VLT is a Company, specialized in the field of High Vacuum
and Ultra High Vacuum brazing, brazing furnaces, and in all
related services. The specific long term experience of its
engineers and technicians, combined with a flexible and dynamic
organization, allows VLT to comply with personalized client
requirements without relaxing its top level quality.
For the above reasons, in combination with our competitive
prices, VLT is a proud supplier of some among the biggest
Companies and research establishments in Italy and Europe.
Some of the VLT technologies and processes have been developed
in side the Company, which holds also some patents in the
The very large experience of VLT in the field of vacuum
joining, allows successful joining of a large variety
of metals, ceramics and composities together in simple
or complex configurations. In Tab.1 is included a list of
materials which can be joined by VLT with the vacuum brazing
technology, we should underline that some of these joints
require the development of specific techniques and cannot
be provided by all suppliers.
Of course VLT can comply with any major national or international
applicable codes and standards.
In addition, while VLT operates always in accordance with
the procedures described in a Quality Assurance Manual, it
may also provide a complete QA documentation, if required
by the Client.
VLT can reliably supply high quality products at competitive
prices in the following areas:
U.H.V. all metal furnaces
Chemical reactors CVI/CVD
Ceramic – Metal and Metal – Metal Vacuum brazed components
Systems and components for U.H.V.
Amagnetic Multipin feedthroughs as MIL-C-5015
Amagnetic miniature Multipin feedthroughs as MIL-C-26482
Thermocouples Multipin feedthroughs as MIL-C-5015
Components included in this catalogue
In each of the above areas VLT can offer feasibility studies,
design, products and technical assistance.
For any needs, bids VLT is at your disposal feasibility
ABOUT VACUUM BRAZING
Conventional brazing process and technology are one of the
oldest civilization art. However, in the last 50 years brazing
has been applied to joining of a much larger variety of different
materials and, therefore, has required the development of
more complex processes.
Several variations of brazing processes may be considered
mature technology: dip brazing, exothermic brazing, brazing
under an electric blanket, induction brazing etc.
One of the latest, most powerful and innovative way of brazing
is brazing in vacuum.
High temperature vacuum brazing has continuously developed
in the last 30 years in an impressing way and together with
the development of vacuum equipment. Therefore applications
of vacuum brazing have increased considerably and new applications
have been found for this joining technique.
High performance vacuum furnaces can be supplied by VLT.
Ask for details
A few words about vacuum brazing. This technology allows
to join and to seal metals to different metals and metals
to metallised ceramics. Basically the method is to place the
assembly in a vacuum furnace, to heat it and reach a temperature
at which the filler metal (properly placed in the assembly)
will fuse and will diffuse into the base materials by capillary
action. The filler metal (brazing alloy) will diffuse into
the cristallographic structure of the base materials thus
creating a metallurgical bond. The superior quality of this
sealing technique in respect to others methods is now very
well recognized. The most important advantage of vacuum brazing
in respect to other methods is that it allows to joint (seal)
a wide variety of metals and metal alloys and metals to metallized
alumina and ceramics as shown in Tab. 1.
The advantage in respect to brazing atmospheric pressure
is that no flux is required.
The advantage in respect to welding, is that base metals
are not locally melted, introducing metallurgical modifications
and significant residual stresses. In vacuum brazing the entire
assembly is heated up at the fusion temperature of filler
metal or alloy which is lower than the fusion temperature
of base material.
Base metal does not melt. filler metal melts and diffuse
into the base metal by capillary action
Main characteristics of vacuum brazed joint (seal):
- Strength: often the strength of the joint is higher than
that of the joined parts.
- Temperature resistance: joints may reach an operating temperature
close to that of the filler metal. that may be 1200°C - Brazed
joints are ductile and are able to withstand considerable
shock and vibration stresses High temperature vacuum brazing
in furnaces is particularly well suited for joining:
- heat - resistant nickel - and iron – base alloys that contain
aluminum and/or titanium
- reactive metals
- refractory metals A list of metals, which can be joined
by VLT high temperature vacuum brazing techniques is included
in Table 1. We should underline that some of these joints
required the development of specific techniques and cannot
be provided by all suppliers.
