Thermal barrier coating having high phase stability

A device (10) comprising a substrate (22) having a deposited ceramic thermal barrier coating layer (20) characterized by a microstructure having gaps (28) where the thermal barrier coating (20) consists essentially of a pyrochlore crystal structure having a chemical formula consisting essentially of A.sup.n+.sub.2-x B.sup.m+.sub.2+x O.sub.7-y, where A is selected from the group of elements selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and mixtures thereof; where B is selected from the group of elements selected from Zr, Hf, Ti and mixtures thereof; n and m are the valence of A and B respectively, and for -0.5.ltoreq.x.ltoreq.0.5, ##EQU1## and excluding the following combinations for x=0, y=0: A=La and B=Zr; A=La and B=Hf; A=Gd and B=Hf; and A=Yb and B=Ti.

FIELD OF THE INVENTION
 This invention relates generally to the field of thermal barrier coatings,
 and more particularly to a thermal barrier coating for very high
 temperature applications, such as in a combustion turbine engine. In
 particular, this invention relates to the field of ceramic thermal barrier
 coatings having high phase stability at 1400.degree. C. and higher, which
 are resistant to sintering damage, for coating superalloy or ceramic
 components in the hot sections of a combustion turbine, such as turbine
 blades and vanes, transitions, ring segments and combustors.
 BACKGROUND OF THE INVENTION
 The demand for continued improvement in the efficiency of combustion
 turbine and combined cycle power plants has driven the designers of these
 systems to specify increasingly higher turbine inlet temperatures.
 Although nickel and cobalt based superalloy materials are now used for
 components in the hot gas flow path, such as combustor transition pieces
 and turbine rotating and stationary blades, even these superalloy
 materials are not capable of surviving long term operation at temperatures
 sometimes as high as 1400.degree. C.
 It is known in the art to coat a superalloy metal component with an
 insulating ceramic material to improve its ability to survive high
 operating temperatures, for example U.S. Pat. No. 4,321,310 (Ulion et al).
 It is also known to coat the insulating ceramic material with an erosion
 resistant material to reduce its susceptibility to wear caused by the
 impact of particles carried within the hot gas flow path; for example,
 U.S. Pat. Nos. 5,683,825 and 5,562,998 (Bruce, et al. and Strangman,
 respectively).
 Much of the development in this field of technology has been driven by the
 aircraft engine industry, where turbine engines are required to operate at
 high temperatures, and are also subjected to frequent temperature
 transients as the power level of the engine is varied. A combustion
 turbine engine installed in a land-based power generating plant is also
 subjected to high operating temperatures and temperature transients, but
 it may also be required to operate at full power and at its highest
 temperatures for very long periods of time, such as for days or even weeks
 at a time. Prior art insulating systems are susceptible to degradation
 under such conditions at the elevated temperatures demanded in the most
 modern combustion turbine systems.
 U.S. Ser. No. 09/245262, filed on Feb. 2, 1999 (Subramanian, et al.; ESCM
 283139-00491), also related to columnar thermal barrier coatings (TBCs),
 usually of yttria-stabilized zirconia (YSZ), deposited by electron beam
 physical vapor deposition (EB-PVD) with a sintering resistant layer of
 aluminum oxide or yttrium aluminum oxide, deposited as a continuous or
 discontinuous layer between submicron gaps in the TBC columns. This
 material was thermally stable up to about 1200.degree. C. Other columnar
 TBC coatings are described in U.S. Ser. No. 09/393,415, filed on Sep. 10,
 1999, (Subramanian; ESCM 283139-00224) , where TBC columns had a
 composition of (A,B).sub.x O.sub.y and were covered by a sheath of a
 composition of C.sub.Z O.sub.W, where A,B and C were selected from Al, Ca,
 Mg, Zr, Y, Sc and rare earth equal to La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb,
 Dy, Ho, Er, Tm, and Yb. In this application, a reaction between C.sub.z
 O.sub.w and (A,B).sub.x O.sub.y was key to obtain a multiphase TBC system
 which was expected to be sinter resistant and strain tolerant up to
 1400.degree. and higher. The same materials were used as an (A,B).sub.x
 O.sub.y planar based TBC coated with a C.sub.z O.sub.w overlay in U.S.
