mirror finish was achieved using a RAYON™ Velvet polish cloth with 1 μm diamond on it. In both polishing and fine polishing, the diamond abrasive particles were suspended in oil and thus were able to roll or slide across the cloth in order to obtain mirror polished sintered body.

[0053] The mirror polished ZnS polycrystalline ceramic materials were used to collect the percentage IR transmittance. Fourier transform infrared (FT-IR) spectra were recorded on a IS 50 Fourier transform infrared spectrometer.

[0054] FIG. 4 shows the percentage of IR transmittance of the ZnS polycrystalline ceramic samples with 75% transmittance in the long wavelength region (5-9 microns).

[0055] FIG. 5 shows SEM images of the sintered product. The SEM images indicate high density samples without any pores or voids in the samples. The knoop hardness for the nano-grained samples were measured and found to be in the range of 290-320 kg/mm.sup.2. It also shows the grain size is approximately in the range of 1 to 5 μm.

[0056] The sample obtained through SPS process is further Hot-Isostatically pressed (HIP). The HIP process subjects a component to both elevated temperature and isostatic gas pressure in a high pressure containment vessel. The pressurizing gas most widely used is Argon. An inert gas is used, so that the material does not chemically react. The chamber is heated, causing the pressure inside the vessel to increase. Many systems use associated gas pumping to achieve the necessary pressure level. Pressure is applied to the material from all directions (hence the term “isostatic”). The HIP is performed at a temperature within the range of 800-1100° C., and more preferably, at a temperature within the range of 850-1080° C. wrapped in Molybdenum, copper and platinum foils to achieve visually transparent samples also called as water clear and multi-spectral grade ZnS. This sample is transparent in the visible range, mid IR range and long wavelength region. The samples were cut and polished using several grade diamond pastes.

[0057] The FIG. 6 shows the X-ray diffraction analysis of ZnS disk that went through the SPS process. It can be seen that there is a trace level appearance of Wurtzite phase with a minimal peak. All the cubic phases of ZnS as seen in FIG. 2 are retained. The wurtzite phases 332 starts to appear. Wurtzite phase will decrease the transmission of the sample if the peak intensity is substantial. Since the peak intensity is small compared to the cubic phase (example 322 next to 111) the IR transmission is still high (around 75%).

[0058] The sintered and annealed ceramic windows disclosed herein may be supplemented with coatings to further enhance their properties and to provide increased protection. An anti-reflective coating, for example, may be applied to minimize the reflection of infrared radiation and thereby cause more of the radiation to pass through the window. Examples of coating materials for this purpose are low refractive index materials, particularly yttria, silica, magnesium fluoride, calcium fluoride, zinc fluoride, zinc selenide, and Hafnium oxide. Multiple antireflective coatings may also be used. In some applications, a coating that will transmit visible radiation in addition to the infrared radiation may be desired. Examples of coating materials for this purpose are leaded glass and Zinc Selenide. Alternatively or in addition, coatings for scratch or erosion resistance may be applied, particularly for enhanced protection against rain, blowing sand, and particle impacts in general. Materials with a high damage threshold velocity, such as gallium phosphide, sapphire, spinel, and aluminum oxynitride (ALON) may also be utilized.

[0059] The above description of the present invention is illustrative, and is not intended to be limiting. It will thus be appreciated that various additions, substitutions and modifications may be made to the above described embodiments without departing from the scope of the present invention. For example, (i) the mold material can be made of alloy materials of Tungsten-Zirconium-Molybdenum (ii) during SPS process, DC pulse shapes could be designed to inject modulated current (iii) SPS process can replaced by Laser sintering process or RF sintering process or Microwave sintering process or hot-press process or cold iso-static process followed by sintering or vacuum sintering (iv) although the current invention focuses on ZnS, the process can be equally applied to CdS and ZnSe or any falling in the family of Chalcogenides. Accordingly, the scope of the present invention should be construed in reference to the appended claims.