In the pictures below we show some characteristics of a
typical brazed joint.
Photomicrograph (20x) of a AISI 316 P joint; brazing
material is copper
Photomicrograph (250x) of a detail of fig. 3 where
diffusion zones of brazing material into base material
may be identified
Photomicrograph (1000x) of a detail where diffusion
zone may be better identified
HOW TO USE THIS CATALOGUE
This catalogue presents VLT standard Alumina-to-metal brazed
components, manufactured by advanced vacuum brazing techniques,
highly reliable and deliverable shortly after the order.
The catalogue outlines main technical characteristics of
the products in order to facilitate your choices depending
on your needs. If you have any doubts or you cannot find the
component you were looking for, please contact VLT for any
additional information and for more details on technical characteristics.
To help you in browsing the catalogue we have organized it
in separate sheets, one for each series of components. On
each sheet technical performances and main dimensions are
listed together with outline drawings and pictures of the
component, assembled on KF or CF flanges and in addition the
end views of flanges with multiple components.
Of course VLT is ready to supply special order components
for specific needs (e.g. a greater number of components, or
a combination of different components mounted on the same
We remind you that this is not the only VLT catalogue and,
if you are interested, please ask us information on metal-to-metal
brazed components and on complete vacuum brazing furnace systems.
The alfanumeric identification system
VLT has adopted an alfanumeric system for a simple and unique
identification of all standard components.
For example: FT04KF--
FT is the component type
04 is the dimension
KF is the type of vacuum flange
4 is the number of feedthroughs in a single vacuum flange
-- is a code to define the conductor material according
to the Client order.
The catalogue includes the following sections:
GENERAL TECHNICAL CHARACTERISTICS
Metal-Alumina composite structures have been widely used
in several applications, taking advantage from the good mechanical
properties and, at the same time, the outstanding electrical
resistivity of the alumina. More recently leaktight metal-ceramic
sealings have been obtained by several techniques allowing
the use of these composite structures in various laboratory
and industry applications, where in most cases they are now
Figure 1 - relative thermal expansion
A large variety of sealing techniques have been developed
since 1930 to satisfy different needs; however only in the
last years the vacuum brazing technique, after being extensively
tested, became the leading technique to produce the best quality
structures between the widest possible material combinations.
Vacuum brazing is currently used to join ceramic and different
metals, while ususally nickel-iron alloys is used, because
they have expansion coefficients close to that of alumina
(see figure 1). Alloys Trade Marks are Kovar, Vacodil,
Many difficulties have been encountered in joining alumina
with stainless steels. They are, however, particularly interesting
since they are not only mechanically stronger and more corrosion
resistant, but they are also amagnetic. The structures may
be also easily welded to adjacent parts by TIG process.
VLT standard production includes structures made of alumina
and amagnetic metals (such as AISI 304L / 316L); they are
manufactured by processes developed in the VLT research activities
and proved in a number of successful applications.
Field of application
Ceramic - metal composite structure applications are very
wide and they are generally required when a low/high intensity
current supply is needed up to high frequencies, inside a
leaktight volume in vacuum or pressurized conditions through
electrically isolated structures (even at high voltages),
in particular when these structures are loaded by mechanical
stresses and operate at high temperatures.
Examples of industries where such structures are becoming
more and more commonly used are aereonautics, electronics,
nuclear, chemical, biomedical, aereospace and research labs..
Specific application examples are vacuum tubes, mass spectrometers,
ionizations vacuometers, ceramic isolators for particle accelerators,
pace maker feedthroughs, laser heads, electrical feedthroughs
for high voltage and/or high current and/or high frequency,
semiconductors, nuclear power plants multiple electrical penetrations,
advanced nuclear reactor fuel elements, etc. But, generally
speaking, most of the applications are related to critical
conditions, where their use has no alternative, regardless
of their cost.