 Ser. No. 09/393,417, filed on Sep. 10, 1999, (Subramanian; ESCM
 283139-00223). In this application also, a reaction between C.sub.z
 O.sub.w and (A,B).sub.x O.sub.y was key to obtain a multiphase TBC system
 which was expected to be sinter resistant and strain tolerant up to
 1400.degree. and higher. Specific compounds capable for application as
 TBCs are described in U.S. Ser. No. 09/405,498, filed on Sep. 24, 1999
 (Subramanian, et al.; ESCM 283139-00076). There, TBC layers of
 LaAlO.sub.3, NdAlO.sub.3, La.sub.2 Hf.sub.2 O.sub.7, Dy.sub.3 Al.sub.5
 O.sub.12, Ho.sub.3 Al.sub.5 O.sub.12, ErAlO.sub.3, GdAlO.sub.3, Yb.sub.2
 Ti.sub.2 O.sub.7, LaYbO.sub.3, Gd.sub.2 Hf.sub.2 O.sub.7, and Y.sub.3
 Al5O.sub.12 were generally described. These were compounds capable for TBC
 application, due to their inherently superior sintering resistance and
 phase stability.
 A solid, vapor deposition material useful for the EB-PVD method to provide
 heat resistant coatings in aircraft engines and the like, where excellent
 heat resistance and thermal shock resistance is required, is taught by
 U.S. Pat. No. 5,789,330 (Kondo, et al). There, the material is sintered
 zirconia, containing a special stabilizer selected from yttria, magnesium
 oxide, calcium oxide, scandium oxide, or oxides of rare earth elements
 equal to La, Ce, Pr, Nd, Pm, Sm, Eu, Hd, Tb, Dy, fermium, Wr, thulium, Yb
 and ruthenium in the range of 0.1 wt percent to 40 wt percent of the
 material. The sintered material has 25% to 70% monoclinic phase and up to
 3% tetragonal phase, with the rest as cubic phase.
 Some high temperature resistant coatings, as taught in U.S. Pat. No.
 5,304,519 (Jackson, et al), have utilized thermal spraying of zircon plus
 zirconia particles (ZrSiO.sub.4 and ZrO.sub.2 respectively) partially
 stabilized with an oxide selected from CaO, Y.sub.2 O.sub.3, MgO,
 CeO.sub.2, HfO.sub.2 or rare earth oxide, where rare earth equal La, Ce,
 Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. These materials
 are used as refractory, thermal shock resistant coatings for hearth rolls
 for annealing steel, stainless steel and silicon steel sheet at furnace
 temperatures between 820.degree. C. and 1100.degree. C.
 Data regarding sintering rates of single oxides A.sub.x O.sub.y are
 available but only a few publications discuss sintering rates of
 multicomponent oxides. One such publication is by Shinozaki, et al. 1981,
 (9), pp. 1454-1461, where the sintering tendencies of a solid solution of
 mixed Sm.sub.2 O.sub.3 --ZrO.sub.2 were discussed in The Chemical Society
 of Japan, "Sintering Sm.sub.2 O.sub.3 --ZrO.sub.2 Solid Solution." There,
 tablets of the mixed component oxides at various mole % were sintered at
 from 1200.degree. C. to 1600.degree. C. and isothermal linear shrinkage
 was measured. The least amount of sintering, 3% to 10% at 1400.degree. C.,
 was found at ranges of 5 mole % to 50 mole % Sm.sub.2 O.sub.3.
 In "La.sub.2 Zr.sub.2 O.sub.7 --a new candidate for thermal barrier
 coatings", R. Va.beta.en, X. Cao, F. Tietz, G. Kerkhoff, D. Stover, United
 Thermal Spray Conference, 17.-19.3.99, Dusseldorf, Hrsg. E. Lugscheider,
 P. A. Kammer, Verlag Fur Schwei.beta.en und Verwandte Verfahren,
 Dusseldorf, 1999, p. 830-034, plasma sprayed TBC coatings of one specific
 compound, La.sub.2 Zr.sub.2 O7, were discussed. Although this material is
 of the pyrochlore structure, as shown in their FIG. 2, our own results in
 the Example, below, show this specific compound is not good as a TBC.
