Abstract
A method is provided for producing an article which is transparent to near-wave IR, mid-wave and Long-wave multi-spectral and IR wavelength in the region of 0.4 μm to 16 μm. The method includes the steps of (a) Producing ultra-fine powder of CaLa.sub.2S.sub.4 via SPLTS process, (b) followed by pretreatment of the ultra-fine powder under inert and reducing gas conditions including H.sub.2 or Argon or N.sub.2 or H.sub.2/H.sub.2S, H.sub.2S, and mixtures there of (c) followed by sieving the powder in 140 mesh screen and cold pressing the powder at 7000 psi for 7 min. into a disk shaped green body (d) then Cold-Isostatic Pressing (CIP) at 40,000 psi for 5 min in a rubber mold (e) finally sintered article of CaLa.sub.2S.sub.4 disk of 25.4 mm diameter with ultra-high density containing cubic phase of CaLa.sub.2S.sub.4 to yield IR transmission of a peak value of 57% within the IR wavelength range of 2 μm to 16 μm, either by using microwave sintering followed by hot isostatic press or spark plasma sintering followed by hot isostatic press or vacuum sintering at (3×10.sup.−6 torr) followed by hot isostatic press or hot press sintering followed by hot isostatic press and finally followed by mirror polished IR article, is obtained.
Claims
1. A process for producing an article which is transparent to Infrared (IR) in wavelength range of 2 μm to 16 μm, comprising: synthesizing ultra-fine powders utilizing oxidizer comprising metal acetates, metal nitrates with fuels glycine, thiourea, and thioacetamide, through Self-propagating low temperature synthesis (SPLTS) and generation of population of CaLa.sub.2S.sub.4 nanoparticles; pre-treating said CaLa.sub.2S.sub.4 nanoparticles with reducing or inert or neutral gases in a temperature range and time duration to obtain powder of CaLa.sub.2S.sub.4 nanoparticles; sieving said pre-treated powder of CaLa.sub.2S.sub.4 nanoparticles with 140 mesh screens; cold pressing said sieved powder of CaLa.sub.2S.sub.4 nanoparticles in a pressure range and time duration to obtain a green body of an article shaped in to a CaLa.sub.2S.sub.4 disk; cold-isostatic pressing said cold pressed CaLa.sub.2S.sub.4 disk in a range of pressure for a time duration to obtain densification; hot press sintering said cold-isostatically pressed CaLa.sub.2S.sub.4 disk in a range of punch pressure, vacuum and temperature for a time duration to obtain CaLa.sub.2S.sub.4 sintered ceramic disk; hot isostatic pressing said CaLa.sub.2S.sub.4 sintered ceramic disk at temperature range and pressure range in Argon (Ar) gas to obtain highly transparent article in the visible, mid IR and long IR wavelengths; polishing said hot isostatic pressed CaLa.sub.2S.sub.4 ceramic disk to obtain a final ultra-high transparent IR transmitting article; and wherein said polished CaLa.sub.2S.sub.4 ceramic disk with the thickness of 25.4 mm has a peak IR transmission of 57% in the range of wavelengths between 3 μm and 5 μm and about 50% transmission between 6 μm and 16 μm, possessing Knoop hardness of 559 Kg/mm.sup.2.
2. The process for producing an article according to claim 1, wherein the pre-treatment of said CaLa.sub.2S.sub.4 nanoparticles is carried in H.sub.2 or N.sub.2 or Ar or H.sub.2S or mixtures thereof at a temperature in the range of 800° C. to 1000° C. and a time duration in the range of 4 to 12 hours.
3. The process for producing an article according to claim 1, wherein said sieved powder of CaLa.sub.2S.sub.4 nanoparticles are cold pressed in a pressure range of 5000-7000 psi with a hold time of 5-7 min to form green body of CaLa.sub.2S.sub.4 disk.
4. The process for producing an article according to claim 1, wherein said cold pressed CaLa.sub.2S.sub.4 disk is cold-isostatic pressed in the pressure range of 30,000-40,000 psi in a rubber mold with a hold time of 5 to 7 min. to attain 65%-70% of theoretical density.
5. The process for producing an article according to claim 4, wherein said cold iso-statically pressed CaLa.sub.2S.sub.4 nanoparticles are subjected to spark plasma sintering in the temperature range of 850°-1250° C. with punch pressure in the range of 100-120 Mpa, while passing pulse spark plasma current, skipping hot press sintering, followed by said hot-isostatic press in the range of pressure of 15,000-30,000 psi and in a temperature range of 800° C.-1200° C. under Ar gas atmosphere.