Today, however, common use equipment and components (as cars,
home appliances, etc.) may take advantage from the strength
and reliability of metal ceramic structures; large series
productions. available now, may reduce unity costs so that
they can be economically competitive with more conventional
You will find below some parameters describing metal - alumina
structure performances. These values shall be considered typical
of standard production materials, and may be extended for
special applications. In other parts of this catalogue some
examples of specially designed components are provided and
specific values are reported.
Operating temperature: 400 °C
Thermal gradient: Operating temperature up to 450°C
is standard, but a thermal gradient less than 30°C per minute
must be maintained to prevent the seal
Leaktightness: better than 10-10 mbar l/s
Mechanical characteristics: outstanding; in general
structural resistance is limited by that of the weaker material
and not by the brazing. Dielectric and resistivity characteristics:
alumina features better characteristics than glass in general;
they vary depending on the ceramic type and purity, but they
are kept very high even at high temperatures and in a humid
Other characteristics are related to corrosion resistance
(particularly if the metal is stainless steel), radiation
resistance (also neutrons), and the possibility of fine machining.
Additionally, if stainless steel is used, the structure is
Alumina (main alumina properties are reported in table 1).
As reported in the table only high purity alumina (97% and
99,5%) is used in VLT production. All alumina are also glazed
in order to further increase surface electrical resistivity.
Table 1 - Main alumina properties
Quoted values shall be used for reference only
b) Metals parts
As standard metal parts are in AISI 304L or 316L.
For special applications other metals can be used as: Titanium,
Nickel alloy, Copper, Molibdenum etc.
c) Brazing materials
Silver-copper, silver-copper-nickel, silver-copper-palladium,
pure gold, pure copper (see also Table 2).
Special low/high temperature melting alloys (300-1550°C)
may be additionally selected for special applications. On
customer request feasibility studies may be performed to investigate
seal joints between materials not listed above.
Table 2 - Brazing alloys
Alumina-Metal structure are assembled on standard vacuum
flange DN-KF, DN-CF and DN-ISO-K or in any type of flange
on client needs. Flanges are TIG welded as in fig. 2.
All welds are vacuum leak tested by Helium mass spectrometer
to assure a leaktightness better than 5x10-10 mbar/l/sec.
Figure 2 - Typical electrical feedthroughs with KF flange
Amagnetic Power Electrical Feedthroughs
All feedthroughs described in this section are totally amagnetic.
Standard materials include AISI 304 or copper conductors.
On request it is possible to supply other materials such as
AISI 316 or Ni-Mo.
All currents indicated in the following are applicable to
full section conductors. Conductors with diameters above 2
mm may be supplied in the tubular option with forced cooling.
The external surfaces of the ceramics are glazed to maximize
the resistance to the surface discharge.
The maximum allowable voltages are limited by the resistance
to the surface discharge on the ceramic and by the resistance
between the two conductors on the vacuum side. If the vacuum
is at least 10-5 Torr the critical path is the first, which
is related to the lenght of the path and to its cleanliness,
besides a number of other parameters. However, since the maximum
allowable voltage may be reduced by several causes, data included
in the catalogue may only be considered as orientation for
your specific application. VLT will be glad to provide you
with its expertise to help you in the choice.
The maximum voltages indicated in the following are referred
to mean values at 50 Hz RMS at 50% humidity and with an atmospheric
pressure of 760 mmHg.
The components may reach 400 °C and may operate continuously
In the catalogue the highest number of feedthroughs per
flange is 4. This number is generally adequate for standard
applications. However, on request, it is possible to supply
flanges with an higher number of feedthroughs, and even a
combination of different feedthroughs including other components
described in other sections of this catalogue.