 However, introduction of cation excess/defects or oxygen defects change
 the sintering properties and this is not suggested in the paper.
 What is needed is a TBC coating for a device, where the coating will remain
 thermally stable, protective, strain compliant, and resistant to
 substantial sintering of gaps in its grain structure, for use in
 long-term, high temperature turbine applications at temperatures up to
 1400.degree. C. Preferably the TBC will be a new material which itself
 meets the above criteria without the need for extra processing steps or
 additional coating.
 It is a main object of this invention to provide a device which is capable
 of operating at temperatures up to about 1400.degree. C. for extended
 periods of time with reduced component degradation. It is a further object
 of this invention to provide a method of producing such a device that
 utilizes commercially available materials processing steps.
 SUMMARY OF THE INVENTION
 These and other objects of the invention are accomplished by providing a
 device for operating over a range of temperatures, having a deposited
 thermal barrier coating on at least a portion of its surface, the device
 comprising a substrate with a bond coat; and then a deposited ceramic
 thermal barrier layer, the ceramic layer consisting essentially of a
 pyrochlore crystal structure having a chemical formula consisting
 essentially of A.sup.n+.sub.2-x B.sup.m+.sub.2+x O.sub.7-y where A is
 selected from the group of elements consisting of La, Ce, Pr, Nd, Sm, Eu,
 Gd, Tb, Dy, Ho, Er, Tm, Yb, and mixtures thereof; where B is selected from
 the group of elements consisting of Zr, Hf, Ti, and mixtures thereof; n
 and m are the valence of A and B respectively; and for
 -0.5.gtoreq.x.gtoreq.0.5, y=7-([(2-x)n+(2+x)m]/2) or
 y=7-(((2-x)n+(2+x)m)/2), that is:
 ##EQU2##
 and excluding the following combinations for x=0, y=0:A=La and B=Zr; A=La
 and B=Hf; A=Gd and B=Hf; and A=Yb and B=Ti, which describe the following
 excluded compounds: La.sub.2 Zr.sub.2 O.sub.7, La.sub.2 Hf.sub.2 O.sub.7,
 Gd.sub.2 Hf.sub.2 O.sub.7, and Yb.sub.2 Ti.sub.2 O.sub.7. The preferred
 combinations for this invention are A=Sm and B=Zr; A=Eu and B=Zr; A=Gd and
 B=Zr; with the first combination being the most preferred.
 Further, a method according to this invention, for producing a device
 operable over a range of temperatures, includes the steps of: providing a
 substrate; depositing a bond coat and then depositing a ceramic thermal
 barrier layer over the bond coat in a manner that provides a deposited
 ceramic layer consisting essentially of a pyrochlore crystal structure
 having a chemical formula consisting essentially of A.sup.n+.sub.2-x
 B.sup.m+.sub.2+x O.sub.7-y, where A is selected from the group of elements
 consisting of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and
 mixtures thereof; where B is selected from the group of elements
 consisting of Zr, Hf, Ti and mixtures thereof; n and m are the valence of
 A and B respectively; and for -0.5.gtoreq.x.gtoreq.0.5,
 ##EQU3##
 and excluding the following combinations for x=0, y=0:A=La and B=Zr; A=La
 and B=Hf; A=Gd and B=Hf; and A=Yb and B=Ti.
 These compositions will be extremely stable even under long term exposure
 to temperatures up to about 1500.degree. C. and can be deposited by well
 known plasma spray, EB-PVD, and D-gun techniques, HVOF (high velocity
 oxygen fuel deposition) techniques, inductively coupled deposition
 processes, and electron beam directed vapor deposition techniques.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Referring now to FIG. 1, one component device of a turbine is shown.
 Turbine blade 10 has a leading edge 12 and an airfoil section 14, against
 which hot combustion gases are directed during operation of the turbine,
 and which is subject to severe thermal stresses, oxidation and corrosion.
 The root end 16 of the blade anchors the blade. Cooling passages 18 may be
 present through the blade to allow cooling air to transfer heat from the
 blade. The blade itself can be made from a high temperature resistant
 nickel or cobalt based superalloy, such as, a combination of
 Ni.multidot.Cr.multidot.Al.multidot.Co.multidot.Ta.multidot.Mo.multidot.W.