6. The process for producing an article according to claim 4, wherein said cold iso-statically pressed CaLa.sub.2S.sub.4 nanoparticles are subjected to microwave sintering at a frequency of 2.45 GHz and at power level between 800-1100 watts at 1120° C. for 30-40 min at atmosphere, skipping said hot press sintering, followed by said hot-isostatic press in the range of pressure of 15,000-30,000 psi and in a temperature range of 800° C.-1200° C. under Ar gas.
7. The process for producing an article according to claim 4, wherein said cold iso-statically pressed CaLa.sub.2S.sub.4 nanoparticles are subjected to vacuum sintering in a temperature range of 800° C.-1200° C. for 3-6 hrs and in the vacuum range of 10.sup.−5 Torr to 10.sup.−6 Torr, skipping said hot press sintering, followed by said hot-isostatic press in the pressure range of 15,000-30,000 psi and in a temperature range of 800° C.-1300° C. under Ar gas.
8. The process for producing an article according to claim 1, wherein said cold-isostatic pressed CaLa.sub.2S.sub.4 disk is subjected to said hot press sintering in the pressure range of 100-120 MPa and in a temperature range of 900° C.-1200° C. for 6-12 hours in vacuum range of 1×10.sup.−3 to 3×10.sup.−6 Torr, followed by said hot-isostatic press in the range of pressure of 15,000-30,000 psi and in a temperature range of 800° C.-1200° C.
9. The process for producing an article according to claim 1, wherein said SPLTS synthesis can be applied to synthesize CdS and ZnSe family of chalcogenides.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) FIG. 1A is a flow chart depicting the first embodiment of a method for producing sintered article of CaLa.sub.2S.sub.4 in accordance with the teachings herein.
(2) FIG. 1B is a flow chart depicting the second embodiment of a method for producing sintered article of CaLa.sub.2S.sub.4 in accordance with the teachings herein.
(3) FIG. 1C is a flow chart depicting the third embodiment of a method for producing sintered article of CaLa.sub.2S.sub.4 in accordance with the teachings herein.
(4) FIG. 1D is a flow chart depicting the fourth embodiment of a method for producing sintered article of CaLa.sub.2S.sub.4 in accordance with the teachings herein.
(5) FIG. 2 is a high resolution powder X-ray diffraction pattern of a Calcium Lanthanum Sulphide nano-powder pretreated in reducing gas atmosphere powder samples made in accordance with a methodology of SPLTS process disclosed herein.
(6) FIG. 3 is SEM image of CaLa.sub.2S.sub.4 powder obtained through SPLTS process and pre-treated in a reducing gas atmosphere.
(7) FIG. 4 is Energy Dispersive X-ray Analysis (EDAX) spectrum of CaLa.sub.2S.sub.4nano-powder
(8) FIG. 5 is an FTIR spectrum of a CaLa.sub.2S.sub.4 article made in accordance with the teachings herein.
(9) FIG. 6 is an SEM image of a sintered product made in accordance with the teachings herein.
(10) FIG. 7 is the final CaLa.sub.2S.sub.4 disk of 25.4 mm diameter exhibiting the transmission in the visible range when placed over the text of chemical formula ‘CaLa.sub.2S.sub.4’ on a paper.
(11) FIG. 8 shows the physical properties of sintered CaLa.sub.2S.sub.4 disk compared with ZnS IR transmitting disk.
DETAILED DESCRIPTION OF DRAWINGS
(12) FIG. 1A shows the process flow chart of the innovative integrated process of the current invention resulting in CaLa.sub.2S.sub.4 IR window. In step 1, the process commences with CaLa.sub.2S.sub.4 nano-powder obtained through SPLTS process. In step 2 the nano powder undergoes a heat treatment in any one of the gases namely, H.sub.2 or Ar or N.sub.2 or H.sub.2S or mixture of these gases, in a temperature range 800° C.-1000° C. for 12 hours. In step 3, gas treated nano-powders are obtained. In step 4, gas treated nano-powders are sieved through 140 mesh screen and cold pressed to obtain 25.4 mm disk followed by cold isostatic press for densification. The cold press to obtain 25.4 mm green body is done using a Tungsten carbide die sets, in a pressure range of 5000-7000 psi with a hold time of 5-7 minutes. The cold isostatic press is done with a pressure in the range of 30,000-40,000 psi in a rubber mold with a hold time of 5-7 minutes to attain 65%-70% of theoretical density. This is followed by, in step 5, vacuum sintering in a temperature range of 800° C.-1200° C. for 3-6 hrs and in the vacuum range of 10.sup.−5 Torr to 10.sup.−6 Torr and further followed by hot isostatic press in the range of pressure of 15,000-30,000 psi and in a temperature range of 800° C.-1200° C., thereby producing a sintered CaLa.sub.2S.sub.4 disk, which is subsequently, as shown in step 6, mirror polished to obtain the IR disk that is transparent in the visible, mid IR region and long IR wavelengths.