The following feedthroughs types are described in the catalogue:
FT (General Purpose Feedthroughs)
HCF (High Current Feedthroughs)
HFT (High Voltage Feedthroughs)
IMPORTANT NOTE: In the catalogue tables the last letter
indicating the conductor material is missing. The choice is
given to the Client (if necessary, after consultation with
VLT) and the type of material shall be indicated in the Order
GENERAL PURPOSE FEEDTHROUGHS
HIGH CURRENT FEEDTHROUGHS
HIGH VOLTAGE FEEDTHROUGHS
Amagnetic Instrumentation Electrical Feedthroughs
All feedthroughs are completely amagnetic and rated for high-vacuum
and ultra-high-vacuum applications. They are also suitable
for cryogenic temperatures. They are constructed of vacuum
grade materials with high purity alumina insulation on AISI
304 stainless steel weldable mounts, flanges and threaded
Atmosphere side mating connectors are supplied as shown
in the drawings. Other connector types are available on order.
All standard feedthroughs are available in the grounded
The following types are included in the catalogue:
BNC (Bayonet Naval Connector) coaxial
MHV (Miniature High Voltage) coaxial
Examples of VLT amagnetic coaxial instrumentation feedthroughs
Amagnetic Vacuum Breaks
VLT Vacuum Breaks and Electric Isolators (CB) are designed
for applications which require high voltage insulation in
high vacuum and ultra high vacuum use.
Both sleeves are AISI 304 and therefore the entire break
is amagnetic. Sleeves of other materials (such as AISI 316
or KovarTM, Copper) may be supplied on request.
Both KF (ISO-K for larger sizes) and CF flanges may be mounted
on all breaks.
Maximum RMS voltage ratings are listed in the following
In the tables also temperature ratings are indicated
VLT amagnetic Vacuum breakers production
VLT amagnetic Vacuum breakers production
VLT supply a large choise of amagnetic viewport zero length
on standard vacuum flange DN CF, DN KF, DN ISO-K.
Sapphire standard viewport
Plane parallel window up to 50mm free view
Optical surface finish up to 50-20 scoth-dig
Crystal orientation normal to the optic axis
Orientation parallel to window surface available
on request at additional cost
Antireflection coating available on request at additional
cost. The uniformity of AR coating will be not over
the 80% of the window clear aperture.
Viewport with other optical materials available on request.
Special executions on request
Feedthroughs for glow-discharge
VLT presents a model of feedthrough for glow-discharge.
The model features the same characteristics than other feedthroughs
presented in Section A.
The feedthrough is supplied complete with nut and O-ring.
VLT present a large selection of high and ultra-high-vacuum
cladded thermocouples , suitable for temperature measurements
in closed environment, where a perfect leaktightness is required.
The principal feature of the VLT production is that the thermocouple
are supplied already assembled with their flanges and/or feedthroughs.
In this way the thermocouples are not only very compact, but
they are also immediately operable.
Thermocouples are available in 2 standard configurations:
- high vacuum brazed on all types of flanges and transitions
(KF, CF, VCR, etc.)
- high vacuum brazed on electrical feedthroughs and then
joined with flanges (KF, CF, VCR, etc.) by vacuum brazing
or TIG welding.
In the second version the thermocouple cladding is ground
isolated. The thermocouple, therefore, can be utilized directly
on objects under high voltage.
Technical characteristics of standard thermocouple
Type K (Chromel Alumel) temperature range -200 +1200°C
Type J (Iron – Costantan) temperature range 0 +750°C
Type T (Copper – Costantan) temperature range -200 +350°C
Inconel cladding 1,5 mm diam.
Mg O2 99,4% Insulating material
Thermocouple length on request
Miniature standard connectors (200°C)
Miniature ceramic connector (650°C)
VLT can supply multi-type feedthroughs, including both thermocouple
and power feedthroughs
Thermocouple identification system
A simple alphanumeric system is adopted to identify thermocouples
TCK KF 04—where:
TCK is for thermocouple type K
KF is the vacuum flange type
04 is the number of T.C. in one vacuum flange
-- is the thermocouple length
This section is a presentation of a few special components
designed and manufactured on client needs. Components may
be manufactured either with Alumina to Metal or Metal to Metal