 A basecoat (or bond coat) could cover the body of the turbine blade, which
 basecoat (or bond coat) could be covered by a thermal barrier coating 20.
 The barrier layer of this invention, as well as the base coat (or bond
 coat) and other protective coating can be used on a wide variety of other
 components of turbines, such as, turbine vanes, turbine transitions, or
 the like, which may be large and of complex geometry, or upon any
 substrate made of, for example, metal or ceramic, where thermal protection
 is required.
 FIG. 2 shows one example of possible thermal barrier coating system for the
 protection of a turbine component substrate 22 such as the superalloy core
 of a turbine blade. A basecoat 24 of a MCrAlY-type alloy can be used as a
 protection layer, as shown, where M (metal) in the alloy is usually
 selected from the group consisting of Ni, Co, Fe and their mixtures and Y
 is here defined as included yttrium, Y, as well as La, and Hf. This layer
 can be deposited by sputtering, electron beam vapor deposition or one of a
 number of thermal spray processes including low pressure plasma spraying,
 high velocity oxygen fuel (HVOF), and the like to provide a relatively
 uniform layer about 0.0025 cm to 0.050 cm (0.001 inch to 0.020 inch)
 thick. One purpose of this layer is to provide, upon heat treatment, an
 oxide scale 26, predominately aluminum oxide, about 0.3 micrometers to 5
 micrometers thick, in order to further protect the substrate 22 from
 oxidative attack.
 When prior art thermal barrier coating systems are exposed to the high
 temperature environment of the hot gas flow path of a land-based
 combustion turbine power plant, one of the reasons for failure of the
 thermal barrier coating (TBC) is sintering and loss in strain tolerance of
 the TBC. A current state-of-the-art TBC 20 is yttria-stabilized zirconia
 (YSZ) deposited by electron beam physical vapor deposition (EB-PVD). The
 EB-PVD process provides the YSZ coating with a columnar microstructure
 having sub-micron sized gaps 28 between adjacent columns of YSZ normal
 (90.degree.) angle to the substrate material. Alternatively, the YSZ may
 be applied by air plasma spraying (APS), which consists of a series of
 sub-micron sized cracks, also here considered gaps, within the YSZ layer
 and predominantly parallel to the substrate. The gaps provide a mechanical
 flexibility to the TBC layer. During operation at high temperatures, these
 gaps have a tendency to close, and if the device is maintained at the
 elevated temperature, usually above 1200.degree. for 8YSZ, for a
 sufficient length of time, the adjacent sides of the gaps will bond
 together by sintering. The bonding of the ceramic material across the gaps
 reduces the strain compliance of the TBC coating, thereby contributing to
 the potential for failure of the TBC during subsequent thermal transients.
 The new TBC coating 20 disclosed here is a pyrochlore crystal structure
 having a chemical formula consisting essentially of A.sup.n+.sub.2-x
 B.sup.m+.sub.2+x O.sub.7-y, where A is selected from the group of elements
 consisting of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and
 mixtures thereof; where B is selected from the group of elements
 consisting of Zr, Hf, Ti and mixtures thereof; n and m are the valence of
 A and B respectively; and, for -0.5.ltoreq.x.ltoreq.0.5,
 ##EQU4##
 and excluding the following combinations for x=0, y=0:A=La and B=Zr; A=La
 and B=Hf; A=Gd and B=Hf; and A=Yb and B=Ti.
 In the above formula, the values "n" and "m" in the brackets are the charge
 of the A and B elements; for example, if A=Sm.sup.3+ and B=Zr.sup.4+, n=3
 and m=4, then the value of y is:
 ##EQU5##
 The above ranges of x and y clearly indicate that a defective pyrochlore
 structure is also a candidate for a TBC. For example, for A=Sm, B=Zr and
 x=0, y is equivalent to 0 and the formula reduces to Sm.sub.2 Zr.sub.2
 O.sub.7, a preferred embodiment. For x=0.1, y is equivalent to -0.05 and
 the formula is Sm.sub.1.9 Zr.sub.2.1 O.sub.7.05, also a preferred
 embodiment. A listing of other preferred materials
 -0.5.ltoreq.x.ltoreq.0.5 and
 ##EQU6##
 A=Eu and B=Zr; and A=Gd and B=Zr.