(13) FIG. 1B shows the process flow chart of the innovative integrated process of the current invention resulting in CaLa.sub.2S.sub.4 IR window. In step 1, the process commences with CaLa.sub.2S.sub.4 nano-powder obtained through SPLTS process. In step 2 the nano powder undergoes a heat treatment in any one of the gases namely, H.sub.2 or Ar or N.sub.2 or H.sub.2S or mixture of these gases. In step 3, gas treated nano-powders are obtained. In step 4, gas treated nano-powders are sieved through 140 mesh and cold pressed to obtain 25.4 mm disk of green body followed by cold isostatic press to improve densification. The cold press is to obtain 25.4 mm green body done using a Tungsten carbide die sets, in a pressure range of 5000-7000 psi with a hold time of 5-7 minutes. The cold isostatic press is done with a pressure in the range of 30,000-40,000 psi in a rubber mold with a hold time of 5-7 minutes to attain 65%-70% of theoretical density. This is followed by, in step 5 microwave sintering followed by hot isostatic press. The microwave sintering can be carried out at a frequency of 2.45 GHz and at a power level between 800-1100 watts at 1120° C. for 30-40 min under forming gas condition (H2/N2). This is followed by hot isostatic press in the range of pressure of 15,000-30,000 psi and in a temperature range of 800° C.-1200° C., thereby producing a sintered CaLa.sub.2S.sub.4 disk, which is subsequently mirror polished to obtain the disk that is transparent in the visible, mid IR and in the long IR wavelength region. As shown in step 6, the final sintered CaLa.sub.2S.sub.4 disk after polishing is obtained.
(14) FIG. 1C shows the process flow chart of the innovative integrated process of the current invention resulting in CaLa.sub.2S.sub.4 IR window. In step 1, the process commences with CaLa.sub.2S.sub.4 nano-powder obtained through SPLTS process. In step 2 the nano powder undergoes a heat treatment in any one of the gases namely, H.sub.2 or Ar or N.sub.2 or H.sub.2S or mixture of these gases in the temperature range of 800° C.-1000° C. for 4-12 hrs. In step 4, the powder is sieved through a 140 mesh screen. A green body of 25.4 mm diameter, from a population of thus sieved nano-powder, is obtained by cold pressing the powder, using a Tungsten Carbide die sets, in a pressure range of 5000-7000 psi with a hold time of 5-7 minutes. The Spark plasma sintering can now be carried out by inserting the green body of 25.4 mm diameter disk of CaLa.sub.2S.sub.4 inside a specially passivated cavity of the conductive mold and placed inside a spark plasma chamber. The mold die-set materials include those made from the alloy TZM (Titanium-Zirconium-Molybdenum) or Graphite. Preferably, the mold is made of special grade graphite. In all these cases, special care is taken to eliminate contamination of sintered material emanating from the material of the mold. Pressure is applied in the range of 100-120 MPa to the mold from the top and bottom using upper and lower energizing punch electrodes, maintaining a temperature in the range of 850° C.-1020° C. inside the chamber. At the same time, pulsed direct current is allowed to flow through the mold through the energizing punch electrodes. A power supply for generating pulsed direct current may be utilized which is similar to the power supply used for an electrical discharge machine. The pulsed direct current in transition mode may be applied at an initial stage of sintering and continuous pulsed direct current through train of pulses may be applied thereafter or, alternatively, a continuous train of pulsed direct current may be applied throughout the sintering. The spark current passing between the grains weld the grains (sintering). This is followed by hot isostatic press in the range of pressure of 15,000-30,000 psi and in a temperature range of 800° C.-1200° C., thereby producing a sintered CaLa.sub.2S.sub.4 disk, which is subsequently mirror polished to obtain the IR disk that is transparent in the visible, mid IR and in long IR wavelength regions.
(15) As shown in step 6, the final sintered CaLa.sub.2S.sub.4 disk after polishing is obtained.
(16) FIG. 1D shows the process flow chart of the innovative integrated process of the current invention resulting in CaLa.sub.2S.sub.4 IR window. In step 1, the process commences with CaLa.sub.2S.sub.4 nano-powder obtained through SPLTS process. In step 2 the nano powder undergoes a heat treatment in any one of the gases namely, H.sub.2 or Ar or N.sub.2 or H.sub.2S or mixture of these gases in the temperature range of 800 C-1000 C for 4-12 hrs. In step 4, the powder is sieved through a 140 mesh. A green body of 25.4 mm diameter, from a population of thus sieved nano-powder, is obtained by cold pressing the powder, using a Tungsten Carbide die sets, in a pressure range of 5000-7000 psi with a hold time of 5-7 minutes. and then cold isostatic pressed to densify the pellet with a pressure in the range of 30,000-40,000 psi in a rubber mold with a hold time of 5-7minutes to attain 65%-70% of theoretical density. This is followed by hot press sintering, as in step 5, in the pressure range of 50-100 MPa and in a temperature range of 900° C.-1200° C. for 6-12 hrs under a vacuum of 10.sup.3 Torr to 10.sup.−6 Torr. This is followed by hot isostatic press in the range of pressure of 15,000-30,000 psi and in a temperature range of 800° C.-1200° C., thereby producing a sintered CaLa.sub.2S.sub.4 disk, which is subsequently mirror polished to obtain the IR disk that is transparent in the visible, mid IR and in long IR wavelength region. As shown in step 6, the final sintered CaLa.sub.2S.sub.4 disk after polishing is obtained.
(17) FIG. 2 shows high resolution powder X-ray diffraction analysis of cubic crystalline CaLa.sub.2S.sub.4 ultra-high purity (99.999%) nano powder, obtained through SPLTS process followed by reducing gas thermal treatment. All the X-ray reflections could be indexed in terms of cubic CaLa.sub.2S.sub.4 crystal structure. Cubic crystalline CaLa.sub.2S.sub.4 phase structure can be seen through peaks represented by 211, 310, 321, 420, 332 etc. Many reflections indicate highly poly-crystalline material and represents the purity of the cubic crystalline structure of CaLa.sub.2S.sub.4 crystal and bears direct relation to the increased transmission of the crystal in the multi-spectral IR range. The powder X-ray diffraction samples were prepared for this test by mixing CaLa.sub.2S.sub.4 nanopowder with few drops of ethanol, made into a paste, filled in a special sample holder, and dried at 40° C. The dried holder is cooled to room temperature and examined for phase determination. The powder X-ray diffraction data was collected on a Rigaku (RU200B) computer-automated diffractometer with a copper (Cu) target and graphite-mono-chromated Radiation. The X-ray source operates at 50 kV and 180 mA. A single crystal of silicon (Si) and aluminum (Al) were used as calibration standards. The obtained powder pattern of CaLa.sub.2S.sub.4 was compared with the standard JCPDS file card [20-0339] for CaLa.sub.2S.sub.4
(18) FIG. 3 shows SEM (Scanning Electron Microscopy) morphology of CaLa.sub.2S.sub.4are crystalline nanoparticles with ˜300 nm particle size and is clearly visible. The nanoparticles are very crystalline in nature and suitable for sintering to produce ultra-high-density Infra-red ceramic windows.
(19) FIG. 4 shows the Energy Dispersive X-Ray Analysis of the sample of CaLa.sub.2S.sub.4 crystals synthesized through SPLTS process and followed by thermal gas treatment. The elemental analysis shows the peak corresponding to Sulphur, Calcium and Lanthanam.
(20) FIG. 5 shows the IR transmission of CaLa.sub.2S.sub.4 disk in accordance with the process flow of our first embodiment for a disk diameter of 25.4 mm. A peak transmission of 57% is seen as per the measurement.
(21) FIG. 6 shows SEM image of the sintered disk. The SEM image exhibits high density of the sample without any pores or voids. The Knoop hardness for the sintered samples were measured and found to be in the range of 550-559 kg/mm.sup.2. It also shows the grain size is approximately in the range of 300 nm to 2 μm.
(22) FIG. 7 shows the visual transmission exhibited by the CaLa.sub.2S.sub.4 disk fabricated in accordance with the process described under our first embodiment. The disk transmits visually a typed chemical formula CaLa.sub.2S.sub.4 on a sheet of paper when the disk was placed over it.
(23) FIG. 8 tabulates the mechanical and thermal properties of the CaLa.sub.2S.sub.4 disk fabricated in accordance with our process described under first embodiment. The comparison is made against ZnS disk. It can be seen that Knoop hardness (559 Kg/mm.sup.2) is substantially higher than that of ZnS disk (245 Kg/mm.sup.2).
(24) The above description of the present invention is illustrative, and is not intended to be limiting. It will be understood that one skilled in the art could make various additions, substitutions and modifications to the above described embodiments without departing from the scope of the present invention. For example, (i) the die-set material can be made of alloy materials of Tungsten-Zirconium-Molybdenum (ii) sintering parameters could be changed for various sintering methods like, Microwave sintering, spark plasma sintering, RF sintering, Laser sintering, vacuum sintering (iii) parameters for hot or cold isostatic press could be modified (iv) sintering methods can be combined in any preferred sequence (v) SPLTS process parameter could be modified (vi) although the current invention focuses on CaLa.sub.2S.sub.4, but the process can be equally applied to CdS and ZnSe or any material falling in the family of Chalcogenides, such as ZnS, ZnTe. Accordingly, the scope of the present invention should be construed in reference to the appended claims.