 These materials are stable upon long-term exposure at high temperatures.
 Due to their phase stability and high sintering resistance, they are
 potential candidates for thermal barrier coating applications. As
 mentioned previously, conventional TBC coatings are yttria-stabilized
 zirconia (YSZ), preferably 8 wt. % YSZ (8YSZ). YSZ can be described by the
 unit cell of ZrO.sub.2 shown in FIG. 3. The ZrO.sub.2 crystal structure is
 depicted by the arrangement of the cations Zr and the anions O, as shown.
 It consists of a face-centered arrangement of Zr ions, shown as black
 circles. The anions are in the tetrahedral sites within the cube. There
 are eight oxygen atoms, shown as white circles for clarity, and 4 Zr
 atoms, resulting in ZrO.sub.2. Yttrium ions are not shown for clarity. A
 key feature of YSZ is that yttrium ions are randomly distributed in the Zr
 sites, resulting in oxygen vacancies in the ZrO.sub.2 lattice structure.
 These vacancies are also randomly distributed, and are not shown in the
 figure. This is a well known crystal structure.
 Materials with a pyrochlore structure are discussed in detail by M. A.
 Subramanian, et al., in "Oxide Pyrochlores--A Review," Prog. Solid State
 Chem., vol. 15, pp. 55,143, (1983). There, on page 65, a pyrochlore
 structure derived from a fluorite lattice was shown. While this paper
 discusses the crystal structure, no reference is made to TBC applications.
 The pyrochlore structure can be described as a structure with ordered
 oxygen vacancies (as distinguished from the random oxygen vacancies of
 YSZ) located in a crystal structure. The pyrochlore structure can, in
 principle, be shown to be a derivative of the ZrO.sub.2 crystal structure.
 Since the pyrochlore structure is a derivative of the ZrO.sub.2 structure,
 several of the same advantageous properties--such as low thermal
 conductivity, high thermal expansion, and deposition of single crystalline
 columns--are expected.
 This paragraph will now describe, in a simple manner, the relationship of
 the pyrochlore structure to the fluorite, ZrO.sub.2 structure. Doubling
 the Zr.sub.4 O.sub.8 unit cell results in a "Zr.sub.8 O.sub.16 " unit
 cell, where the Zr sites are now occupied by equal amounts of A and B
 cations to form A.sub.4 B.sub.4 O.sub.16. The A and B cations are arranged
 in an ordered manner; the relationship of the arrangement of these cations
 to that of the ZrO.sub.2 crystal structure is shown in FIGS. 4(a) and
 4(b). FIG. 4(a) shows the structure, without oxygen present, consisting of
 two ZrO.sub.2 unit cells, one on top of the other, A.sub.4 B.sub.4
 O.sub.16 (shown for the sake of clarity to identify A and B anions). FIG.
 4(b) shows the entire pyrochlore structure, with oxygen atoms shown as
 white circles and oxygen vacancies shown as circles with a Y inside them
 and labeled 10. The pyrochlore structure of FIG. 4(b) consists of missing
 oxygen atoms (of which there are two) in specific locations. Therefore,
 this results in a formula of A.sub.4 B.sub.4 O.sub.14 --or, actually,
 A.sub.2 B.sub.2 O.sub.7 --a common formula for a pyrochlore structure.
 This crystal structure could be maintained with more oxygen
 vacancies/excess, in combination with A and B cation excess/vacancies.
 These defects can be represented by A.sub.2-x B.sub.2+x O.sub.7-y, where x
 can range from 0.5 to -0.5 and y depends on x as follows:
 ##EQU7##
 where A.sup.n+ and B.sup.m+ are the ions in the formula A.sub.2-x B.sub.2+x
 O.sub.7-y. This non-stoichiometry can result in significant increases in
 sintering resistance. The preferred materials result when A is Sm and B is
 Zr.
 The main advantages of the pyrochlore structure over the fluorite structure
 are: (1) the atomic oxygen defects are key for a low thermal conductivity,
 since the defects result in phonon scattering during thermal conduction;
 (2) the presence of defects also results in a higher thermal expansion, a
 feature important for reducing thermal expansion mismatch between the
 substrate and the ceramic coating; (3) the similarity of the pyrochlore
 crystal structure to the fluorite crystal structure is also key for the
 growth of single crystal columns during EB-PVD growth (as the growth of
 single crystal columns is directly related to the crystal structure); (4)
 the crystal structure could also be important for the formation of the
 vertical columns due to solidification within the splat in APS coatings;
 (5) the pyrochlore structure is a stable crystal structure without
 crystallographic transformations with changes in temperature; and (6)
 sintering resistance of the pyrochlores could also be higher than that of
 YSZ (in YSZ, the oxygen defects are very mobile and can contribute to
 sintering, whereas in the pyrochlore structure, the oxygen defects are
 ordered and, hence, can be more resistant to sintering).
 These TBC coatings can be applied by APS and/or EB-PVD. This ceramic TBC
 coating can be applied as a top coat to an MCrAlY or other bond coat,
 diffusion coating; or directly to the substrate material; or to a standard
 base TBC as a top TBC coating. These ceramic thermal barrier coatings can
 be used on rotating components, such as blades, and stationary components,
 such as vanes, in gas turbine engines to maintain the underlying metallic
 components below a critical temperature. Utilization of this thermal
 barrier coating will also reduce the cooling air requirements and
 subsequently increase the engine efficiency. These materials are complete
 replacements for YSZ TBCs and, in columnar form, need not be coated with
 any other material to maintain their resistance to sintering.
 The following example is presented to help illustrate the invention, and
 should not be considered in any way limiting.
 EXAMPLE
 A sample of Sm.sub.2 Zr.sub.2 O.sub.7 powder (Sm.sub.2 O.sub.3 +2ZrO.sub.2)
 was made and analyzed to insure that the sample had a pyrochlore
 structure. A graph of the x-ray diffraction data for the sample is shown
 in FIG. 5. A first layer of fluorite crystal, 8 wt. % yttria-stabilized
 zirconia (8YSZ), was deposited, shown as 40 in FIG. 6, on top of a
 superalloy substrate 22 having a MCrAlY basecoat 24 and oxide scale 26.
 The 8YSZ had a well known columnar structure and was deposited by well
 known electron beam physical vapor deposition (EB-PVD) techniques and was
 about 225 micrometers thick. Using the top 42 of the 8YSZ to provide
 nucleation sites, a top TBC columnar layer 30 of Sm.sub.2 Zr.sub.2
 O.sub.7, about 225 micrometers thick, was also deposited by well known
 EB-PVD technologies. This provided a dual TBC system. X-ray diffraction
 data confirmed that the coating had a pyrochlore crystal structure, with a
 result similar to that shown in FIG. 5. The TBC layers both provided a
 columnar TBC with minute microcracks or gaps between the columns.
 With a limited amount of material and a goal of thermal stability at
 1400.degree. C., the sample was placed in an oven at 1400.degree. C. for
 500 hours. The Sm.sub.2 Zr.sub.2 O.sub.7 layer resisted substantial
 sintering and there was no loss of inter-columnar spaces.
 The successful Sm.sub.2 Zr.sub.2 O.sub.7 sample contained about 33 mole %
 Sm.sub.2 O.sub.3. It is expected that a range described by the values of x
 and y are all preferred within the pyrochlore structure. As a comparative
 sample, powder compacts of 8YSZ were compared to powder compacts of
 La.sub.2 Zr.sub.2 O.sub.7 after sinter aging the powder compacts at
 1400.degree. C. for 1 and 10 days. Photomicrographs showed that, after 10
 days, 8YSZ still had a significant amount of porosity, however La.sub.2
 Zr.sub.2 O.sub.7 had almost no porosity remaining. This suggests that the
 specific La.sub.2 Zr.sub.2 O.sub.7 compound cannot withstand high
 temperatures and is very likely to lose its strain tolerance. It should be
 noted that stoichiometric La.sub.2 Zr.sub.2 O.sub.7 was utilized in this
 comparison. If cation excess/vacancies or oxygen excess/vacancies were
 introduced, they could significantly change the sintering properties.
 The present invention may be embodied in other forms without departing from
 the spirit or essential attributes thereof, and accordingly, reference
 should be made to both the appended claims and the foregoing specification
 as indicating the scope of the invention.